Results 1 to 7 of 7

Thread: Motor Overvoltage Rule of Thumb

  1. #1
    Join Date
    Apr 2015
    Location
    IL
    Posts
    182

    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. #2
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    37,504
    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.
    I live for today, I'm just a day behind.

  3. #3
    Join Date
    Apr 2015
    Location
    IL
    Posts
    182
    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.

  4. #4
    Join Date
    Feb 2013
    Location
    India
    Posts
    2,456
    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. #5
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    37,504
    Quote Originally Posted by adamscb View Post
    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.
    I live for today, I'm just a day behind.

  6. #6
    Join Date
    Jul 2006
    Location
    San Francisco Bay Area, CA, USA
    Posts
    7,886
    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.

    Name:  image.jpg
Views: 128
Size:  16.0 KB
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  7. #7
    Join Date
    May 2010
    Location
    Washington
    Posts
    95
    Quote Originally Posted by adamscb View Post
    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.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •