12_Y NGM MSKM-TS.ppt

1
New Generation &
Old Generation
Electric Motors

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Electric Motors
Introduction
Types of electric motors
Assessment of electric motors
Energy efficiency opportunities

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Introduction
• Electromechanical device that converts
electrical energy to mechanical energy
• Mechanical energy used to e.g.
• Rotate pump impeller, fan, blower
• Drive compressors
• Lift materials
• Motors in industry : 70% of electrical
load
What is an Electric Motor?

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Introduction
How Does an Electric Motor Work?
1
2
3
4

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Introduction
Three types of Motor Load
Motor loads Description Examples
Constant
torque loads
Output power varies but
torque is constant
Conveyors, rotary kilns,
constant-displacement
pumps
Variable torque
loads
Torque varies with square
of operation speed
Centrifugal pumps, fans
Constant
power loads
Torque changes inversely
with speed
Machine tools

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Electric Motors
Introduction

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Type of Electric Motors
Classification of Motors
Electric Motors
Alternating Current
(AC) Motors
Direct Current (DC)
Motors
Synchronous Induction
Three-Phase
Single-Phase
Self Excited
Separately
Excited
Series Shunt
Compound

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• Field pole
• North pole and south pole
• Receive electricity to form
magnetic field
• Armature
• Cylinder between the poles
• Electromagnet when current goes through
• Linked to drive shaft to drive the load
• Commutator
• Overturns current direction in armature
DC Motors – Components

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• Speed control without impact power
supply quality
• Changing armature voltage
• Changing field current
• Restricted use
• Few low/medium speed applications
• Clean, non-hazardous areas
• Expensive compared to AC motors
DC motors

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• Relationship between speed, field
flux and armature voltage
DC motors
Back electromagnetic force: E = KN
Torque: T = KIa
E = electromagnetic force developed at armature terminal (volt)
 = field flux which is directly proportional to field current
N = speed in RPM (revolutions per minute)
T = electromagnetic torque
Ia = armature current
K = an equation constant

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• Separately excited DC motor: field current
supplied from a separate force
• Self-excited DC motor: shunt motor
• Field winding parallel
with armature winding
• Current = field current
+ armature current
Speed constant
independent of load
up to certain torque
Speed control:
insert resistance
in armature or
field current
DC motors

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Self-excited DC motor: series motor
DC motors
• Field winding in series
with armature winding
• Field current =
armature current
• Speed restricted to
5000 RPM
• Avoid running with
no load: speed
uncontrolled
Suited for high
starting torque:
cranes, hoists

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DC compound motor
DC motors
Field winding in
series and
parallel with
armature winding
Good torque and
stable speed
Higher %
compound in
series = high
starting torque
Suited for high
starting torque if high
% compounding:
cranes, hoists

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Classification of Motors
Electric Motors
Alternating Current
(AC) Motors
Direct Current (DC)
Motors
Synchronous Induction
Three-Phase
Single-Phase
Self Excited
Separately
Excited
Series Shunt
Compound

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• Electrical current reverses direction
• Two parts: stator and rotor
• Stator: stationary electrical component
• Rotor: rotates the motor shaft
• Speed difficult to control
• Two types
• Synchronous motor
• Induction motor
AC Motors

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• Constant speed fixed by system
frequency
• DC for excitation and low starting
torque: suited for low load applications
• Can improve power factor: suited for
high electricity use systems
• Synchronous speed (Ns):
AC Motors – Synchronous motor
Ns = 120 f / P
F = supply frequency
P = number of poles

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• Most common motors in industry
• Advantages:
• Simple design
• Inexpensive
• High power to weight ratio
• Easy to maintain
• Direct connection to AC power source
AC Motors – Induction motor

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Components
• Rotor
• Squirrel cage:
conducting bars
in parallel slots
• Wound rotor: 3-phase, double-layer,
distributed winding
• Stator
• Stampings with slots to carry 3-phase windings
• Wound for definite number of poles

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How induction motors work
• Electricity supplied to stator
• Magnetic field generated that moves around
rotor
• Current induced in rotor Electromagnetics
Stator
Rotor
• Rotor produces second
magnetic field that
opposes stator magnetic
field
• Rotor begins to rotate

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• Single-phase induction motor
• One stator winding
• Single-phase power supply
• Squirrel cage rotor
• Require device to start motor
• 3 to 4 HP applications
• Household appliances: fans, washing
machines, dryers

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• Three-phase induction motor
• Three-phase supply produces magnetic
field
• Squirrel cage or wound rotor
• Self-starting
• High power capabilities
• 1/3 to hundreds HP applications: pumps,
compressors, conveyor belts, grinders
• 70% of motors in industry!

