AC Motor Speed Controller

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This AC motor speed controller can handle most universal type (brushed) AC motors and other loads up to about 250W. It works in much the same was a light dimmer circuit; by chopping part of the AC waveform off to effectively control voltage. Because of this functionality, the circuit will work for a wide variety of loads including incandescent light bulbs, heating elements, brushed AC motors and some transformers. The circuit tries to maintain a constant motor speed regardless of load so it is also ideal for power tools. Note that the circuit can only control brushed AC motors. Inductive motors require a variable frequency control.


Schematic of the AC Motor Controller


Total Qty.
R1127K 1W Resistor
R2110K 1/4W Resistor
R31100K 1/4W Resistor
R4133K 1/4W Resistor
R512.2K 1/4W Resistor
R611K 1/4W Resistor
R7160K Ohm 1/4W Resistor
R813K Linear Taper Trim Pot
R915K Linear Taper Pot
R1014.7K Linear Taper Trim Pot
R1113.3K 1/4W Resistor
R121100 Ohm 1/4W Resistor
R13147 Ohm 1W Resistor (See Notes)
C1, C320.1uF Ceramic Disc Capacitor
C21100uF 50V Electrolytic Capacitor
D116V Zener Diode
Q112N2222 NPN Transistor2N3904
TR11TRIAC (See Notes)
U11DIAC Opto-Isolator (See Notes)
BR1, BR225A 50V Bridge Rectifier
T11Transformer (See Notes)
MISC1PC Board, Case, Line Cord, Socket For U1, Heatsinks


  1. TR1 must be chosen to match the requirements of the load. Most generic TRIACs with ratings to support your load will work fine in this circuit. If you find a TRIAC that works well, feel free to leave a comment.
  2. U1 must be chosen to match the ratings of TR1. Most generic DIAC based opto-isolators will work fine. If you have success with a specific part, feel free to leave a comment.
  3. T1 is any small transformer with a 1:10 turns ratio. The circuit is designed to run on 120V so a 120V to 12V transformer will work. Alternately, you can wind T1 on a transformer core using a primary of 25 turns, a secondary of 200 turns, and 26 gauge magnet wire.
  4. R9 is used to adjust motor speed. R10 is a trim pot used to fine tune the governing action of the circuit. R8 fine tunes the feedback circuit to adjust for proper voltage at the gate of SCR1. It should be adjusted to just past the minimum point at which the circuit begins to operate.
  5. R13 must be chosen to match the load. Generally, larger loads will require a smaller value.
  6. Since this circuit is not isolated from mains, it must be built in an insulated case.

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AC Motor Speed Controller
Friday, September 26, 2014 10:20:34 PM
please im also interested in the above project. ac motor speed controller. can anyone provide notes on how the circuit works. thanx in advance
Mohammed Ali
AC Motor Speed Controller
Thursday, December 20, 2012 7:58:28 AM
The project is very good and i like it but it lacks notes on how the circuit operates,can any one pliz sent me the circuit operation. Thank u in advance.
Bakos Gy
R13 AC Motor Speed Controller
Thursday, November 01, 2012 7:12:29 PM
Is R13 not 0.47Ohm? I mean on full steam the written 250W motor consumes more than one amper, counting that through with this 1ampere U=I*R=1*47=47Volt , P=U*I=47*1=47Watt ????!!!
AC Motor Speed Controller
Saturday, May 19, 2012 8:17:06 AM
The project is ok and i like it but it lacks notes on how the circuit operates,can u pliz sent me the circuit operation notes on my e-mail. Thank u in advance.
AC Motor Speed Controller
Saturday, March 12, 2011 9:51:30 PM
wonderful circuit. is it feasible to use this circuit for 220V?
AC Motor Speed Controller
Tuesday, March 01, 2011 10:02:27 PM
I suppose number of turns for T1 transformer should be half for 220V. Pls correct me if I'm wrong. I trying to build for 220V...
AC Motor Speed Controller
Thursday, February 17, 2011 6:03:42 AM
Someone asked a question re this project on converting circuit to 220v. Any suggestions on component values?
AC Motor Speed Controller
Tuesday, January 04, 2011 12:14:40 PM
dear sir,i have an ac induction motor drive and also have a input circuit (i use previousely to operate a dc motor),but the problem is it not works on ac drive.can u suggest on it. i can provide technical detail and pictures of entire system. waiting for ur early reply.
wind tech from Pine Ridge
AC Motor Speed Controller
Saturday, July 31, 2010 11:54:57 PM
About the diac used in the speed controller( to the person looking for a match to the triac) Diacs are not much different depending on the part # they only severe to allow the triac to trigger symetrically in both polarities of gate signal. They will not pass current until the gate trigger signal rises above the mid twenty volt range in either +/- voltage.
AC Motor Speed Controller
Tuesday, March 02, 2010 8:26:13 AM
this site is very useful for all the persons not only for techno grads but also for every one who are interested in electrical machines which is one of the challenging subject it also helps in improving knowledge
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