motor protection requiriments EMERSON EDUARDO RODRIGUES

Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s
employees. Any material contained in this document which is not already
in the public domain may not be copied, reproduced, sold, given, or
disclosed to third parties, or otherwise used in whole, or in part, without
the written permission of the Vice President, Engineering Services, Saudi
Aramco.
Chapter : Electrical For additional information on this subject, contact
File Reference: EEX21607 W.A. Roussel on 874-1320
Engineering Encyclopedia
Saudi Aramco DeskTop Standards
Motor Protection Requirements

Engineering Encyclopedia Electrical
CONTENTS PAGE
TYPICAL FACTORS THAT ARE SPECIFIED ON A MOTOR
NAMEPLATE .......................................................................................................1
Rated Volts.................................................................................................2
Full-Load Amperes.....................................................................................2
Service Factor (S.F.)...................................................................................3
Horsepower ................................................................................................3
Temperature Factors...................................................................................4
Temperature Rise............................................................................4
Insulation Class and Ambient Temperature....................................4
Time (Duty)................................................................................................5
Locked-Rotor Codes...................................................................................5
Miscellaneous Information.........................................................................6
Maker’s Name.................................................................................6
Frequency and Number of Phases ..................................................6
Speed ..............................................................................................6
ANSI/IEEE DEVICES AND FUNCTION NUMBERS THAT RELATE
TO AC INDUCTION MOTOR PROTECTION....................................................7
Purpose.......................................................................................................7
Standard Device Function Numbers...........................................................7
Device 2RS .....................................................................................7
Device 27........................................................................................7
Device 46........................................................................................9
Device 47........................................................................................9
Device 49........................................................................................9
Devices 50/50G/50GS ..................................................................10
Device 51LR.................................................................................10
Device 86M ..................................................................................10
Device 87M ..................................................................................10

T/C CHARACTERISTIC CURVES OF AC INDUCTION MOTORS ...............11
Thermal Capability Curve ........................................................................11
Stall Time Vs Locked Rotor Current ............................................11
Motor Starting Curve................................................................................13
Locked-Rotor Current...................................................................13
Starting Time ................................................................................13
Full-Load Current .........................................................................13
THERMAL PROTECTION FUNDAMENTALS OF AC INDUCTION
MOTORS.............................................................................................................15
Thermal Overload Protection ...................................................................15
Replica-Type Relays.....................................................................15
Resistance Temperature Detectors (RTDs)...................................17
Protection Versus Stall Time ........................................................18
Thermal Locked-Rotor Protection............................................................18
Induction Disc Relays...................................................................19
Protection Versus Stall Time ........................................................22
Combined Protection................................................................................23
Underprotection - Device 49.........................................................23
Overprotection - Device 51...........................................................23
FUNDAMENTALS OF FAULT PROTECTION FOR LOW AND
MEDIUM VOLTAGE AC INDUCTION MOTORS...........................................24
Introduction ..............................................................................................24
Phase Faults..............................................................................................24
Current Limiting Fuses .................................................................25
Circuit Breakers ............................................................................26
Ground Faults...........................................................................................31
Residual Connection.....................................................................31
Zero Sequence Connection ...........................................................32
OTHER TYPES OF MOTOR PROTECTION FUNDAMENTALS FOR
AC INDUCTION MOTORS................................................................................36

Undervoltage Protection...........................................................................36
Purpose and Thermal Effects........................................................36
Time-Delay Relays - Device 27....................................................37
Coordination .................................................................................37
Phase Unbalance Protection .....................................................................39
Purpose and Thermal Effects........................................................39
Voltage Unbalance Relays - Device 47 ........................................39
Current Unbalance Relays - Device 46.........................................40
Voltage Unbalance (Low Voltage Motors)...................................46
Miscellaneous Protection..........................................................................47
High Speed Reclosing...................................................................47
Repetitive Starting - Device 2RS..................................................47
Protection Scheme One-Line Diagrams ...................................................48
Low Voltage Motors.....................................................................48
Medium Voltage Motors...............................................................51
SOLID-STATE MOTOR PROTECTION PACKAGE (MPP)
FEATURES .........................................................................................................54
General Description..................................................................................54
Features and Capabilities ..............................................................54
Benefits.........................................................................................54
Multilin MMR 269 Plus ...........................................................................55
Single-Line Drawing.....................................................................56
Protection Features .......................................................................58
Communication Features ..............................................................59
Diagnostic Features.......................................................................60
Other Features...............................................................................62
Westinghouse IQ-1000II ..........................................................................63
Block Diagram..............................................................................63
Protection Features .......................................................................65
Communication Features ..............................................................67

Diagnostic Features.......................................................................68
Other Features...............................................................................73
GLOSSARY ........................................................................................................74
LIST OF FIGURES
Figure 1. Typical Ac Motor Nameplate ................................................................1
Figure 2. Ac Motor Voltages ................................................................................2
Figure 3. Nema Temperature Ratings ...................................................................4
Figure 4. Locked-Rotor Kva Codes ......................................................................5
Figure 5. Ac Motor Protection One-Line Diagram...............................................8
Figure 6. Motor Curves.......................................................................................12
Figure 7. Motor Starting Current ........................................................................14
Figure 8. Bl-1 T/C Curves...................................................................................16
Figure 9. Dt-3 Relay ...........................................................................................17
Figure 10. O/L Relay Protection.........................................................................18
Figure 11. Starting Time Ts < 20 Seconds..........................................................19
Figure 12. Starting Time 20 < Ts < 70 Seconds .................................................20
Figure 13. Starting Time Ts > Tlr .......................................................................21
Figure 14. L/R Relay Protection .........................................................................22
Figure 15. Combined Protection.........................................................................23
Figure 16. Current Limiting Fuses (R-Rated) .....................................................25
Figure 17. Fuse Protection..................................................................................26
Figure 18. Mcp Protection ..................................................................................27
Figure 19. Phase Faults: Device 50....................................................................28
Figure 20. Partial Differential Protection.............................................................29
Figure 21. Full Differential Protection................................................................30
Figure 22. Residual Connection..........................................................................31
Figure 23. Zero Sequence Feeder Breaker..........................................................32
Figure 24. Three-Wire Circuit.............................................................................33
Figure 25. Four-Wire Circuit ..............................................................................33