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Speed and slip
• Motor never runs at synchronous
speed but lower “base speed”
• Difference is “slip”
• Install slip ring to avoid this
• Calculate % slip:
% Slip = Ns – Nb x 100
Ns
Ns = synchronous speed in RPM
Nb = base speed in RPM

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Relationship load, speed and torque
At start: high
current and
low “pull-up”
torque
At start: high
current and
low “pull-up”
torque
At 80% of full
speed:
highest “pull-
out” torque
and current
drops
At full speed:
torque and
stator current
are zero

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Electric Motors
Introduction

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Assessment of Electric Motors
Motors loose energy when serving a load
• Fixed loss
• Rotor loss
• Stator loss
• Friction and rewinding
• Stray load loss
Efficiency of Electric Motors

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Factors that influence efficiency
• Age
• Capacity
• Speed
• Type
• Temperature
• Rewinding
• Load

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Motor part load efficiency
• Designed for 50-100% load
• Most efficient at 75% load
• Rapid drop below 50% load

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• Motor load is indicator of efficiency
• Equation to determine load:
Motor Load
Load = Pi x  HP x 0.7457
 = Motor operating efficiency in %
HP = Nameplate rated horse power
Load = Output power as a % of rated power
Pi = Three phase power in kW

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Three methods for individual motors
• Input power measurement
• Ratio input power and rate power at 100%
loading
• Line current measurement
• Compare measured amperage with rated
amperage
• Slip method
• Compare slip at operation with slip at full
load
Motor Load

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Input power measurement
• Three steps for three-phase motors
Step 1. Determine the input power:
Motor Load
Pi = Three Phase power in kW
V = RMS Voltage, mean line to
line of 3 Phases
I = RMS Current, mean of 3 phases
PF = Power factor as Decimal
1000
3
x
PF
x
I
x
V
Pi 

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Input power measurement
Step 2. Determine the rated power:
Step 3. Determine the percentage load:
Motor Load
r
r x
hp
P

7457
.
0

%
100
x
P
Pi
Load
r

Load = Output Power as a % of Rated Power
Pi = Measured Three Phase power in kW
Pr = Input Power at Full Rated load in kW
Pr = Input Power at Full Rated load in kW
hp = Name plate Rated Horse Power
r = Efficiency at Full Rated Load

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Result
1. Significantly
oversized and
under loaded
2. Moderately
oversized and
under loaded
3. Properly sized
but standard
efficiency
Motor Load
Action
→ Replace with more efficient,
properly sized models
→ Replace with more efficient,
properly sized models when
they fail
→ Replace most of these with
energy-efficient models when
they fail

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Electric Motors
Introduction

How to Improve Efficiency
Efficiency = Output
Input
=
Output + Losses
Since output is going to remain constant,
what we can reduce are LOSSES.
Output

55 - 60% Stator
Rotor
I2R
I2R
Suitable selection of Copper Conductors for Max
Active Material
Specially designed Al-die cast Rotors
20 - 25% Core Losses Low watt Loss Material
Thinner Laminations
Burr height
2 - 12% Friction & Windage Optimum design of Fan
4 - 5% Stray Optimum Slot Geometry
Minimum Overhang Length
Losses in Motors

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1. Use energy efficient motors
2. Reduce under-loading (and avoid
over-sized motors)
3. Size to variable load
4. Improve power quality
5. Rewinding
6. Power factor correction by capacitors
7. Improve maintenance
8. Speed control of induction motor
Energy Efficiency Opportunities

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• Reduce intrinsic motor losses
• Efficiency 3-7% higher
• Wide range of ratings
• More expensive but
rapid payback
• Best to replace when
existing motors fail
Use Energy Efficient Motors

