# 6V motor powered by a 24V battery

6V motor powered from a 24V battery

Is it possible to power a 6V motor from a 24V battery if I switch on an off the power with a certain frequency and filling factor. If yes, how can I calculate the optimum frequency and filling factor of the PWM signal that drives the switch?

How about just having a 25% (or less) duty cycle applied to a PWM output signal?

Lefty

If the motor were a pure resistive load a duty cycle of 1/16 (P=U^2/R) would do the job.

Is it possible to power a 6V motor from a 24V battery if I switch on an off the power with a certain frequency and filling factor.

Yes.

If yes, how can I calculate the optimum frequency and filling factor of the PWM signal that drives the switch?

To answer that question, you have to define "optimum" for us.

I mean the duty cycle and frequency for which the motor reaches an efficiency as close as possible to that of the case when it is powered from a 6V battery.

simplex: If the motor were a pure resistive load a duty cycle of 1/16 (P=U^2/R) would do the job.

True, but not useful as DC motors are not resistive loads (unless stalled!).

I know motors are not pure resistive loads. My initial question is related to motors not resistive loads.

That's been answered by retrolefty - 25% - well that's a good starting point as it depends on PWM rate and how the free-wheeling is done. If you are using a half-H-bridge switching from +24V to 0V then the behaviour in theory is linear with duty cycle. However real-world behaviour won't be perfect.

Honestly, that 25% duty cycle seemed to me a miscalculation done by somebody that took a pure resistive load as model but overlooked the fact that P in not U/R but U^2/R.

In general, motors don't want very high pwm. I would stay within 1khz if I were you.

I will also add that much more than 4x energy goes into the motor at 24v than at 6v, particularly when the motor is revolving at speed closer to its no-load rpm at 6v. As such, I would put 25% @ 24v as the upper end as an approximation to 100% @ 6v.

If you want to find out the exact answer, you may have to experiment.

Without knowing the inductance of the motor it is impossible to answer the question in the manner you want.

For a motor like this: http://www.pololu.com/catalog/product/1117

Fine but you still donâ€™t know the inductance.

Consider the inductance is L, a parameter, and calculate duty_cycle_optimum and f_optimum as a function of L. That would be much more useful for me because the results can be easily particularized to any motor as soon as I find its inductance.

I would suggest you figure this out empirically. Attach a load to your motor -- like a propeller or something -- and run it from 6V. Measure the RPM and current draw. Now run it from PWM 24V and finesse the duty cycle and frequency until you reach a similar RPM and current draw.