Closed Loop Single Phase AC Motor Speed Controller Circuit

The article presented here explains a very simple 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 Description:

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) × o.6 / R4

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.

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

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

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