no, you can't control an AC motor with a single transistor that way. you'd
need several in a bridge configuration. There are actually special power
bricks made for this (e.g POWEREX). And don't forget, he's asking for a 240V
AC motor! There's factor of sqrt(2), not including any transients. At a bare
minimum, you'd need a 600V device for this. So i guess the 2N3055 is right
out... so is the IRF511 from radio shack (which is rated at 80V).

-----Original Message-----
From: jhancock [mailto:jhancock@CWV.NET]
Sent: Friday, February 16, 2001 4:08 PM
To: PICLIST@MITVMA.MIT.EDU
Subject: [EE]: URGENT Power Control help needed


You might consider building an emitter follower circuit and using a
potentiometer in the base to control the base voltage.  I don't know how
that would work with an AC motor.  I used something like that on an old auto
dc fan motor to get a variable speed control - worked OK.

+12VDC source would go the (NPN) collector and the emitter to the motor.
Other motor lead goes to -12V.  The power potentiometer (I used 1000 ohms
several watt rated potentiometer) was connected from +12V to -12V with the
wiper going to the base of transistor.  This would provide a variable output
voltage between zero and about 11.3 volts.  Actually, I used two transistors
in parallel, a couple old obsolete 2N274 PNP types (polarity reversed from
above).  You can get a NPN (I think) 2n3055 power transistor at Radio Shack
which is good for several amps if heat sinked.  If you don't want the full
voltage range, put a resistor between +12 V and the top of the potentiometer
to limit the maximum base voltage.

You need to mount the power transistor on a heat sink - I used scrap
aluminum or aluminum chassis or sheet of copper pcb.

An IRF 510 mosfet also available at RS might also work and would not draw
base current allowing a smaller power rated potentiometer.  Not sure how the
output voltage would vary with gate voltage.

Jess
-----Original Message-----
From: Andy Faulkner <andyf97@YAHOO.COM>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Friday, February 16, 2001 6:44 AM
Subject: Re: [EE]: URGENT Power Control help needed


>Has anyone got a complete working circuit for
>controlling the speed of a 240V AC electric motor such
>as a hand drill from a low dc voltage say 0 to 12V or
>0 to 5V.
>
>I need one urgently for a show were doing this
>weekend.
>
>Andy Faulkner
>
>
>
>
>
>=====
>
>Andy Faulkner andyf97@yahoo.com
>Pyrosync International http://www.pyrosync.co.uk
>
>Tel +44 778 896  6087.....................UK Dial 0778 896 6087
>
>Fax +44 870 132 3275.....................UK Dial 0870 132 3275
>http://www.event-supplies.co.uk http://www.spectacular-fireworks.co.uk
>
>
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