Where to get stepper motor power supplies for oddball voltages like 3.6 volts?

ElectricAye

Until recently I had never wired up a stepper motor from "scratch" and I've been surprised to find that many stepper motors run off of very odd voltages. The motor I wanted to use works on 3.6 volts, 1.4 amps per channel in unipolar mode. Problem is, I haven't been able to find any relatively cheap power supplies that operate at that voltage. What's a poor boy to do?

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Watching the world pass me by, one photon at a time.

W9GFO

I run my steppers at 24 volts yet they are "rated" for something like 2 volts. There's a reason why you can do that - but I don't remember it.

Rich H

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The Simple Servo Tester, a kit from Gadget Gangster.

ElectricAye

W9GFO said...
I run my steppers at 24 volts yet they are "rated" for something like 2 volts. There's a reason why you can do that - but I don't remember it.

Rich H

Rich,
thanks for giving me a clue here but now I'm really confused. In agreement with your above statement, Wikipedia has this to say:

"Stepper motors nameplates typically give only the winding current and occasionally the voltage and winding resistance. The rated voltage will produce the rated winding current at DC: but this is mostly a meaningless rating, as all modern drivers are current limiting and the drive voltages greatly exceed the motor rated voltage." en.wikipedia.org/wiki/Stepper_motor#Stepper_motor_ratings_and_specifications

What worries me is that I guess I am making my own "modern driver" using the Propeller and some IRF3708 Mosfets, but I don't have any in-built current limitations in that, so unless current limitations are somehow built into the motor, I don't know what could be limiting the current.

Other tidbits I found on the internet suggest that you can run the supply voltage higher than what's rated because the voltage rating pertains to keeping the motor from overheating and not to "normal" operations. One website even suggested you should run your motor voltage at least 3 times higher than its rated voltage. So now I'm really feeling lost.

Why does life have to always be so confusing and painful?

kwinn

NPN transistor with the appropriate resistor on the base will give a constant current thru the stepper (as long as the voltage on the stepper is high enough) when the prop drives the base. For 1A a 3.3 ohm resistor will be close.

The alternative is an adjustable regulator to provide the correct voltage.

Phil Pilgrim (PhiPi)

The reason the rated voltage is practically meaningless is due to the inductance of the motor coils. If you try to run a stepper with a constant voltage supply, it's performance at higher speeds will be very poor. This is because, before the coil can even reach its rated saturation current, it will be either switched off or reversed. The way to speed up the coil's current buildup, and hence its running torque, is to increase the voltage drive to several times the coil's rated steady-state voltage. But this entails monitoring the current and "chopping" the drive voltage when the rated current is reached.

A simpler way to get the same performance, albeit much less efficiently, is to add resistance in series with the motor coils. For example, if the motor is rated for 3V steady state at 1.5 amps, it's coil resistance will be 2 ohms. By adding another 10 ohms in series with the coil and boosting the drive voltage to 18V, the motor will still draw 1.5 amps and "see" 3V in its steady state. But, when first energized, since its current draw is zero, it will "see" the full 18V, which will force a quicker current build-up. While effective, this "L-5R" arrangement wastes a lot of energy, which the resistors will have to dissipate as heat -- lots of it. In this age of constant-current PWM drives, it's almost never used any more.

-Phil

Leon

That's what I did recently when I was experimenting with a stepper motor. I used a large WW resistor and it got rather hot, unlike the MOSFETs, which stayed cool.

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Leon Heller
Amateur radio callsign: G1HSM

Peter Jakacki

You can drive these in open-loop PWM where you are not monitoring the current and just set the duty cycle to the equivalent of say 3.6/12 for a 12V supply. One problem is that while you are developing and testing software that the power to the windings may stay on and possibly burn out the motor.

So during testing you can just put a ~20W/12V halogen lamp in series with the supply to the motor. What happens is that the lamp's resistance is very low when it is cold but once you start drawing enough current for the filament to glow it will increase in resistance. Knowing that an 20W 12V globe will draw around 1.5A when 12V is applied will have the same effect when the series coil is left energized (appearing as a very low DC resistance) and thus limit the current to around 1.5A. So anytime you see that little sucker light up you know your software is trash. The other way to handle the fault current is to get hold of a polyswitch which is a lot more compact than a lamp plus you can leave it in the completed circuit. Once you draw too much current the polyswitch goes high resistance and draws just enough current to keep the polyswitch warm until you removed the power, then it cools almost immediately and you are back in business.

One other method is to make sure the excess current causes the processor to reset in which case the load is disconnected by virtue of the fact that the I/O lines revert to inputs.

The PWM object should have a frequency that suits the inductance of the motor but if it's too high you will have trouble driving MOSFETs hard enough directly from the processor. In fact it will be too high so it's actually safer to use a bipolar NPN, especially those with low Vce(sat) and high gain (Hfe).

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*Peter*

ElectricAye

Phil Pilgrim (PhiPi) said...
The reason the rated voltage is practically meaningless is due to the inductance of the motor coils. ...

A simpler way to get the same performance, albeit much less efficiently, is to add resistance in series with the motor coils. ... While effective, this "L-5R" arrangement wastes a lot of energy, which the resistors will have to dissipate as heat -- lots of it. In this age of constant-current PWM drives, it's almost never used any more.

-Phil

Phil, Peter, kwinn, Leon,

Wow. Suddenly this all makes sense to me now. It's nice to know there's a quick and dirty "fix" (this L-5R approach) and a less inelegant solution down the road (PWM).

you guys are great.
Many, many thanks for saving my sanity today,
Mark

ElectricAye

A quick little overview I found on this topic:

dkc1.digikey.com/us/en/tod/STMicroelectronics/StepperFundamentals_NoAudio/Stepper_Fundamentals_NoAudio.swf