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  #1  
Old Wed 08 April 2009, 10:31
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Why Mariss uses 32 X SQRT(Inductance) to calculate voltage of power supply

Marris Friemannis, the owner of Geckodrive and the designer of the Gecko stepper drivers that many of us use, published this on his forum (geckodrive group on yahoo.com).

Motor power increases proportionately with supply voltage while iron losses
increase as the square of the voltage.

The cause of iron losses is ripple current which induces eddy currents in the
motor's laminations. Ripple current amplitude is proportional to the supply
voltage and inversely proportional to inductance (V/L). Eddy currents are
proportional to ripple current.

The induced eddy currents generate heat as they circulate through the resistance
of laminations equal to I^2 R.

Putting all this together shows that heating is proportional to the SQRT of
inductance.

Motors have a maximum rated case temperature. The number '32' was empirically
derived from thermocouple measurements of motor temperatures to find the
multiplier that resulted in an 85C motor temperature when the applied voltage
was equal to the SQRT of mH and this multiplier.

Mariss
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  #2  
Old Wed 08 April 2009, 11:26
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
I am using the formula, but I certainly don't see 85C case temperatures. He must have measured the highest temperature rise and added that to a very high assumed ambient air temp of maybe 45 to 50C to reach the 85C figure?
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  #3  
Old Wed 08 April 2009, 11:46
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Gerald, I agree that I rarely get 85C case temperatures; however, when I run the motors at maximum Amps AND at maximum voltage, I get temperatures very close to 85C (above 80C).

With normal operation (meaning that the motors are pulling 50% to 75% of their rated current), case temperatures rarely get above 65C.

With the power supply voltage set to 75% of maximum OR with the current limiting resistors set to allow about 75% of the motor's maximum rated current (even with the power supply set to maximum voltage), I get about 65C maximum temperatures.

All the motors that I use from Oriental Motor can safely handle 85C, but I prefer to run them at 65C to 70C maximum. (That is strictly my personal preference. I have no data showing that running a motor at 65C would extend the motor's life or that running a motor at 85C would shorten the motor's life. I'm probably too old to start that test. I might expire before the test completes. )
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  #4  
Old Wed 08 April 2009, 12:05
bradm
Just call me: Brad #10
 
Somerville(MA)
United States of America
Gerald, are you factoring in the reduced current mode (71%?) of the Geckos when a motor is stopped? I bet Mariss chose the worst thermally performing motor he had, and then ran it continuously to get a conservative estimate.
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  #5  
Old Wed 08 April 2009, 12:51
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Brad, when I first got mixed up with stepper motors about 5 yrs ago, everyone was talking of the worst motor heating being at very low speed or standstill. However, that picture has changed with more intelligent drives becoming available. Also, in our application we have low torque (power) demands on the motors at low speeds. We have learnt that we see the most heat (and motor current draw) when the the motors are pumping out torques close to the point where they want to slip.

We only run motors near to the slip point when re-surfacing the table top, because a slipped step won't ruin the job and we want the job finished quickly with a big cutter. And we can feel this is where the motors get the hottest. I recall figures of 55-60C being about worst case. I can't recall ever touching a motor and saying "ouch" . . . . which would be case if they were around 85C. Maybe our summers are not hot enough. :-)

Anyway, this is all moot because steppers are quite happy to run hot and even 85C is okay - it is us humans who have the bad reaction when touching them.

Lurking around many forums, I cannot recall even one case where someone burnt a stepper motor because of overvoltage. Not even with the cheapo motors.
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  #6  
Old Wed 08 April 2009, 20:50
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
It suddenly struck me this morning that Mariss may have been finding his empirical constant with motors wired bipolar parallel, and that he may have come to a different conclusion had he wired them half-coil.

Also, he assumes a 100% duty cycle (continuous use at full load 24/7) which is very rare for our application. Most tools in our workshops are rated below 50% duty cycle. (a bit of a fuzzy concept, but definitely applicable to electrical motors used in tools)
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