PWM Driving motors

Started by arhodes19044 December 6, 2003
I am driving motors via the hardware PWM. I have the H-bridge and
all the electronics all set and it works great. I had heard that
higher frequency driving is superion to low frequency. Like 7KHz vs
14Hz. I set up a test program to drive the motor ar a variety of
frequencies at 50% pulse width.

I found that there was a steep drop in torque (and speed) with
increasing driving frequency. 14 and 56 Hz seemed to work OK, but at
3600 and 7200 (50% duty cycle) the motor nearly stops. (I can hear
an induction whine in the motor, too).

So, Is there something I am supposed to do to make the higher
frequency drive work? I presume there was a good reason that for
someone to have written that the higher frequencies are superior,
therefor I'd like to use the superior technique.

-Tony Rhodes



here is a thread that briefly discusses advantages / disadvantages of pwm
frequency.

http://micromouse.cs.rhul.ac.uk/mail-archive/msg00779.html

"at high frequencies current never really starts to flow before the pulse
cuts off. With little current flowing you get poor torque."

"one downside to large current pulses is the amount of EM interference
generated playing havoc with delicate electronics" (lower frequency
disadvantage)

hope this helps.

-Trevor

At 08:25 PM 12/6/2003 +0000, you wrote:
>I am driving motors via the hardware PWM. I have the H-bridge and
>all the electronics all set and it works great. I had heard that
>higher frequency driving is superion to low frequency. Like 7KHz vs
>14Hz. I set up a test program to drive the motor ar a variety of
>frequencies at 50% pulse width.
>
>I found that there was a steep drop in torque (and speed) with
>increasing driving frequency. 14 and 56 Hz seemed to work OK, but at
>3600 and 7200 (50% duty cycle) the motor nearly stops. (I can hear
>an induction whine in the motor, too).
>
>So, Is there something I am supposed to do to make the higher
>frequency drive work? I presume there was a good reason that for
>someone to have written that the higher frequencies are superior,
>therefor I'd like to use the superior technique.
>
>-Tony Rhodes >">http://docs.yahoo.com/info/terms/





Yes, I had thought that the inductance (reactance?) of the windings
of the motor were the culprit. I guess that I will go with
the "best" drive frequency that I can determine by fingers and eye.
14Hz seems do do fine. 56Hz seems to lose a slight amount of torque,
and does not seem to gain anything useful in the low duty cycle range.

I find that the motor has excessively low torque at less than 20%
duty cycle (even at 14hz) and this effectively is the lower limit of
motor speed.

-Tony --- In , Trevor Pinkney <tpinkney@s...> wrote:
> here is a thread that briefly discusses advantages / disadvantages
of pwm
> frequency.
>
> http://micromouse.cs.rhul.ac.uk/mail-archive/msg00779.html
>
> "at high frequencies current never really starts to flow before the
pulse
> cuts off. With little current flowing you get poor torque."
>
> "one downside to large current pulses is the amount of EM
interference
> generated playing havoc with delicate electronics" (lower frequency
> disadvantage)
>
> hope this helps.
>
> -Trevor
>
> At 08:25 PM 12/6/2003 +0000, you wrote:
> >I am driving motors via the hardware PWM. I have the H-bridge and
> >all the electronics all set and it works great. I had heard that
> >higher frequency driving is superion to low frequency. Like 7KHz
vs
> >14Hz. I set up a test program to drive the motor ar a variety of
> >frequencies at 50% pulse width.
> >
> >I found that there was a steep drop in torque (and speed) with
> >increasing driving frequency. 14 and 56 Hz seemed to work OK, but
at
> >3600 and 7200 (50% duty cycle) the motor nearly stops. (I can hear
> >an induction whine in the motor, too).
> >
> >So, Is there something I am supposed to do to make the higher
> >frequency drive work? I presume there was a good reason that for
> >someone to have written that the higher frequencies are superior,
> >therefor I'd like to use the superior technique.
> >
> >-Tony Rhodes
> >
> >
> >
> >
> >
> >">http://docs.yahoo.com/info/terms/