Help me save my new motor!

[Darthpaul]

After I suspect frying my first hub motor I’m after some advice/confirmation to save my new one before I kill it again!
I’ve got a 250w rear hub motor (AKM) rated as 36v. It’s mated to a 48v battery through a 9 mosfet controller. What I’ve learned recently is that although my controller seems to put out 22a, little 250w motors seem to have a max of 17a. So I’ve set my controller to limit the ampage by a factor of 1.5 so should now only be putting out 15amps. Does this sound ok? Also more research has shown that there’s a sweet spot of motor speed to current input with lots of talk of not going at less than 50% of the motor speed or excess current is turned to heat. Am I right in thinking this is only an issue when giving it max current but struggling up a hill at say 10mph? If I’m going 6mph on the flat and only using a few watts am I ok?

 

[WheezyRider]

After I suspect frying my first hub motor I’m after some advice/confirmation to save my new one before I kill it again!
I’ve got a 250w rear hub motor (AKM) rated as 36v. It’s mated to a 48v battery through a 9 mosfet controller. What I’ve learned recently is that although my controller seems to put out 22a, little 250w motors seem to have a max of 17a. So I’ve set my controller to limit the ampage by a factor of 1.5 so should now only be putting out 15amps. Does this sound ok? Also more research has shown that there’s a sweet spot of motor speed to current input with lots of talk of not going at less than 50% of the motor speed or excess current is turned to heat. Am I right in thinking this is only an issue when giving it max current but struggling up a hill at say 10mph? If I’m going 6mph on the flat and only using a few watts am I ok?

Increasing the volts is not a problem, but running a small motor at 22A is a bad idea. 15A should be fine, but I would test the output with a power meter to be sure you have 15A. How have you done the modification to get 15A from the controller?

I'm running a Bafang SWX 250W 36V at 59V and 15 Amps on one bike and that runs fine. My other motor is a Voilamart 250W 36V, which is set up at stock voltage at the moment, but previously I've run that as high as ~72V without problems, but again, only 15A. To improve cooling in these little motors, some people have added automatic transmission fluid (ATF) to the hub. It improves the heat transfer from the coils and magnets to the case and ATF is low viscosity, so drag is not increased too much. It also helps with the lubrication of the clutch, which will often seize on these little motors due to poor lubrication from the factory applied grease. Furthermore, it helps lubricate and cool the nylon cogs, so you are less likely to get cog stripping. I haven't tried this yet, but I will do at some point.

What heats the motor is rapid acceleration from standing starts, steep hill climbing etc. With the number of Watts you'll have available at higher voltage, you will in most cases hit 25kph quickly and then the controller will cut the power anyway, then give top up pulses as and when needed, so the motor isn't likely to get too hot.

 

[Nealh]

Unless you use PAS5 or throttle you won't get near the 22a current output, though in general yes if upping the V's reduce the current.

 

[Darthpaul]

I think that’s what did my last one in. I had that initial thrill of ebiking and was hitting the throttle and never coming out of PAS5!
I’ve cut the amps through the display in C5 if I remember.
That’s interesting about the ATF fluid. I’m going to attempt a repair job on the old motor so may drop some fluid in that if I can revive it.
So theoretically although it’s a 250w nominal motor, having 48v at 15a sending 700w to the motor for a sustained time isn’t too much of an issue? Particularly if it’s running at a decent speed and not struggling.

 

[Nealh]

Most 250w can handle 700w but not for a continuous period, for ATF try about 25/30mls. To much and it will slosh around and likely leak out.
Drill a tapped hole in to the motor center core or the LHS plate, screw in an oil nipple for filling/maintenance.

 

[WheezyRider]

Most 250w can handle 700w but not for a continuous period, for ATF try about 25/30mls. To much and it will slosh around and likely leak out.
Drill a tapped hole in to the motor center core or the LHS plate, screw in an oil nipple for filling/maintenance.

How much power a "250W" motor can handle continuously will depend on the type and manufacturer. Motors I've played with have had no problem handling 700 to 800W for significant periods and over 1kW for short runs. It's also important to remember that the rules say it is 250W output, not input and these motors are about 70% efficient, so you can put 350W in to get around 250W mechanical power out. If you have "cooked" your motor, also think about the magnets, not just the wiring, if heated above a certain temp, they will lose some of their magnetisation permanently.

