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Motor speed control - operates well at low speed only

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Aug 1, 2009
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Motor speed control help

I have this circuit attached below. I have built it identical except for the 1.5M pot. All I had was a 1M pot and I am using 18 VAC instead of 12. A couple of problems is that I have to turn the pot about 3/4 of the way until I get a response from the motor. I am using this as a speed control for a model train engine operating at 18 VAC. This other problem is it operates at low speed fairly smooth but as I increase the speed the train eventually stops and the motor just hums. I have read just about everything I could get ahold of on the internet about phase control of a triac. I have not come across how to calculate the R/C circuit for the gate control. Everything you find is for 120 VAC or 220 VAC operation. Any ideas or suggestions for 18 VAC operation. Thanks.

Re: Motor speed control help

This form of speed control, can be finicky. Your trigger
circuit is playing against the line (60Hz) phase. The
SCR (or TRIAC) trigger event -must- be applied at an
angle prior to the zero-crossing because the zero-
crossing is the only way these things shut off. Applying
before the peak can also be bad. You would like the
range of control to be from peak to zero and limit the

You may want to change the 0.1uf cap to 0.15, to
get to a time constant more like the original design.

The DIAC breakover voltage might be too large a
portion of the available line, to get you a good
trigger event. This may be part of why you need so
much potentiometer to start it.

Do you have a 2-channel 'scope that can show you
how the trigger phase and input AC waveform line up?
R1*C1 should be setting your max conduction angle
(min delay) and (R1+R2_full)*C1 your min angle
(max delay, equals next-zero-crossing or 8.3mS).

I haven't touched SCR motor controls since the '80s
(early '80s), and don't remember much more than
this. Or maybe, not even.

Motor speed control help

Thanks for your reply, I do have a scope and will check what you suggested.

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