Stepper motor input power/ efficiency calculation

I hooked one up once to see how well it worked as a generator. I was not impressed. Some of these things are not exactly efficient when they have been designed for efficiency in one mode of operation. I believe that the limiting factor is the field strength and arrangement of the PM's in the rotor. The poles are very close together which effectively slows down the rate at which the field cuts the wires. Maybe ok if you ran it at a really high speed but I'm just guessing.

A stepper used as a generator/alternator will produce a pretty low voltage, but should work fine if all you want to do is light a few LEDs or something equivalent.

One quick way to get a rough idea of the efficiency is to compare the torque required to spin the motor with all its leads open, with the torque required to spin the motor at the same speed with all its leads shorted together. A good motor will require significantly more torque with its leads shorted.

Of course a better way would be to choose some standard load (an appropriately sized, both value and wattage, resistor), then connect the shaft to a constant speed source and measure the voltage and current.

Thanks for your reply. I know from the electro-mechanical energy conversion class here at OIT that e=- dphi/dt which relates the output voltage to the rate at which the magnetic flux is cut by the loops of wire in the stator. Since the stepper motor I am looking at has 1.8 degrees per step, the number of flux reversals per revolution is 400 (200 Hz). So even at low rpm (30-90 rpm) the voltage generated should be significant. I know the value of the coil resistance so I could terminate the motor with the same value and see what the output power (voltage & current) is for this range of rpm. I am looking at the Qmot QSH 6018 series of steppers from Trinamic. These look like 90 watt motors. I need 10-15 watts @ 30-90 rpm as a generator. DC pancake motors might work but I believe the rpm would be too low. However, in another post someone mentioned that since the poles are so close together the magnetic flux level (pole to pole) is relatively low. Again, thanks for your comments.

Are you referring to designing stepper motors, or designing machines powered by stepper motors?