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.

Simulation and Working:

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.

closed loop single phase AC motor speed controller circuit

Courtesy: Elektor Electronics

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


My Dear Mr. Swagatam,

I have gone though your designed circuit. Since I am not an Electrical Engineer  but a Production Engineer, I do not know much about Electronics. 

I would like to know from you that I want to operate tow (2) HP single phase
AC motor with your designed circuit by using the same parts that you have already listed. IS IT POSSIBLE? If not, would you kindly be pleased to guide me how I could make it possible? Kindly provide revised part list with their exact value.

Thanking you and look forward to hearing from you as soon as possible.

Syed Asghar Bokhari

My Reply:

Dear Syed,

This circuit was not designed by me actually. It was designed by the elektor electronics engineers. For 2 HP motor the circuit will remain the same except the SCR which will need to be upgraded to a 40 amp scr such as NTE5536

R1 may also require some change, but I am not sure exactly to what value. If you find the SCR not triggering well, you can try reducing it to some lower value such as 33K, 1 watt and check the response.


40 amp SCR Specifications:

Need Help? Please send your queries through Comments for quick replies! And please Bookmark my site :)


S M OMAR Faruk said…
I would like to ask, What is this P1?
what kind of motor (HP or Wattage) can you aplly this circuit to?
Swagatam said…
you can use any wattage AC motor provided the SCr is rated appropriately for handling the current.
pathak said…
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
Thanks, & Regards,
Swagatam said…
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
Arkhantos said…
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?
Swagatam said…
yes it will as long as long as the input voltage does not fluctuate....
Arkhantos said…
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?
Swagatam said…
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.
Arkhantos said…
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!
Swagatam said…
L1/C1 are not crucial, they are placed for 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.
Does it control motor speed precisely without a use of tachogenerator?
Arnold said…
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,
Swag said…
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
Svetoslav said…
does this cirquit work with 220v or is designed for 110v thanks
Swag said…
Hi, it can be used for both 220V and 120V inputs
Doms said…
Are all diodes are Zener or not? Thanks in advance.
Swagatam said…
They are 1N4007 rectifier diodes.

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