Newsgroups: comp.robotics
Path: brunix!sgiblab!swrinde!gatech!howland.reston.ans.net!agate!dog.ee.lbl.gov!newshub.nosc.mil!nosc!pegasus!uunet!portalarioch
From: arioch@shell.portal.com (Kip J Mussatt)
Subject: Re: PWM DC motor control
Message-ID: <Cnr8oz.C30@unix.portal.com>
Sender: news@unix.portal.com
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Organization: Portal Communications Company
References: <Cn7H4o.By2@unix.portal.com> <8syXJc1w165w@sfrsa.com>
Date: Mon, 4 Apr 1994 21:50:59 GMT
Lines: 45

eds@sfrsa.com (Ed Severinghaus) writes:

>arioch@shell.portal.com (Kip J Mussatt) writes:

>> pjg@parint.esl.com (Paul Gyugyi) writes:
>> >same average value.  The reason is that you get less heating of the motor
>> >with a PWM signal, and so you can go to a higher torque/speed without 
>> >overheating the motor.
>> 
>> I believe the heating would be the same either way, if any difference.
>> Heat is a biproduct of power dissapation.
>> 
>> P = V^2 / R;  assuming R stays the same since the coefficient for 
>> thermal conductivity for copper is + not - , the copper will reach
>> a given temp. under a given load; R is relatively constant.  The V
>> is Vrms.  Which for PWM would be the same as direct linear DC.
>> 6Vrms is 6Vrms.  I do not see any way of more power being used to 
>> supply a direct linear control (not that I think it is the best, PWM
>> is much superior). 
>> 
>> -Kip 
>> arioch@shell.portal.com

> 
>But the issue is not jsut the heating of the motor.  Rather, the 
>"heating" issue is that of the heat dissipated in the control cirucit, 
>i.e. power transistor, which supplies the voltage to the motor.  In PWM 
>this can be negligible because current and voltage are not present at the 
>same time in the control circuit (in the ideal model), whereas, in the 
>"linear" model the control limits the voltage to the motor by adding its 
>(varying) resistance in series with the motor.  for a given *average* 
>voltage on the motor, say 6 VDC in a 12 VDC system, the PWM control will 
>typically use 5-10% of the power of the motor, but the linear system will 
>waste the same power in the control as is delivered in the motor. 
> 
>P.total =  V.supply x I  not V.motor x I

Did you read the original post?   The issue was "overheating the motor"
as in "burning it up"

Lets all move on to a new subject because this has seemingly turned into a
flame war of sorts not the answer and question forum it should be.

-Kip
arioch@shell.portal.com 
