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Topic: Stepper Motor Voltage (Read 1 time) previous topic - next topic

focalist

May 01, 2011, 05:16 pm Last Edit: May 01, 2011, 05:18 pm by focalist Reason: 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...
When the testing is complete there will be... cake.

RuggedCircuits

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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Grumpy_Mike

Quote
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.

Quote
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.


bubulindo


Quote
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.
Eu não sou o teu criado. Se respondo no fórum é para ajudar todos mediante a minha disponibilidade e disposição. Responder por mensagem pessoal iria contra o propósito do fórum e por isso evito-o.
Se realmente pretendes que eu te ajude por mensagem pessoal, então podemos chegar a um acordo e contrato onde me pagas pela ajuda que eu fornecer e poderás então definir os termos de confidencialidade do meu serviço. De forma contrária toda e qualquer ajuda que eu der tem de ser visível a todos os participantes do fórum (será boa ideia, veres o significado da palavra fórum).
Nota também que eu não me responsabilizo por parvoíces escritas neste espaço pelo que se vais seguir algo dito por mim, entende que o farás por tua conta e risco.

Dito isto, mensagens pessoais só se forem pessoais, ou seja, se já interagimos de alguma forma no passado ou se me pretendes convidar para uma churrascada com cerveja (paga por ti, obviamente).

focalist

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. 






When the testing is complete there will be... cake.

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