Figure 26. Zero-Sequence Connection................................................................34
Figure 27. Ground Fault Protection - Mv System...............................................35
Figure 28. Effects Of Voltage Variation .............................................................36
Figure 29. Time Curves - Undervoltage Relay ...................................................38
Figure 30. Cvq Relay..........................................................................................39
Figure 31. Cm Relay...........................................................................................40
Figure 32. Cm Relay Operating Characteristics..................................................41
Figure 33. Primary Open (Three-Line Diagram) ................................................42
Figure 34. Phasor Diagram (Primary Open) .......................................................43
Figure 35. Secondary Open (Three-Line Diagram) ............................................44
Figure 36. Phasor Diagrams (Secondary Open)..................................................45
Figure 37. Voltage Unbalance Derating Factors.................................................46
Figure 38. Protection: 0.75 Kw (1.0 Hp) Or Less...............................................48
Figure 39. Protection: Greater Than 0.75 Kw To 75 Kw (1.0 To 100
Hp) .......................................................................................................49
Figure 40. Protection: Greater Than 75 Kw (100 Hp) .......................................50
Figure 41. Protection: Class E2 Controllers (<1125 Kw) ..................................51
Figure 42. Power Circuit Breaker (<7500 Kw)...................................................52
Figure 43. Protection: Power Circuit Breaker (>7500 Kw) ...............................53
Figure 44. Multilin 269 Plus: Faceplate.............................................................55
Figure 45. Multilin 269 Plus: Legend ................................................................56
Figure 46. Multilin 269 Plus: Single-Line Drawing...........................................57
Figure 47. Multilin 269 Plus: Protection Features .............................................58
Figure 48. Iq-1000ii: Faceplate..........................................................................63
Figure 49. Iq-1000ii: Block Diagram.................................................................64
Figure 50a. Iq-1000ii: Protection Features.........................................................65
Figure 50b. Iq-1000ii: Protection Features (Cont’d)..........................................66
Figure 51a. Iq-1000ii: Monitor Data..................................................................68
Figure 51b. Iq-1000ii: Monitor Data (Cont’d)...................................................68
Figure 51c. Iq-1000ii: Monitor Data (Cont’d) ...................................................69
Figure 51d. Iq-1000ii: Monitor Data (Cont’d)...................................................69

Figure 52. Iq-1000ii: Alarm Conditions.............................................................70
Figure 53a. Iq-1000ii: Trip Conditions ..............................................................71
Figure 53b. Iq-1000ii: Trip Conditions (Cont’d) ...............................................72
Figure 54. Iq-1000ii: Internal Diagnostics.........................................................73

Saudi Aramco DeskTop Standards 1
TYPICAL FACTORS THAT ARE SPECIFIED ON A MOTOR NAMEPLATE
NEC Article 430-7 states a motor nameplate must be marked with the following information:
• volts, full-load amperes, service factor, horsepower
• temperature factors, time (duty), locked-rotor codes
• maker’s name, frequency, number of phases, speed
Figure 1 is an example nameplate that contains the NEC minimum required nameplate
information; 16-SAMSS-503 also requires the nameplate to contain additional information
pertaining to insulation class, winding temperature rise, type of bearings, rotor Wk
2
, types of
enclosure, etcetera.
Figure 1. Typical AC Motor Nameplate

Rated Volts
The voltage marked on the motor nameplate is the rated motor terminal voltage per NEMA
MG-1. The nominal three-phase system voltage that matches the rated three-phase voltage is
listed in Figure 2.
Figure 2. AC Motor Voltages
Full-Load Amperes
The full-load amperes marked on the motor nameplate are based on the rated voltage,
horsepower, and frequency. Overload protection, as specified by NEC Art 430-32, is based
on the marked full-load amperes.

Service Factor (S.F.)
When the voltage and frequency are maintained as per the nameplate markings, the motor
may be overloaded up to the hp obtained by multiplying the rated hp by the service factor
shown on the nameplate. When the motor is operated at the higher service factor, efficiency,
power factor, and speed may be different than at rated load, but locked rotor torque and
current, and breakdown torque remain unchanged. For example, a 100 hp, 1.15 S.F. motor
may be safely loaded to 115 hp.
Horsepower
Horsepower is the rated output mechanical power that may be applied to the motor shaft. IEC
motors are rated on output kW vice output hp, where 1hp equals 0.746 kW. For example, a
NEMA MG-1 rated 1000 hp motor is equivalent to a nominal 750 kW (746 kW actual) IEC
rated motor.

Temperature Factors
The temperature rise or the insulation class and ambient temperature must be marked on the
motor (See Figure 3). Note: Saudi Aramco motor specifications (17-SAMSS-502 and 503)
require Class F insulation. However, for fractional horsepower motors, SAES-P-113 permits
a minimum Class B insulation.
Figure 3. NEMA Temperature Ratings
Temperature Rise
The temperature rise shown in the above Figure is based on motor operation at altitudes of
1000 meters (3300 ft) or less, ambient temperatures of 40
o
C, and rated horsepower for 1.0
S.F. motors or 1.15 times rated horsepower for 1.15 S.F. motors.
Insulation Class and Ambient Temperature
The insulation class as shown above (Figure 3) is based on a 40
o
C ambient, but if the motor is
operated at higher ambients, the motor temperature rise must be calculated in accordance with
NEMA MG 1-12.43.

Time (Duty)
The time ratings for motors, per NEMA MG1-10.36 are 5, 15, 30 and 60 minutes, and
continuous. Note: Saudi Aramco specifications call for continuous duty motors only.
Locked-Rotor Codes
Both NEMA MG 1-10.37 and the NEC require the locked-rotor indicating code letters to be
marked on the motor nameplate. The letter designations are based on full voltage and rated
frequency (See Figure 4).
Figure 4. Locked-Rotor kVA Codes

Miscellaneous Information
Maker’s Name
The NEC requires the motor manufacturer’s name to be marked on the nameplate. Most
manufacturers also include additional markings such as serial numbers, model numbers,
bearing numbers, etcetera.
Frequency and Number of Phases
The motor frequency (50 or 60 hertz) as well as the number of phases (1 or 3) are required
markings on the motor nameplate. Virtually all other ratings are based on loadings at rated
frequency. All AC motors are required by NEMA MG1-12.44 to operate successfully under
running conditions at rated load and voltage and at plus or minus 5 percent frequency.
Speed
NEMA MG1-10 lists the synchronous speed of motors (Nrpm = 120f/p), whereas the
nameplate speed for induction rotors includes slip (rotor speed).

ANSI/IEEE DEVICES AND FUNCTION NUMBERS THAT RELATE TO AC
INDUCTION MOTOR PROTECTION
Purpose
The devices in switching equipment are referred to by numbers, with appropriate suffix letters
when necessary, according to the functions they perform. Exercise caution when interpreting
the letter suffix: There are often dual meanings. For example, the suffix G can mean ground
(50G) or generator (87G).
The numbers are based on a system, and adopted as standard for automatic switchgear by
ANSI/IEEE Std.C37.2. The system is used in connection diagrams, one-line diagrams,
instruction books, and in specifications. Figure 5 is a one-line diagram showing application
of standard ANSI/IEEE device numbers.
Standard Device Function Numbers
Device 2RS
A time-delay starting, or closing relay is a device that functions to give a desired amount of
time delay before or after any point of operation in a switching sequence or protective relay
system, except as specifically provided by device functions 48, 62, and 79. Saudi Aramco
uses this device to block repetitive starting (RS) of large motors rated at 3750 kW(5000 hp) or
larger.
Device 27
An undervoltage relay is a device that functions on a given value of undervoltage. Saudi
Aramco uses this device to detect undervoltage on a motor bus or individual motors.

