The Shop > Electronics & IC Programing
how to make a speed controller for a washing machine motor
AdeV:
--- Quote from: picclock on December 18, 2016, 10:35:21 AM ---A simple controller can be made by using the dc voltage generated by the motor rotation to trigger the thyristor when the rotation, and generated voltage falls.
--- End quote ---
A couple of questions... please tell me to go google it if it's easier...
What's the purpose of the 2 capacitors? I presume the large one on the RHS is to reduce ripple, and the small one between the DIAC and the pot is to negative bias the diac?
Approx what size should the capacitors be?
As the pot is at up to 400v potential, presumably a boggo carbon track pot can't be used... Firstly, what component would you recommend there, and second, how hard would it be (famous last words...) to replace it with some kind of computerised pot?
Thanks! :thumbup:
seadog:
I'd imagine the one on the trigger is to reduce the chance of spikes causing false switching Ade.
philf:
A simpler circuit still. I made several many years ago for drill speed controllers. They regulate the speed very well.
I still have some of the BT151 sensitive gate thyristors.
Phil.
picclock:
Hi
The capacitor on the pot wiper is to allow a large pulse trigger current to flow when the diac voltage collapses. This means you can use thyristors with a relatively large trigger current, say up to 25mA. Larger capacitor = more trigger current. 0.1uF is good for most cases. Can also have capacitor low end connected to thyristor anode (motor side). Not a lot in it except capacitor voltage is then vdiac max say 40V - probably better to do this if you are making one. The other capacitor on the RHS is to smooth the rectified AC, and will need a voltage of 400V. 10uf is plenty, 1 uF is adequate. You can work out the ripple voltage. For a 50K pot current is around 6mA. CV=IT - near enough in this case. In the 10mS before the next pulse with 1uF 6*10-3 * 10-2 /10-6 =0.6v ripple - more than adequate in 340V (1.414*240). With a 50K pot you will need a 2W wirewound controller. There is no reason not to use a 100k pot with a 1W rating which might make things simpler.
To replace the pot with a DAC would not be difficult, but the output voltage would have to be very high so likely a simple transister buffer/amplifier would do. BF338? video deflection transistors may work for this.
Circuit speed regulation is excellent, and will allows full control from zero up to peak voltage - vdiac (~310V).
@Philf - Circuit above only gives half wave control with limited voltage swing so much less power. Do not see the need for the lower diode which appears to do nothing.
Best Regards
picclock
philf:
--- Quote from: picclock on December 19, 2016, 03:08:21 PM ---
@Philf - Circuit above only gives half wave control with limited voltage swing so much less power. Do not see the need for the lower diode which appears to do nothing.
--- End quote ---
It must have been over 40 years ago that I built the last one - it worked very well giving a very good speed range. I must have binned it as my drills are now all variable speed. I worked for Mullards who then manufactured thyristors at our factory in Hazel Grove. I think it was one of our application engineers who gave me the circuit. The BT151 had a gate current of <5ma. I can't comment on the need for the diode. (I was a mechanical engineer.) I might try to cobble one together to see if it works without.
Phil.
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