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PWM AC Motor Speed Controller

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Well-Known Member
The circuit loks like a dimmer. You got a triac together with something that looks like a simple diferential amplifier circuit. But there is no sign of voltage levels or other component values.


New Member
The circuit controls both low voltage (from 24VAC to 240VAC at 5.5A) AC motors and mains voltage (110VAC or 220VAC) loads.

R1: 4.7K resistor
R2: 3.3K resistor
R3: 680 ohm resistor
R4, R5: 22K resistors
R6: 1.5K resistor
R7: 47K (50Hz), 10K (60Hz)
R8: 100K resistor
R9: 100 ohm resistor
R10: 120 ohm resistor
R11: 68 ohm resistor
R12: 1K resistor
R13: 27 ohm resistor
R14: 390K resistor
RV1, RV2: 100K trim potentiometers
C1: 470uF electrolytic capacitor
C2: 1uF electrolytic capacitor
C3: 100nF capacitor
C5: 47nF capacitor
C6: 100nF/400V capacitor
C7: 47nF/400V capacitor
D1, D2, D7: 1N4007 diodes
D3-D6: 1N4148 diodes
T1-T3: BC547B transistors
T4: BC517 transistor
TRI1: BT137F-600 Triac
F1: 250mA slow fuse
F2: 5A slow fuse
L1: 50uH/1kHz coil
Trafo1: 2x6V/1x0.3A
Heatsink for the triac
terminal blocks (VAC, load and mains)
J1, J2 mains voltage jumpers


Well-Known Member
T1 is just a half cycle 60 Hz amp.

When T2 shuts off the variable resistors and cap C2 determine the turn-on cycle delay to trigger the Triac.

T3 & T4 is a voltage comparitor with R12 & R3 providing the reference.

The Triac shuts down on zero current crossing.

L1 & C7 just EMI filter. R13 C6 is snubber across Triac to prevent false trigger by inductive load.


What does D7 do?

Also, doesn't appear to be any bias on Q4 collector circuit. I don't think it will actually work.

Even it it did. There should be a much easier way to do this.
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New Member
thanks for the reply..
so if there is a better to do it..
can u pls help me?
i need help in the AC Motor Speed Controller

thanks in advance..


The only thing you have to do is delay the trigger signal to the triac. Look at this linked webpage. This is a very simple circtuit. The resistors, capacitors and diac determine the trigger phase, and thus the power delivered to the motor.

I reached this site by goggling "triac motor control" There are lost more.

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New Member
i need to make some changes to the AC Motor..
i need to change from 240V to 48V
can i still use the circuit from the website "AC Motor Control"?


New Member
For an AC motor may I suggest an incandescent lamp (about 100W) as a parralel load to improve performance.
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