Figure 5. AC Motor Protection One-Line Diagram

Device 46
A reverse-phase, or phase-balance current relay is a relay that functions when the polyphase
currents are of reverse-phase sequence, when they are unbalanced, or when they contain
negative phase-sequence components above a given amount. This relay is primarily used to
protect motors against single-phasing (primary or secondary opens).
Device 47
A phase-sequence voltage relay is a relay that functions on a predetermined value of
polyphase voltage in the desired phase sequence. This relay, in conjunction with a Device 27
relay, is used to detect undervoltage, reverse phasing, and single- phasing of a motor.
Device 49
A machine, or transformer thermal relay, is a relay that functions when the temperature of a
particular element exceeds a predetermined value. These elements consist of a machine
armature, or other load-carrying winding or element of a machine, or a power rectifier or
power transformer (including a power rectifier transformer). Thermal relays are used to
overload protect all Saudi Aramco motors; however larger motors require more sophisticated
(capable) Device 49 relays. For example, a small 7.5 kW (10 hp) motor may be protected by
a simple solder-pot overload device, whereas a large 3750 kW (5000 hp) motor would require
use of a much more sophisticated ABB type BL-1 thermal overload relay.

Devices 50/50G/50GS
An instantaneous overcurrent, or rate-of-rise relay, is a relay that functions instantaneously on
an excessive value of current or on an excessive rate of current rise, thus indicating a fault in
the apparatus of the circuit being protected. Saudi Aramco uses this device for both phase
and fault protection of motors. The suffix G is the abbreviation for ground and GS is the
abbreviation for ground sensor. Module EEX216.04 will describe the different applications
of Device 50.
Device 51LR
An AC time overcurrent relay is a relay with either a definite or inverse time characteristic
that functions when the current in an AC circuit exceeds a predetermined value. Saudi
Aramco uses this device to provide thermal locked-rotor (LR) protection for medium voltage
motors.
Device 86M
A locking-out relay is an electrically-operated hand or electrically reset, relay that functions to
shut down and hold a piece of equipment out of service on the occurrence of abnormal
conditions. Saudi Aramco uses this device to lock out large motors (M) after occurrence of a
fault. This device is activated by Device 87. Note: Device 86 requires manual reset.
Device 87M
A differential protective relay is a protective relay that functions on a percentage, phase angle,
or other quantitative difference of two currents or other electrical quantities. Saudi Aramco
uses this device for fault protection of motors (M) rated greater than 4 kV.

T/C CHARACTERISTIC CURVES OF AC INDUCTION MOTORS
Thermal Capability Curve
Heating characteristics of motors are very difficult to obtain and vary considerably with motor
size and design. These heating characteristics are modeled as curves, and are an approximate
average of an imprecise thermal zone, where varying degrees of damage or shortened
insulation life may occur.
Figure 6 shows a typical motor capability curve, which is the motor designer’s estimate of the
amount of load current that may flow in the motor without exceeding permissible
temperatures.
Stall Time Vs Locked Rotor Current
Cold Start - The locked-rotor time (tLR) shown in Figure 6 depicts the time (capability) of the
motor (current versus time), which is based on starting the motor cold (the motor windings,
rotor, etcetera are at ambient temperature).
Hot Start - If the motor’s duty cycle permits hot starts - the motor windings, etcetera are at an
elevated temperature, the manufacturer must be consulted to determine a permissible starting
time (ts) to prevent motor damage.

Figure 6. Motor Curves

Motor Starting Curve
Locked-Rotor Current
The starting current is represented by the curve (solid line) as previously described in Figure 6
and the current (labeled LRAa and LRAs) shown in
Figure 7.
Asymmetrical (DC Transient) -The asymmetrical starting current (LRAa) exceeds the
symmetrical locked rotor current (LRAs) during the first few cycles because of the transient
direct current. This transient current appears, as it does under fault conditions, because the
series reactance (inductance) prevents an instantaneous change in the magnitude of the
alternating current. The magnitude of the asymmetrical starting current is approximately 1.5
LRAs for low voltage motors and 1.6 LRAs for medium voltage motors.
Symmetrical - After the transient current decays, the starting current hovers near the
symmetrical starting current (LRAs). The magnitude of this starting current is typically 4 to 6
times the motor’s full-load amperes (FLA). The exact amount is based on the subtransient
reactance (X”d) of the motor, which ranges from 16.7 to 25 percent.
Starting Time
The starting time (ts) of the motor is the approximate time it takes the motor to approach rated
running speed. For purposes of this course, it is assumed that the starting time (ts) is less than
the locked-rotor (stall) time (tLR).
Full-Load Current
After the motor reaches rated speed, it acquires its normal rated value (full-load amperes),
assuming rated load, voltage and frequency.

Figure 7. Motor Starting Current

THERMAL PROTECTION FUNDAMENTALS OF AC INDUCTION MOTORS
Thermal Overload Protection
Overload (O/L) protection is always applied to motors to protect them from overheating. NEC
Article 430-38 requires an O/L device in each phase except “where protected by other
means.” This requirement (one-per-phase) is necessary because single phasing of the primary
in a delta-wye configuration results in a 2:1:1 three-phase motor current relationship. This
protection is provided by replica-type relays for small kW-rated motors and by resistance
temperature detectors (RTDs) for larger motors.
Replica-Type Relays
Replica-type relays operate directly from motor circuit current. They receive their name
“replica” because they tend to “replicate” the heating characteristics of the motor. For very
small motors, this type of relay is simply a bimetallic element that operates within a heater
unit. For large kW-rated motors, they are truly a type of overcurrent relay. For instance,
Saudi Aramco specifies an ABB-type BL-1 O/L relay for motors rated 4 kV and larger and
less than 7500 kW. Figure 8 is a T/C characteristic curve of a BL-1 relay.

