Need Help with PWM DC motor speed control overheating at low RPM

[Paul Hickman]

Hello,

I am stumped by a problem with a DC brushed motor speed control project. The motor is a 24VDC 2.8A brushed motor with a 5:1 gear reduction. I have a very stable 24VDC power supply with 4.2A output. The circuit I am using is a microcontroller outputting a PWM at 940Hz to a TIP142G (125W)

View attachment 60594


My Problem when trying to run the motor at low speeds(200 RPM) the tip142 gets real hot even with a heatsink. But at high speeds(900 RPM and above) it is cool.

Any Ideas how to keep this cool??? I have tried with and without a protection diode across the motor. no effect.

Thanks in advance.

 

[Diver300]

I would suggest you need an oscilloscope to see what is happening.

If you are trying to run a motor on PWM, you need a freewheel diode across the motor. It should be a schottky diode for fast switching. Without it, the transistor is likely to fail.

The inductance of the motor should keep the current largely constant, so you would expect the current to flow in the transistor for more of the time at high speed. The current will be approximately proportional to the load torque, and without knowing what the load is, I have no idea how that will change with speed. However the load torque, and therefore current, is unlikely to increase at low speed, so the current will be no more at low speeds, and the reduced on-time for the transistor means that there should be less heat.

The fact that the transistor gets hotter at low speeds makes me think that there is more current at low speed. That could be because the PWM speed is too low, and the transistor is on for too long and the current has time to build up to near the stall current. That could take the transistor out of saturation, which will result in huge amounts of heat.

 

[alec_t]

At low speeds the motor current will be higher than at high speeds and it's likely the MCU can't provide enough base current to keep the transistor turned on fully; hence the overheating. Try using an NPN driver transistor connected to the TIP142 in a Darlington configuration. Use a series resistor between the MCU and the base of the NPN.