Locking stepper motor?

I was moving the carriages slowly up and down by hand to see if I could notice any obstructions from cabling etc. Of course the power was disconnected during this. I noticed my x tower motor offers very little resistance when moving the carriage. What's more if I move the carriage several times it intermittently locks so no movement is possible until you move the carriage in the opposite direction. Their is no pattern to this, can happen in either direction. My other to carriages on y and z do not feel the same as they offer more resistance and do not appear to lock up. This got me thinking... when I am printing or calibrating I occasionally hear a clunk sound I have come to realise my print will come out awful when this happens or my calibration figures go eerie. Just trying to figure out what is normal...

Stepper motors generate a voltage/current when driven externally, so when you move them fast enough, that voltage will (partly)revive the stepper drivers, which may put them in a conducting state where moving the motor requires much more force. You will feel a much larger cogging, but it should not be impossible to move all together.
If the carriage is refusing to move at all I think you have a mechanical problem, probably not related to the stepper motors.

If the X tower motor offers very little resistance when moving the carriage compared to the other ones, perhaps the pulley grub screw is loose?

Stepper motors occasionally contain small fragments of debris which can cause them to lock up. It's usually possible to dismantle the motor and clear the debris.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks Jelle and dc42.

I decided to take the motor off the printer and test it to be sure it wasn't anything else.
The motor locks very easily when rotating the shaft. I have ordered another motor today from RepRapPro do see if it cures my x-axis problems.
Fingers crossed...

I've only had that happen to me after opening a motor to look inside. It is probably quicker to open and clean than to wait for shipment. A bit of tape stuck on and removed from the rotor will remove most magnetic debris that is in there, the non-magnetic stuff will just fall out after opening. I can be a bit fiddly to reassemble the motor properly, but you can allow yourself a few tries. A smoothly turning rotor is proof of correct assembly.