This relay is a temperature-compensated relay, which means it has different T/C curves
depending on the motors’s temperature. For instance, if the motor is at room ambient (just
turned on) when the O/L occurs, the relay responds to the 0 percent curve. If the motor has
been running continuously, the relay would respond to the 100 percent curve. Because
replica-type relays only respond to current, they will not typically protect for blocked
ventilation.
Figure 8. BL-1 T/C Curves

Resistance Temperature Detectors (RTDs)
RTD-type relays operate from exploring coils embedded by the manufacturer directly in the
motor windings. They are commonly used in industrial applications in motors rated above
1125 kW (1500 hp). Note: Saudi Aramco (17-SAMSS-502) requires RTD applications in
motors rated above 150 kW (200 hp). RTDs respond to temperature alone, and they will
protect against blocked ventilation. Figure 9 is an ABB DT-3 type relay used to detect
overtemperature (overloads) with RTDs in a large motor.
Figure 9. DT-3 Relay

Protection Versus Stall
Time
Thermal-type relays
offer very good
protection for light
overloads as shown in
Figure 10, but provide
inadequate protection
(shaded area) for heavy
overloads or during
starting.
Figure 10. O/L Relay
Protection
Thermal Locked-Rotor
Protection
Thermal locked-rotor
(L/R) protection, similar
to O/L protection,
involves the matching of a relay to the motor’s thermal capability curve, and at the same time
remembering that the capability curve is at best an approximation.
A motor with a locked-rotor condition is particularly vulnerable to damage because of the
large amount of heat generated (I
2
R). Also, remember that a motor at standstill cannot
dissipate the heat as well as a rotating motor.

Induction Disc
Relays
The type of L/R
protection
depends on
comparison of the
starting time (ts)
of the motor to its
permissible
locked-rotor time
(tLR).
If the starting time
(ts) is less than or
equal to 20
seconds, and less
than L/R time
(tLR), it is best to
use an extremely
inverse relay
similar to types
ABB CO-11 or
GE IFC 77 (see
Figure 11).
Figure 11.
Starting Time ts
< 20 Seconds

If ts is between 20 and 70 seconds and less than tLR, it is best to use a relatively flat relay
similar to types ABB CO-5, CO-6 or GE IFC-95 (see Figure 12).
Figure 12. Starting Time 20 < ts < 70 Seconds

If ts is greater than tLR (see Figure 13), a mechanical zero-speed switch may be used. This
device supervises an overcurrent unit (Device 51) and prevents its operating a timer when
rotation is detected. Note: This scheme will not detect a failure to accelerate to full speed nor
pullout with continued rotation.
Figure 13. Starting Time ts > tLR

Protection Versus Stall Time
The overcurrent relay offers excellent protection for heavy overloads as shown in Figure 14,
but overprotects (shaded area) for light overloads.
Figure 14. L/R Relay Protection

Combined Protection
The best (recommended) thermal protection for large motors is to combine both O/L and L/R
protection as shown in Figure 15.
Figure 15. Combined Protection
Underprotection - Device 49
A typical scheme is to provide two overload protective devices (i.e. BL-1 relays) in phases A
and C, which underprotects (thermally) for heavy overloads (i.e. locked-rotor conditions), but
adequately protects for light overloads.
Overprotection - Device 51
To complement Device 49 thermal protection, one locked-rotor device (i.e. CO relay) is
applied to phase B, which overprotects for light overloads, but adequately protects for heavy
(i.e. locked-rotor conditions).

FUNDAMENTALS OF FAULT PROTECTION FOR LOW AND MEDIUM
VOLTAGE AC INDUCTION MOTORS
Introduction
As with thermal protection, the size of the motor and the type of service will influence the
type of fault protection required to protect the motor.
Although NEC Article 430-52 and Table 430-152 dictate phase and ground fault protection
for low voltage motor circuits, the type of protective device is a designer’s choice. There are
five types, each having different benefits depending on the size of the motor, cost of
protection, etcetera. The five types used for low voltage motor protection are:
• non-time delay fuse (non-current limiting).
• time delay fuses (current limiting).
• inverse time circuit breaker.
• magnetic only circuit breaker.
• motor circuit protector (MCP).
Medium voltage motors, typically large and expensive, are fault protected by NEMA Type R
current limiting fuses or differential relays. Ground fault protection can be provided by a
residual scheme, but zero sequence protection is the preferred scheme.
Phase Faults
Although the NEC permits current limiting fuses for low voltage motor phase fault protection,
Saudi Aramco SAES-R-114 specifies magnetic-only molded case circuit breakers or MCPs
for protection of motors rated below 75 kW (100 hp), and devices 50 or 87 for all other
motors (low and medium voltage).

Current Limiting Fuses
R-rated (NEMA Type R) current limiting fuses are used in NEMA Class E2 controllers to
provide short circuit fault protection up to 350,000 kVA. Note: Class E2 controllers will be
discussed in detail in EEX216.05. Figure 16 lists the continuous current ratings of NEMA
Type R fuses, while Figure 17 shows a typical T/C coordination scheme for protecting a
medium voltage motor. Note: SAES-P-114 permits Class E2 controllers with current limiting
fuses for motors rated 4.0 kV, 1125 kW (1500 hp) or smaller sized motors.
Figure 16. Current Limiting Fuses (R-Rated)

Figure 17. Fuse Protection
Circuit Breakers
SAES-P-114 requires: a) magnetic-only or MCP fault protection for low voltage motors rated
less than or equal to 75 kW (100 hp); b) low voltage power circuit breakers (LVPCBs) for low
voltage motors rated above 75 kW; and c) medium voltage power circuit breakers for motors
rated greater than 1125 kW (1500 hp).

Low Voltage Motors -The T/C characteristics of an MCP (or magnetic-only molded case
circuit breaker) are shown in Figure 18.
Figure 18. MCP Protection

Medium Voltage Motors - Phase fault protection for medium voltage motors rated above
1125 kW (1500 hp) is provided by a power circuit breaker controlled by relays.
• Instantaneous trip units (device 50) are recommended where the ratio I3φ/LRAs
is greater than 5 and the kVA rating of the motor is less than 50 percent of the
kVA rating of the transformer (see Figure 19).
Figure 19. Phase Faults: Device 50

• Partial differential (Device 87M) is the preferred phase fault protection for
large motors, and is recommended when I3φ is approximately equal to LRAs,
which varies from 4-6 times the full load, three-phase current. The advantage
of this scheme is that it has excellent sensitivity, the starting currents cancel,
and only three current transformers (CT) are required. The biggest problem
with this protection scheme is a “physical limitation” based on the CT size (see
Figure 20). Note: Saudi Aramco specifies differential protection (87M) only for
medium voltage motors.
Figure 20. Partial Differential Protection

• Full differential (Device 87M) is recommended whenever I3φ is
approximately equal to LRAs, which varies from 4-6 times the full load three-
phase current, and a partial differential scheme does not work. The only
advantage of a full differential scheme over the partial differential scheme is
that it offers cable protection. Obvious disadvantages are that six CTs are
required, and the scheme is often oversensitive (nuisance trips) to high starting
currents because of unequal CT saturation. (See Figure 21). Note: Saudi
Aramco specifies differential protection (87M) only for medium voltage motors.
Figure 21. Full Differential Protection

