# Thread: Motor Overvoltage Rule of Thumb

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## Motor Overvoltage Rule of Thumb

Forum,

I've read about the 10% rule for overvolting motors, and the question I had is does this 10% rule apply to the nameplate voltage of the motor, or the nominal system voltage?

For example, motors rated at 460v (system nominal of 480v), running off of a transformer supplying 495 volts:

460 x 1.10 = 506
480 x 1.10 = 528

Quite a difference in the two scenarios...let me know, thanks!

2. Motor should be fine with + or - 10% of nameplate rating of 460 volts.

The nameplate motor amps, efficiency, power factor, speed, etc. are all based on what those values would be if supplied at rated volts and frequency and output load is right at the rated horsepower.

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Is it safe to assume then that during an overvoltage condition, the amps would then decrease? Something is telling me that's not the case.

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If the motor manufacturer mentioned the 10℅ voltage variation allowance on name plate, it is okay. Otherwise, even such 10℅ voltage increase may result in loss of motor insulation life.

5. Originally Posted by adamscb
Is it safe to assume then that during an overvoltage condition, the amps would then decrease? Something is telling me that's not the case.
Induction motors the general answer is yes. Presuming frequency remains constant, they have a tendency to try to want to run at their designed speed regardless of load. Drop the voltage and they produce less torque, which results in speed reduction - but then because of the nature of an induction motor current goes up to try to maintain same output if the source can deliver the current it demands. Only can tolerate so much before things overheat though. That is the simple explanation, you want more details of what all goes on there are other members (Jraef is excellent with motors) that are better at explaining this.

But yes if you are only running a few percent over rated voltage - the current will be a little lower then it would be at rated voltage. Too high of voltage and you get into terms like high excitation or saturation and those also result in excessive heating and eventual winding insulation failure.

6. To further what kwired said, this is a fairly decent graphic example of the effects of voltage variation on 3 phase AC induction motors. It follows similarly for single phase in general, but because there are several different types of single phase motor designs, it's not as simple.

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Originally Posted by adamscb
Forum,

I've read about the 10% rule for overvolting motors, and the question I had is does this 10% rule apply to the nameplate voltage of the motor, or the nominal system voltage?

For example, motors rated at 460v (system nominal of 480v), running off of a transformer supplying 495 volts:

460 x 1.10 = 506
480 x 1.10 = 528

Quite a difference in the two scenarios...let me know, thanks!
Don't discount the voltage drop between the transformer and the motor terminals as well. You are likely a lot safer from motor overvoltage than you think.

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