RMS versus Peak Amps - Stepper Driver

[kochevnik]

I was wondering if someone with more knowledge than I have can clear something up for me - a lot of the stepper drivers give peak amps and/or RMS amps ratings, while the stepper motors themselves are rated in amps/phase.

Let's say for example, that I have a 2.8 amps/phase stepper motor (bipolar) - exactly WHAT amp rating (RMS or peak) should I be looking for to get a driver capable of efficiently driving this motor ?

Any experts shed some light onthis subject ? Thanks.

[HFOSM]

Radial mean square is Peak X .717. Most everything I am aware of is in RMS however, RMS and Peak are relative to voltage and not current draw. If the stepper is 2.8 amps and your concern is current draw would you not just make sure you can supply a 150% of rated current. I typically go overboard with safety margines.

[gar]

061011-1528 EST USA

kochevnik:

I can not answer your question, but I want to clarify RMS and peak for you.

RMS stands for "Root Mean Square". This can be applied to any measurement --- voltage, current, surface finish, light intensity, etc.

There is some implication that you are dealing with a steady state signal and in the case of a random signal that it is a stationary process.

The RMS value is calculated by taking the instantaneous square of the signal, averaging this over some adequate period, and taking the square root of the average. You would normally do this using calculus, but you could approximate the value graphically.

In the case of a sine wave signal with an average value of zero the result over an integral number of whole cycles for the RMS value is 1/2 square root of 2, or 0.707 times the peak. If the signal is a square wave with zero average value, then the RMS value equals the peak value. A DC signal has an RMS = peak.

The RMS value of a sine wave provides the same heating effect as the same value of a steady DC signal.

It should be noted that meters such as a Simpson 260 are calibrated for AC to read the RMS value of a sine wave. But the Simpson 260 is actually measuring the full wave rectified average value of the sine wave. This average value is 0.636 times the peak. Thus, a 260 in AC position, but not output, will read higher than in the DC position. This ratio should be about 0.707/0.636 = 1.12. An experiment on my 260 produced 5.4 in the AC position with 4.95 DC input.

The real concern with your motor is it's maximum internal hot spot temperature. RMS current to your motor in combination with the motor resistance is part of the heat source in the motor

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[mcpltd]

This has been discussed in another thread, but to make it easy for you you should set it to the peak value.
In other words if you motor is rated at 2.8A you should set your driver to 2.8A peak.

[bunalmis]

You can use label value for max current if the motor stopped or low speeds.

But if motor turn at high speed and if you use microstep label value must rms.