Answer MET Question 39
Question : Explain the effect of reduced voltage on standard cage motors with respect to EACH of the following:
(i) Burn out; (ii) Starting current; (iii) Starting torque; (iv) Speed.
Answer: Effects of low voltage.
When you subject a motor to voltages below the rated voltage the effect is increase in current to provide same amount of power. An increase in current is a danger to the motor only if that current exceeds the motor's current rating. When amps go above the rating, heat begins to build up in the motor, this heat will damage the motor. The more heat and the longer the exposure to it, the more damage to the motor.
Efficiency drops with either high or low voltage. Power factor improves with lower voltage and drops sharply with higher voltage. Inrush current goes up with higher voltage.
The existing load is a major factor in determining how much of a decrease in supply voltage a motor can handle.If the voltage decreases, the current will increase in roughly the same proportion that the voltage decreases. For example, a 10% voltage decrease would cause a 10% amperage increase. This would not damage the motor, if the current stays below the rated value.
Low voltage can lead to overheating, shortened life, reduced starting ability, and 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.
$\displaystyle \small \mathrm{Torque, T\ \alpha\ (Voltage,V)^2 }$
$\displaystyle \small \mathrm{T'\ \alpha\ (0.9V)^2 }$
$\displaystyle \small \mathrm{T'\ \alpha\ 0.81V^2 }$
Thus, a 10% reduction from rated voltage (100% to 90%, 230V to 207V) would reduce the starting torque, pull-up torque, and pullout torque. The resulting values would be 81% of the full voltage values. Now it is clear that it's difficult to start loads if the voltage happens to be low. Similarly, the motor's pullout torque would be much lower than it would be under normal voltage conditions.
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.
Effects of high voltage.
This is not true that "since low voltage increases the amperage draw on motors, then high voltage must reduce the amperage draw". High voltage on a motor tends to push the magnetic portion of the motor into saturation. This causes the motor to draw excessive current in an effort to magnetize the iron beyond the point where magnetizing is practical.
When you subject a motor to voltages below the rated voltage the effect is increase in current to provide same amount of power. An increase in current is a danger to the motor only if that current exceeds the motor's current rating. When amps go above the rating, heat begins to build up in the motor, this heat will damage the motor. The more heat and the longer the exposure to it, the more damage to the motor.
Efficiency drops with either high or low voltage. Power factor improves with lower voltage and drops sharply with higher voltage. Inrush current goes up with higher voltage.
The existing load is a major factor in determining how much of a decrease in supply voltage a motor can handle.If the voltage decreases, the current will increase in roughly the same proportion that the voltage decreases. For example, a 10% voltage decrease would cause a 10% amperage increase. This would not damage the motor, if the current stays below the rated value.
Low voltage can lead to overheating, shortened life, reduced starting ability, and 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.
$\displaystyle \small \mathrm{Torque, T\ \alpha\ (Voltage,V)^2 }$
$\displaystyle \small \mathrm{T'\ \alpha\ (0.9V)^2 }$
$\displaystyle \small \mathrm{T'\ \alpha\ 0.81V^2 }$
Thus, a 10% reduction from rated voltage (100% to 90%, 230V to 207V) would reduce the starting torque, pull-up torque, and pullout torque. The resulting values would be 81% of the full voltage values. Now it is clear that it's difficult to start loads if the voltage happens to be low. Similarly, the motor's pullout torque would be much lower than it would be under normal voltage conditions.
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.
Effects of high voltage.
This is not true that "since low voltage increases the amperage draw on motors, then high voltage must reduce the amperage draw". High voltage on a motor tends to push the magnetic portion of the motor into saturation. This causes the motor to draw excessive current in an effort to magnetize the iron beyond the point where magnetizing is practical.
(i).
Burnout - Motor burn out occurs because of overheating of the windings.
Reduced voltage on squirrel cage motors will draw more current to
sustain the same load, hence causes overheating of the windings and burn
out. Overheating causes the motor winding insulation to deteriorate at
a very fast rate. For every 10°C rise in temperature, the insulation
life is reduced to half which causes further shorts circuits and damage
to the motor.
(ii) Starting Current - Starting current will reduce if the motor is started with low supply voltage. Motor starting current depends upon the value of supplied voltage and stand still impedance of stator winding. The starting torque of an induction motor is proportional to the square of supply voltage, therefore there is corresponding and greater drop in starting torque and in starting current.
(iii) Starting Torque - Torque is Proportional to square of the applied voltage. Therefore if Voltage is reduced the starting torque is also reduced. The run up time depends, on the starting torque developed & the load on the motor; therefore there is a possibility of motor stalling.
(iv) Speed - With reduction of voltage, the motor draws more current from the supply to sustain the load almost at the same speed. However with considerable reduction of voltage, the speed of motor drops, when the developed torque is unable to sustain load torque. If the speed drops to such a level that it crosses the maximum torque point, then the motor falls back to very low speed and stalls.
(ii) Starting Current - Starting current will reduce if the motor is started with low supply voltage. Motor starting current depends upon the value of supplied voltage and stand still impedance of stator winding. The starting torque of an induction motor is proportional to the square of supply voltage, therefore there is corresponding and greater drop in starting torque and in starting current.
(iii) Starting Torque - Torque is Proportional to square of the applied voltage. Therefore if Voltage is reduced the starting torque is also reduced. The run up time depends, on the starting torque developed & the load on the motor; therefore there is a possibility of motor stalling.
(iv) Speed - With reduction of voltage, the motor draws more current from the supply to sustain the load almost at the same speed. However with considerable reduction of voltage, the speed of motor drops, when the developed torque is unable to sustain load torque. If the speed drops to such a level that it crosses the maximum torque point, then the motor falls back to very low speed and stalls.
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