# Checking the Back EMF of a Motor

Gold Member
Summary:
Checking the backemf of BLDC motor on an oscilloscope.
I want to check the back emf of motor as shown in the article below

My question is does a normal probe will be sufficient or do I need a differential probe to check the back - EMF waveform?

Is a normal probe a 10x single ended passive probe like this?

Do you know what the expected EMF is? I think it would be mostly worried about the input voltage range spec and grounding. For grounding the main thing to worry about is what ground the other probes and scope are using.

If there is any concern or unknown there, and I suspect there is because you asked here, you'll want an isolated probe, not a differential probe.

This is what you're trying to avoid

@PhysicsTest Technically if the motor is designed for low input voltage then the possible spikes in the output if the motor is excited and driven in generator mode shouldn't be too high , but on the safe side you could just instead of a differential probe if you haven't got one just use a resistor voltage divider which is a crude analogy of the differential probe.
The you can be safe to limit the current and voltage but preserve the waveform. then calculate the values of the divider, measure the voltage drop across and calculate the real value of back EMF.

sophiecentaur
Gold Member
2020 Award
The suggested method is ever so complicated, compared with just using a small series resistor and a DVM. When the motor is running, the voltage across the resistor will be equal to the supply volts minus the back emf. The lower the resistor, consistent with the sensitivity of your volt meter, the more accurate answer you will get.
But wait; a small value resistor with a voltmeter across it is actually an ammeter. Just another way of looking at the problem.

But wait; a small value resistor with a voltmeter across it is actually an ammeter. Just another way of looking at the problem.
In theory that works but I think in practice one will get a poor measurement that way. For example I think the smallest integration window my 87V supports is 250uS, that will be pretty slow to get a good view of the peak. And the resistor will just attenuate it further. For example: if the purpose of the measurement was to make sure the reverse breakdown on a diode was correctly spec'ed I don't think that measurement would be sufficient. But it is the simplest for sure, and we weren't given the requirements of the measurement, so maybe it's golden. The nice think about a DVM is it floats so it's the safest possible thing. :)

sophiecentaur
Gold Member
2020 Award
one will get a poor measurement that way.

It depends what you want and what you mean by "poor". A DVM is not necessary; a voltage amplifier across the resistor (connected at the earthy end of the motor so no need for a differential measurement) can feed an oscilloscope. You haven't specified what aspect of the back EMF that you need to measure. Do you actually want the commutator spikes?

Do you actually want the commutator spikes?

The OP mentioned bldc motor so no commutator.

sophiecentaur