Ground Faults
SAES-P-114 requires ground fault protection for all motors rated 22.5 kW (30 hp) and larger.
Residual protection is permitted only on induction motors rated above 7500 kW (10,000 hp),
where high cable charging currents would cause false operation of zero sequence (50GS)
protection.
Residual Connection
The residual connection is not very sensitive because it “sees” current through the “eyes” of
the phase CTs. This connection often causes nuisance trips as well because of the unequal
saturation of the three CTs (see Figure 22).
Figure 22. Residual Connection

Zero Sequence Connection
The zero sequence connection (Device 50GS) is the preferred ground fault protection scheme.
Low voltage motors use a static trip (solid-state) device to trip the breaker, whereas a relay
(ABB Type SC or GE Type PJC) trips the breaker via a lockout relay (Device 86M) for
medium voltage motors. Note: Saudi Aramco typically specifies zero sequence CTs for
ground fault protection.
Low Voltage Motors - Figure 23 shows the zero sequence connection for protecting a low
voltage motor. Figures 24 and 25 show alternate connection schemes with the zero sequence
CT connection being the preferred Saudi Aramco connection.
Figure 23. Zero Sequence Feeder Breaker

Figure 24. Three-Wire Circuit
Figure 25. Four-Wire Circuit

Medium Voltage Motors - Figure 26 shows the ground fault protection scheme for medium
voltage motors. The primary advantages of this type of system are increased sensitivity (no
current flows under normal conditions), which eliminates false tripping during motor starting
and the lowest CT cost (only one required). The primary disadvantage is CT saturation,
especially when induction disc (Device 51) relays and/or solidly-grounded systems are used.
Figure 26. Zero-Sequence Connection

Figure 27 is a typical one-line diagram and accompanying coordination scheme using zero
sequence ground fault protection schemes.
Figure 27. Ground Fault Protection - MV System

OTHER TYPES OF MOTOR PROTECTION FUNDAMENTALS FOR AC
INDUCTION MOTORS
Undervoltage Protection
A low voltage condition will prevent motors from reaching their rated speed on starting, or
cause them to lose speed and draw heavy overload current. While overload relays (Device
49) will eventually detect this condition, the motor should be quickly disconnected when
severe low voltage conditions exist. Where continuous operation is essential, such as station
auxiliary service or continuous manufacturing processes, an undervoltage relay is used for
alarm purposes only.
Purpose and Thermal Effects
The primary purpose of undervoltage relay protection (Device 27) in Saudi Aramco
applications is as a backup device for locked rotor protection (Device 51). Device 51 is
applied to phase B, while Device 27 is applied to phases A and C. Because power (I
2
R) is
directly proportional to the current squared and any decrease in voltage (see Figure 28) results
in an increase in current, Device 27 will eventually remove the motor if Device 51 fails,
although some damage may occur as a result of the increased temperature (approximately 17
percent for just 10 percent low voltage).
Figure 28. Effects of Voltage Variation

Time-Delay Relays - Device 27
Time-delay voltage relays, similar to time delay overcurrent relays (Device 51), use induction
disc relays for their time-undervoltage characteristics (see Figure 29).
Coordination
Device 27 relays must be coordinated with upstream fault relays to prevent tripping the motor
for any upstream faults that cause voltage dips on the system. Additionally, caution must be
exercised to ensure the relay does not trip due to voltage sags as a result of the motor, or
adjacent large motors starting on the same bus.

Figure 29. Time Curves - Undervoltage Relay

Phase Unbalance Protection
Purpose and Thermal Effects
The purpose of phase unbalance protection is to prevent motor overheating damage. Motor
overheating occurs because increased phase currents flow in order that the motor can continue
to deliver the same kW (hp) as it did with balanced voltages. Negative-sequence voltages
also appear and cause abnormal currents to flow in the rotor. Because a motor’s negative
sequence impedance (Z2) approximates a motor’s locked rotor impedance, a small negative
sequence voltage produces a much larger negative sequence current.
Voltage Unbalance Relays - Device 47
SAES-P-114 recommends use of an ABB Type CVQ relay (see Figure 30) for voltage
unbalance protection. This relay protects against system undervoltage (a Device 27 function),
single-phasing of the supply, and reversal of phase rotation of the supply (100 percent
negative sequence). No settings are required for the CVQ relay.
Figure 30. CVQ Relay

Phase Reversal protection is primarily protection for the process instead of protection for the
motor. Imagine reversing the phases for a pump. The motor begins “sucking” the fluid
instead of pumping the fluid.
Current Unbalance Relays - Device 46
SAES-P-114 recommends use of an ABB Type CM current unbalance relay for motors rated
above 1125 kW (1500 hp). This relay is used to detect phase unbalance or open phase. It
consists of two mechanically independent disc units. Phase A and B currents energize the
upper electromagnets, while phase B and C currents energize the lower electromagnets.
When phase currents are balanced, the electromagnets create equal and opposing torques on
each of the discs (see Figure 31).
• The relay contacts are electrically common and connected in parallel. Closing
of any one contact on either the upper or lower disc completes the trip circuit.
• Because the CM relay is calibrated for one ampere sensitivity and is set to
operate on an unbalance, no setting of this relay is required.
• Note: If this relay is applied on a multi-motor bus, an unbalance on any motor
could trip the entire bus. The best recommendation is to apply one CM relay
per motor.
Figure 31. CM Relay

Figure 32 describes the CM relay’s operating characteristics.
Figure 32. CM Relay Operating Characteristics

Single-Phasing is caused by the opening of either a primary or secondary conductor feeding a
motor. Figures 33 through 36 describe the three-wire and phasor diagrams for these
conditions.
Figure 33. Primary Open (Three-Line Diagram)
Primary open phasor diagrams and equations:
• = + = ∠ °+ ∠ °=
• = + = ∠ °+ ∠ ° = ∠ °
• = + = ∠ °+ ∠ °= ∠ ° = −
I I I
I I I p u
I I I p u I
A A A
B B B
C C C B
1 2
1 2
1 2
1 120 1 300 0
1 0 1 60 3 30
1 240 1 180 3 210
. .
. .
• = + = ∠ °+ ∠ °= ∠ °
• = + = ∠ °+ ∠ ° = ∠ ° =
• = + = ∠ °+ ∠ ° = ∠ °
I I I p u
I I I p u I
I I I p u
a a a
b b b a
c c c
1 2
1 2
1 2
1 90 1 330 1 30
1 330 1 90 1 30
1 240 1 240 2 240
. .
. .
. .

