AC Motor Speed Controller Circuits using Back EMF

In this post I have explained a few circuit diagrams for building AC motor speed controller circuits using back EMF concept.

Working Principle of AC Motors

So now, all those home type appliances like electric drill machines, sander machines, sewing machines, food mixer machines, etc. they all are mostly using that AC motor which is called series-wound type motor, right?

Then, when this motor starts working, then it creates something called back-electromotive force or just back-emf. That back-emf always goes up or down depending on how fast that motor is rotating. So if speed is more then back-emf also becomes more and if speed is less, then back-emf also becomes less.

Now the actual voltage which really goes inside that motor and runs it, that is not full supply voltage. It becomes supply voltage minus back-emf. So whatever is remaining after subtracting back-emf, that only goes to run the motor.

This thing is very interesting because this back-emf thing makes the motor kind of self-controlling type. We do not need any extra circuit or sensor for controlling the speed. It adjusts automatically.

Like suppose motor is doing some work and suddenly load becomes more then speed will drop, right? But when speed drops then back-emf also becomes less. So because back-emf got reduced, now more voltage enters the motor. That extra voltage helps to push the speed back to original level.

So in this way, motor always tries to keep its speed more or less stable by itself without any extra controller or feedback system.

Using Triac

Now most of the AC motors which we normally use, they are usually designed in such a way that they can run with only one fixed speed. They do not have any built-in speed variation feature.

But if we want to make their speed variable then we can easily use one simple method. That method is using Triac based phase-control circuit.

One good example of this kind of circuit is shown in the below figure. This type of circuit becomes very useful mainly for handling light load appliances like food mixer machines and sewing machines. For such light duty loads, this Triac circuit works very nicely for changing the motor speed as we want.

But still, it is important to understand one thing, that this type of Triac speed control setup has one small limitation. The speed control range is not very wide. It can control the speed, but only within a limited range.

Now if we compare with some other heavy duty appliances like electric drill machines and sanding machines then situation becomes different.

These machines face heavy changes in their load while working. Load keeps increasing and decreasing too much. Because of this they do not work well with the above simple AC motor speed controller circuit. That circuit cannot handle such big load fluctuations properly.

Using SCR

So for these kinds of high-load appliances, a better and more reliable option is to use an AC variable speed regulator circuit which works using SCR instead of Triac. One such good circuit is shown in the following diagram. This type of circuit can manage high loads and still control the speed nicely.

So now in this type of circuit setup, we are using one SCR as the main control device. This SCR gives half-wave power to the motor which means only one half cycle of the AC goes to the motor. Because of this, the motor speed and power both drop down a little, around 20 percent reduction happens.

Then during those off half-cycles where the SCR is not conducting, that time it quietly checks or senses the back-emf coming from the motor.

This back-emf information helps the circuit to automatically adjust the next gate triggering pulse for the SCR. That way the circuit tries to maintain some level of automatic speed control depending on how the motor is behaving.

Now there is one more important part. The small network made using R1, RV1, and D1, this thing is designed for adjusting the phase angle up to 90 degrees.

This phase control makes sure that every power pulse going to the motor is always minimum 90 degrees long. Because of this, the motor is able to get strong torque even during lower speed operations.

Then when the speed becomes low, this circuit behaves in a different way. It enters one special mode which we can call skip-cycling mode. In this mode, the circuit gives power pulses to the motor only sometimes, not in every cycle.

This skipping action helps to match the power delivery with the motor's actual load requirement.

Delivering High Torque

So this kind of AC motor speed controller circuit is very good in giving high torque even at lower speeds. That is one big advantage. But still there is one small issue. Sometimes the motor may show some light stuttering or vibration, especially at low speeds.

Same like the earlier circuit, here also we must understand that this system also has one limitation. The speed control range is not very wide. It works nicely but within a limited area of speed adjustment.

Closed Loop AC Motor Speed Controller using Back EMF

The next article presented below 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 ratio between resistors R3 and R2 + P1.

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 + R2 + R3 / R3) × 0.6

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

• R1 = 39K,
• R2 = 12K,
• R3 = 22K,
• R4 = 68K,
• P1 = 220K,
• All diodes = 1N4007,
• C1 = 0.1/400V,
• C2 = 100uF/35V,
• T1 = BC547B,
• SCR = C106
• L1 = 30 turns of 25 SWG wire over a 3mm ferrite rod or 40 uH/5 watt

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.

Back EMF Drill Speed Controller Circuit

This circuit is mainly used to control the steady speed of smaller series wound motors, as found in several electric hand drills, etc.

The torque and the speed is controlled by P1 potentiometer. This potentiometer configuration specifies how minutely the triac could be triggered.

When the speed of the motor drops just under the preset value (with load connected), then the motor 's back EMF decreases.

As a result, voltage around through R1, P1, and C5 rises so that the triac is activated earlier and motor speed tends to increase. A certain proportion of speed stability is achieved in this manner.