# Torque from high voltage PWM to DC motor vs. lower DC voltage

I have a treadmill motor I'd like to use for a future project. Voltage across the 0.9 Ohm motor is ~14Vrms at lowest speed, and it's powered by a ~10% duty 120V PWM (I don't know why the numbers are off... perhaps my digital oscilloscope is inaccurate?)

I measured no-load current to be around 4.5A, and 7-12A with me standing/walking on it (the treadmill is rated at 12A).

My question is, if I were to supply 14VDC to the motor with no PWM, what kind of current could I expect to get? The speed would be the same.. but could the current reach the same levels?

My project involves being able to reverse the motor but motor drivers and DPDT relays rated for 120VDC and > 15A are expensive.

Any help is greatly appreciated.

PWM hits the motor with the rated voltage and hence full power for a short time then turns off and repeats.

The average voltage is 14v but the motor requires 120v to work. If you apply 14v, it does not get enough voltage to turn under any load if at all.

Weedpharma

Actually, I can make the motor run off a 9V battery :)

edit: thanks for giving me that idea... I just found out 9V can move a 10lb weight

Fast or slow decay mode?

http://www.robotshop.com/ca/en/cytron-13a-single-dc-motor-controller.html

I was looking at this motor controller... it doesn't say. But let's say I build my own H-bridge circuit and use fast decay mode?

In fast decay mode, if the current doesn't decay to zero, then you should expect a linear relationship and active braking. Synchronous rectification is even better, it will always have a linear relationship and active braking.

Fast decay will decay to zero much more readily with slow PWM than fast PWM frequency of course (obvious really). An oscilloscope will show you what's happening.

The current in a DC motor is mainly the result of the load and frictional torques, the decay modes mostly affect the ratio between PWM duty cycle and effective drive voltage (ie motor speed), and active braking.

If you want 4-quadrant control, go with synchronous rectification for an easy life.