Motor inductance variations

ccmdr

100 W
Joined
Jul 5, 2015
Messages
204
Location
East or South UK, depends
Hello again all,

After my Adaptto randomly blew a FET I've been chasing down the cause. There are no cuts in insulation, no cuts, internal breaks or shorts to earth from phase/hall/temp etc... I suspect it was moisture/condensation in the motor, which has now been completely coated in Scotch 1603 used to protect works motors after a gasket failure.

So I'm now checking the inductance via 2nd hand el cheapo-ish inductance meter for £20. Most info out there is how or what inductance is, not what the max/min % variations between phases are.

This Cro V3 not QS has
1: 303uH
2: 300uH
3: 290uH

Does anyone more knowledgeable in the subject know if a >3% difference in inductance makes it harder for the controller to recognise? Even a point into a paper with the right info would be good, 'google powers' have failed :).

Thinking of a Max-E when Allex re-stocks so don't want to blow up the Mini-E again to test if Max-E is safe to go on.

Cheers again for an insight into this guys.
 
try measuring from the controller. might be cables that bess stuff up.

I cant see that inductance to blow the ass out of a fet.
 
Phase inductance will vary a bit depending on alignment with the magnets. Typically a motor at rest will have two phases with similar values and one that's different. If you make several measurements at different points of rotation, you'll see the outlying value move from one phase to another.
 
Thanks for your info flippy, cycborg :),

The motor is currently apart to apply the Scotch 1603 so all measurements are through uncoiled 1.2m cables without any induced magnetic inductance from the outrunner :).

Did think such a small variation wouldn't make a difference, but wondered how much of a discrepancy is expected in a std serviceable BLDC compared to a measured % 'toleranced' unserviceable BLDC.

Just out of interest, I'll take readings when I rebuild the motor. Yet to source some decent epoxy to secure the magnets to minimise anymore points of failure :).

If others have more info or links to citation it'd be appreciated :).

Cheers,
 
At the time the FET blew, 30A battery and 55A phase in the settings. Although what the controller actually puts out has always over run quite badly so probably 35-39A battery, indicated.

Literally went at 3/4 WOT from a standstill.
 
for an accurate measurement the rotor needs to be removed
 
Hi madin88,

Yeah, the measurements have been taken with the rotor removed, only way to lacquer the rotor, check hall leg insulation wear/breaks and for cable chaffing from axle to outrunner :).

Cheers though keep it coming :)
 
I don't think a 3% difference is going to cause a problem. Probably just variation caused by the way the wire is wound. If there was a shorted turn or something, the readings would be way off.
 
Phase-to-phase inductance that high makes the motor easy to drive for a controller. Depending on the controller as motor inductance drops below 100uH then they become increasingly difficult for many controllers.
 
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