View Full Version : Motor resistance, why is it important?

02-13-2009, 01:47 PM
I notice most brushless motor specs include the winding resistance in ohms. Why is this a useful spec? How can I use this spec?


02-13-2009, 03:09 PM
If you mean the effective resistance (Ri or Rm), which isn't quite the same as the resistance of the windings, then that's one of the 3 parameters needed by all the motor modelling programs like Motocalc to do their calculations. The other two are the motor constant Kv and no-load current (Io). With just those 3 numbers you can work out what an electric motor will do under various loads, voltages etc.

But it's not really of much direct interest to mere humans, unless you particularly enjoy doing the sort of complex calculations much better suited to computers ;).


02-13-2009, 04:11 PM
Thanks Slip...
I never tried but can you just hook an Ohm meter to a pair of the 3 leads to test this effective resistance or is it an impedience you have to calculate? Is it a way to check the condition of the windings, say for instance you were buying a used motor?

02-14-2009, 04:02 AM
Resistance is futile.

02-14-2009, 04:22 PM
The lower the resistance (a.o.), the higher motor efficiency, the more power the motor can handle. You can only do this comparison when all the rest of the motor has not changed!

Prettig weekend ;) Ron

02-15-2009, 03:39 AM
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.)

02-15-2009, 04:28 AM
This is great information. Give me some time to work with it, no doubt there will be more questions later.
Thanks all.

02-15-2009, 07:02 AM
A brushless motor has more than resistance. Because the voltage is rising and falling it will have inductance and capacitance which act similar to resistance but are frequency dependant. The brushless motor is closer to an AC 3 phase motor than a brushed DC motor. The DC resistance of the wire is in many cases insignificant and will tell you nothing about power or current when operating. Speed is based solely on the frequency of the DC pulses so, as speed changes, so does total reactance (oposition to current flow) because induction and capacitance are frequency dependant.

02-15-2009, 07:07 AM
oh, no, numbers....aaahhhhh....LOL


02-15-2009, 03:02 PM
I thought a brushless motor was the same as a brushed motor except it has electronic brushes(rather than mechanical) located remotely.

02-15-2009, 06:44 PM
DC brushed motor has inductance and capacitance as well. Very different construction though. Brushed windings are multiples of 2, brushless is multiples of 3. Brushed motors might not use permanent magnets, brushless always will.

02-15-2009, 07:24 PM
The brushed motors and brushless motors are apples and oranges. They both provide power output to their loads, but are totally different in operation. The brush type motors have a pair of brushes (Sometimes four brushes!) that ride on what's called a commutator on the spinning part of the motor, the rotor. This causes a voltage drop on the commutator/brush connection, as well as sliding friction. It gets hot at high power levels. The brushless motor does all of its switching electronically.

For our application, the brushless motors are more efficient, resulting in more power and/or flying time on the same battery.

These brushless motors are what you could call a sychronous three phase AC motor. These motors turn over at an RPM dependent on the frequency of the AC supply.

And, you can call our ESC controls a "variable AC frequency drive".

02-17-2009, 02:05 AM
I love motors with lower resistance. It is an often overlooked factor when shopping for brushless motors. It benefits motor efficiency drastically. Take notice how much better such motors like Neu motors , Mega, Himaxx motors are in comparison from the chinese imports out of Hong Kong. Much better resistance and increased efficiency = cooler running motor = longer lifespan.

02-17-2009, 02:22 AM
All I fly are Hacker motors. They include the A30-12XL, A40-12S, A40-10L, A50-12S, A50-16S motors. All are flown at or slightly over their maximum current ratings. All models land with the motor only luke warm, as measured by an infra-red thermometer aimed at the motors windings.

As you indicate, motor winding resistance is significant on these motors. Hacker motors typically have 0.02 ohms or less, some of the cheaper motors have three or four times that value. Take the Hacker A50-12S motor, with its 0.016 ohm winding resistance, multiply that times 50 Amperes, you wind up 0.8 volts strictly to IR (Amps times Resistance) loss. Take one of the cheaper motors with 0.06 ohms at the same 50 Amperes, you wind up with a 3 volt loss in the windings. Since power loss is equal to I times I times R, the cheaper motor has 0.06 / 0.016 or over three times more watts loss due to winding resistance alone. And, the motor will run hotter.

You run some of these motors at their supposed rated power levels through the Motocalc software program, and Motocalc suggests these motors will burn up!

Dr Kiwi
02-21-2009, 02:44 AM
You run some of these motors at their supposed rated power levels through the Motocalc software program, and Motocalc suggests these motors will burn up!

And run 'em on the test stand to determine the level of ignorance/deception/optimism/flat out lying of the manufacturers and suppliers... and they DO burn up!