Motor maximum temps

JackFlorey

100 kW
Joined
Feb 19, 2020
Messages
1,573
Location
San Diego
Question about hub motor temperatures -

What do people use as a general maximum, and what's the never-exceed? And what limits that? I assume it's a combination of the temperature limit of the winding insulation and the Curie temperature of the magnets (although the magnets are always airgapped from the windings - if you're not using Statorade that is.)

I'm currently using a 750 watt DD Shengyi motor, and am running it at 1000 watts (power limited.)
 
If I installed a temp sensor on the hottest part of the stator, I'd limit temps to 140F, and it should last until the two shaft bearings wear out. 140F is hot enough to burn your hand, but the outer shell will only feel warm.

Hot copper windings have more resistance than cool windings, so its a situation where the hotter the motor gets, more of the max watts are converted to waste-heat, and less of the incoming watts are converted to work. Between 140F and 200F, your motor might survive just fine, but doing that is a very inefficient way to run the system.

Plenty of builders have run hotter than that, take on the level of risk you are comfortable with. 90C / 200F is a pretty common "shut it down just before it dies" setting.

Astro motors used high-temp Samarium-Cobalt magnets, extra-thin laminations, and high-quality insulation lacquer along with top-level QC, so I would be comfortable running those a bit warmer...

Hot motors can have any one of several failure modes. One weak generic Hall sensor dies, junk solder melts at a low temp, clear wire-insulation deforms at a thin spot leading to a short, magnets partially de-magnetized (as you mentioned), the shaft seals on the bearing deform, leading to an early bearing-death, junk bearing lube boils away at too low a temp, etc...

Generic motors have multiple weak areas. Quan Shun / QS seems to use quality materials with good QC...
 
With a kit supplied by Grin the Cycle Analyst was preconfigured for the Shengyi SX geared hub motor (a 3kg 500w class motor). The temperature settings were 90C and the power starts to be limited and 130C max (complete overtemp).
 
I was going to say 250 F is about where you start cooking off the winding varnish, which is around 130C

But about 200F measured through a hole in the cover, directly on the winding is doable for a fairly long time. Winding will turn darker brown, but not totally black.

But at temps lower than that, I've had hall sensors fry, after the temp spikes when you stop riding, and the air cooling wind stops.

So yeah, its good to stick with what Spinningmagnets said, since replacing halls is a pain.

One thing I learned about how to melt a motor. Overload it works really good. So if you weigh 200 pounds or less, not much to worry about. But if you weigh 300 and pull a 200 pound trailer, better not climb any hills.
 
I think doing 40 mph on that flat front tire did overload the motor some that night. Having to slow to 10 mph in the corners was about like stopping each corner. And the 110v 40 amps into a 500w motor.
 
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