testing ohm resistance on BL motors

[sjcrss]

Does any one know how to test the ohms resistance accross a Bl motors 3 wire connectors......Im looking to find out if a motor has issues, and would like to know the smartest way to test this....
thanks

[Serum]

You need a meter that is capable of measuring VERY low resistance.

But if you just want to test the motor you can shorten any of the 3 wires. If you shorten one on one, you will feel a jumping resistance while turning the shaft. If you short all three of them, you feel a consistent resistance when you turn the shaft.

[sjcrss]

I have a volt meter that can measure real low resistance..and what do you mean by shorten...does that mean short accross the plugs....I quite dont understand that part...

[Rtsbasic]

Shorting the 3 plugs/cables together should give even resistance while turning it, is what he means. DO NOT do this while connected to a battery, for obvious reasons :)

[sjcrss]

thanks.. that gives me a starting point

[sjcrss]

and thanks about the battery tip...I wasn't planning on doing it with a battery connected anyways....

[sjcrss]

anyone else have any input....

[Rtsbasic]

Give it time..they're probably all off biking or something daft :D

[sjcrss]

i know, and i will thanks

[BrianG]

Low resistances are better measured with a conductance meter. Conductance is simply 1/resistance, but is measured a different way for really low resistances. You're going to have a hard time measuring coild resistance even with a decent meter. Most ohm-meters can display only as low as 1/10 of an ohm. If that is the lowest value, the resolution won't be great.

Don't forget to account for meter lead resistance. To get a baseline "0", short the leads together and read the resistance value. Subtract this from whatever you measure on the coils.

Whatever you do measure will be VERY VERY low because you are just measuring DC resistance since it's just a length of wire. Most of the impedance will come from the inductive reactance properties of the coil. To take this into account is a little complex. You'd have to take an inductance reading, know the frequency the ESC operates at, calculate the inductive reactance, then combine this value with the DC resistance. You can't just add these values since inductive reactance is 90 degrees out of phase with pure resistance, so the result would be a vector.

[sjcrss]

wow...you lost me there briang...can you put it in more lahmen terms.....sorry

[BrianG]

Sorry. I do get carried away. :dft001:

I guess my point is that it's not as easy as simply measuring the resistance and be able to figure out the current draw, especially since BL is three phase. If you had a capable meter, you could measure the current on one of the windings while running. But you'd have to have a meter capable of measuring fairly high frequency since the pulses are in the kHz. Most meters assume 60Hz as the frequency which changes things. Some are "true RMS", but you don't want that either since the signals going to the motor are square wave.

Much easier to measure the current and voltage on the battery pack. Even that changes depending on the load (take-off/acceleration vs maintaining speed). That's where the eagletree device comes in handy. It samples the voltage and current periodically to get a waveform.

[sjcrss]

that makes sense.....