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I am new to PWM control. Is it advisable to operate a motor at higher than rated voltage with the "PWM average voltage" more than the nominal voltage specified in the datasheets? After some researching, the majority of the information that I kept seeing is that it should be OK (with a recommendation to operate at high PWM frequency). Secondly, if the "average PWM voltage" is kept less than the nominal voltage (with the supply voltage much higher) is it correct to refer to it as over-voltage operation? Or is it only when the average voltage is higher than nominal voltage?

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  • \$\begingroup\$ Just a small point regarding "From other forums, ...": This site is not a forum, it is a volunteer-expert driven question & answer site. \$\endgroup\$ May 21 '13 at 15:57
  • \$\begingroup\$ Apart from the insulation/arcing issues Olin explained, it is the power transferred to the motor that needs to be limited to a safe value. When pulsing the supply "quick enough", as with PWM or for stepper motors, the max. PWM voltage may often well be above the allowed average DC voltage because the current and thus the power actually drawn by the motor will be limited by its own inductance. In other words: A (fast) PWM signal of a given calculated average DC voltage will usually transfer much less power to the motor than an actual DC voltage of the same magnitude would. \$\endgroup\$
    – JimmyB
    May 21 '13 at 16:13
  • \$\begingroup\$ @HannoBinder how do you figure that PWM transfers "much less" power? I suppose there are some switching losses, and some losses due to the current ripple, but assuming a reasonable implementation, these are so small I'd say "just a little less", not "much less". \$\endgroup\$
    – Phil Frost
    May 21 '13 at 16:49
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As long as the PWM frequency is fast enough, it's average voltage is what counts. No, the average PWM voltage should not exceed the motor's rated voltage, at least not for long. This is no different that applying a DC voltage to the motor.

Using a high voltage supply and then less than 100% PWM to compensate is a perfectly legitimate way to run a motor, again, as long as the PWM freuqency is fast enough. In effect you are creating a switching power supply that converts the high voltage to the lower one used to drive the motor. It may not look that way because the induction of the motor widings are a integral part of this power supply.

It can be useful in controlling a motor to occasionally for short periods of time drive it with a higher than rated voltage. If using this kind of control scheme, care should be taken to guarantee that the overvoltage does not persist. You could go so far as measuring the temperature or calculating the energy dissipated by the motor along with the assumed decay to ambient to calculate the maximum drive you are allowed to apply at any instant.

You can't go too far with this. Generally 1.5x the specified drive voltage will be OK for short periods of time, especially when heating is taken into account. At higher voltages you will hit other limits, like arcing and insulation breakdown. Note that these apply to the peaks of the PWM waveform, not the average.

For example, a "5 V" motor run from 30 V with a PWM duty cycle of 1/6 at 25 kHz will likely be fine. 30 V insulation capability pretty much happens by applying any insulation at all. However, the same factor of 6 applied to a 110 V motor would be a bad idea without consulting the manufacturer. 660 V applied to a "110 V" motor could cause damage, even if this is only at the peaks of the PWM and the average is still within specs.

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    \$\begingroup\$ related: I made a demonstration of how a PWM driven motor is a switching power supply: electronics.stackexchange.com/questions/56170/… \$\endgroup\$
    – Phil Frost
    May 21 '13 at 16:46
  • \$\begingroup\$ 1. When you talk about 1.5x the specified voltage are you referring to the avg voltage or the peak? 2. www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/… Excerpt from this site: "Motor full-load efficiency is at a maximum between nominal voltage and about 10%" I have been told slight overvoltage usage is quite common. could this be a reason? \$\endgroup\$
    – Rustin
    May 22 '13 at 12:38
  • \$\begingroup\$ @Rustin: 1: The average. This is the same as driving the motor with a DC voltage 1.5 times its rating. 2: Not sure what you are asking, not following link. Generally the reason for driving with a modest overvoltage for short periods of time is that some kind of servo system is driving the motor and this allows a little positive pull even at full voltage. Again, you can only do this for a short period. Doing it long term can overheat the motor. Sophisticated systems actually model the temperature to know how much overdrive they can get away with at any time. \$\endgroup\$ May 22 '13 at 13:00
  • \$\begingroup\$ I was trying to understand constraints under which one would want to use overvoltage for a motor. The link above came up during a google search regarding the same. From the excerpt that I mentioned above, it seemed that slight overvoltage operation is actually desirable for higher efficiency at full loads. \$\endgroup\$
    – Rustin
    May 22 '13 at 13:26
  • \$\begingroup\$ How do you figure out what PWM frequency is fast enough? How did you arrive at 26 kHz in your example? \$\endgroup\$
    – dca
    Feb 25 at 5:41

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