Rear Hub motor magnetically locked

I was out riding my newly converted e-bike with a 1500w Amazon rear hubmotor kit. It was running really well, until it came to a grinding halt. I was barely able to walk the bike home as there was a tremendous resistance int he hubmotor. I disconnected the controller and tried to turn the rear wheel. I was barely able to move it. My first thought was a seized bearing. I removed one cover plate and checked the sprocket side bearing. It was ok. I then checked the wire-side bearing. Also OK. I then checked the phase wires for damage and continuity. All good. The rotor appears to be magnetically locked in the stator. I was able to remove it with a lot of pushing. The magnets seem good and there is no sign of burning.
I then cut the phase wires at the solder connection to the wiring harness. Still no movement. I re-assembled the motor and it is nearly impossible to turn.
I'm worried that the motor is shorted in the coils. Can you point me in the right direction to attack this? I have a new hub-motor coming as a spare, but I would like to fix this one if I can.

Welcome to the forum.

If you had a motor with a short in the coils, it would be fairly easy to turn at low speeds, but the faster you turned it, the harder it would be to turn. The motor needs to move to generate enough power in the coil to cause a magnetic resistance. That magnetic field only works when the coil is in motion relative to the magnets, so it's actually impossible for it to ever lock up.

It sounds like you've had a magnet break lose from the ring, and jam up the motor. Their just glued on to the ring in most motors.

You have a mechanical issue but don't worry about the coils, they tend to short to the stator eventually on all those cheap hubmotors.

Yes, one of the winding is shorted.

Have experienced this twice in last weeks - this on lower powered system going uphill - at exact same location and high temp (+30°C ambient air). After hub seems permanently magnetized - creating extra roll resistance and making for a very sweaty ride home. Swapping controller (cheap Chinese of exact same type) - no change.
After disassembly of hub motor to check for issues - (& repairing bad solder on a phase wire) - magnetic resistance disappeared and motor ran again on same controller - until yesterday that is - same cut-out and magnetic resistance again. While I assume that the controller can use better cooling to prevent cut-out the fact that the electro-magnets appear to retain their magnetic force has me puzzled.
Will continue TS at first opportunity but any input from members on this forum would be highly appreciated.

Sounds like a shorted winding. When a winding shorts, the motor can move fairly easily at very low speeds, like by hand but will be very hard to turn quickly.
If you disconnect the 3 phase wires from the controller and you still have this resistance, then the motor is toast. More typically the controller shorts and the motor will spin freely once you disconnect it from the controller.

You can also get phase wire insulation damaged or even melted inside teh axle, so that a bit more stress, or a temperature change (like heat expanding the metals) can cause a temporary short.

Pulling the wire into the motor thru the axle a few inches to examine it might be useful.

@fechter thanks for thinking with me - to clarify - rolling resistance does not seem related to speed - you can distinctly feel the magnetic resistance as you turn the axle - as you rotate you feel it going from field to field (double checked the bearing - all fine).
I've also checked the wiring again - and measured the electrical resistance of the coils - (very low but not zero) but did not get any wiser. Controller completely disconnected - no difference. Rotor taken out and reinstalled - no difference.
@amberwolf also thanks - reading the above can you think of any other way to establish beyond reasonable doubt that the motor is toast (remember that first time it recovered - i wished i knew what i did to make the magnetic resistance all but disappear)
If its not to hot later in the evening ( +40°C forecast ed) i'll look into this further - above all thanks to both for your input!

You could try taking apart the motor and disconnecting the windings from the section going through the axle. Most motors will have a junction between the two parts of the wire. Then see if you still have turning resistance. If you are lucky, just the section going through the axle is shorted and can be replaced. If you still have turning resistance, then one of the coils is shorted. While it is possible to re-wind all the coils, it's probably not worth the effort.

cjan said:

@amberwolf also thanks - reading the above can you think of any other way to establish beyond reasonable doubt that the motor is toast

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To eliminate axle wire shorts as a problem, you can open the motor cover on the side where the phase wires connect inside, and cut the axle phase wires from the motor phase windings. I'd do this with a few inches of wire left on the motor phase winding side so you can resplice them if that turns out to be the problem, without having to resolder to the actual motor windings (PITA).

If the problem still exists with the axle wires cut, then the motor is toast (windings shorted somewhere).

if you've already got the stator out, then it's easy to pull the wires in a few inches and examine them first if you like. Or just pull them in, then cut off the whole section that used to be inside the axle, and splice the wires back together inside the motor, without bothering with the examination.

The most common three ways a motor shorts:

-- the axle wires being twisted and insulation damaged, because of no or insufficient torque arms to prevent axle rotation

-- the axle wires being damaged at the axle exit by impact, scraping, shipping damage, etc.

-- the windings (or axle wires) insulation melting from severe overloading / overheating of the motor, causing shorting between phases. Often the windings are dark brown or black, somewhere if not everywhere, and there's a smell you can't miss.

The windings shorting are not repairable without rewinding it, in most cases, and it's easier and often cheaper to just replace the whole motor.

Axle wires are easy to replace, other than getting them thru the axle in the first place, which is why I recommend pulling the existing wire in from the inside of the motor; it's easier to do that than run it out from inside. If you need more axle wire than you're left with, http://ebikes.ca sells it by the meter.

(remember that first time it recovered - i wished i knew what i did to make the magnetic resistance all but disappear)

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If you moved the cable around at all, then if it was shorted internally that can break the short temporarily, until it heats up again enough to short again.

If its not to hot later in the evening ( +40°C forecast ed) i'll look into this further - above all thanks to both for your input!

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Yeah, it's hot here, too, and I don't do that well in the heat anymore, so I don't get much done in the summer.

Torn apart the hub motor and checked the wires going through the axle - no damage to be found. Reassembled and tested with an identical controller to the one I used - no difference. Looks like I'll be on the lookout for another 36 volt hubmotor that can take 9 gears

Very odd you can't see damage at the wires. The motor is definitely shorted. Disconnecting the wires entirely from the windings would show you if the short really is in the coils.


Most shorted coils are easy to diagnose, they are black, stink like hell, and clearly had the varnish burned off the coil wires.

Lots of effort needed to remove the core from the magnet ring is normal. And by now, you learned how carefully you need to put it back. Snip off a fingertip easy.