How hot can a brushed motor get before destruction?

Just wondering what temperature limits I should ensure for a brushed motor. Like, what kind of temperature should I make sure if never exceeds if measured at the case.

sw,

I would think that brushed motors would share the same temperature constraints as imposed by brushless since they are based on the magnet temp before demag and also winding temp before shorting thru insulation...

Sadly none of this transfers to case temp, since until you measure internal stator temp against case temp to determine the proper delta and response time... I would think figure out the magnet type, assume 45 degrees F above ambient and subtract it from the motor temp max - then attach a probe, better to err on the side of caution right?

-Mike

Generally I agree. The only potential difference between heat damage in a brushless and brushed is there are no halls to die in a brushed motor.

However, there is a curious thing that can happen to a brushed commutator when it's too hot and you push too much current thru it, especially at high RPM: A Plasma Fire.

You don't want that to happen.

I dont' know if it makes a difference temperature-wise, but many brushed motors still use ceramic magnets, instead of rare-earth, probably because of their size. So if the ceramic magnets can run hotter before demag, then so can the rest of the motor, but if they have a *lower* demag temp, well....

I know I used to run those little radiator fan motors on the original DayGloAvenger until they were hot enough on the outside case to instantly turn ice water into steam, and that's with the motors bolted to the large aluminum plate (4x rackmount faceplate) that was also bolted to the aluminum bike frame, with airflow from riding and from the rear tire/spokes cooling it.

I don't want to know what the armature temp was. Yet, they still worked apparently just fine, and never even melted the plastic encasing the armature windings themselves (they were brushed 4-pole axial flux motors, coreless, with the windings flat-mounted and cast into black plastic disks).

Though, I think if I'd had a battery pack capable of *sustaining* those power levels for very long, they probably would have turned into slag inside.

amberwolf said:

However, there is a curious thing that can happen to a brushed commutator when it's too hot and you push too much current thru it, especially at high RPM: A Plasma Fire.

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AW - isnt that caused by already present sparks of the brushes combined with the semi metallic dust that forms in all Brushed motors? I've seen the flash over before (in test tanks) and when we watched it slow mo, it seemed the "dust" was being ignited by the arching which already exists.

Granted this is all far above the temperatures and currents the motor was rated for... I don't know the breaking point but it should be above 2x the rated power (current) of the motor at a given voltage.

All this is imho - Im a novice compared to guys like Miles and Jermey Harris = )_

-Mike

Could be that, too, but I'd bet that the hotter it is the worse the problem gets.

I have a 250 watt brushed motor on hand and I originally intended to run it at 50% duty cycle max because it was originally designed for 12 volts of lead and I'm now powering it at 30 volts of lifepo4, but it just seems so pokey slow, so I was wondering what limits I should remain in as I increase the duty cycle to get more speed.

After sustained riding, the case of the motor gets to about 100 degrees fahrenheit with the outside temperature being 60 degrees. I'm wondering if I can push it to 140 degrees safely.

While I would like to err on the side of caution, I'm not going to do so for long if doing so really means it's *this* slow. I'll probably increase it until the case reaches 130 degrees or so and if it blows, oh well, time to upgrade! 600 watt currie motor, here I come!

(Yes, I know about RC motors and I own a couple, but I compare the cost of brushed motors to an effective brushless setup and it seems like a no-brainer for a 20 mph scooter. If I wanted 30 mph continuous, it'd be a different story.)

sw,

Personally I would run it temporarily on 12v at the same current you intend for 30v of LiFePo and monitor the temps at direct connection (12v) - then apply that temperature range to the limit for 30v operation.. sounds like your running now at what I would expect the motor to run at or 40 degrees F above ambient temp.

In either case, without specs - the best solution is to monitor at 12v and extrapolate the proper temperature allowable for 30v operation....

Is this one of the geared currie scooter style 250w motors? I have one too with a #10 sprocket.

-Mike

Generally brushed motors can take a bit more heat than brushless. It will depend on the magnets as well. If it has NdFeB magnets, they can be damaged by excessive heat. Otherwise the armature and commutator and brushes are the other weak points. You can't really judge their temerature by the outside case. If you're moving along so there is some wind, the case can be relatively cool while the insides are cooking. When you stop, you'll notice a substantial temperature rise on the case.

It sounds like you can push yours a bit more. A case temp of 140-150 is not too high usually.

I would imagine you sacrifice brush life in going up that high, but otherwise theres less to burn...

grindz145 said:

I would imagine you sacrifice brush life in going up that high, but otherwise theres less to burn...

Click to expand...

If the brushes are cheap, and someone wants to squeeze a bit more power , i don't think brush life would be a factor.. 8)