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[DPRG] Inductors and PID

Subject: [DPRG] Inductors and PID
From: Ed Koffeman edk at kinetric.com
Date: Thu Nov 14 08:21:00 CST 2002

Hi, Kip.

The word "chopping" is usually used to describe very low frequency PWM, like old R/C controllers or older industrual controllers,
SCR controllers etc. that have very long ON times.  These cause the motor to have very high current ripple if there's not enough
inductance.  If it's low like that you also get the current dropping to zero during the OFF times. The motor is getting more like
bursts of full power and there are very large losses.

If you are actually doing PWM at e.g. 20 KHz the current is constant, with a triangle wave ripple - it never stops and doesn't spike
to high values.  You get the same effect as if you have attached a lower (ON time % of the supply) DC voltage.

So, tell him you're doing PWM at x Khz and unless you are using a slotless or other non-iron-core winding (very low inductance) I
doubt he'll maintain you're short on inductance.

To check, just measure the % of current ripple.  In general, if the current doesn't drop to zero during the OFF time (with e.g. 10%
of max current flowing) then you're not hurting anything.

Ed Koffeman

----- Original Message -----
>From: "Kipton Moravec" <kip at kdream.com>
To: "DPRG List" <dprglist at dprg.org>
Sent: Thursday, November 14, 2002 7:05 AM
Subject: [DPRG] Inductors and PID

> I was talking in passing to an older electrical engineer about my 10 amp 90V
> motor controller.
> I told him I was chopping and he said don't forget to add an inductor, or
> you will burn out your motor's brushes faster.
> I always thought the motor windings were a huge inductor, and why would I
> want more?  I put a 3000V 2000 pF capacitor on the motor output to help
> attenuate the spikes I think he is talking about.
> I have never seen it on h-bridges, or is it only an issue with the high
> voltage?   Can anyone point me to some sources?
> Kip
> Kipton Moravec  kip at kdream.com
> Custom Electronics Design and Manufacture
> http://www.kdream.com
> _______________________________________________
> DPRGlist mailing list
> DPRGlist at dprg.org
> http://nimon.ncc.com/mailman/listinfo/dprglist

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