Stepper motor ratings

[thomasholley]

I am somewhat confused on stepper motor specs. I see a motor rated at say 5V and 1A. Most threads say run it at higher than rated voltage and you get more speed but less torque. More current gives more torque.

How do I calculate this? How do I know what my set points should be to get the best results from my particular motor?

In selecting a reasonable voltage and current for an application am I just to concern myself with the VA rating of the stepper, sort of like a transformer? If so then the ratings on the stepper seem ambiguous to me.

I just want to figure out how to go about selecting the right motors and drive system for a particular application, I'll ask torque questions later.

Thanks,
Thomas

[ESjaavik]

Higher voltage give higher torque except for very slow stepping where there is no difference. That is assuming the same current. You choose as high voltage as you can afford, and a driver that can deliver the current your motor is rated for.

Your assumed motor will draw 1 A if you put 5V over it at standstill then it is DC current and the resistance of the windings are what counts. Use Ohm's law. As you start to move the motor you don't apply DC but AC. Then the inductance of the windings come to play. An inductance don't like changes in current and oppose that, so you have to apply a higher voltage to counteract this. So Ohm's law does not work any more as the current lags the voltage. Increase the speed and at some point the voltage you have available will not be enough to drive the 1A current through the motor. At that speed the current and thus torque starts to fall rapidly with further increasing speed.

You can use as high voltage as the motor insulation is designed for, a chopping drive will at all times chop the current in exactly large enough chunks that your 1A will flow. Until the speed is too high for the available voltage to give that current. Then the chopper will not be chopping anymore, but give all it have. It's the "throttle to the floor" condition. Compare voltage to HP and current to torque.

I have tried steppers with 70V drives, and they can reach far beyond the speed you normally think of with steppers. At these speeds they run almost as smooth as any other motor. No knacking or growling.

[thomasholley]

ESjaavik,
Thanks so much for your reply. Please excuse my ignorance but I still have some questions.

When you say choose the highest voltage I can afford, does this refer to the motor or the driver, or both?

I can see Ohm's Law becomes more difficult under AC conditions since one must consider frequency and indictance of the windings, etc. in calculating the impedence of the stepper. It is good to know a chopper drive will ballance current requirements.

Thanks,
Thomas

[ger21]

You can give a motor up to 20x to 25x the rated voltage. You also need to make sure your driver can handle that same voltage. I think 70V is the max rating of Gecko 201's, which are probably the most popular drives you'll see here. (rated over 30V, anyway) There is some good information in the white papers at http://www.geckodrive.com/support.htbml?order_id= .

[thomasholley]

Gerry,
Thanks for the link. Those white papers answer a lot of my questions about sterrer motors.

Thomas

[Bent]

I´ve fund these steppers for sale. I´m pretty new at this so what kind of torque can i expect out of these. What kind are they BI or UNI polar. They are SANYO STEP SYN rated at DC 3V 2A 1.8 DEG step 8 wires go into them. I can´t read the type nr. due to bad picture. I can get 4 for approx. 50 dollar. Would that be a fair price.

Bent

[pigifly]

Originally posted by ger21
You can give a motor up to 20x to 25x the rated voltage.

So you say that if I have a 2.46V 1.82A unipolar motor, I can drive it with a tension up to 60V without damaging ?

In effect I'm actually searching for some diy controller that could handle 2,5 V without success, instead it's plenty of controller that can handle 50V and 5/10A.

Are this good enough for my motor ?

Pigi

[mikegaylor]

pigifly

Yes You can drive the motors with a higher voltage it is the current that is important not to exceed. The higher the voltage the higher the torque a high speed. You should not howerver drive the motor with more than about 35 X the motors rated voltage.

Regards

Mike

[ger21]

I'd stick to about 20x. jmo

[Fixxit]

You CAN run stepper motors at higher voltages, but you MUST have some way of limiting the current. Just connecting a 5 volt stepper to 30 volts is a sure route to smoke and disaster.

The better stepping motor drivers have a "chopper" circuit that will limit the average current to something that your stepper can handle. This is a manual (potentiometer), or software adjustment. It is not an automatic plug and play thing.

The other way of running at higher voltages is to have a resistor in series with the winding to limit the current.

[mikegaylor]

Hi Fixxit,

Yes you are correct that is why I said the current must not be exceeded.

Whilst were on the subject I'm looking for some beta testers my software.

The software is in two parts.

1. LPT port Driver which uses our own command language(FL language). This is used to drive the 3 axis router from the printer port in 95/98/xp

2. HPGL reader and simple drawing creation program. (Creates FL code)

As it stands at the moment documentation is very limited as I'm still writing the help file.

If you or you know anybody that is interested please let me know. contact me : mgaylor@flaminglamps.co.uk

Regards Mike Gaylor