Power loss in the motor, phase gets very hot

[Allex]

Have a 36 V500W geared hubmotor. Was running fine untill I was riding it on 74V. Now when going back to 36V it is very weak, When going up for a very small hill phase wires and controller gets very hot (not the motor) The speed is only about 5km/h It was so hot that the phase wires has melted, I have replaced them.
Tried with a new controller but same thing there. Opened up the motor but nothing unusual there to notice.
Is the motor toasted?

 

[dnmun]

no, you damaged the controller. there a very few controllers made for 36v which will handle 74V and they would indicate if they are able to handle 74v if they can.

 

[Allex]

No the controller is fine. I have checked with a new 36V controller on the 36V battery. And when running on 84V I was using Lyens 84V Infineon. What else could it be then?

 

[fechter]

Possibly a shorted motor winding. If a winding is shorted there will be a lot of resistance to turning (backwards) even with the controller disconnected.

 

[Allex]

Tried to spin the wheel forward and back, it is smooth, no resistance at all!

 

[Trackman417]

Maybe your magnets got weaker from all the heat? That would be my guess.

 

[fechter]

Next you could try shorting the phase wires one pair at a time and see how this changes the resistance to spinning the motor. You should see a big increase in resistance to turning with some cogging when the phase wires are touching. If the magnets are toasted, you will only see a slight increase in resistance. Keep in mind the turning resistance of a shorted motor increases dramatically with speed, so you need to give it a good spin when testing. All 3 combinations of phase wires should give the same amount of resistance. Most geared hub motors have a freewheel so there will never be resistance in the forward direction. If there is no freewheel, it should feel the same in both directions.

If that checks out OK, I would suspect a fried hall sensor, but those controllers are supposed to detect a bad hall sensor and prevent the motor from getting power.

 

[hydro-one]

sounds like a shorted winding to ground, phase wire shorted to ground or each other, bad phase connection, bad hall connection or sensor. probably

 

[Allex]

Next you could try shorting the phase wires one pair at a time and see how this changes the resistance to spinning the motor. You should see a big increase in resistance to turning with some cogging when the phase wires are touching. If the magnets are toasted, you will only see a slight increase in resistance. Keep in mind the turning resistance of a shorted motor increases dramatically with speed, so you need to give it a good spin when testing. All 3 combinations of phase wires should give the same amount of resistance. Most geared hub motors have a freewheel so there will never be resistance in the forward direction. If there is no freewheel, it should feel the same in both directions.

If that checks out OK, I would suspect a fried hall sensor, but those controllers are supposed to detect a bad hall sensor and prevent the motor from getting power.

Thank you. Now when you say cogging I remember that when the motor was new I felt this cogging when spinning it in the air, when I spin now I font feel this at all. When shorting phase wires I can feel the wheel coggs but a bit more compared when it was new. And as you say, when I try to spin it more I feel more resistance.
So my guess would be that the magnets are gone, because I dont feel the cogging as when it was brand new?

 

[fechter]

I guess the other test is to try running the motor with the wheel up and see how fast it goes. With weak magnets, it will try to go very fast but not have any torque.

Most NdFeB magnets will be destroyed over some temperature around 100C. Some will not even take 80C. With the right equipment, they could be re-magnetized, but this is probably not a DIY project.

 

[Allex]

Spot on, The wheel have good speed and I can reach my maximum speed on flat ground but It takes ages to get there because of lack of torque.
That it, I need a new motor... Thank you all!

 

[Dauntless]

What are the chances of someone replacing magnets in that situation? I remember with slot cars you used to be able to get your batteries "Rezapped" as we called it. Any way for that with a bike motor?

 

[fechter]

What are the chances of someone replacing magnets in that situation? I remember with slot cars you used to be able to get your batteries "Rezapped" as we called it. Any way for that with a bike motor?

That is certainly possible, but it would be hard to find magnets that are exactly the right size to fit.
Re-zapping NdFeB magnets is also possible but takes a really big setup.

Another thing I've seen people do for wind generators is to use narrow rectangular magnets and tile them together. This is challenging, as they repel each other strongly and you have to use clamps to hold them in place while the glue hardens. If they are narrow enough, they can follow the curve of the rotor nicely.

 

[John in CR]

It would be easier and cheaper to find someone with the same motor with some other damage, and cannibalize it for the rotor with magnets.