I've played with steppers and stepper drivers before and got them working nicely but I'm no expert. Now I've bought a new driver and looking at a new motor. The motor says "Rated current 4.2A & resistance 0.9ohms ". My driver has different current settings one of which is "current 3.5 peak 4.0" and the voltage input calls for 9-42vdc. My question is how do you determine what input voltage to use? If I can supply 9V @4amps am I good? What reasons are there to use higher voltage?

Higher voltage = higher speeds at high torques possible. You got quite a big motor, rated at 4.2A.

My advice: Get a power supply up to 35..36V @ 5..6A (-> 24V minimum if 35..36V not available) (to stay a little bit under the maximum voltage of your driver) The bigger amperage is ok, guarantees that your power supply doesn't have to operate at its limits, which will guarantee a longer life for the supply.

Set the driver current limiter to a value close to a value of up to 4A. So the driver gets powered by 35 .. 36V and takes care in every operational situation, that the stepper doesn't get more than 4A (current limit); the motor's electrical limits will be taken care of by the driver.

9V @4A would work but you don't use the existing power of your motor.

rpt007: 9V @4A would work but you don't use the existing power of your motor.

How would I know this? Should the motors have a power rating?

Here's a link to the motor... amazon

Here's the driver... amazon

freakdaddy: How would I know this? Should the motors have a power rating?

The important number for a stepper motor is the current - in that case it is 2.8amps.

Within reason (i.e. below maybe 100v or wherever the motor insulation breaks down) the higher the voltage the better the motor will perform. PROVIDED that the stepper driver is set so that it does not allow more than 2.8amps to flow.

Good to know. Thanks!

Robin2: Within reason (i.e. below maybe 100v or wherever the motor insulation breaks down) the higher the voltage the better the motor will perform.

Scroll to section 6 where Mr. Gecko describes the appropriate maximum. For OP's motor that would be 32 * sqrt(5.4) = 74V.

freakdaddy: How would I know this? Should the motors have a power rating?

Stepper motors have a current, resistance and inductance, that's what matters.

There will be graphs of torque v. speed for particular supply voltages in the datasheet, which are worth checking on.

There is also a voltage rating for the winding insulation, which you may have to worry about if wanting to run at 80 or 100V or large voltages like that (typically only a worry for large industrial stepper motors driving heavy machinery).

Steppers top speed is limited by losses due to the very high pole-pair count, I don't think you have to worry about top speed, as far as I'm aware.

"For OP's motor that would be 32 * sqrt(5.4) = 74V.

???? Did the OP actually give the motor Inductance?"

Please show how you derived your formula. Mr. Gecko also talks of a voltage clamp on the Power Supply in terms of a Zener. It would have to be a "bigly" Zener to grab around 4 amps. A MOSFET and Zener making an active clamp, yes. Not just a Zener.

Hi, This has more info on your stepper motor including voltage ratings.

http://www.omc-stepperonline.com/nema-23-cnc-stepper-motor-28a-19nm269ozin-23hs302804s-p-25.html

Tom... :)