Newsgroups: comp.robotics
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From: ORBAN@amt00.ime.nrc.ca (Peter Orban)
Subject: Re: looking for a SCR speed control unit....
In-Reply-To: mgrstk@NeXTwork.Rose-Hulman.Edu's message of 24 Jun 1993 03:21:18 GMT
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References: <HATTORI.93Jun24102855@athena.csrd.nkk.co.JP> <20b6jeINNfdn@master.cs.rose-hulman.edu>
Date: Fri, 25 Jun 93 14:45:20 GMT
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In <20b6jeINNfdn@master.cs.rose-hulman.edu> mgrstk@NeXTwork.Rose-Hulman.Edu writes:

> In article <HATTORI.93Jun24102855@athena.csrd.nkk.co.JP>  
> hattori@csrd.nkk.co.JP (Mato Hattori) writes:
> > Hi! Aaron & Steve, 
> > 
> > If my understanding is correct, SCR is for controling AC current, and
> > FET is for switching DC voltage.  The difference is that SCR can
> > control AC motors in variable speed, but FET can turn on or off DC
> > motors in fixed speed.  If you want to control speed of DC motor, like
> > a RC vehicle dose, you have to add PWM circuit to control switching
> > duty of the FET.
> > 
> >
> Yes you are correct. In fact, this is how RC controllers work. Sorry, I  
> should have been a little bit more clear.
> 
> Steve

SCRs are also used for controlling the speed of DC motors. The SCRs
form a rectifying bridge (usually three phase) and the turn on
time is controlled. Like your dimmer at home. SCR drive units got
superseeded by the PWM and most recently by AC drives, as those 
have higher bandwidth. (SCR units work with 50 or 60 Hz, depending 
where you are, while PWM units have the switching frequency in the 
KHz range.) SCR units are still used where high bandwidth is not a
criteria, as they are a lot cheaper.

Hope this helps, Peter
--
Peter Orban
National Research Council of Canada
Internet: orban@nrcamt.ime.nrc.ca

