Motor circuit


Thread Starter


my problem has to do with reducing the speed of my motor but i want it to have the same current. A friend suggested PWM and i tried building such but it doesnt work. Any other PWM circuits would be helpful!
Use a Vector type Variable Frequency Drive Inverter. They have PWM built in and can maintain current down to zero hertz with the proper paramaters programmed in. If you are trying to build your own, the parts alone will cost more than a manufactured unit.

Hakan Ozevin

Dear Ollie,
"Reducing the speed, but having the same current" (which means a constant current source) is not the right approach to an AC motor control system.

The motor will draw the unique current to run the load at that speed. You cannot have it to be at a constant current and run the load without problem at the same time.

Therefore, instead of looking for a constant current source, look for a VFD/VSD, which is available on the market.
If you need to control the speed of a motor
you have to vary the applied voltage, (and frequency for a ac motor). You can not control the speed of any motor (ac or dc) with by maintaining a constant current. The current will vary with the applied voltage. In small permanent magnet dc motors inject a constant current to maintain the motor speed and as the load increase or decrease the applied voltagee will change (to maintain constant current) and the motor motor back emf.
I think you mean you have a mostly constant torque load, thus same current requirement for a PWM motor driver. In general, you can't simultaneously control speed and current (torque). If you control speed, the current
(torque) will be "whatever" it needs to be to match required load torque at that steady-state speed. Likewise, if controlling current (torque)

There are several manufacturers of motor-drive modules/ICs for low power/simple control applications. Motorola, ST come to mind. Or how about a simple voltage dropping resisitor if a dc motor?