Wednesday, December 7, 2011

AC Motor Speed Controller

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.


Part:
Part
Total Qty.
Description
Substitutions
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
SCR11ECG5400
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

Notes :

  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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