Figure 34. Phasor Diagram (Primary Open)

Figure 35. Secondary Open (Three-Line Diagram)
Secondary open phasor diagrams and equations:
• ∠ ° ∠ ° ∠ °
• ∠ °+ ∠ ° ∠ °
• ∠ ° ∠ ° ∠ °
• ∠ ° ∠ °
•
I = I + I = 1 120 + 1 240 = 1 180 = -1.0 p.u.
I = I + I = 1 0 1 0 = 2 0 = 2.0 p.u.
I = I + I = 1 240 + 1 120 = 1 180 = -1.0 p.u. = I
I = I + I = 1 90 + 1 270 = 0
I
A A1 A 2
B B1 B2
C C1 C2 A
a a1 a 2
b = I + I = 1 330 + 1 30 = 3 0 = 3 p.u.
I = I + I = 1 210 + 1 150 = 3 180 - 3 p.u. = -I
b1 b 2
c c 1 c2 b
∠ ° ∠ ° ∠ °
• ∠ ° ∠ ° ∠ °

Figure 36. Phasor Diagrams (Secondary Open)

Voltage Unbalance (Low Voltage Motors)
Protection of low voltage motors using voltage unbalance relays is usually not cost effective.
As previously discussed, increased heating occurs as a result of the voltage unbalance, and the
only other practical means to reduce the thermal effects is to reduce the shaft kW (hp) loading
in accordance with the following formula and Figure 37.
Percent NEMA unbalance =
Max deviation from average voltage
average voltage
Figure 37. Voltage Unbalance Derating Factors

EXAMPLE A: Given the following data, what is the maximum safe connected shaft kW
(hp) to avoid thermal overheating of the motor?
Motor Ratings: 150 kW (200 hp), 3-phase, 460V
Voltages: Vab = 449V, Vbc = 459V, Vca = 421V
ANSWER Vavg = (449 + 459 + 421)/3 = 1329/3 = 443V
Maximum voltage deviation from average = 443 - 421 = 22V
Percent NEMA unbalance = (22/443) X 100 = 4.97%
Per Figure 37, the motor should be derated approximately 75% to 112.5
kW (150 hp) for a 5% voltage unbalance. Note: Derating the motor is
not the preferred method to avoid overheating. The preferred method is
to correct the causes of the voltage unbalance. For example, removing
single-phase loads from the motor bus, balancing the single-phase loads
on the bus, etcetera.
Miscellaneous Protection
High Speed Reclosing
If a motor is reenergized before it has stopped rotating, high transient torques can develop (T
α V
2
), and possible damage (e.g. broken shafts) can occur. The most probable cause of
reenergization is utility high speed reclosing (10-36 cycles) after a fault. The simplest
protection schemes are a timing relay that allows the motor to coast to a stop before restarting,
or delaying restart using an undervoltage permissive relay in the starting control circuit set at
25-33% of normal voltage.
Repetitive Starting - Device 2RS
Restarting motors with insufficient cooling time, or operating with extreme load variations
(jogging) can result in dangerously high motor temperatures. Timing circuit protection
schemes based on manufacturer-recommended starting cycles (e.g., 2 hot/1 cold per hour), or
temperature sensitive relays, such as the CT relay just previously discussed, are also used to
protect the motor against repetitive starting. Use of this type of relay requires very careful
analysis of the motor and its projected operating cycles.

Protection Scheme One-Line Diagrams
SAES-P-114 (Chapter 6) very clearly lists the preferred protection scheme for the various
types of induction motors used in Saudi Aramco industrial applications. Figures 38 through
43 are one-line diagrams developed to describe the SAES-P-114 motor protection
requirements.
Low Voltage Motors
Voltage motor protection is separated based on the following motor rating categories:
• 0.75 kW (100 hp) or less
• Greater than .75 kW to 75 kW (1.0 to 100 hp)
• Greater than 75 kW (100 hp)
0.75 kW (1.0 hp) or Less - This category of low voltage motor is protected by thermal
magnetic molded case circuit breakers (MCCB) with three-pole thermal magnetic trips
(Figure 38a), or combination controllers with overloads, a contactor, and a magnetic-only
MCCB or thermal-magnetic MCCB as shown in Figure 38b.
Figure 38. Protection: 0.75 kW (1.0 hp) or Less

Greater than 0.75 kW to 75 kW (1.0 to 100 hp) -This category of motor protection permits
use of motor circuit protectors (MCP), and requires window-type CT ground fault protection
for motors rated 22.5 kW (30 hp) and larger. Overload and contactor requirements are the
same as the less than 0.75 kW (1.0 hp) category. The one-line diagram for this category is
described in Figure 39.
Figure 39. Protection: Greater Than 0.75 kW to 75 kW (1.0 to 100 hp)

Greater Than 75 kW (100 hp) to a maximum of 185 kW (250 hp) - This category of motor
requires a low voltage power circuit breaker (LVPCB), drawout type, electrically-operated,
with shunt-trip device. Undervoltage protection (Device 27), in addition to ground fault
protection (Device 50GS), is required for the larger, low voltage motors. SAES-P-114
permits individual or common bus undervoltage protection (see Figure 40).
Figure 40. Protection: Greater Than 75 kW (100 hp)

Medium Voltage Motors
Medium voltage motor protection is separated into the following two motor rating categories:
• 150 kW (200 hp) through 7500 kW (10,000 hp)
• 7500 kW (10,000 hp) or greater
150 kW (200 hp) through 7500 kW (10,000 hp) - SAES-P-114 further breaks this category
of motor protection into two sub-categories. Power circuit breakers are the typical protective
devices with Class E2 controllers permitted for motors rated 1125 kW (1500 hp or less).
Figure 41 is the recommended protection scheme using Class E2 controllers.
Figure 41. Protection: Class E2 Controllers (<1125 kW)

Figure 42 is the recommended protection scheme using a power circuit breaker for motors
rated 150 kW (200 hp) through 7500 kW (10,000 hp) ranges.
Figure 42. Power Circuit Breaker (<7500 kW)

7500 kW (10,000 hp) or Greater - This category of motor requires differential (Device 87M)
protection versus Device 50 short circuit protection, and temperature (Device 49T) protection
as opposed to thermal overload protection using a BL-1 relay (Device 49). Additional
overload protection for this motor category is also provided by using an ABB COM relay (see
Figure 43).
Figure 43. Protection: Power Circuit Breaker (>7500 kW)

SOLID-STATE MOTOR PROTECTION PACKAGE (MPP) FEATURES
General Description
Solid-state motor protection packages (MPPs) are typically self-contained, door-mounted,
motor protection devices. Saudi Aramco (SAES-P-114) permits use of MPPs for motors 4.0
kV or greater in any kW (hp) rating.
Features and Capabilities
Solid-state MPPs (latest generation), such as the Multilin 269 Plus or Westinghouse IQ1000-
II, are the best or preferred method of protecting medium voltage motors in today’s industrial
environment. These MPPs develop very accurate thermal models of the motor, and,
therefore, the protection set points (for example, locked-rotor and thermal protection) can
better match the thermal characteristics of the motor. In contrast, conventional relays are set
to protect based on an estimate of the motor’s thermal capabilities. Algorithms, used in the
MPPs for the motor’s I
2
t thermal characteristics, are calculated based on the motor’s actual
load amps. Most MPPs also continuously calculate positive and negative sequence currents
as well. The primary features of a typical MPP are:
• Protection
• Communication
• Diagnostics
Benefits
The key benefit of an MPP is that these types of relays offer, for all practical purposes,
unlimited motor protection. Another and often overlooked benefit is that they extend a
motor’s life, because the protection set points are based on much more accurate thermal
models of the motor. Note: Because all of the motor protection is in a single, self-contained
package, a designer must consider backup electromechanical relays when fail-safe tripping is
not allowable for an MPP failure.

