ETAP 5.
Motor Acceleration
Copyright 2003 Operation Technology, Inc.
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 2
Motor Types
• Synchronous
• Salient Pole
• Round Rotor
• Induction
• Wound Rotor (slip-ring)
• Squirrel Cage (brushless)
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 3
Typical Rotor Construction
• Rotor slots are not parallel to the shaft but
skewed
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 4
Wound Rotor
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 5
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 6
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 7
Motor Torque Curves
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Acceleration Torque
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 9
Operating Range
• Motor, Generator, or Brake
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 10
Rated Conditions
• Constant Power
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 11
Starting Conditions
• Constant Impedance
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 12
Voltage Variation
• Torque is proportional to V^2
• Current is proportional to V
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 13
Frequency Variation
• As frequency decreases, peak torque shifts toward lower
speed as synchronous speed decreases.
• As frequency decrease, current increases due reduced
impedance.
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 14
Number of Poles Variation
• As Pole number increases, peak torque shifts toward lower
speed as synchronous speed decreases.
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 15
Rotor Z Variation
• Increasing rotor Z will shift peak torque towards lower
speed.
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 16
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 17
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 18
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:
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 19
Circuit Model I
• Single Cage Rotor
– “Single1” – constant rotor resistance and
reactance
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 20
Circuit Model II
• Single Cage Rotor
– “Single2” - deep bar effect, rotor resistance and
reactance vary with speed [Xm is removed]
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 21
Circuit Model III
• Double Cage Rotor
– “DB1” – integrated rotor cages
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 22
Circuit Model IV
• Double Cage Rotor
– “DB2” – independent rotor cages
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 23
Characteristic Model
• Motor Torque, I, and PF as function of Slip
– Static Model
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 24
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 25
Calculation Methods II
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 26
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 27
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 28
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
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 29
Starting Devices Types
• Auto-Transformer • Y/D Winding
• Stator Resistor • Partial Wing
• Stator Reactor • Soft Starter
• Capacitor at Bus • Stator Current Limit
• Capacitor at Motor – Stator Current Control
Terminal – Voltage Control
• Rotor External Resistor – Torque Control
• Rotor External Reactor
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 30
Starting Device
• Comparison of starting conditions
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Starting Device – AutoXFMR
• Autotransformer
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 32
Starting Device – Stator R
• Resistor
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 33
Starting Device Stator X
• Reactor
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 34
Transformer LTC Modeling
• LTC operations can be simulated in motor
starting studies
• Use global or individual Tit and Tot
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 35
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 statusÆclosing operation
– Close statusÆopening operation
• For studies, MOV can only be started once
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 36
MOV Modeling II
• Five stages of operation
Opening Closing
Acceleration Acceleration
No load No load
Unseating Travel
Travel Seating
Stall Stall
• Without hammer blow Æ Skip “No Load” period
• With a micro switch Æ Skip “Stall” period
• Operating stage time extended if Vmtr < Vlimit
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 37
MOV Closing
• With Hammer Blow- MOV Closing
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 38
MOV Opening
• With Hammer Blow- MOV Opening
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 39
MOV Voltage Limit
• Effect of Voltage Limit Violation
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Motor Acceleration Slide 40
