Voltage drop issues for motors
Voltage drop issues for motors
(OP)
Earlier in one of the threads I have asked the effect of voltage drops on motors. I have learned much that if the temrinal voltage of a motor is rated for a specific voltage and current, and if we run a motor at lower voltages it can cause excessive heat. Is this heat asscoiated with running the motor at reduced voltages which will draw higher amps through the motor wiring which cause the heat? Does it only happen during starting, where the in-rush is 6x the full load amps? Or at any time, when the motor is running at full load? How about the use of reduced voltage starting? Doesn't that cause problem as well?






RE: Voltage drop issues for motors
The reduced voltage starting situation is a balance between the longer time required to accelerate the load under reduced voltage and the heating that occurs during this extended time. Severely reduced voltage will result in LESS current, but will also do little work to accelerate the load, so the voltage cannot be reduced so much that start times are extended enough that heating becomes damaging.
Starting voltage is reduced to limit the torque produced to an acceptable level if that is a concern or the starting current is limited to an acceptable level if that is the major concern.
Jim
RE: Voltage drop issues for motors
Yes, if you reduce the voltage so far that there is insufficient torque to accelerate the load, the motor overheats, but that cannot really be considered because that is NOT a successful application to begin with.
As to voltage drop when running, the reason for the extra heating is that torque is reduced by the square of the applied voltage (at a fixed frequency). So if your load remains the same, and the motor does not develop it's rated torque, the motor will slow down thus increasing slip. Increased slip increases current for the same amount of work, and excess slip represents additional loses that are lost as heat in the stator and rotor.
By the way, if on the other hand your motor was seriously over sized for the load so that the loss of torque is immaterial, you may not in fact see any significant overheating of the motor.
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RE: Voltage drop issues for motors
So a reduction in iron loss and an increase in copper loss in both the rotor and the stator.
Provided that the torque reduction is not too severe, the losses during start are not significantly higher with reduced voltage starting.
Best regards,
Mark Empson
http://www.lmphotonics.com