Multilin MMR 269 Plus
This relay is primarily a current (CT secondary) sensing relay. Voltage functions (metering
and/or relaying) are optional. Figure 44 is a description of the MMR 269 Plus faceplate.
Figure 44. Multilin 269 Plus: Faceplate

Single-Line Drawing
Figure 45 is the legend describing the relays shown on the single-line drawing (Figure 46).
Figure 45. Multilin 269 Plus: Legend

Figure 46. Multilin 269 Plus: Single-Line Drawing

Protection Features
The protection features (relay functions) listed in Figure 47 are no different than their
electromechanical counterparts. The differences are that all of the functions are self-
contained in one case, and the user enters the set points via a keypad. Note: SAES-P-114
requires a separate undervoltage relay (Device 27) if the meter option is not selected.
Figure 47. Multilin 269 Plus: Protection Features

Communication Features
The communication features of the relay provide motor data/status to a remote device such as
a computer. The following list describes the communication features of the Multilin MMR
269 Plus.
• Overload alarm
• Stator RTD alarm
• Ground fault alarm
• Undercurrent alarm
• Unbalance alarm
• Bearing RTD alarm
• Broken RTD alarm
• Undervoltage alarm (meter option only)
• Power factor alarm (meter option only)
• Self test alarm
• Alphanumeric display
• Actual motor values displayed
• Status indication
• RS485 port
• Analog output load amps
• Analog output motor thermal capacity
• Analog output stator temperature
• Analog output (average RMS amps) (meter option)
• Analog output kW (meter option)
• Analog output kVAR (meter option)
• Analog output p.f. (meter option)

After the relay has been programmed and the motor is running, operations personnel, using
the keypad, can demand the following list of actual values.
• Three-phase average current
• Individual phase currents
• Hottest stator RTD temperature
• Individual stator RTD temperature
• Maximum stator RTD temperature since last access
• Unbalance ratio (% In/lp)
• Ground leakage current
• Individual motor bearing RTD temperatures
• Individual drive bearing RTD temperatures
• Individual maximum bearing temperatures since last access
• Thermal capacity remaining/ Estimated time to trip at present overload level
• Motor load as a % of full load
• Phase-to-phase voltage
• kW, kVAR, MWH, p.f., frequency
Diagnostic Features
The diagnostic features of the relay include the following:
• Learned motor parameters
• Pre-trip values
• Motor operation historical data
• Latched fault indications
Several of the learned motor parameters include cool down time from run to stop, cool down
time from run-overload to run-normal, learned negative sequence contribution, and
acceleration time.
To assist in fault diagnosis, the relay will identify the cause of trip and the following values
can be recalled for rapid fault diagnosis.

• Average motor current
• Unbalance ratio
• Ground fault current
• Maximum stator RTD temperature
• Phase voltage
• kW
• Power factor
• Frequency
To assist in fault diagnosis, maintenance and operations monitoring, the relay will display the
following list of statistical values.
• Running hours since last commissioning
• Number of starts since last commissioning
• Number of trips since last commissioning
• Number of overload trips
• Number of unbalance trips
• Number of ground fault trips
• Number of RTD trips
• Number of short circuit trips
• Number of start trips
• Total watt-hours

Other Features
Numerous other features as shown in the following list are also available on the relay.
• Emergency restart
• Learned acceleration time
• Start inhibit
• Single shot restart
• Output relays
• Draw-out case option (extra)
• Optional DC control supply (extra)
• Exponential running cool down
• Anti backspin timer

Westinghouse IQ-1000II
The IQ-1000II is also a current (CT secondary) sensing relay. Figure 48 is a description of
the IQ-1000II faceplate.
Figure 48. IQ-1000II: Faceplate
Block Diagram

The IQ-1000II receives motor current sensing derived from 3 separate current transformers,
each of which monitors one phase of an AC line to the motor (see Figure 49). If an optional
zero sequence ground fault transformer is used, the IQ-1000II monitors ground fault current
levels and compares them to a user-selected setpoint. If optional RTDs are used, the IQ-
1000II gathers winding temperature data from six RTDs embedded in the stator windings of
the motor. Four RTDs associated with the motor and load bearings can also be monitored for
temperature levels. Additionally, one auxiliary RTD, such as motor case temperature, can be
monitored.
Figure 49. IQ-1000II: Block Diagram

Protection Features
The IQ-1000II protection features are current sensitive only (see Figures 50a and b). SAES-
P-114 would, therefore, require a separate undervoltage relay (Device 27) to protect the
motor.
Figure 50a. IQ-1000II: Protection Features

Figure 50b. IQ-1000II: Protection Features (Cont’d)

Communication Features
IMPACC, a Westinghouse Local Area Network, can be used to communicate with one or
multiple IMPACC-compatible devices. These devices include the following: IQ-500,
IQ1000, IQ-1000 II, IQ Data Plus, IQ Data PlusII, IQ Data, IQ Generator, Digitrip,
Addressable Relay II Advantage Motor Starters, and IQ Energy Sentinels. Up to 1000
devices can be connected on a network via shielded twisted pair wire. Four different
communication levels are available and are described below:
• IMPACC Series I: Standardized software package that runs on a 100% IBM-
compatible computer. The software is packaged with a computer interface card
(CONI). Series I offers the following features: System Monitoring, Data
Logging, Event Logging, Remote On/Off Control, Dialup Capabilities and
Gateway Interface.
• IMPACC Series II: Customer-written software for special applications.
Custom software is required in situations where (1) Westinghouse software
does not provide feature(s) desired by the customer or (2) the customer wants
to communicate to a non-IBM compatible computer or a programmable
controller. A MINT translator module converts device-messages into 10-byte
ASCII RS-232 signal. An RS-232 Protocol Manual is included with each
MINT.
• IMPACC Series III: Standardized software package that runs on most 100%
IBM-compatible computers. Series III requires a CONI or a MINT to operate.
Series III runs in the Microsoft Windows environment and includes the
following features: System Monitoring, Data Trending, Event Logging,
Spreadsheet-compatible Trend and Log files, Remote On/Off Control, Gateway
Interface, Device/System Alarming, Analog Alarming, Security (password
protection) and Enhanced Graphics.
• IMPACC Driver Software (Third Party Vendors): Data acquisition software
written by third party vendors. Software drivers are available to gather data
from systems such as IMPACC, Programmable Controllers and/or Energy
Management Systems. The IMPACC Driver for ICONICS’ Genesis (real-time
graphics interface program) offers the following features: System Monitoring,
Data Trending, Event Logging, Remote On/Off Control Device/System
Alarming, Customized Graphics and Communications to other Genesis-
compatible systems (PLC’s), Energy Management Systems

