Single Phase Induction Motor
Hi friends, I hope you all are fine and doing great with your lives. Today, I am gonna give you an Introduction of a Single Phase Induction Motor. As I previously mentioned in my article on Introduction to Induction Motors, there are two main types of Induction Motors, single phase induction motor and 3 phase Induction Motor. For information on three phase, you can read my article on 3 Phase Induction Motors. Now let’s talk about the definition of single phase induction motor.
A single phase induction motor is the one which operates on a single phase AC power source. This motor is used in the applications where requirement of power is low. It is generally used in domestic applications because of limited size and less power. Some of the uses include fans, washing machines, pumps, toys, vacuum cleaners, refrigerator compressors and in machine tools.
First I will tell you about its construction, followed by working principle and advantages. And in the last sections of this article, I will share some knowledge about the starting problem of a single phase induction motor and give a comparison of 3 phase and single phase induction motors. Let’s now take a look on construction of a single phase induction motor.
Construction of Single Phase Induction Motor
- The basic construction of Single Phase Induction Motor is similar to all other motors.
- A rotor and a stator are the two main components of Single Phase Induction Motor.
- We will have a look at both Stator and Rotor one by one below.
- So, first of all have a look at the functionality of Stator of Single Phase Induction Motor:
Stator of Single Phase Induction Motor
- The stator of a single phase induction motor is the stationary part as in other motors.
- The stationary stator of the motor is supplied with an AC power supply which is single phase.
- Stator is a cylindrical frame having slots.
- Inside the stator frame, cylindrical core of rotor is placed with a little air gap in between them.
- The purpose of making slots is to carry winding circuit.
- The winding circuitry of stator is called stator winding which in the case of this particular motor is single phase.
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Rotor of Single Phase Induction Motor
- The second basic part of motor is the rotor.
- As in all other motors, the rotor is the rotating part.
- Rotor is cylindrical in shape and it is connected to the output shaft of the motor.
- In other words, this is the part which supplies rotation at the output. Load is connected to the shaft of rotor.
- Rotor of a single phase induction motor is similar in construction with a squirrel cage 3 phase induction motor. I have explained its construction in my article on 3 phase induction motor.
- Rotor has slots on all over its surface to carry conductors that are copper or aluminum bars.
- These conductor bars are short circuited with each other by two end rings. One end ring is on each side of rotor.
- Just like rotor of a squirrel cage 3 phase induction motor, it has no slip ring and brush assembly.
- That was all about basic structure of stator and rotor, now I am moving towards next section of this article.
Working of Single Phase Induction Motor
The working principle of a single phase induction motor is based on Faraday’s law of electromagnetic induction. AC supply is given to stator windings which is single phase, the current flowing through the winding will produce a magnetic field which is called stator magnetic field. The flux lines of this magnetic field will cross the conductors of rotor. As the flux is changing due to changing magnetic field of AC supply with time, an emf and current will be induced in the rotor. The induced current will give rise to another magnetic field which is called rotor magnetic field. A single phase induction motor differs in operation from a 3 phase induction motor in the sense that this motor cannot generate a rotating magnetic field. Instead of a rotating magnetic field, it produces a magnetic field which pulsates due to AC oscillations between 0 and 180 degrees. In other words, the magnetic field does not rotates but reverses 180 degrees. The interaction of the two magnetic fields or magnetic fluxes, one from stator and second from rotor will produce torque.
Advantages of Single Phase Induction Motor
In this section, I am highlighting some of the advantages of a single phase induction motor which are as under.
- A single phase induction motor is more economical where less power is required.
- Such motor is simple in construction because of absence of slip rings and brushes.
- Because of simple construction, it is very easy to maintain and repair.
- This motor is cheap in cost.
- Also, single phase induction motors are reliable and robust.
Starting Problem of a Single Phase Induction Motor
The initial torque or starting torque of a single phase induction motor is very low so this motor cannot take a self start. For 3 phase induction motor the starting torque is high so it can start on its own. Now to overcome this starting problem, capacitor can be used to build the starting torque. This capacitor creates a phase difference between the flux of rotor and flux of stator. The capacitor is used with a starting winding which is switched off once the motor is started.
Comparison of Single Phase and 3 Phase Induction Motors
In this segment of my article, I am going to give a little comparison of single phase and 3 phase induction motors so you can get an idea which one is best for your application. Both are compared on the basis of their features, construction, supply and uses.
- The output produced by a single phase induction motor is about half as produced by 3 phase induction motor.
