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DPI Binder Rev 1.0

IP-PROOFING

TECHNOLOGIES INC.

Power Quality Paper #3

Page 1

The Effect of Voltage Dips On Induction Motors

by: M D McCulloch

1. INTRODUCTION

Voltage depressions caused by faults on the system affect the performance of

induction motors, in terms of the production of both transient currents and transient

torques. It is often desirable to minimize the effect of the voltage dip on both the

induction motor and more importantly on the process where the motor is used. In

order for the user to achieve optimum protection, he must prioritize the importance

of the different processes, and then choose the appropriate method to minimize the

effects of voltage dips, within the constraints of the supply and budget. The worst

case scenario is that of a three phase fault which occurs electrically close to the

motor. It is this scenario which will be examined in this paper.

When a voltage dip lasts longer than a cycle, many AC contactors will fall out,

disconnecting the motor from the supply (and the fault). The user has the following

options as to how the plant will react to such a situation.

1. Restart all the motors once the fault has cleared.

2 Restart the critical motors once the fault has cleared, then restart the

remaining motors in a sequenced start so as to avoid excessive current being

drawn from the supply .

3. Keep the critical motors connected to the supply; even under fault conditions.

(The motor may drop out after a time when the process cannot be rescued)

Each of the above options has certain capital cost implications, as well as causing

different stresses on the electrical system, the mechanical system and the motor.

This paper will examine the effects which stress the motor, and then presents ways

of determining the amount of these stresses.

2. CRITERIA FOR SAFE RESWITCHING

This section examines what conditions must prevail in order to switch the motor back

on to a healthy supply assuming that flux still remains in the machine. Generally

speaking, re closing (or bus transfer) should be avoided when the residual voltage

and the oncoming system voltage exceed about 125% - 135% of the rated voltage

of the motor. The American National Standards Institute (ANSI) standard 050.41-77

(Polyphase Induction Motors for Power Generating Stations) and a proposed

National Electric Manufacturing Association (NEMA) standard would permit a

maximum of 1.33 pu V/Hz (the units V/Hz gives a measure of the flux) for

out-of-phase re switching. Thus, for example if the flux of the motor has not yet

decayed, then the maximum phase difference between the voltage should be 83

electrical degrees. For a phase difference of 180 degrees, the residual voltage

should be less than 0.33 pu V/Hz. It must be remembered that the flux will decay with

a given time constant, (shown later) and that the frequency of the residual voltage

is proportional to the speed of the motor.

3. WHAT HAPPENS WHEN A THREE PHASE FAULT OCCURS

This paper makes the assumption that the voltage dip is caused by a three phase

fault which is electrically close to the motor which is normally valid for most plant. If

this assumption is not valid, i.e. the impedance of the fault as seen by the motor, is

of the same order as the leakage inductance of the motor, then the fault impedance

DPI Binder Rev 1.0

IP-PROOFING

TECHNOLOGIES INC.

Power Quality Paper #3

Page 2

can be added to the stator series impedance in the following analysis in order to

obtain meaningful results.

When a three phase fault occurs close to the motor no electrical energy can enter

or leave the motor. It is obvious that current can still flow in both the stator and the

rotor windings, thus torque can be produced. The energy stored in the magnetic field

can therefore be dissipated either as mechanical energy or as heat due to copper

losses in the motor.

As the fault is applied, there is a large transient current of the order of the starting

currant but with a DC offset. The peak current of this combined effect can be as large

as eight times the rated RMS current. Both the AC and the DC component of the

current die away very rapidly. The time constant of this decay can be approximated

by the time constant associated with the sum of leakage inductances of the stator

and rotor in series with the stator and rotor resistances. Typically the flux will decay

to less than 30% of rated flux in 5 to 6 cycles. Since this transient is dominated by

the decay in the main flux, the loading of the machine has only a small effect on the

size of the transients. Associated with the transient current there

is a torque transient

with a characteristic initial large negative torque impulse of a typical magnitude of -8

pu. This torque impulse reaches its maximum within two cycles, but decays rapidly.

Since the flux decays rapidly, normally to less than 30% within 5-6 cycles, clearing

of the fault and the subsequent re-establishment of the terminal voltage, produces

transients of the same order as at startup which do not excessively stress the

mechanical load or the motor. Matters are helped if the inertia is high and the speed

has not decreased significantly. However if many motors are left connected to the

system their combined effect could lead to a voltage depression due to large currents

(a brown out), and all the motors might not be able to accelerate back to full speed.

Thus it is common practice to allow the contactors to the non-essential motors to drop

out, and only to be re closed at a later stage and in an orderly manner. However

additional control equipment must be purchased in order to affect this.

4. WHAT HAPPENS WHEN A MOTOR IS OPEN CIRCUITED & RECONNECTED

In this scenario, a motor has experienced a fault and the contactors are allowed to

drop out, effectively open circuiting the motor. At a time later, once the fault has been

cleared, the motor is reconnected to the supply. This section explores what the

effects are of the last two operations especially if the motor needs to be operated as

soon as possible. When the motor terminals are disconnected from the supply, once

again no energy can enter or leave the magnetic field via the electrical terminals.

Further as there is no current in the stator, torque cannot be produced, thus energy

cannot be transferred to the mechanical load.

At the instant when the stator current is interrupted, the net mmf in the motor must

remain constant. This mmf balance is achieved-by an instantaneous increase in the

current in the rotor windings (cage). The energy in the rotor must now decay due to

the resistance of the rotor. The time constant for this process is that of the open rotor

resistance in series with the rotor leakage inductance and the mutual inductance.

This time constant is therefore much longer than for the case of the short circuited

terminals and is typically in the order of seconds. Thus the flux remains much longer

in the motor in this scenario and it will also remain stationary with respect to the rotor.

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