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simplex

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?

retrolefty

#1
Oct 22, 2012, 01:23 am
How about just having a 25% (or less) duty cycle applied to a PWM output signal?

Lefty

simplex

#2
Oct 22, 2012, 01:41 amLast Edit: Oct 22, 2012, 01:43 am by simplex Reason: 1
If the motor were a pure resistive load a duty cycle of 1/16 (P=U^2/R) would do the job.

dhenry

#3
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.

simplex

#4
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.

MarkT

#5
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!).
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

simplex

#6
Oct 22, 2012, 05:15 pm
I know motors are not pure resistive loads. My initial question is related to motors not resistive loads.

MarkT

#7
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.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

simplex

#8
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.

dhenry

#9
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.

Grumpy_Mike

#10
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.

simplex

#11
Oct 23, 2012, 12:39 am
For a motor like this:
http://www.pololu.com/catalog/product/1117

Grumpy_Mike

#12
Oct 23, 2012, 01:12 am
Fine but you still don't know the inductance.

simplex

#13
Oct 23, 2012, 04:57 amLast Edit: Oct 23, 2012, 05:05 am by simplex Reason: 1
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.

gardner

#14
Oct 23, 2012, 05:43 am
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.

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