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Power Loss Area Efficiency Improvement
1. Fixed loss (iron) Use of thinner gauge, lower loss core steel reduces eddy current losses.
Longer core adds more steel to the design, which reduces losses due to
lower operating flux densities.
2. Stator I2R Use of more copper & larger conductors increases cross sectional area of
stator windings. This lower resistance (R) of the windings & reduces losses
due to current flow (I)
3 Rotor I2R Use of larger rotor conductor bars increases size of cross section, lowering
conductor resistance (R) & losses due to current flow (I)
4 Friction & Winding Use of low loss fan design reduces losses due to air movement
5. Stray Load Loss Use of optimized design & strict quality control procedures minimizes stray
load losses
Use Energy Efficient Motors

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• Reasons for under-loading
• Large safety factor when selecting motor
• Under-utilization of equipment
• Maintain outputs at desired level even at low
input voltages
• High starting torque is required
• Consequences of under-loading
• Increased motor losses
• Reduced motor efficiency
• Reduced power factor
2. Reduce Under-loading

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• Replace with smaller motor
• If motor operates at <50%
• Not if motor operates at 60-70%
• Operate in star mode
• If motors consistently operate at <40%
• Inexpensive and effective
• Motor electrically downsized by wire reconfiguration
• Motor speed and voltage reduction but unchanged
performance
2. Reduce Under-loading

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• Motor selection based on
• Highest anticipated load: expensive and risk
of under-loading
• Slightly lower than highest load: occasional
overloading for short periods
• But avoid risk of overheating due to
• Extreme load changes
• Frequent / long periods of overloading
• Inability of motor to cool down
3. Sizing to Variable Load
X

Motors have
‘service factor’
of 15% above
rated load

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Motor performance affected by
• Poor power quality: too high
fluctuations in voltage and frequency
• Voltage unbalance: unequal voltages
to three phases of motor
4. Improve Power Quality

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Keep voltage unbalance within 1%
• Balance single phase loads equally
among three phases
• Segregate single phase loads and
feed them into separate
line/transformer
4. Improve Power Quality

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• Rewinding: sometimes 50% of motors
• Can reduce motor efficiency
• Maintain efficiency after rewinding by
• Using qualified/certified firm
• Maintain original motor design
• Replace 50 HP, >15 year old motors instead
of rewinding
• Buy new motor if costs are less than 50-65%
of rewinding costs
5. Rewinding

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• Use capacitors for induction motors
• Benefits of improved PF
• Reduced kVA
• Reduced losses
• Improved voltage regulation
• Increased efficiency of plant electrical system
• Capacitor size not >90% of no-load
kVAR of motor
6. Improve Power Factor (PF)

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Opportunities
Checklist to maintain motor efficiency
• Inspect motors regularly for wear, dirt/dust
• Checking motor loads for over/under loading
• Lubricate appropriately
• Check alignment of motor and equipment
• Ensure supply wiring and terminal box and
properly sized and installed
• Provide adequate ventilation
7. Maintenance

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Opportunities
• Multi-speed motors
• Limited speed control: 2 – 4 fixed speeds
• Wound rotor motor drives
• Specifically constructed motor
• Variable resistors to control torque
performance
• >300 HP most common
8. Speed Control of Induction Motor

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Opportunities
• Variable speed drives (VSDs)
• Also called inverters
• Several kW to MW range
• Change speed of induction motors
• Can be installed in existing system
• Reduce electricity by >50% in fans and pumps
• Convert 50Hz incoming power to variable
frequency and voltage: change speed

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Direct Current Drives
• Oldest form of electrical speed
control
• Consists of
• DC motor: field windings and armature
• Controller: regulates DC voltage to armature
that controls motor speed
• Tacho-generator: gives feedback signal to
controlled

Aspects of selection decision
1. Process--
• Load requirements
• Area of operation
• Application
2. Site conditions
• Ambient conditions
• Application & criticality
• Duty factor
3. Product
• Performance parameters
• Feature including high Efficiency

The 4 conditions why motors need to
be relocated or repaired or replaced
1. Motors need to be sized to load
2. Improved motor efficiency by
new technology can be adopted
3. Existing motor found less efficient
4. The existing motor winding or rotor
is unreliable