- A single phase induction motor is used where less power is required as compared to 3 phase induction motor.
- The efficiency and power factor is also low in case of single phase induction motor.
- Single phase motors are simple and cheaper for small rating as compared to 3 phase induction motors.
- Starting torque is low in single phase motor as compared to 3 phase induction motor.
- Single phase induction motors are used for domestic applications while 3 phase induction motors are used in industrial applications.
- Maintenance of a single phase motor is very easy in contrast with 3 phase induction motor.
- Another important point is the construction. It’s easy to construct a single phase induction motor as compared to 3 phase induction motor.
- Single phase induction motor is reliable and economical if compared to 3 phase induction motor.
- 3 phase induction motor is self starting while single phase induction motor is not a self starting motor.
Introduction to Induction Motor
Hi fellows! Hoping everyone is fine and doing great. Today, I am going to give you an Introduction to Induction Motors, the term which is very common and familiar but still many of us do not know it’s working and difference from other motors. Induction Motor is an AC electric motor, having a stator and a rotor just like other motors, but the working principle is a little different which would be discussed further. There are two types of Induction Motors, one is named as Single Phase Induction Motor, while the second one is named as 3 Phase Induction Motor.
An Induction motor is also called asynchronous motor, because the speed of rotation of its rotor is less than stator. In other words, it does not run at its synchronous speed. Before going into the details of the working principal of induction motor, I wanna first summarize the basic concepts of Rotor and Stator:
Rotor
- Rotor is the rotating part of the induction motor, which is actually the shaft of the motor.
- Rotor of an Induction motor is a laminated cylindrical core.
- Moreover this laminated cylindrical core has slots that carry aluminum or copper conductors, which are joined at ends.
- So, let's now have a look at the stator of the induction motor.
Stator
- In the previous section, we have discussed the rotor details, now I am gonna throw some light on stator.
- Unlike rotor, stator is the stationary part of the induction motor.
- While rotor is a small cylindrical core which rotates in the outer cylindrical frame of the motor. This outer cylindrical frame is named as stator.
- Stator has slots to carry the winding circuit which is supplied by an AC power.
Both stator and rotor are made up of an electric circuit to carry current and a magnetic circuit to carry magnetic flux.
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Working of Induction Motor
The working of an Induction Motor can be summarized as, “the stator winding produces magnetic field and due to electromagnetic induction a current is induced in rotor which produces torque”. No electrical connection exists between rotor and stator which differentiates it from other motors (DC, Synchronous), as it works on Faraday’s law of Electromagnetic Induction.
The AC power supplied to the stator creates a magnetic field which is changing over time due to AC oscillations. This changing magnetic field produces changing magnetic flux that induces a current in rotor windings in accordance with Faraday's law. This current will generate a magnetic field to oppose the stator magnetic field in accordance with Lenz’s law. To oppose this change, the rotor will start rotating in the direction of changing magnetic field of the stator. Relative speed of the stator’s magnetic field and rotor is the driving factor. Therefore, the speed of rotor is always maintained less to keep it moving.
Let’s take a look on some parameters of an Induction Motor like poles, synchronous speed, slip and power factor.
Poles
- Number of poles of an induction motor is denoted by “p”.
- A single phase AC machine has 2 poles of opposite polarity set at 180 degree apart.
- A simplest 3 phase machine has 6 poles which are set at 60 degree apart.
Synchronous Speed
- It is the speed of rotation of stator’s magnetic field denoted by “n_s”.
- n_s= 2f/p
Where,
f = frequency of AC supply
p = no. of poles
Slip
- Slip is the difference between stator's magnetic field and rotor's mechanical speed of the induction motor.
- We can also say that its the difference between synchronous speed and operating speed. expressed as a ratio.
- s=(n_s-n_r)/n_s
Power Factor
- The power factor of a motor is the ratio of real power to the apparent power.
- At full load power factor of an induction motor ranges from 0.85 to 0.90 and at no load it is approx. 0.12.
Types of Induction Motors
Now lets just take a brief look on types of induction motor. Single phase and three phase are the two types of an AC Induction Motor. Single phase motors are used for smaller loads while 3 phase motors are used in high load applications.
According to the type of rotor, an induction motor is divided into two types; squirrel cage and slip ring /wound type motor. The most commonly used is the squirrel cage induction motor.
Concluding, Advantage of Induction Motor is its simple operation, reliability and low cost. It is used in a number of applications in home appliances and commercial uses such as pumps, compressors, fans, mixers, conveyors, crushers, machine tools, cranes, etc.