Stepper motor: voltage vs. current ratings

[cnczane]

I've seen posts here like, "Those are 3V motors and ought to work fine with ~brandname~ controller."

The PowerMaxII steppers I have don't spec a voltage rating. Rather, they give "rated current" and a phase resistance.

However, depending on how they're wired, the "Voltage ratings" (which I'd describe to be "the voltage needed to get the phase current up to the rated current") are different, not what I expected.

For example:
Bipolar parallel 5.6A rated current into 0.23ohm ph resistance = 1.288V
Bipolar series 2.8A into 0.92ohm = 2.576V
Unipolar 4.0A into 0.46A = 1.84V.
ratings
Based on what's been written here, are these (approx.) "2V motors?" Or are they 1V, 2V, or 2.5V motors depending on how connected?

[Mariss Freimanis]

The only purpose of a "voltage rating" is to give an idea of what is the maximum power supply voltage the motor should be run at.

The ultimate limit is the motor case temperature. This limit will be approached (85C) when the supply voltage is around 20 times the motor's rated voltage.

This is an empirical number based on lab tests; it may not exactly apply to your motor but it will be close.

To simplify, the higher the supply voltage the greater the the power output from the motor. However, the higher the supply voltage the hotter the motor will be. Again, the limit to all this is the rated maximum temp of the motor.

The overdrive ratio (20 times the motor's rated voltage) is based on the unipolar voltage. I based it on that since most motors are 6-wire and I thought that would be the most helpful for the majority of people.

When I get the time (when the G2002 firmware is ready for prime time), I will run a new series of detailed lab tests on the new octagonal and cruciform motors. My data is based on the old, round motors. These new tests will take frame-size into account as well.

Until then, figure your motor as being 1.84V in parallel (unipolar I*R) and 3.68V in series (twice the unipolar value). This yeilds 36.8VDC and 73.6VDC using the present 20:1 ratio for max supply voltage.

Mind that PacSci rates their motors at a 65V max limit. PacSci is a class act that never indulges in specmanship. This limit is to guarantee the windings will not arc at max temp and destroy a drive in the process. PacSci motors are the Rolls-Royces of step motors; they give realist specs and limits most other mfgs won't to make their motors look better then they are. Observe that limit.

Mariss

[cnczane]

"PacSci motors are the Rolls-Royces of step motors; "

Nice to hear I got one decent component in this Trabant project... They are sweet.

Another interesting stat would be "conversion efficiency" which is to ask, "What percentage of power supply energy is lost as heat?"

I presume there is a heat-performance tradeoff along the lines of "lower heat losses means lower performance." It would be helpful to see that quantified.

Thanks for the insight.

[gmac]

Mariss,
thanks for the top quality reply, thats a great piece of work and its backed up by real-world facts, knowledge, and hands-on experience.
its great to see your interest and your ability to do destructive testing at this level. the result is a bridge to reality for both the engineer and the edge-riders.

there will always be hotrod ideas but the engine blows when the limits of the laws of physics and strengths of materials are exceeded -same on every high-performance track whether its a circuitboard or a hot-air-powered machine. ( no politics intended).

Now a question- this is for your OPINIONATED response so feel very free to opine
what should a mfg user apply as the realistic limit of a system that wants indefinite longevity (OK, 5 years of noncontinuous use so say 15% runtime on 5 days per week basis- gives us 200 straight 24 hour days running.) we think a hobbyist would get much less use on his system but then he may want it to run a lot harder than a production setup. does the 200 days compared to say a 30-day lifecycle for a hobbyist mean an operating environment increase of an order of magnitude, 100%, more? ( within the component limits, always) or is it much less. understanding that the failures increase nonlinearly with temp and other factors, where should the hobbyist draw his line at 'highest reasonable performance ' for component longevity while still squeezing performance ? After all- the hobbyist doesnt usually care about component cost because he will probably never replace them with the same part unless he really needs to - he probably will seek an upgrade or other replacement unless he is in a production environment when a critical part turns to smoke.