resistance of motors

Thread Starter


Joined Nov 13, 2011
I am now dealing with my Final Year Project project to optimize the braking of a motor, in order to obtain high regenerative power.

I would like to know whether anyone could answer my following questions:

1. Will a dc motor's resistance change during braking?
2. Are there any method to determine the impedance of the dc motor duration its braking / operation?

3. Are there any method to determine / measure the impedance of an induction motor during its operation / braking?

Thanks for your helps


Joined Feb 19, 2009
Unless it is sold as a motor/generator, you will need to find out experimentally.

Bench test it, add a small flywheel then remove power and replace with a power resistor for braking. Measure voltage across resistor to get voltage and current output of motor during braking. If you do not connect a resistor, there will be no braking action, it will coast to a stop.

The "brake" function in motor controllers is often simply removing power and tying the motor leads together for maximum inductive braking on a permanent magnet motor.

You can still stop fast or slow, depending on how you are controlling the motor.


Joined Oct 26, 2011
1. The resistance of a DC motor is unchanged when it goes into the generation mode--the current simply swaps direction. Regeneration implies that the power is restored to the AC mains--that can be accomplished via a thyristor full converter bridge.

2. When motoring, the terminal voltage will be a little higher than the CEMF due to the armature resistance--conversely, when generating, the terminal voltage will be a little lower than the CEMF.

One way to measure armature resistance dynamically while generating, is to couple two motors together and run the drive motor at a constant speed (perhaps tacho feedback) and measure the CEMF of the generating motor under no-load conditions--then connect a resistor across the generating motor terminals and make a new voltage measurement. The change is voltage is simply a function of the armature resistance and load resistor current--use ohms law.

If you do not have a constant speed setup, simply measure the speeds under both conditions (unloaded and loaded). Then assume that the change in CEMF is directly proportional to the change in speed and factor that into your solution.

Note that in large motors (e.g. 40HP), there may be a series field that interacts with the shunt field flux--if connected, it complicates the problem problem.

3. To determine the AC motor impedance motoring and generating requires a similar motor-generator dynamometer setup--this time with a DC motor that can be used either as a drive or load and your DUT (3 phase AC induction motor) (I do not know what would happen with a single phase motor).

Operate the AC motor from the AC mains and load the motor via the DC motor (generator) to its rated slip and measure the stator current--calculate impedance from line voltage and stator current. While you are at it, measure power factor as well (stator current & line to neutral voltage).

For impedance during regenertion, drive the AC motor (now generator) via the drive motor at the synchronous speed--connect the AC motor to the AC mains (it may thump severely)--increase the speed of the drive motor to obtain the same slip above synchronous speed and measure the stator current--calculate impedance in the same way. Measure power factor too.

If you have access to an old power meter (KWH), it is fun to watch the disk reverse direction when regenerating.

Thread Starter


Joined Nov 13, 2011
Besides, I would like to know whether the impedance of the synchronous would change during its braking operation?And any method I could obtain the impedance of the synchronous motor on-line during its braking operation?