# Peak power



## major (Apr 4, 2008)

mrmeseeks said:


> When motors have peak powers stated without a time is there a typical amount of time that it can run for before overheating or damaging the motor. Also is there some sort of rule of thumb I could use to calculate how long I could run the motor at a certain percentage of peak and or a certain percentage of the difference between peak and continuous power? It would also be helpful to have a similar rule of thumb for motors that have stated times for their peak powers but more so for the ones that don't.





major said:


> Yes, peak is universally used for an instantaneous value. Certain manufacturers may choose to further quantify it with a duration. If you see it used without a time duration, it is instantaneous. With a duration, use that time.


I did address that for you. We'll see if anyone has a better answer. Unless you have a power vs time rating curve from the motor manufacturer, you're just guessing. Don't assume anything. There are really no rules of thumbs which will be honored with warranty. You damage the motor due to overload, it's your problem.


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## mrmeseeks (Jun 14, 2014)

I think it's pretty safe to say that reaching the peak power will not instantaneously damage the motor, it may start overheating immediately but hopefully overheating will not immediately destroy anything. Similarly running the motor above the continuous power level but not reaching the peak power should allow for some running time before the motor begins to overheat.

If anyone has any actual knowledge about how long electric motors can be run above their continuous levels that would be very helpful.


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## major (Apr 4, 2008)

mrmeseeks said:


> If anyone has any actual knowledge about how long electric motors can be run above their continuous levels that would be very helpful.


Not all motors are the same in this regard. It can be vastly different from one motor model to the next. Here is a discussion by members concerning a similar topic on a DC motor. Maybe that will give you some insight. 

http://www.diyelectriccar.com/forums/showthread.php/kostov-11-250v-wiring-construction-96269p2.html


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## mrmeseeks (Jun 14, 2014)

That is what I figured unfortunately


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## sunworksco (Sep 8, 2008)

A heat sensor can be installed near the motor windings and an appropriate motor controller can be used that can save the motor from overheating.
Overheated motors usually fail when the heat melts the winding resin and the fields sort-circuit.


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## kennybobby (Aug 10, 2012)

i occasionally do acceptance and characterization testing of electric motors using a hysteresis dynamometer. Many years ago before the tubes were invented i talked to the design engineers at Inland Motors, later known as Kohlmorgen, later as Danaher, about the time duration for testing at peak current (stall torque) for BLDC PM motors. Their reply was that the motors were designed to survive 10 seconds at peak and recommended not to exceed 10 seconds. So the rule of thumb that i derived for closed-loop operation with current and speed control was to not test at the peak rating for more than 5 seconds, and knock-wood, i have never burned up any customer's motor.

The brushed DC series-wound motors (and AC induction) being used in diy EVs can be driven to nearly infinite currents with 1 MW controllers! and there are plenty of sad stories of southern-fried motors. These motors seem to be copies of old designs and have little detailed technical specification and probably no heritage design knowledge available to know what the peak rating might actually be. 

European motor ratings (S1, S2, etc.) are based upon duty cycle and time to reach temperature and may be a good place to look for developing rule of thumb factors. 

Otherwise rules of thumb are based upon experience. But this is new ground to be developed and pushing the envelope is the pioneering spirit needed at the foreskin of technology.


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## PStechPaul (May 1, 2012)

As mentioned, the limit is determined by the temperature at the hot spot of the motor, and the temperature rating of the insulation. The time required to reach the critical temperature depends on the power dissipated and how fast the heat can be removed by whatever methods are provided, which can be convection, forced air, and liquid cooling. For transformers, such as those I use in circuit breaker test sets, the continuous rating is true continuous, and above that it is duty-cycle rated and time limited, based on I^2. So it may go like this:

1x continuous
1.4x 50% 30 min ON, 30 min OFF
2x 25% 10 min ON 30 min OFF
3x 10% 1.5 min ON 15 min OFF
10x 1% 0.1 sec ON. 10 sec OFF

Similar ratings may apply to motors, although generally with shorter ON times. Induction motors are generally limited to a maximum of 2x to 4x because of breakdown torque limit, but series wound motors have essentially no limit and may possibly put out 10x power but only for a few seconds and probably with significant damage to brushes and commutator.


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