Stepper Motor Voltage

Other Hardware Motors, Mechanics, Power and CNC
1

This may seem a silly question, but here we go:

Though steppers are rated for specific voltages (with associated currents) per winding, typically 12v, 24v, or 6v.. is there (within reason) a problem with running a stepper at other than it's specified voltage, other than the current draw will of course change in accordance with the winding's resistance?

Here's where Ohm's Law tells me it shouldn't matter if I run a 24v stepper at 12v, but it will draw more current. I'm also assuming it may not have full torque operating at other than it's design voltage... but we're talking about a hack here.

Here's the situation:

I've ripped apart a couple of printers for the motors, and am working on hack based upon the harvested steppers. I have two sets of motors which are roughly similar in terms of size (assuming torque is somewhat equivalent, high torque isn't needed for the project), one set that might be up to the job, which are smaller, and have voltage ratings of 24v and 12v. The second pair are a lot heavier duty, but have ratings of 6.5v and 24v respectively. They are much more capable motors, especially since I may increase the load later and want more guts.

I've cannibalized MTA001M Darlington Arrays from the control circuitry, these are good for a couple of amps per output, so I figure I'll be good in terms of current capability.

The question is really which set of motors I should choose, if I drive them from the same voltage? Oddly, the PS didn't provide 6.5v directly for the big stepper, and I don't want to overcomplicate things. I realize I'm going to end up using two MTA001M's, as I'm going the two-line stepper control route (requiring six channels) which is fine since I salvaged two.. so I could use different motor power feeds.. but KISS and all..

SO, is it okay to run steppers at a LOWER voltage than designed, and is it okay to run them at a HIGHER voltage than designed.. as long as you are not look for optimal power? Assume we are talking about 12v motor supply power for this purpose...

2

It is OK to run steppers at lower-than-rated voltage. They will simply carry less current and deliver less torque. It is NOT OK to run them at higher-than-rated voltage (continuously) as the current will be excessive and you can destroy the coils if the motors are left to overheat.

It is, however, common to run steppers at higher-than-rated voltage but using PWM (e.g., analogWrite function) to deliver a lower average current, within specifications. So a 12V motor run at 24V should have its PWM duty cycle limited to 50% (e.g., analogWrite(127)).

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The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons

3

Here's where Ohm's Law tells me it shouldn't matter if I run a 24v stepper at 12v, but it will draw more current

Odd my version of ohms law says that if you run a 24V motor at 12V you gel LOWER current.

It is, however, common to run steppers at higher-than-rated voltage

Just to explain why this is. The faster the motor runs the less torque it has. Running it at a higher voltage increases the torque at higher speeds.

4

Grumpy_Mike:

Here's where Ohm's Law tells me it shouldn't matter if I run a 24v stepper at 12v, but it will draw more current

Odd my version of ohms law says that if you run a 24V motor at 12V you gel LOWER current.

Mine says the same too... We must be on to something.

5

LOL, my bad... realized that once I ran a few numbers and it just didn't look right. See, I told you I have an unhealthy disregard for the law...

I take it that using PWM is the method I saw referenced as using a "chopper". Though the overvoltage causes more heat, the intermittent pulsing compensates for that, effectively "dimming" the motor like you might dim an LED.. I should read more on that I assume.

Right now have a prototype in progress and have built a stepper controller for two steppers (two line method) using eight 3904 transistors, four for each stepper. I've got it running at the moment, started with 5v to test it and then have switched to the 9v "Vin" to drive a single stepper. Though rated for 24v, the stepper runs well, doing stepping at 20RPM at 9v... though very low torque. I'll test at 12v shortly but I foresee no problems.. that will come from a decent external power supply capable of a few amps at 12v. The 3904's are only rated for 200mA each or something like that.. but they were in the bin. I'm hoping I can fully test the proto before I burn them out and replace them with a TIP120 or other gutsy NPN.

6

Though the overvoltage causes more heat, the intermittent pulsing compensates for that

Yes that's right although the over voltage causes over current and it is the over current that causes over heating.

There are two forms of current chopping. One where you simply put a switched PWM signal into the motor control. The other that actually measures the current through the coil through measuring the voltage across a small resistor in the ground return path. Then when it sees that the current has reached the set value turns off the voltage. This is the better one but harder to implement.

7

Alpha test of wall/whiteboard plotter :

2n3904's were used to make the stepper controller.. I plan to re-use the darlington arrays salvaged from the printers for the finished version, but I wanted to build the stepper controller from discrete components to help teach myself a bit more. Tom Igoe's stepper motor reference and the linked info from the Stepper library section of the Playground have been tremendously useful. Using a two-wire control setup, I'm only using Digital 2-5 to control both steppers.. which is good because I plan to add another axis as well as a print head of sorts.

For this test, the steppers (one rated for 12v, the other 24v) are just being powered by the 9v "Vin" on the Arduino, drawn from a wall wart.

Even underpowered that much, they seem quite up to the job. They get a bit warm and hum a bit, and I suspect the motion isn't what it could be in terms of smoothness.. but it does work. I think adding an external 12v supply makes sense, the wall wart isn't a very big one, and on general principles I don't like drawing real power across Arduino board traces...

8

The key thing to avoid with stepper motors is to avoid over current - older motors would be demagnetized by over current (or even by disassembly), even if only briefly. In the long term over current will cause overheating too.

For high performance switch-mode drivers are used that provide whatever voltage is needed to overcome back-EMF without exceeding the max current - this means much faster speeds from the motor (and because the controller is switch-mode it doesn't waste power from the high-voltage supply to do this).

However this is more complexity which isn't generally required for low speed use.