Step motor question

Stepper motors are locked when not powered. If it does turn by hand when powered it means there´s something wrong with it. The driver which controls the motor can be programed and tuned to trip the motor when an overcurrent is sensed that is equivalent to back torque.

My experience with a (4 wire bi-polar) stepper is:

It turns freely if not connected at all but you can feel the steps,

Very hard to turn if wires are tied together.

I think locking is obtained by shorting opposing wires. (will measure it tomorrow)

Cant speak for every stepper Motor out there but the few I have had anything to do with No. I would imagine a stepper motor wouldnt be much use if it could freewheel

Most stepper motors employ a permanent magnet in the rotor. The poles or pole-pieces of this magnet attract the iron tips of the electromagnet stators. This attraction requires some torque to overcome, and prevents the rotor from free-wheeling under light load, and can certainly be felt if you rotate the motor by hand, but I would hardly call it a locked rotor. Over time the rotor magnet may become weakened (especially if the motor is kept and/or operated near large transformers, welders, etc.), and you can certainly feel the difference between a good and a really bad motor. I use a series of weights on a standard length lever arm to measure both the un-energized and energized torques. Generally, larger motors will have higher torques under both conditions.

Dick

Yes, a stepper motor will turn freely if there is no power (just like any other motor), that is assuming that the motor is disconnected from the drive. If the drive is connected induced current may be able to circulate, setting up an opposing torque.

To test the windings short out one pair (ie one coil) and try to rotate, you should feel a difference from the open circuit condition. A high quality S.M. will have a high opposing torque. They have permanent magnets inside, with very small air gaps, to keep a high flux and taking them apart can often seriously damage them. Jeff