Single phase induction motors, and their starting methods
Hi, do you have a good memory? If you; you will remember that single-phase induction motors are the most common in every place around us and the most used types of single phase motors are single phase induction motors and it’s the focus of our attention today working principle of single phase induction motors and types of single phase induction motors. If not you should first read the article about Working principle of single phase induction motor.
Also, Single phase induction motors are AC motors where they can convert electric power to mechanical power to perform some physical tasks. They require only one power phase for operation so they are used in low power applications which we mentioned in the last article.
Construction of single phase induction motors:
Of course, single phase induction motors have the same construction of single phase motors, but don’t apprehensive I’ll repeat it to add small Vital additions; the single-phase induction motor consists of:
It’s the stationary part which has a laminated stamping used to reduce eddy current losses on its External surface. Stamping also made of silicon steel to reduce the hysteresis losses.
The stamping is provided with slots which carry two windings the main winding and an auxiliary (starting) winding which is placed perpendicular to the main winding. The windings are distributed.
When we apply a single phase AC supply to the stator winding it produced a magnetic field and the rotor rotates at a speed somewhat less than the synchronous speed which is
Ns=(120 f )/P
Ns: is the synchronous speed.
f: the frequency of supply voltage.
P: number of poles of the motor.
It is the rotating part consists of a cheap and low power rating squirrel cage rotor which has an aluminum, brass or copper bars (conductors) which are placed on and they are closed or semi-closed slots.
The slots have short-circuited aluminum bars shorted by a ring at both ends.
The slots are a bit skewed to each other to prevent magnetic locking of stator and rotor teeth.
And also make the working of induction motor more smooth and quiet.
The rotor and the mechanical load are connected together through the shaft.
The working of single phase induction motor requires a lot of focus.
Take a breath and let’s start.
When we supply the stator with single phase supply an alternating flux produced in the stator winding.
And this alternating flux causes induced current in the rotor bars.
According to Faraday’s law of electromagnetic induction, this induced current will also produce alternating flux.
Despite both alternating fluxes are the motor fails to start.
And this makes us delve deeper why single phase induction motors aren’t self- started and types of starting use.
Why single phase induction motor isn’t self -starting?
We can clearly describe this by two methods:
- Double field revolving theory and it’s easier to understand and it’s what we will explain.
- Cross-field theory.
Double field revolving theory:
Double field revolving theory state that any alternating quantity can be resolved into two components and each component has a magnitude equal to the half of the maximum magnitude of the alternating quantity and also both components rotate in the opposite direction to each other.
Don’t worry; I’ll explain easily, but first, we need to know the symbols we use where:
Qm: the pulsating field in the motor.
ω: the speed of rotation of pulsating fields (rad/Sec).
ϕ1,ϕ2: alternating flux produced by the stator.
We can resolve the pulsating flux Qm into two components:
Qm/2 and -Qm/2 (the negative sign only explains the direction of rotation).
Each of these components rotates in opposite direction.
And as we explained before, when we apply a single phase AC supply to the stator winding it produced Qm.
And as double field said this alternating flux divided into two components Qm/2.
Each component rotates in opposite direction with the synchronous speed Ns.
If we notice the resultant of these two components of flux at any instant of time we will have the value of instantaneous stator flux at that particular instant.
At the starting of the motor both components of magnetic flux will be equal in magnitude and oppose each other so they will cancel each other, hence the net torque will be zero and that’s the reason which makes single phase induction motor, not self-starting motors.
Of course, we have to solve that.
It’s what we made with the types of starting of single phase induction motors. Keep attention.
We illustrated before that single phase induction motor has no starting torque.
But when it rotates at any other speed, except synchronous speed, there is a resulting torque.
Here we introduced an auxiliary winding in the stator with an addition to the main winding and we put the auxiliary at a space angle of 90° and that what produced a starting torque.
And to produce a maximum starting torque the current at the main and auxiliary windings must be at the angle of 90°.
We don’t only depend upon the auxiliary winding which can be arranged in some ways to change the characteristics of the machine and give us some subdivided types of single phase induction motors and this types classified according to the starting method we use; so types of single phase induction motor an also starting methods of single phase induction motors are:
- Split phase induction motor.
- Capacitor start induction motor.
- Capacitor start capacitor run induction motor.
- Permanent split capacitor motor.
- Shaded pole induction motor.
Split phase induction motor:
It’s one of the most widely used types of single phase induction motors, it also is known as resistance start motor.
We use it in markets and domestic applications like fans, blowers, washing machines, grinder, lathes because it’s low in cost and has a low starting current and torque.
In addition to the main winding a centrifugal switch connected in series with the auxiliary to disconnect the auxiliary winding from the circuit when the motor attains a speed up to 75 to 80 % of the synchronous speed.
The nature of the main running winding is inductive so we add a high resistance to the starting winding to make the phase difference between the two windings.
The current for highly resistive winding (starting winding) is in phase with the voltage or vary by a little small angel and the current for the highly inductive winding (running winding) lag the voltage by a large angle, so the resultant of these two current produce rotating magnetic field rotates in one direction.
The starting and main current get split from each other by some angle, hence it gained its name split phase induction motor.
Capacitor start induction motor:
In this motor, we add more turns to the auxiliary winding and we placed an electrolytic capacitor serially with the auxiliary winding.
Of course, there is no change in the angle between the main winding and auxiliary winding it is 90°.
In this case we connect a capacitor to the starting winding to keep the current flowing in the capacitor leads the voltage applied by some angels, and as the running winding is inductive in nature, there will be a large phase angle difference between these two currents and they produce a resulting current, the resultant current will produce a rotating magnetic field which will produce by turn a very high starting torque.
It’s high in cost and has a power rating from 120W to 7KW.
So, we use this type in applications requires high starting torque.
Capacitor start capacitor run induction motor:
we can say it’s similar to capacitor start induction type.
But here we use two capacitors in parallel and we also use a centrifugal switch.
And without Controversy, We use the capacitors to improve the power factor and the running conditions of single-phase induction motor.
This motor is simple in operation, have a better efficiency, able to start large loads so the domestic and industrial applications achieve High utilization efficiency of this motor.
Permanent split capacitor motor:
In this case, we provide only one capacitor in series with the auxiliary winding and this capacitor work in both running and starting conditions because there isn’t a centrifugal switch.
This motor has the advantages of; good efficiency, low start current, large torque, uses simple capacitors and the more important it hadn’t a centrifugal switch. These benefits make it the most suitable for fans, blowers, heaters, air conditioners and so on.
Shaded pole induction motor:
The difference in this motor is the stator as it consists of salient poles with an exciting coil. We wrapped each pole by a shading coil so we call it as the shaded pole.
This motor is very economical, reliable, has a simple construction. But In contrast, it has a low power factor, has a poor starting torque, and has a low efficiency and high copper losses.
The advantages and disadvantages of this motor make it the most suitable for small instruments,
Toys, record players, small fans, electric clocks…etc. Substantially it is available in a range of 1/300 to ½ KW.