This document provides an overview of motor acceleration studies, including: - The purpose of motor acceleration studies is to ensure motors can start under voltage drops and are adequately sized. - Common motor types include synchronous, induction, and wound rotor motors. - Motor acceleration is modeled either statically using locked rotor impedance or dynamically using characteristic curves. - Starting devices like auto-transformers, resistors, and reactors can be modeled to study their effect on motor starting.

1. ETAP 5.0ETAP 5.0
Copyright 2003 Operation Technology, Inc.
Motor Acceleration

2. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 2
Why to Do MS Studies?
• Ensure that motor will start with voltage drop
• If Tst<Tload at s=1, then motor will not start
• If Tm=Tload at s<sr, motor can not reach rated speed
• Torque varies as (voltage)^2
• Ensure that voltage drop will not disrupt other loads
• Utility bus voltage >95%
• MCC bus voltage >80%
• Generation bus drop <7%
• Ensure motor feeders sized adequately

3. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 3
Motor Types
• Synchronous
• Salient Pole
• Round Rotor
• Induction
• Wound Rotor (slip-ring)
• Squirrel Cage (brushless)

4. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 4
Typical Rotor Construction
• Rotor slots are not parallel to the shaft but
skewed

5. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 5
Wound Rotor

6. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 6
Operation of Induction
Motor
• AC applied to stator winding
• Creates a rotating stator magnetic field in air gap
• Field induces currents (voltages) in rotor
• Rotor currents create rotor magnetic field in air gap
• Torque is produced by interaction of air gap fields

7. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 7
Slip Frequency
• Slip represents the inability of the rotor to
keep up with the stator magnetic field
• Slip frequency
S = (ωs-ωn)/ωs where ωs = 120f/P
ωn = mech speed

8. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 8
Motor Torque Curves

9. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 9
Acceleration Torque

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Operating Range
• Motor, Generator, or Brake

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Rated Conditions
• Constant Power

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Starting Conditions
• Constant Impedance

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Voltage Variation
• Torque is proportional to V^2
• Current is proportional to V

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Frequency Variation
• As frequency decreases, peak torque shifts toward lower
speed as synchronous speed decreases.
• As frequency decrease, current increases due reduced
impedance.

15. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 15
Number of Poles Variation
• As Pole number increases, peak torque shifts toward lower
speed as synchronous speed decreases.

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Rotor Z Variation
• Increasing rotor Z will shift peak torque towards lower
speed.

17. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 17
Modeling of Elements
• Switching motors – Zlr, circuit model, or
characteristic model
• Synch generator - constant voltage behind
X’d
• Utility - constant voltage behind X”d
• Branches – Same as in Load Flow
• Non-switching Load – Same as Load flow
• All elements must be initially energized,
including motors to start

18. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 18
Motor Modeling
1. Operating Motor
– Constant KVA Load
2. Starting Motor
– During Acceleration – Constant Impedance
– Locked-Rotor Impedance
– Circuit Models
Characteristic Curves
After Acceleration – Constant KVA Load

19. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 19
Locked-Rotor Impedance
• ZLR = RLR +j XLR (10 – 25 %)
• PFLR is much lower than operating PD.
Approximate starting PF of typical squirrel
cage induction motor:

20. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 20
Circuit Model I
• Single Cage Rotor
– “Single1” – constant rotor resistance and
reactance

21. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 21
Circuit Model II
• Single Cage Rotor
– “Single2” - deep bar effect, rotor resistance and
reactance vary with speed [Xm is removed]

22. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 22
Circuit Model III
• Double Cage Rotor
– “DB1” – integrated rotor cages

23. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 23
Circuit Model IV
• Double Cage Rotor
– “DB2” – independent rotor cages

24. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 24
Characteristic Model
• Motor Torque, I, and PF as function of Slip
– Static Model

25. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 25
Calculation Methods I
• Static Motor Starting
– Time domain using static model
– Switching motors modeled as Zlr during starting and
constant kVA load after starting
– Run load flow when any change in system
• Dynamic Motor Starting
– Time domain using dynamic model and inertia model
– Dynamic model used for the entire simulation
– Requires motor and load dynamic (characteristic) model

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Calculation Methods II

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Static versus Dynamic
• Use Static Model When
– Concerned with effect of motor starting on other
loads
– Missing dynamic motor information
• Use Dynamic Model When
– Concerned with actual acceleration time
– Concerned if motor will actually start

28. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 28
MS Simulation Features
• Start/Stop induction/synchronous motors
• Switching on/off static load at specified loading
category
• Simulate MOV opening/closing operations
• Change grid or generator operating category
• Simulate transformer LTC operation
• Simulate global load transition
• Simulate various types of starting devices
• Simulate load ramping after motor acceleration

29. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 29
Automatic Alert
• Starting motor terminal V
• Motor acceleration failure
• Motor thermal damage
• Generator rating
• Generator engine continuous
& peak rating
• Generator exciter peak rating
• Bus voltage
• Starting motor bus
• Grid/generator bus
• HV, MV, and LV bus
• User definable minimum time
span

30. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 30
Starting Devices Types
• Auto-Transformer
• Stator Resistor
• Stator Reactor
• Capacitor at Bus
• Capacitor at Motor
Terminal
• Rotor External Resistor
• Rotor External Reactor
• Y/D Winding
• Partial Wing
• Soft Starter
• Stator Current Limit
– Stator Current Control
– Voltage Control
– Torque Control

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Starting Device
• Comparison of starting conditions

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Starting Device – AutoXFMR
• Autotransformer

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Starting Device – Stator R
• Resistor

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Starting Device Stator X
• Reactor

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Transformer LTC Modeling
• LTC operations can be simulated in motor
starting studies
• Use global or individual Tit and Tot

36. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 36
MOV Modeling I
• Represented as an impedance load during
operation
– Each stage has own impedance based on I, pf,
Vr
– User specifies duration and load current for each
stage
• Operation type depends on MOV status
– Open status closing operation
– Close status opening operation
• For studies, MOV can only be started once

37. Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 37
MOV Modeling II
• Five stages of operation
Opening Closing
Acceleration Acceleration
No load No load
Unseating Travel
Travel Seating
Stall Stall
• Without hammer blow Skip “No Load” period
• With a micro switch Skip “Stall” period
• Operating stage time extended if Vmtr < Vlimit

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MOV Closing
• With Hammer Blow- MOV Closing

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MOV Opening
• With Hammer Blow- MOV Opening

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MOV Voltage Limit
• Effect of Voltage Limit Violation