# Arduino Forum

## Using Arduino => Motors, Mechanics, Power and CNC => Topic started by: simplex on Oct 22, 2012, 01:09 am

Post by: simplex on Oct 22, 2012, 01:09 am
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?
Post by: retrolefty on Oct 22, 2012, 01:23 am
How about just having a 25% (or less) duty cycle applied to a PWM output signal?

Lefty
Post by: simplex on Oct 22, 2012, 01:41 am
If the motor were a pure resistive load a duty cycle of 1/16 (P=U^2/R) would do the job.
Post by: dhenry on Oct 22, 2012, 02:33 am
Quote
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.

Quote

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.
Post by: simplex on Oct 22, 2012, 04:03 am
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.
Post by: MarkT on Oct 22, 2012, 03:59 pm

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!).
Post by: simplex on Oct 22, 2012, 05:15 pm
I know motors are not pure resistive loads. My initial question is related to motors not resistive loads.
Post by: MarkT on Oct 22, 2012, 05:50 pm
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.
Post by: simplex on Oct 22, 2012, 09:04 pm
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.

Post by: dhenry on Oct 22, 2012, 11:52 pm
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.
Post by: Grumpy_Mike on Oct 23, 2012, 12:04 am
Without knowing the inductance of the motor it is impossible to answer the question in the manner you want.