Sunday, July 17, 2016

Effects of Higher and lower voltage on Induction motors

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.