I am testing a BLDC motor by coupling its shaft with another motor that runs at constant speed. There are no electrical connections to the tested motor. Measuring the voltage between the phases looks close to a perfect sine wave. I expect to see a trapezoidal waveform.
Is this expected in some cases for bldc?
Am I measuring the back emf wrong?
I appreciate any guidance,
Gerard
Hi Gerard Knight,
You are measuring the back-EMF wrong, because your BLDC is in generator mode if it's shaft is coupled with a motor that runs at constant speed.
You can check this link: https://electronics.stackexchange.com/questions/54997/how-can-i-measure-back-emf-to-infer-the-speed-of-a-dc-motor
Hi Gerard Knight
Yes it's right what sir Alexandru-Ionel Constantin answered.
You also observe current waveform of stator of BLDC motor.
Dear Gerard
The MMF shape induced in Your BLDC motor, which in this case is working as a genarator, depends on two factors.
The first one is the spatial shape of magnetic induction produced by the machines
permanent magnets (PMs), and the second factor is how this magnetic field produced by PMs is received by distributed windings of Your motor.
The second one, receiving the magnetic field depends of two factor of the windings, depends on constructionn of the windings and more precisely: on distribution factor of the windings (kd) and the coil span factor (kc).
In effect these factors multiply - and finally we have winding factor kw=kd*kc.
These factors (both) are different for consequtive space harmonics of the magnetic field. Normally a constructor of the machine chooses thye factos in such way that the winding as a whole 'sees' the sinusoidal shape of PM magnetic field and finally EMC induced is closely sinusoidal.
That is in a short way.
Poptr WAch, Poland
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