PWM motor causing PWM drive to overheat

I have used KB PWM drives for many years on 1/2 Hp and up SCR rated motors for my machine tools. I now have a project for which I got a small Bodine 1/3 Hp 130 volt PWM motor with the expectation to use a 220 VAC KB PWM drive (12 AMP output - way oversized). There are two large resistors on the KB drive that get very hot with this small motor that did not get hot with the larger Hp motors that I had previously used this drive for. I experimented by attaching this 1/3 Hp motor to 4 other KB drives, both PWM and SCR types, that are powering large DC motors on my machine tools, all 4 had the same results, the large resistors got extremely hot with the small motor.

The small motor only pulls about .7 amp.

This is the first PWM 130 volt rated motor I have bought, all my others are 90 vlt SCR rated. Is there some characteristic of PWM motors that causes the drives to run hot?

Thanks

There is no such thing as a PWM motor, generally either DC brushed or BLDC, although occasionally you could come across a 3ph synchronous P.M. motor.
But for the first, DC brushed, you either need a DC drive or in many cases a BLDC drive will run a DC motor also..
The KB SCR and PWM drives, typically run a DC brushed motor,
You have to confirm the motor details of the one that runs hot, or post a pic.
PWM is a characteristic of the drive, not the motor.
Max.

Thanks for writing. I was using the KB terminology. This motor is a typical permanent magnet DC brushed motor, purchased surplus, little wear on the brushes, smooth rotation. The surprise is the small size 130 volt motor causing the overheating when larger (1/2, 3/4, 1 Hp 90 volt PM motors) do not cause their respective drives to run hot. This is with both PWM and SCR drives.

Try the motor on a Automotive battery and see how it runs.
Also it should generate a DC voltage when back-fed (rotated), the voltage will be directly relative to the RPM at a ratio for the voltage/rpm rated motor plate values.
Max.

I did some testing. I used an optical tachometer.

240 VAC PWM drive, 130 VDC output, RPM 2550 - rated RPM 2496, pretty good match
120 VAC SCR drive 90 VDC output, RPM 1880 - not far off of 90/130*2550
12 volt car battery RPM 235 - pretty good match, at bottom end of tach usability

driving at approx 1250 RPM off a lathe, generates 58 volts, .44 rated voltage at .49 rated RPM - again pretty good numbers

It could be that the small motor has much less inductance. When a motor is run with PWM, the frequency is usually high enough that the current doesn't change much during one PWM cycle. That is messed up if the inductance is small and the current changes quickly. The peak current could be being limited by the resistors not the inductance of the motor.

That makes sense. To test that I could put an inductor in line. What are you thoughts on the Hammond 159ZE at the bottom. The motor is rated at 2.3 amps.

In my experience, I have never known a DC brushed motor that is fed from a PWM drive to require a Inductance, the way it is desirable with a SCR bridge version.
Did you measure the current when using the drive?
In a DC motor, the current is limited be the BEMF or generated voltage tending to oppose the applied voltage, resulting in very small current, of which then is dependent on load, which tends to decrease the resultant generated voltageand increases current proportianlly , I have monitored the off-load current of a 2.5HP DC motor from zero to max RPM and the result was a constant extremely low current throughout the RPM range.
Max.

It is sitting on my workbench with no load, it only draws about .7 amps. That is why it is surprising it is getting the drive resistors so hot.

Do you know the PWM frequency?

bll230 said:

it only draws about .7 amps.

Click to expand...

How are you measuring that? Is that average or peak current?

Do you have a link or pic of the motor, Bodine part No.?
Max.

This is the motor:


I did some more checking. It is drawing .4 amp measured on my old standby non-Chinese Radio Shack analogue meter and my giveaway Chinese Harbor Freight digital meter. My Triplett RMS clamp meter had trouble with the PWM wave form.

I have a very large 20 amp, 20 MH choke from one of my machine tools. I tried using it and it made no difference, resistors got just as hot.

From the KB manual, the operating frequency is greater than 16 KHz.