Approach to relocating motors-1
Sizing the motor
• Evaluate cumulative mechanical and
electrical efficiency and fix load factor
• Best efficiency of the driven
equipment can be matched with
appropriate motor at highest efficiency
at that load

• Over Sized motor
Motor out put 37 kW,4P
Name plate Eff 92%
Measured in put 19 kW
Measured losses 2.1 kW
Calculated out put 16.9
kW
Load factor 45%
Motor efficiency 89%
Efficiencies at partial loads
always poor
• Sized SEE motor
Motor out put 22 kW, 4P
Name plate Eff 93%
Measured in put 18.1 kW
Measured losses 1.2 kW
Calculated out put 16.8 kW
Load factor 76%
Motor efficiency 93%
Energy saved 0.9 units per
hour;
@8000 hours/annum is 7200
units
Case Study-1

Approach to replacing motors- 2
Efficiency level 1 & 2 motors
• Annual energy cost of motors is in excess
of 10 times their initial purchase cost.
• Standard efficiency motors are nearly 3
to 5% inferior
• Each 1% increase in efficiency results in
saving of Rs.400/ kW in energy cost
p.a

Case Study- 2
Attributes Old Motor Eff level 2 Eff level 1
Out put 15 kW 15 kW 15 kW
Load factor 75% 75% 75%
Efficiency % 88% 90.5% 93%
In put 17 kW 16.6 kW 16.1 kW
Purchase
cost
Rs.4000/-RW Rs.26000 Rs.30000
Running cost
8000hrs@Rs
4/
Rs.5.44 L Rs. 5.31 L Rs. 5.15 L
Savings Nil Rs.13000 Rs.29000

Approach to replacing motors- 3
Field Efficiency test on motors
• It is possible to estimate energy losses
and arrive at the approx. motor
efficiency at the site
• Any LT induction motor can be
included in the efficiency evaluation
program to suggest retrofit

kW Volts Amps PF Hz Rpm mΩ /
phase
Name
plate
22 415 40 0.86 50 1460
No
load
1.2 410 18 0.08 49.8 1494
Load
Eff
%
19.7
87%
409 38 0.84 49.8 1460

Approach to repairing motors- 4
Predictive maintenance
• Detection and measurement of vital
machine conditions can be employed
to assess the reliability
• Vibration, temperature, resistance, PF,
harmonics and winding impedance
data can be measured and motor
reliability assessed

Location kW / rpm Rated Ranking on a scale of 1 to 10 to fix priority Category
Efficiency Age Rewound Load Factor Hours/year Total Type
Example
ID Fan 22/ 1450 91 2 2 2 1 7 A Rank Marks
A Total marks > 7
B Marks 4 to 7
C Marks < 3
Marks for standard motors
Age Marks
Repairs or
rewound Marks
Load
factor Marks Hours Marks
> 16 yaers 4 Class of Insulation B More than 3 times 3 >75% 1 6000 to 8000 3
8 to 16 years 3 Class of Insulation B 2 times (Major) 2 40 to 60% 2 4000 to 5999 2
2 to 8 years 2 Class of Insulation B or F Once (Major) 1 < 39% 3 < 3999 hours 1
< 2 years 1 Class of Insulation F None 0
Key motor Identification

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Care for Bearings
Nearly all VFD-driven motors are vulnerable
to bearing damage, but for too long the
importance of shaft grounding to protect motor
bearings has been ignored or underestimated.
To make the savings generated by VFDs
sustainable, an effective long-term method of
shaft grounding is essential.

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Pitting of a bearing race wall (magnified)

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For motors up to 100 hp, where common mode voltages could
cause bearing damage, adding a shaft grounding ring to the
motor, either inside the motor or externally, provides effective
protection against bearing currents for motor bearings as well
as attached equipment.
Additional Care

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Taken from a failed motor, the “fluted” bearing race wall
(left) resulted from VFD-induced bearing currents.
Protected by Bearing Protection Ring, the race on the right
is undamaged.

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For motors above 100 hp, both circulating currents and
common mode voltages can cause bearing damage.
Combining an insulated bearing on one end with a shaft
grounding ring on the opposite end provides the best
protection from electrical bearing damage.
Additional Care

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THANK YOU
FOR YOUR ATTENTION


12_Y NGM MSKM-TS.ppt

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