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Closed Loop Single Phase AC Motor Speed Controller

Closed Loop Single Phase AC Motor Speed Controller

The article presented here explains a very simple closed loop AC motor speed controller circuit that may be used for controlling single phase AC motor speeds.

The circuit is very cheap and uses ordinary electronic components for the required implementations. The main feature of the circuit is that it’s a closed loop type, that means the speed or the torque of the motor can never get affected by the load or the speed of the motor in this circuit, on the contrary the torque is indirectly proportional to the magnitude of the speed.

Circuit Operation:

Referring the circuit diagram of the proposed single phase closed loop AC motor controller, the involved operations may be understood through the following points:

For the positive half cycles of the input AC, the capacitor C2 is charged through the resistor R1 and the diode D1.

The charging of C2 persists until the voltage across this capacitor becomes equivalent to the simulating zener voltage of the configuration.

The circuit wired around transistor T1 effectively simulates the operation of a zener diode.

The inclusion of the pot P1 makes it possible to adjust the voltage of this “zener diode”. Precisely speaking, the voltage developed across T1 is literally determined by the combined values of P1, R3 and R4.

The voltage across the resistor R4 is always maintained equal to the 0.6 volts that’s equal to the required conducting voltage of T1’s base emitter voltage.

Therefore it means that the above explained zener voltage should be equal to the value that may be acquired by solving the expression:

(P1 + R3 + R4) × 0.6 / R4

How the Load is Positioned for a Special Reason

A careful investigation reveals that the motor or the load is not introduced at the usual position; rather it’s wired up just after the SCR, at its cathode.

This causes an interesting feature to be introduced with this circuit.

The above special position of the motor within the circuit makes the firing time of the SCR dependant on the potential difference between the back EMF of the motor and the “zener voltage” of the circuit.

That simply means that the more the motor is loaded, the quicker the SCR fires.

The procedure quite simulate a closed loop type of functioning where the feedback s received in the form of back EMF generated by the motor itself.

However the circuit is associated with a slight drawback. The adoption of an SCR means the circuit can handle only 180 degrees of phase control and the motor cannot be controlled throughout the speed range but only for 50% of it.

Another disadvantage associated due to the inexpensive nature of the circuit is that the motor tends to produce hiccups at lower speeds, however as the speed is increased this issue completely disappears.

The Function of L1 and C1

L1 and C1 are included for checking the high frequency RFs generated due to the rapid phase chopping by the SCR.

Need less to say the device (SCR) must be mounted on a suitable heatsink for optimal results.

Parts List for the above closed loop AC motor speed controller circuit

  • R1 = 56K,
  • R2 = 33K,
  • R3 = 10K,
  • R4 = 22K,
  • VR1 = 330K,
  • All diodes = 1N4007,
  • C1 = 0.1/400V,
  • C2 = 100uF/25V,
  • T1 = BC547B,
  • L1 = 30 turns of 25 SWG wire over a 3mm ferrite rod


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

22 thoughts on “Closed Loop Single Phase AC Motor Speed Controller”

  1. What maximum power of the motor it can control and what components should be used for the speed controller for motor 1000 – 1200 watt, 115 v,

    • upto 300 watts can be controlled with the shown set up, for higher wattage you may have to upgrade the SCR with a higher value, and also reduce the gate resistor value appropriately

  2. Do you have the inductance value of L1? May i use a toroidal inductor instead?
    By the way, amazing blog you have, and i really appreciate that you answer!

    • L1/C1 are not crucial, they are placed for PFC….to reduce RF interference in the atmosphere.

      still if you want to include them , you can try 200 turns of 24 SWG magnet wire on any iron core such as an iron bolt.

      I am glad you liked my website…please keep reading.

  3. Will this circuit provide a stable frequency of revolution (RPM) even under load?
    Meaning the motor will keep spinning at the same speed even when a load is aplied?

    • Thanks!
      Im thinking to use it to control a washing machine universal motor in order to make a belt sander.
      Will a 10amp thyristor be enought?
      Also, what power dissipation capability will be needed in the resistors?

    • The design was originally intended to control drill machines, so probably any similar motor can be controlled with this circuit, although the capability is restricted to 180 degrees only….The resistors can be all 1/4 watt rated, a 10amp SCR will do if the motor max consumption does not exceed this value.

  4. Sir , How AC supply to motor is completed? Means Both the half cycles. My another Q is can I connect Transfomer pri to control 230 ac v , so that sec volage automaticaly get controlled. Sir Expecting your reply. my email vijayraopathak@yahoo.co.in
    Thanks, & Regards,

    • Pathak, the above circuit will control only one half cycles of the AC and therefore will be able to provide a 180 degree phase control only….for full control you can make any regular dimmer switch circuit….the transformer can be also controlled by using a dimmer switch circuit in by connecting it series with the trafo primary

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