instantaneous current limits in dc motors?

For AC induction motors, without brushes, the maximum current is self-limited to about 3-5 times the rated current, and a thermal motor controller will limit that "stall current" to about 5 seconds.

But for series wound or permanent magnet DC motors, much higher currents can occur. You are probably safe to use a circuit breaker or fuse rated at the normal current value, although a slow blow fuse or a time delay circuit breaker could allow 3x current for as long as 60-90 seconds, which may be too much.

If a motor has an overload spec, you might be able to use that for an I^2t rating. So a motor that is rated at 10 amps, and can run at 20 amps for 10 seconds, has an I^2t of 4000. So you can run 32 amps for 4 seconds, and 63 amps for 1 second. You must also allow a cooling off period between bursts, and this is also a square law function. At 2x the duty cycle, ton/(ton+toff), is 1/2^2 = 25%, at 3x it is about 10%, and 5x it is 4%.

But the brushes might limit the safe overload to a lower value. At some point there will be excessive arcing and localized heating which could cause damage and reduced life of the brushes and commutator. These figures are related to I^2R heating effects, and are commonly applied to transformers.

Brushless DC motors have their own limitations due to the solid state components, and should have overcurrent protection built into the controller.

BTW, there is no such thing as a DC motor. It must be converted to AC by some means to make it continue to run. The only true DC machine is a solenoid. There is also no such thing as DC. The current had to start at zero at some point in time, and then it took a finite time to reach some level, and eventually it will return to zero. So it's really a rectangular waveform of (perhaps very long) period. :)

Paul

"Brandon"

You should not exceed the max voltage rating at all.

High current will flow if the motor is stalled or overloaded.

Just how much and for how long is OK depends on the motor.

...... Phil

I can't tell you details, because getting such information from the motor manufacturers can be like pulling teeth (or perhaps finding hen's teeth).

I can tell you that graphite brushes are more tolerant of overload than precious metal, and that AlNiCo magnets will demagnetize with too much current, probably instantaneously.

I can also tell you that in one application we routinely subjected some of the Maxon 16mm graphite-brushed motors to the full stall current with a slow PWM voltage. The only failure mechanism that I know about was the magnetic encoder wheels spinning off of the shaft from the angular acceleration.

It's specified on the motor..

Well, there are homopolar motors, but they are rare except as lab demonstrations.