A Brushless Challenge for you...

[mattsedgley]

Now... I was playing with my cars tonight.. as i do.. sit at the table make make formula one sounds with them etc etc...

anyway I got thinking about motors.. I was under the impression that you could test a motors resistance to know what winding it was? Correct? EG a 21.5 would have more resistance 'cause there's more wire in there than a 3.5 for example, or vice versa whichever way round it goes..

Now.. I'm not exactly the stupidest tool in the draw - but I couldn't for the life of me get a resistance, or any other reading for that matter from the motor other than when spinning the rotor which gave a voltage read out just like an alternator would, which for obvious reasons couldn't be used to tell the difference between the windings!

So here comes the challenge..

I challenge you to show me how you tell the difference between one motor and another motor using only a Multimeter!

Matt
(Confused, puzzled, and not looking forward to the first day of school tomorrow)

[mark robinson]

Whats the prize if I can tell you? ;D

[Jazzler]

Calculate DC motor resistance.

To calculate the DC resistance of a typical brushless motor takes two digital multimeters, a power resistor on the order of 10 ohms, 20 watts, and a 12 volt battery.

Hook up the power resistor IN SERIES with the battery and one winding of the motor. Insert in this connection, the digital multimeter on its 10 Ampere range. So, the connection from battery plus goes through the resistor, the multimeter, the motor under test, with the other motor lead going back to the negative connection of the battery. MAKE DANG CERTAIN NOT TO CONNECT THE BATTERY DIRECTLY ACROSS THE MOTOR WINDINGS. YOU WILL BURN UP YOUR MOTOR!

Now, take the second multimeter, and measure the voltage drop directly across the motor windings.

The 10 ohm resistor will limit the current through your motor to about 1.2 Amperes.

Now, to calculate the motor winding resistance, divide the voltage measured across the motor by the current through the motor, as R=E/I. That will be the motor resistance.

Example, if you measure the current as 1.12 amperes, and the voltage across the motor as 0.04 volts DC that calculates out to a winding resistance of R=E/I or R=0.04/1.12. Thats 0.035 ohms. This will be as accurate as your measuring meters.

Note, do measure the voltage across the motor directly across the motor windings, NOT across an alligator clip or similar. Measuring across the alligator clip will result in your measurement being the total of the motor resistance PLUS the alligator clip.

Is their an easier way to measure this value?? Sure, buy a commercial micro-ohmmeter. But be prepared for sticker shock! These specialized meters can easily run in the thousands of dollars!

(Before I retired, we used these commercial micro-ohmmeters to measure contact resistance of circuit breakers rated at 800 Amperes three phase, and 38,000 Volts.)

Source: www.wattflyer.com/forums/archive/index.php/t-43839.html
Disclaimer: I accept no responsibility for this information... Didn't write it, just found it. If you try it and it anything bad happens, Don't blame me!

[mattsedgley]

MJR - You'll have to wait and see!

Jazzler - agreed you seem to have found a solution,

Perhaps let me re-phrase my question slightly - according to an event organiser it is possible to check what motor wind is being run simply, with just a multimeter, it's quick and simple enough to do track side, at an SLCC for example - particularly in a case such as when MH was accused of cheating at Aldershot....

[gwesty]

the rotors are colour coded , three screws and the rotors out so u can measure the "stack"

[keitheroonie]

Sept 2, 2009 15:54:52 GMT @mattsedgley said:

MJR - You'll have to wait and see!

Jazzler - agreed you seem to have found a solution,

Perhaps let me re-phrase my question slightly - according to an event organiser it is possible to check what motor wind is being run simply, with just a multimeter, it's quick and simple enough to do track side, at an SLCC for example - particularly in a case such as when MH was accused of cheating at Aldershot....

I would be very surprised if you were able to test the windings of a motor trackside, yes you should be able use a low reading ohmmeter to test the windings of a motor off the car but connect it to a speedo and you've then got to take in to account the internal resistance of that particular speedo, 2 parallel resistances would give you a lower reading than just the motor alone.

I've got a low reading ohmmeter if you wanted a play one day

[mattsedgley]

So how would Mr Newton Test a motor at a meeting??