Diagnostic Features
The IQ-1000II has 52 set points, most of which can be used for diagnostic purposes
(maintenance, troubleshooting, etcetera). Figures 51a, b, c, and d list the monitor data.
Figure 51a. IQ-1000II: Monitor Data
Figure 51b. IQ-1000II: Monitor Data (Cont’d)

Figure 51c. IQ-1000II: Monitor Data (Cont’d)
Figure 51d. IQ-1000II: Monitor Data (Cont’d)

Figure 52 lists the alarm conditions data.
Figure 52. IQ-1000II: Alarm Conditions

Figures 53a and b list the trip conditions.
Figure 53a. IQ-1000II: Trip Conditions

Figure 53b. IQ-1000II: Trip Conditions (Cont’d)

Other Features
In addition to load-associated protection, the IQ-1000II relay also, through the use of special
algorithms, provides rotor temperature protection. The relay continuously measures/monitors
both the positive and negative sequence currents, and incorporates their combined effect into
an algorithm that effectively tracks rotor temperature.
Another unique feature of the relay is the internal diagnostics failure message capability as
shown in Figure 54.
Figure 54. IQ-1000II: Internal Diagnostics

GLOSSARY
American National An organization whose members approve various standards
Standards Institute (ANSI) for use in American industries.
analog (output) One type of continuously variable quantity used to represent
another; for example, in temperature measurement, an electric
current output represents temperature input.
asymmetrical (current) The combination of the symmetrical component and the
direct-current component of the current.
current-limiting (fuse) A fuse that, when it is melted by a current within its specified
current-limiting range, abruptly introduces a high arc voltage
to reduce the current magnitude and duration. Note: the
values specified in standards for the threshold ratio, peak let-
through current, and I
2
t characteristic are used as the
measures of current-limiting ability.
diagnostic Pertaining to the detection and isolation of either a
malfunction or mistake.
duty A variation of load with time, which may or may not be
(rotating machinery) repeated, and in which the cycle time is too short for thermal
equilibrium to be attained.
horsepower (shaft) (hp) The mechanical output (shaft) rating of a motor. One
(1) hp equals 746 watts. See kilowatt (shaft).
hottest-spot A conventional value selected to approximate the degrees
temperature allowance of temperature by which the limiting insulation temperature
rise
exceeds the limiting observable temperature rise.
induction motor An alternating-current motor in which a primary winding on
one member (usually the stator) is connected to the power
source and in which a polyphase secondary winding or a
squirrel-cage secondary winding on the other member (usually
the rotor) carries induced current.
instantaneous (relay) A qualifying term applied to a relay indicating that no delay is
purposely introduced in its action.
Institute of Electrical and A worldwide society of electrical and electronics engineers.
Electronics Engineers (IEEE)

jogging The quickly repeated closure of the circuit to start a motor
from rest for the purpose of accomplishing small movements
of the driven machine.
kilowatt (shaft) (kw) The mechanical output (shaft) rating of a motor. See
horsepower (hp).
locked-rotor The condition existing when the circuits of a motor are
(rotating machinery) energized, but the rotor is not turning.
locked-rotor current The steady-state current taken from the line with the rotor
locked and with rated voltage (and rated frequency in the case
of alternating-current motors) applied to the motor.
locked-rotor indicating Code letters marked on a motor nameplate to show motor
kVA
code letter per hp under locked-rotor conditions.
low voltage Voltage levels below 1000 volts usually called utilization
level outages.
medium voltage Voltage levels greater than or equal to 1000 volts and less
than 100,000 volts.
motor protection A solid-state, self-contained motor protection relay, such as
the
package (MPP) Multilin 269 Plus or the Westinghouse IQ-1000II.
National Electric Code An electrical safety code developed and approved every three
(NEC) years by the National Fire Protection Association (NFPA).
National Electrical A nonprofit trade association of manufacturers of electrical
Manufacturers Association apparatus and supplies, whose members are engaged in
(NEMA) standardization to facilitate understanding between users and
manufacturers of electrical products.
negative sequence Three balanced current phasors equal in magnitude, displaced
current components from each other by 120
0
in phase, and having the phase
sequence
opposite to that of the original set of unbalanced phasors.

positive sequence Three balanced current phasors equal in magnitude, displaced
current components from each other by 120
0
in phase, and having the same phase
sequence as the original unbalanced phasors.
relay An electrically controlled, usually two-state, device that opens
and closes electrical contacts to effect the operation of other
devices in the same or another electric circuit.
replica temperature A thermal relay whose internal temperature rise is
proportional
relay to that of the protected apparatus or conductor, over a range of
values and durations of overloads.
residual (current) The sum of the three-phase currents on a three-phase circuit.
The current that flows in the neutral return circuit of three
wye-connected current transformers is residual current.
rotor (rotating machinery) The rotating member of a machine with shaft.
service factor (S.F.) A multiplier that, when applied to the rated power, indicates a
permissible power loading that may be carried under the
conditions specified for the service factor.
single-phasing (motor) An abnormal operation of a polyphase machine when its
supply is effectively single-phase.
starter (motor) An electric controller for accelerating a motor from rest to
normal speed and for stopping the motor.
starting current The current drawn by the motor during the starting period.
(rotating machinery) It is a function of speed or slip.
stator The portion that includes and supports the stationary
(rotating machinery) active parts. The stator includes the stationary portions of the
magnetic circuit and the associated winding and leads. It may, depending on the design,
include a frame or shell, winding supports, ventilation
circuits, coolers, and temperature detectors. A base, if
provided, is not ordinarily considered to be part of the stator.
symmetrical (current) A periodic alternating current in which points that are one-half
a period apart are equal and have opposite signs.
synchronous speed The speed of the rotation of the magnetic flux, produced by or
linking the winding.

temperature rise A test undertaken to determine the temperature rise
(rotating machinery) above ambient of one or more parts of a machine under
specified operating conditions. Note: The specified conditions may refer to current, load,
etcetera.
time-current The correlated values of time and current that
characteristics designate the performance of all or a stated portion of the
functions of a protective device. Note: The time- current characteristics of a protective
device are usually shown as a curve.
time-overcurrent relay An overcurrent relay in which the input current and operating
time are inversely related throughout a substantial portion of
the performance range.
total current See asymmetrical current.
zero sequence Three balanced current phasors equal in magnitude
current components and with zero displacement from each other.

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