Induced EMF

[SPIGUY1]

Is measuring induced EMF in a 12 VDC PM motor a good way to measure motor performance? I have sketchy info for now, but we have a customer using a 12 VDC PM motor with voltage dropped down from a 24 VDC system. Brush life has dropped from 1000 hours to less than 100. Improvements were made to the brush holder card by increasing the brush clearance and the coiled spring pressure. The brush shunt was changed from a side to rear mount to prevent the shunt catching on the BHO slot.

What is a simple way to measure back (induced) EMF?

 

[DanDel]

When you say: "voltage dropped down from a 24 VDC system", how was that done?

 

[SPIGUY1]

This 12VDC PM motor is part of a fuel pump assembly. Changes were made to the motor brush clearance when it was originally in a 12 volt system. This improved motor life 15X (15k hours). This motor/fuel pump assembly was then adapted to work in a new 24 volt system by providing a 24 volt regulator. +24V is applied to the motor positive supply with the negative supply chopped between +24V and 0 to provide 12volts to the motor on a 50% duty cycle with a PWM output @ 1300 Hz.
In field testing one regulator failed. The customer is concerned that the motor could generate enough back EMF into the regulator to damage it. The motor is only rated at 2 A. with 14VDC applied. I don't believe that this is the case. The customer did say that a power MOSFET was fried in the regulator. They did not know the condition of the motor.

 

[IRstuff]

you do realize that 24 VDC at 50% duty cycle isn't really 12VDC?

You're now running way more current through the windings and the brushes. At the minimum, you're running at least double the current, which means 4 times the power.

The chopping of the 24VDC potentially means that you are cutting the voltage in the middle of a commutation. Won't there be a sizable inductive EMF generated? Normally, wouldn't the inductive EMF show up after the brushes have commutated to the next winding? But if you're chopping asynchronously, then you'll have inductive EMF generated while the brushes are still on the same winding. TTFN

 

[DanDel]

That's what I was afraid of...

 

[Skogsgurra]

IRstuff: Oh yes. 24 V at 50% and 1300 Hz is very much 12 V in a highly inductive DC motor. Just as any PWM scheme can supply any voltage from zero to full DC link voltage to servo motors. They that do all the time, and no problem with the brushes. So this way of controlling a DC motor is just right.

I would check the free wheeling diode first. Seems to be faulty (the transistor was fried). Or is it missing?