As far as choosing volts/amps the starting point will be winding resistance and inductance. A good multimeter with a 0 - 10ohm and 0 - 10mH ranges would do the trick here. Coil voltage isnt an issue, steppers are rarely run at the rated voltage, its current thats the key thing.

Estimating the maximum current allowable is harder. I believe the best solution is to energise the motor in the locked state (2 windings) and measure the temperature rise, gradually increasing current and allowing the temperature to stabilise The maximum RMS current allowable is reached when the case temperature gets to 60degC or so.


This might also help... how to estimate torque from an unknown stepper from Mr Gecko himself...

1) Step motors are essentially 'constant power' motors. That means the product of torque times RPM is constant at higher speeds, meaning torque is the inverse (1/x) of RPM.

2) What matters is power because it's what gets things done. Power is torque times RPM. Since motor power is a constant at higher speeds, all you have to do is measure power accurately at a single speed to computationally derive what your torque will be at any speed. How do you do that?

2a) What you need is a paper towel, a pair of channel-lock pliers (we call them mole-grips) and a multimeter.

2b) Dismount the motor. Take a paper towel and fold it over as many ways as you need to until it is about 2" long, 1" wide and at least 0.25" thick (50mm by 25mm by 13mm).

2c) Fold the 2" (50mm) length around your motor shaft ('U' shaped). Wet the paper towel very slightly if more than 50W is expected from the motor.

2d) Place your channel-lock pliers around the folded towel on your motor shaft. The pliers will be the brake calipers, the towel will be the brake 'shoe'. The towel will dissipate the motor's mechanical power (by boiling the water used to wet it it) while protecting the shaft from being gouged by the pliers.

2e) Connect only one motor (the test motor) to the driver. Set your multimeter to 'DC Amps' and put it in series with your power supply to the driver.

2f) Run your test motor up to 1,000 or so RPM. Do it without the paper towel on the shaft. Measure the power supply current and write it down as 'Amps no-load'.

2g) Place the towel and channel-lock pliers on the motor shaft. Very gradually apply pressure to the plier handles; watch the current as you do, it will increase. Keep increasing pressure while watching the current until the motor stalls. Catch the reading at the instant of stall. This may take several tries until you can sneak up to that point accurately.:-) Mark that current as 'Amps stall'.

2h) Subtract 'Amps no-load' from 'Amps stall'. Multiply the result by your power supply voltage; the result is your motor mechanical power output in Watts (Watts = VDC * (Istall - Ino-load)).

2i) The results from (2h) allows you to compute torque at any speed where the result is less than the holding torque (low-speed torque) of the motor. The general equation is:

in-oz = Watts * 1351 / RPM. If you prefer Nm, divide in-oz by 141 to get torque in Nm (Nm = 9.58 * W / RPM). If the test is carefully done, the accuracy will be within +/-5% of what you would get from a dynamometer.
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