Re: DC Motor Control - Heat Issue
From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 01/14/05
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Date: Fri, 14 Jan 2005 11:06:38 -0800
"Andrew" <xxragexx@gmail.com> wrote in message
news:1105727510.704266.174860@z14g2000cwz.googlegroups.com...
> All,
>
> I am new to PWM signals and DC motor control, so I am bascially
> learning as I go.
...
> Larry, could you please provide me some further information on the LC
> filter? This sounds like a great alternative if it takes up less
> space, and mainly, less weight. I am not familiar with designing LC
> filters for this application, so any information on this would be
> great.
I don't have further information other than basic AC
circuit theory and knowledge that inductor sizes are
related to the amount of energy they can store. To
see why a multistage filter would be more volume
efficient, consider this sequence:
1. Start with series inductor large enough to bring the AC
PWM voltage imposed across the moter to Epwmax.
Call its inductance 'L'.
2. Split the inductor into 2 series inductors, sized L/2.
3. Put a shunt capacitor at the new junction, with a value
C several times greater than C = 1/(L*(f*2*pi)^2)
4. Observe marked reduction in AC across motor.
5. Reduce L accordingly to obtain original Epwmax.
> Also, Larry, you mention that you would predict the heat problem based
> on the eddy current induced. What exactly is eddy current?
It is a name for current that circulates within the iron
laminations in response to a changing B field. The
reason laminations are used in iron core AC devices
is to reduce eddy currents to acceptable levels.
The effect of eddy current is two-fold. It heats the
material in which it flows. It causes a corresponding
current in the the winding which induced the B field.
You can think of eddy current, crudely, as flowing in
a shorted turn that acts like an unintended secondary
of a transformer, where the excitation winding is the
primary. Less crudely, eddy current can be analyzed
as arising in a composite of secondary windings formed
by thin (or infinitesimal) layers within the lamination.
> You
> indicate that the frequency seems to be the cause of the high eddy
> current, would a higher or lower PWM frequency help this, and do you
> suggest changing motor drivers rather than trying an LC filter or
> choke? What frequency would be optimal for this configuration, and
> why?
I cannot address optimal frequency. Your 7+ KHz seems
low, and I wonder if it cannot be increased. You will still
need a low-pass filter, but it can be smaller as you increase
the PWM frequency. At some point in that progression,
losses in the switching elements will outweigh the diminishing
other return from increased frequency. I don't know how to
express that optmization process.
As for changing your driver, I would sure have a go at
a workable filter first. I presume this is not a high-volume,
cost sensitive application, so, from an engineering cost
viewpoint, the filter is going to be your cheapest solution.
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
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