On one occasion, I had my Powabyke fitted with a Voilamart 250W front drive motor. I got a puncture on the rear with another 6 miles to get home. I had no tools or spares, it was raining and I still had the original Powabyke motor in the back wheel at that point, which is huge and very heavy. No way was I going to walk home and I couldn't pedal much as I only had one gear and that was top gear and my legs are dodgy. The rear tyre shredded and the vibration caused the rear steel mudguard to break free and rub continuously on the shredded tyre (I looked a complete clown!).

I was pulling 15A continuously all the way home, so about 600W. I got home and the motor was barely warm. I fixed the bike and found a wheel I could put on the back to get rid of the now destroyed original wheel and run that motor at up to 72V, 15A and it ran several miles and it barely got warm. I have since fitted a rear hub drive to the bike and I opened up the old front motor to see what damage I might have done and yet it was fine. The Bafang SWX I have on my other bike is running at 59V, 15A and that will run for long periods, no problem at full power.

One important issue is that the case is isolated by two air gaps from the coils, so heat conduction out of the motor is poor and if the case is too hot to touch, your coils will probably be on fire! Hence the use of ATF. Most people who use AFT drill one of the brake disc holes on the side plate so it goes all the way through, so you can use that as a fill port. The brake disc holes on the Voilamart 250W motor already go all the way through, so it doesn't even need modification. You will probably want to add more than 30 ml of ATF, but as Nealh says, if you add too much it will leak out and you seriously do not want the stuff on any disc brakes!

Here is an interesting link:

Electric Hub Motor Tuning

Tuning an electric hub motor is a relatively straightforward thing. The biggest obstacle to overcome is excessive heat build-up in the hub motor. Most hub motors are cheaply made in great volume. T…

etrike.wordpress.com

It's an entertaining and informative read

 

[Darthpaul]

Interesting read. I’m wondering now if my original motor had more issues than just pumping too much juice into it? It only had 120 miles on it when it went. Although I was on PAS5 pretty much constantly I remember keeping an eye on the watt meter and wasn’t hitting big numbers that often as I didn’t hit many big hills. I did one big hill once and it hit 1k for a few mins, but I checked it after and it wasn’t warm. Ironically the day it went I’d dropped it to PAS2 as I was on towpaths and didn’t want to be too quick as it was busy. It was warm when it died though so guess the damage was done.

 

[WheezyRider]

Interesting read. I’m wondering now if my original motor had more issues than just pumping too much juice into it? It only had 120 miles on it when it went. Although I was on PAS5 pretty much constantly I remember keeping an eye on the watt meter and wasn’t hitting big numbers that often as I didn’t hit many big hills. I did one big hill once and it hit 1k for a few mins, but I checked it after and it wasn’t warm. Ironically the day it went I’d dropped it to PAS2 as I was on towpaths and didn’t want to be too quick as it was busy. It was warm when it died though so guess the damage was done.

Have you opened up your dead motor yet and looked inside? How did it die? Did it just go dead, make lots of noises?

 

[Darthpaul]

I’ve got the tool on order to open it up so I’ll know more once that arrives. It died for a minute, came back for a few mins, went again, came back then went for good. No weird noises, it was warm but not hot, and it was the warmest day of the year so far. Weirdest thing about it is with the motor plugged in, my battery bars are empty, unplug it and they instantly come back. Not sure what that means!

 

[WheezyRider]

I’ve got the tool on order to open it up so I’ll know more once that arrives. It died for a minute, came back for a few mins, went again, came back then went for good. No weird noises, it was warm but not hot, and it was the warmest day of the year so far. Weirdest thing about it is with the motor plugged in, my battery bars are empty, unplug it and they instantly come back. Not sure what that means!

Hmmm, sounds like it could be shorted.

Someone on here might have a good idea what the normal resistance of the motor coils should be. It might be so low that a normal multimeter won't give a sensible reading. But you could look at the resistance of the different phases and see if there is any variation.

 

[Sturmey]

Weirdest thing about it is with the motor plugged in, my battery bars are empty, unplug it and they instantly come back. Not sure what that means!

The above looks like mosfet short circuit in controller causing current to flow through motor winding when idle. Are you sure the controllers ok?

 

[Darthpaul]

Yep, controller is working fine. I’ve got two controllers (one bought in error) and both do the same thing. Also the new motor is running fine when hooked up. Will say, the resistor nearest the power input on the controller is blackened, but everything is all working ok with the new motor.