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LC Meter to check motors


Senior Member
If you follow Bruce at RC Model Reviews on youtube, you might have seen this before:

I have 4 motors I bought at Flite Fest and when I went down in the beans and forgot to disarm I smoked the ESCs (actually my quad caught fire, it was awesome!)

Anyway, I replaced the ESCs at home and fired them up. just on the test bench with no props I had the ESCs blowing FETs and shooting sparks. Not good.

So I am testing the motors like Bruce explains and none of them have all three matching. Problem is I am not sure how close they are supposed to be. Is within .1 uH acceptable? .01?

Motor 1:
AB 10.22
BC 09.97
AC 10.13
This one has the furthest separation (AB-BC) and also the highest measurement in coil AB. Most likely it's toast.

Motor 2:
AB 10.02
BC 10.00
AC 10.09
These are all so close that if I didn't see that 10.22 on motor 1 I might think it's ok. Are all three just evenly shorted?

Motor 3:
AB 9.71
BC 9.70
AC 9.80
Same here, but obviously wrong compared to motor 1. So all three became evenly shorted maybe?

Motor 4:
AB 10.05
BC 10.11
AC 10.03
So close to motor 2 and so close to each other that again I would be tempted to call this one good, except I don't know how close they need to be. Also, that 10.22 on motor 1 makes me think this motor is also evenly broken.


Some guy in the desert
Kind of hard to say. For example on cheap HK motors I'd expect to see more variance from motor to motor and even winding to winding than on something like a Cobra or SunnySky. So it's kind of a relative thing. What kind of variance you see would also depend on how the motor is wound.

Motor 1 you have enough variance I'd say you've probably got a short in a winding.

The other three their values are all fairly close. I wouldn't worry about the difference from motor to motor - that could be manufacturing variance. But the difference within the motor I just don't have enough experience to say for sure.

Also keep in mind that whether the motor is delta or wye wound will affect just what you're reading.

Here's the electrical diagram of the two types of windings:

On a star wound motor there should be a little pigtail where all three windings come together - then each winding will go to one lead. On a Delta each motor lead will go to two windings:

So when you take a reading off a Delta wound motor all three windings are in the circuit. But when you're reading off a wye wound motor only two are.

So you're never actually reading just one winding unless you disconnect them from each other.

Motor 4 seems the best. You've only got .04 difference between the most different windings. Motor 2 you've got a .09 difference and motor 3 you've got a .1 difference. Not nearly as good as .04 but still a lot better than the .25 difference on motor 1.

.04 I wouldn't worry about. .25 I would. .09 and .1 I'd probably be wary of and may want to test things out with an automotive taillight in series as a current limiter first just to be safe.


Senior Member
Thanks for that insight, jhitesma!

It's hard to tell just from looking at them but I think they are Delta configured. They are RMRC-branded T-Motor MN2208 2300kv

The ESCs are DYS SN20 pre-flashed with BLHeli with oneshot and damped light. Both purchased from the RMRC booth at Flite Fest.

Looking at the delta wiring diagram it makes more sense now. Each time you measure, you are measuring one winding in parallel to a two-winding group in series. I'm guessing that the single winding contributes more to the reading that the other two (individually), which is why you can get three different readings to indicate the health of three different windings, despite the fact that they are bonded together.

The whole situation is weird though. I mean, my last group of 4 ESC blew up on the bench just being test run with no props! I would expect from that to find that my motors are all shorted-out esc-killing paper weights, but at least 2 seem to test fine, and the "worst" one doesn't even appear that bad unless on the tester.

I've got 4 new motors coming and new 30-amp escs. The first thing I'm going to do is test out the new motors for their LC readings right from the factory. That will give me a baseline. I'm actually hoping that testing the new motors reveals that the others are shot because if there is no explanation for my ESCs just blowing up then I don't know what to do. I already blasted through 12 of them in a week and maybe 3 minutes of flying!

First set: Motor shafts rubbing on booms: Let out magic smoke.
Second set: Crashed in the beans and didn't disarm: Let out the magic fire.
Third set: Bench testing with no props and POOF! Begin to think motors are esc-killers

Being scared that your motors are killing your ESCs while at the same time worrying about your ESCs fusing shut and melting your motors is just no fun.


Some guy in the desert
Got my meter over lunch today. Was quite surprised at how quick the Chinese ebay seller shipped it, under $20 including shipping - ordered it the same day I ordered some bits from the FT store - both shipped the same day - both arrived today. Of course the FT bits should have been here last Thursday (according to the USPS tracking) but the USPS apparently lost them somewhere outside of San Antonio and shipped them back and forth between two TX locations a few times before letting them continue their journey to me here in AZ. Go USPS :black_eyed:

Anyway, hooked it up and it seems to be working...but the readings I'm getting off motors aren't making a lot of sense. I was in a rush (trying to eat lunch and play with the meter at the same time) so I may have done something wrong...but it's pretty simple to zero a meter and take readings off 3 leads in various combinations.

But - even on a known good motor I was getting readings that were almost identical on two leads but then if I tested off the 3rd lead I got very different results just like I got and expected to get off a bad motor. So either the good motor was damaged on it's way from Ohio to AZ in my luggage or I'm understanding something wrong. Will have to test with a few more known good motors and known bad motors later tonight to figure out what's going on...maybe test the meter on a few known value capacitors and inductors as well.