[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Re: Dimmer with AC motor?

From: Jeff Mann   jefmanutcc.utoronto.ca
Date: Fri, 27 Oct 2000 19:23:36 -0700

A! Sent via the Art & Robotics Group mailing list: arg-list@xxxxxxxxxxxxxxx
R! Use your "Reply All" to  reply to the list, "Reply" for private response
G!

Hi all -

> Rob Cruickshank wrote:
> 
> Dimmers may have some problems with an inductive load, such as a
> motor. Essentially, the dimmer works by chopping bits out of the AC
> waveform. Inductors, like the coils in a motor, dislike having the
> current through them changed abruptly, and produce large voltage
> spikes. These can fry the triac in the dimmer. They can also, if
> severe enough, break down the insulation in the windings of the coil.

The problem isn't so much in burning out the triac, but in the fact that
it can cause it to self-trigger. You can protect against the inductive
kickback by putting a 100 ohm 1/2W resistor in series with a 0.22uF 200V
capacitor, across the triac power leads.

This should work fine for motors with brushes. The only problem might be
the hysterisis effect in cheap lamp dimmers, which can cause the motor
to misbehave at  low speeds. The circuit to overcome this is very simple
although my ascii schematic skills aren't good enough to draw it here!
Look for an "electric drill speed control circuit" on the net; otherwise
I can try to scan it in.

If the motor doesn't have brushes, it could be one of many types of
induction motor designs. Some of these will work to some extent with
this kind of dimmer circuit. However, they will rapidly overheat and
burn out at slower speeds. This is because - correct me if I'm wrong -
most of these motors work by creating a rotating magnetic field at the
60Hz line frequency, and the further the rotor departs or "slips" from
this speed, the more current it will draw. Fan motors are supposed to be
best-suited to this type of speed control, but I guess as Mike found
out, even they will burn out at slower speeds or draw enough current to
burn out the dimmer. The best way to control the speed of induction
motors is with a variable-frequency power supply, but that's quite complicated.

In case you're interested, I recently built an 8-channel MIDI-controlled
AC dimmer circuit using a PIC chip and triacs. It could be used for
computer-control of stage lighting etc., but I wanted to use it to
cushion the effect of reversing the direction of a reversing AC
induction motor, i.e. to turn off the power and then slowly ramp it back
in the opposite direction instead of slamming it into reverse at full
speed and grinding the gearbox. It seems to work well, and since it is
only slowed down for a very short period of time, the current and
overheating doesn't seem to be a problem.

A good reference for this stuff is "Electric Motors and Control
Techniques" by Irving Gottlieb from TAB books.

cu later <Jeff
-- 
Jeff Mann - Information Consumer ___O___O__= -- >
"Tapping one's toe in time with a piece of music while sitting on 
a modern carpet can induce +/-10 volt potential change on a can of
Spam five feet away."  - The Amateur Scientists' Bulletin

A!
R!      messages saved at http://www.interaccess.org/arg/arg-list.html
G!      unsubscribe/help requests to mailto:Majordomo@xxxxxxxxxxxxxxx