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From: email@example.com (Bob Wilson)
Subject: Re: Triac Hack?
Date: Mon, 23 Sep 2002 04:41:20 -0000
Organization: Your Organization
X-Newsreader: WinVN 0.99.9 (Released Version) (x86 32bit)
In article , EskWIRED@panix.SPAMBLOCK.com
>[ This is a repost of the following article:
>[ From: EskWIRED@panix.SPAMBLOCK.com
>[ Subject: Re: Triac Hack?
>[ Newsgroups: sci.electronics.basics,sci.electronics.circuits
>>> : EskWIRED@panix.spamblock.com wrote:
>>> : I use a standard 600 watt dimmer switch in an unconventional way - to
>>> : control the speed of a fan motor in a hot air popcorn popper which I
>>> : to roast coffee. It works well.
>> The circuit you will need is a switch mode power supply. By switching the
>> motor on and off rapidly and adjusting the 'on' and 'off' timing a very
>> efficient motor control system can be made. I know what you are
>> Bloody complex. It really isn't though.
>But doesn't my light dimmer do exactly that? It uses a triac to switch
>the power off every time the AC waveform passes the zero line, and then
>turns on again later in the waveform.
Yes it does. You are absolutely correct. Not sure why he suggested a SMPS,
since, in essence, you already have the rudiments of one.
The problem you have is that there is no motor load sensing. A triac light
control is simply a "dumb" PWM device that outputs a constant duty cycle
(well it isn't the normal squarewave PWM, but rather a portion of the AC
sinewave, but the result is much the same). When the motor gets loaded down
by your coffee or whatever the heck you are using a popcorn popper for (!),
the triac control doesn't increase the duty cycle to make up for the load.
The problem is especially noticeable at light motor loads, where the motor
is easy to stall.
A proper motor speed control will sense motor back emf and make a
first-order correction. When the motor load increases (i.e. the back emf
decreases), it will increase the duty cycle to attampt to keep motor RPM
(and therefore the sensed back emf) approximately the same.
Many circuits are published that detail what you are looking for (I even
have several that came from 1960s RCA and GE SCR and Triac manuals). In fact
they are very similar to a light dimmer circuit, with the only difference
being the way they can sense motor load. Have a look for "motor speed
control" or something similar either on the internet or in your library.
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