*Motor name plate consists of voltage range for operating motors. Operating the motors both lower and higher than motor name rating will leads to reduced efficiency and may also leads to premature failure of motors.*

__Effects of low voltage on Induction motors__

When
voltage applied to motor get lower than name plate lower range of voltage than
for fixed amount of power at lower voltage current drawn by motor get
increased. Higher current will leads to exceeding motor name plate current
rating that will leads to heating up the motor and even burning out the motor
if over-current persists.

Power
in motor is = Voltage X Current X Power factor

So
lower the voltage higher will be current drawn by motor as power factor is same
for motor.

__Effects of high voltage on Induction motors:-__
There is often assumption made from Power
relation that Since low voltage increases the current drawn by motors, then
high voltage must reduce the current draw this lower current will leads to
lower heating effect of motor. But results are somewhat different High voltage
on a motor tends to push the magnetic portion of the motor into saturation.
Usually this happens when motor voltage exceeds certain limit of voltage level
of motor. Thus Higher voltage will cause the motor to magnetize the iron beyond
the point where magnetizing is practical by drawing higher current.

This
higher current drawn by motor will leads to overheating of motor and thus will
leads to shortening life of motor.

Usually motors rated at 220 and 440V with voltage
tolerance band of 5-10% . If voltage increases on decreased than this band
level then there will be drastic effects on performance and efficiency of
motors. Also this will not means that you will continue to operate motors at
extreme levels of this voltage level otherwise this will shorten life of
motors.

Low
voltage can lead to following:-

a. Overheating

b. Shortened
life

c. Reduced
starting ability and

d. Reduced
pull-up and pullout torque.

The
starting torque, pull-up torque, and pullout torque of induction motors all
change, based on the applied voltage squared.

A 10%
reduction from nameplate voltage would reduce the starting torque, pull-up
torque, and pullout torque by a factor of .92.9.

The
resulting values would be 81% of the full voltage values.

At 80%
voltage, the result would be .82.8, or a value of 64% of the full voltage
value.

Let us
assume that the load torque remains constant. With reduction in voltage the
torque produced, T will reduce ( T is proportional to V^2). So the torque slip
(T-s) characteristic at reduced voltage will have all its amplitudes reduced.
So in order to to get the same torque required by the load the slip s will
increase.

There
are following facts about Higher and lower voltages effects on motors:-

1. Effects
on Single and Three Phase motors

a. It has
been found that Single phase motors are more sensitive to overvoltage than
three phase.

b. U-frame
motors are less sensitive to overvoltage than are T-frames.

c. Premium
efficiency motors known as Super-E are less sensitive to overvoltage than are
standard efficiency motors.

2. Effects
of No. of poles

6 and 8 Pole motors are more sensitive to higher voltages than Two-
and 4-pole motors.

3. Effect
of over-voltage at Light loaded Motors

Over-voltages can cause higher current and temperature of motors
even at light loaded motors. It means that even at lighter loads motor life get
reduced.

4. Effect on efficiency

Motor Efficiency drops with both high or low voltages.

5. Effects
of voltages on Power factor

Motor Power factor improves with lower voltage and drops sharply
with higher voltage.

It has
been found that current will increase in same proportion to voltage decrease.
This means that 5% decrease in voltage will cause 5% increase in current. Motor
will not get damaged until motor current exceeded the rated current of motor.
If this current exceeded the rated current of the motor than this will damage
the motor.

On
lightly loaded motors with easy-to-start loads, reducing the voltage will not
have any appreciable effect, except that it might help reduce the light load
losses and improve the efficiency under this condition. This is the principle
behind some add-on equipment whose purpose is to improve efficiency.