Voltage drop issues for motors 1

[a10jp]

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?

 

[ccjersey]

Reduced voltage under full load will result in extra heating from increased current drawn by the motor.

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

 

[jraef]

I would not over characterize RV starting as absolutely having an appreciable increase in motor heating. As long as the acceleration is acceptable within the motor design parameters, the heating efect is not that much different. The energy curve for starting a motor from a dead stop is a fixed area. DOL starting has the curve very high but short. RV stating changes the shape of the curve to be shallower but longer. Yet the area remains the same, meaning energy going in to the motor is the same either way.

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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[Marke]

In addition to the good information above, if we consider the loaded motor case where the voltage drop is not large enough to cause the motor to stall, as you reduce the voltage applied to the motor, you will reduce the flux in the iron. This will reduce the iron loss, but will increase the rotor slip. The increased slip results in increased slip losses in the rotor of the motor. Additionally, if we consider the woprk component of the current, the reducing voltage will be countered by an increasing current to provide the same KW out the shaft. This will cause in increasing copper loss in the stator propotional to current squared.
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