3 Phase Induction Motor
Hi fellows! Hoping everyone is fine and doing great. Today, I am going to write about 3 Phase Induction Motor. As I mentioned earlier in my article on Introduction to Induction Motors, there are two types of Induction Motors, Single Phase Induction Motor and 3 phase Induction Motor.
3 phase induction motor is the one which operates on three phase AC supply. It is the most commonly used motor for high load and industrial applications. I would discuss advantages of a 3 phase induction motor later in this article. Three phase induction motors are further divided into two types; Squirrel Cage Induction Motor and Wound Rotor Induction Motor based on the construction of rotor. First of all, I am going to discuss about the construction of 3 Phase Induction Motor and then I will throw some light on its working.
Construction of 3 Phase Induction Motor
Talking about its construction, like any other motor a 3 phase induction motor consists of a rotor and a stator. Let’s revise the basic knowledge about rotor and stator first.
- Rotor is the rotating part while stator is the stationary part of motor.
- Rotor is separated from the stator by a small air gap.
- Stator is actually a cylindrical frame inside which the cylindrical core of rotor rotates.
- The stator of motor has slots on its internal side to carry the winding circuitry. This circuit is supplied with AC power. This winding is called stator winding.
- As I told earlier, rotor is a cylindrical core which is laminated and it acts as the output shaft of motor.
- Rotor of motor has also slots to carry conductors. These conductors make up rotor winding.
- Now, I am going to discuss types of rotor.
Types of a 3 Phase Induction Motor
Based on the rotor construction, a 3 phase induction motor is further classified into two types:
- Squirrel Cage Induction Motor
- Wound Rotor Induction Motor
Squirrel cage induction motor is cheaper than wound type, and because of the absence of brush assembly it requires less maintenance due to less wear problems.
Squirrel Cage Induction Motor
- First I will talk about squirrel cage type of induction motor.
- The rotor of a squirrel cage type motor consists of bars of copper.
- These copper bars or conductors extend out from the rotor length and are fixed in slots of rotor core.
- The bars which are extended out are short circuited together by means of copper rings on each sides as shown in figure.
- This type of construction of bars and end rings is similar to a squirrel cage, from where the name comes. Next is the construction of a wound type induction motor.
Wound Rotor Induction Motor
- In wound rotor induction motor, there is a 3 phase winding which is similar to stator’s winding.
- This winding is placed uniformly on slots of the rotor.
- The endings of these windings are connected to 3 slip rings on the shaft.
- Each phase out of three phase is connected to one slip ring.
- The slip rings of each phase rotate with rotor and are further associated with brushes which are stationary. The construction is shown in figure.
- That’s all about construction and classification of 3 phase induction motor. Now let’s discuss about working principle.
Working Principle of 3 Phase Induction Motor
In this section of the article, I am going to talk over the working principle and operation of a 3 phase induction motor. Before diving into the details, let’s revise some basic concepts and laws that govern its operation.
Faraday’s Law:
- Faraday’s law is the basic law of electromagnetic induction which plays the most important part in the working of a 3 phase induction motor.
- According to which an emf is induced in a conductor when it is placed in a varying magnetic field or if the conductor is rotated in a magnetic field. In other words, emf is induced whenever there is a relative motion of the conductor and magnetic field.
- Also, if the conductor is a closed circuit, a current is produced termed as induced current.
Lenz’s Law:
- Following Faraday’s Law as explained earlier in the previous point, when an emf is generated in a conductor, its polarity would be such that it will produce current whose magnetic field will oppose the change that is producing current. This is called Lenz’s law.
Lorentz Force:
- Lorentz force is the force which is responsible to move the conductor in the magnetic field.
- Whenever, a current is flowing in a conductor in the presence of a magnetic field, the conductor will experience this force.
That was all about the basics, now I am going to throw light on its working and development of torque.
First of all, a 3 phase AC voltage is supplied to the stator windings due to which a 3 phase current starts flowing in it. These currents flowing in windings will produce a magnetic field that rotates due to AC oscillations. Now, the flux of this rotating magnetic field will cross the rotor conductors and a voltage is induced in them which depends on three factors; the relative speed of conductor and magnetic field, magnetic flux density and length of the conductor. The result of this induced voltage is the generation of induced current in rotor, which lags behind the rotor’s induced voltage. This leads to the production of rotor’s magnetic field. Finally, the torque induced is the scaled product of stator magnetic field and rotor magnetic field and the rotor of 3 phase induction motor starts rotating.
The rotor of induction motor never achieves the synchronous speed to keep itself rotating as I discussed in my article on Induction Motors. Next, I am going to give some equations and formulas for calculating synchronous speed, slip and rotor frequency of 3 phase induction motor.
- Let’s first talk about Synchronous speed. Synchronous speed is the speed of rotating magnetic field of stator. It is represented as n_s. If “f” is the frequency of AC supply and “p” is the number of poles then n_s = 2*f/p.
- Second term on the list is slip of induction motor. Slip is denoted by “p” and it is the difference of synchronous speed and rotors mechanical speed. If synchronous speed is n_s and rotor speed is n_r then s = (n_s - n_r). Slip is also expressed as a ratio per unit or as percentage of synchronous speed. This expression is used to find rotor speed, when slip of motor is known.
- Rotor frequency is also discussed for 3 phase induction motors. This frequency is directly proportional to the slip. Represented by f_r, it is expressed by the following formula: f_r = s * f
That was all about the working of a 3 phase induction motor, so before telling the advantages I am going to give a brief idea on its equivalent electric circuit.
Equivalent Circuit of a 3 Phase Induction Motor
A 3 phase induction motor is also called a rotating transformer as working principle of both are quite similar. So, the equivalent circuit is also similar to that of a transformer as shown in image. Right side of the image shows rotor circuit and left side is stator side.
- On the stator side, there is resistance and self-inductance in stator windings. R1 is the resistance of stator and X1 is stator reactance.
- E1 is the stator voltage. Er is the induced voltage in rotor windings while Vp is the applied voltage to the machine.
- On the rotor side, Rr is the rotor resistance and Xr is the rotor reactance.
- A_eff denotes the turn ratio of a transformer which associates E1 and Er.
- Xm is the magnetizing reactance.
- I1 is the stator current while Ir is the current in rotor.
In the last section of my article, I am gonna give some advantages of a 3 phase induction motor.
Advantages
- Let’s now take a look on the advantages of a 3 phase induction motor.
- 3 phase induction motors are simple and require easy maintenance.
- They are preferred because of low price.
- They can be used in rugged environments.
- A 3 phase induction motor is brushless so the problem of maintenance and wear is avoided.
- Speed of this motor can also be controlled.
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.