Re: ac induction motor speed control problem
- From: "Fox one! Fox one!" <asd@xxxxxxxxxx>
- Date: Thu, 21 Jun 2007 16:56:45 GMT
"mike theodore" <mixkef_removeme_@xxxxxxxxxxxx> wrote in message
news:f45k7q$6kg$1@xxxxxxxxxxxxxxxxxxxxxx
Hi
I'm experimenting with a single phase ac 220V ac induction motor.
I want to regulate speed and for this i'm using a triac together with
an optocoupler to drive it.
I'm using PWM to control firing angle and hence the speed of the motor .
I see two strange things as speed is reduced
1. Motor overheats. Is this normal ?
2. I hear a buzzing sound coming from the motor .From what is it?
I want to mention that the motor has a primary and a secondary winding
The primary has the triac circuit and the seconadry is connected directly
to
mains throug a capacitor.
Any help would be greatly appreciated .
Michail
I have made a speed control like you have described.
First you need to know what sort of motor you are driving, this will
completely change the design of the speed controller. THe controller you
have described will work for single phase induction motors such as
'shaded-pole' and 'capacitor run' (also known as 'permament split capacitor'
i think?). The motor you describe sounds like a capacitor-run (two
windings, one in series with a capacitor). I do not understand however why
you say one winding is controlled by the triac and the other is connted to
mains? When it is run normally at full speed off the mains, are both these
windings connected together on the mains?
Assuming it is a capacitor run or shaded pole, the only real problem I had
was:
* Drive the optoisolator with a square wave of about 2kHz rather than just
turning the optoisolator on when you want the triac to fire. If your drive
circuit is copied from a triac manufacturers application note(using an
opto-triac to fire the main triac by discharging a capacitor into the gate),
you need to keep sending pulses until the load starts to conduct enough
current (the latching current) to keep the triac turned on. Otherwise thr
triac will only stay on on some cycles and not others. To make it worse it
the triac will only stay on on positive or negative half-cycles so your
motor will get a lot of DC, this is what is making it hot.
This is the application note I copied:
http://www.fairchildsemi.com/ds/MO/MOC3020-M.pdf, figures 7, 8, 9.
The buzzing sound is probably caused by the sharp edges of the waveform.
Normallly it gets a nice smooth sinusoid, but you will be driving it with
sudden voltage/current increases. If you have an EMI filter on the output
(an inductor and capacitor) this might help this problem but I don't have
enough experience to comment.
.
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- From: mike theodore
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