09 - motor acceleration

ETAP 5.0ETAP 5.0

Copyright 2003 Operation Technology, Inc.

Motor Acceleration

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


Motor Types

• Synchronous• Salient Pole• Round Rotor

• Induction• Wound Rotor (slip-ring)• Squirrel Cage (brushless)


Typical Rotor Construction

• Rotor slots are not parallel to the shaft but skewed


Wound Rotor


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


Slip Frequency

• Slip represents the inability of the rotor to keep up with the stator magnetic field

• Slip frequencyS = (ωs-ωn)/ωs where ωs = 120f/P

ωn = mech speed


Motor Torque Curves


Acceleration Torque


Operating Range• Motor, Generator, or Brake


Rated Conditions• Constant Power


Starting Conditions• Constant Impedance


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


Frequency Variation• As frequency decreases, peak torque shifts toward lower

speed as synchronous speed decreases.• As frequency decrease, current increases due reduced

impedance.


Number of Poles Variation• As Pole number increases, peak torque shifts toward lower

speed as synchronous speed decreases.


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

speed.


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


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


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:


Circuit Model I

• Single Cage Rotor– “Single1” – constant rotor resistance and

reactance


Circuit Model II

• Single Cage Rotor– “Single2” - deep bar effect, rotor resistance and

reactance vary with speed [Xm is removed]


Circuit Model III

• Double Cage Rotor – “DB1” – integrated rotor cages


Circuit Model IV

• Double Cage Rotor– “DB2” – independent rotor cages


Characteristic Model

• Motor Torque, I, and PF as function of Slip– Static Model


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


Calculation Methods II


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


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


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


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


Starting Device

• Comparison of starting conditions


Starting Device – AutoXFMR

• Autotransformer


Starting Device – Stator R

• Resistor


Starting Device Stator X

• Reactor


Transformer LTC Modeling

• LTC operations can be simulated in motor starting studies

• Use global or individual Tit and Tot


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


MOV Modeling II• Five stages of operation

Opening ClosingAcceleration AccelerationNo load No loadUnseating TravelTravel SeatingStall Stall

• Without hammer blow Skip “No Load” period

• With a micro switch Skip “Stall” period

• Operating stage time extended if Vmtr < Vlimit


MOV Closing

• With Hammer Blow- MOV Closing


MOV Opening

• With Hammer Blow- MOV Opening


MOV Voltage Limit

• Effect of Voltage Limit Violation

09 - motor acceleration

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