Asynchronous Motor

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One of the most commonly used electrical motors is induction motor. We also call this motor as asynchronous motor because it runs at speed less than its synchronous speed.

Synchronous Speed

Here we need to define what is synchronous speed. Synchronous speed is the speed of rotation of the magnetic field in a rotary machine, and it depends upon the frequency and number poles of the machine.

An induction motor always runs at speed less than synchronous speed. Because the rotating magnetic field produced in the stator will create flux in the rotor and hence will make the rotor to rotate. Due to the lagging of flux current in the rotor with flux current in the stator, the rotor will never reach it’s rotating magnetic field speed, i.e. the synchronous speed. Induction Motor

There are basically two types of induction motors. The types of induction motors depend upon the input supply. The single phase induction motor and three phase induction motor. Single phase induction motor is not a self-starting motor, and three phase induction motor is a self-starting motor.

Working Principle of Induction Motor

We need to give double excitation to make a DC motor to rotate. In a DC motor, we give one supply to the stator and another to the rotor through brush arrangement. But in induction motor, we give only one supply, so it is really interesting to know how an induction motor works. It is very simple, from the name itself we can understand that here, induction process is involved. When we give the supply to the stator winding, a magnetic flux is produced in the stator due to the flow of current in the coil. The rotor winding is arranged in such a way that each coil becomes short-circuited in the rotor itself.

The flux from the stator cuts the short-circuited coil in the rotor. As the rotor coils are short-circuited, according to Faraday’s law of electromagnetic induction, current will start flowing through the coil of the rotor. When the current through the rotor coils flows, another flux gets generated in the rotor. Now there are two fluxes, one is stator flux, and another is rotor flux. The rotor flux will be lagging with respect to the stator flux. Because of that, the rotor will feel a torque which will make the rotor to rotate in the direction of rotating magnetic field. This is the working principle of an induction motor of either type – single and three phase.

Types Induction Motor

  1. Single Phase Induction Motor

    1. Split Phase Induction Motor
    2. Capacitor Start Induction Motor
    3. Capacitor Start and Capacitor Run Induction Motor
    4. Shaded Pole Induction Motor

    Three Phase Induction Motor

    1. Squirrel Cage Induction Motor
    2. Slip Ring Induction Motor

    We have already mentioned above that single phase induction motor is not a self-starting and three phase induction motor is self-starting. So what is self-starting? When the machine starts running automatically without any external force to the machine, then it is called as self-starting. For example, we see that when we put on the switch the fan starts to rotate automatically, so it is self-starting. Point to be noted that fan used in home appliances is single phase induction motor which is inherently not self-starting. How? A question arises how it works? We will discuss it now.

    Why is Three Phase Induction Motor Self Starting?

    In three phase system, there are three single phase lines with 120° phase difference. So the rotating magnetic field has the same phase difference which will make the rotor to move. If we consider three phases a, b, and c when phase a is magnetised, the rotor will move towards the phase a winding a, in the next moment phase b will get magnetised and it will attract the rotor and then phase c. So the rotor will continue to rotate.

Why Single Phase Induction Motor is not Self Starting?

It has only one phase still it makes the rotor to rotate, so it is quite interesting. Before that, we need to know why single phase induction motor is not a self-starting motor and how we overcome the problem. We know that the AC supply is a sinusoidal wave and it produces a pulsating magnetic field in the uniformly distributed stator winding. Since we can assume the pulsating magnetic field as two oppositely rotating magnetic fields, there will be no resultant torque produced at the starting, and hence the motor does not run. After giving the supply, if the rotor is made to rotate in either direction by an external force, then the motor will start to run. We can solve this problem by making the stator winding into two winding, one is main winding, and another is auxiliary winding. We connect one capacitor in series with the auxiliary winding. The capacitor will make a phase difference when current flows through both coils. When there is phase difference, the rotor will generate a starting torque, and it will start to rotate. Practically we can see that the fan does not rotate when the capacitor gets disconnected from the motor, but if we rotate with the hand, it will start rotating. That is why we use a capacitor in the single phase induction motor.
There are several advantages of induction motor which make this motor to have wider application. It has good efficiency up to 97%. But the speed of the motor varies with the load given to the motor which is a disadvantage of this motor. The direction of rotation of induction motor can easily be changed by changing the phase sequence of three-phase supply, i.e. if RYB is in a forward direction, the RBY will make the motor to rotate in reverse direction. This is in the case of three phase motor, but in single phase motor, the direction can be reversed by reversing the capacitor terminals in the winding.

We use the single-phase power system more widely than three phase system for domestic purposes, commercial purposes and some extent in industrial uses. Because, the single-phase system is more economical than a three-phase system and the power requirement in most of the houses, shops, offices are small, which can be easily met by single phase system. The single phase motors are simple in construction, cheap in cost, reliable and easy to repair and maintain. Due to all these advantages, the single phase motor finds its application in vacuum cleaners, fans, washing machines, centrifugal pumps, blowers, washing machines, etc.

The single phase AC motors are further classified as:

  1. Single phase induction motors or asynchronous motors.
  2. Single phase synchronous motors.
  3. Commutator motors.

This article will provide fundamentals, description and working principle of single phase induction motor.

Construction of Single Phase Induction Motor

Like any other electrical motor asynchronous motor also have two main parts namely rotor and stator. Stator: As its name indicates stator is a stationary part of induction motor. A single phase AC supply is given to the stator of single phase induction motor.Rotor: The rotor is a rotating part of an induction motor. The rotor connects the mechanical load through the shaft. The rotor in single phase induction motor is of squirrel cage rotor type. The construction of single phase induction motor is almost similar to the squirrel cage three-phase induction motor. But in case of a single phase induction motor, the stator has two windings instead of one three-phase winding in three phase induction motor.

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Stator of Single Phase Induction Motor

The stator of the single phase induction motor has laminated stamping to reduce eddy current losses on its periphery. The slots are provided on its stamping to carry stator or main winding. Stampings are made up of silicon steel to reduce the hysteresis losses. When we apply a single phase AC supply to the stator winding, the magnetic field gets produced, and the motor rotates at speed slightly less than the synchronous speed Ns. Synchronous speed Ns is given by  Where, f = supply voltage frequency, P = No. of poles of the motor.The construction of the stator of single-phase induction motor is similar to that of three phase induction motor except there are two dissimilarities in the winding part of the single phase induction motor.

    1. Firstly, the single-phase induction motors are mostly provided with concentric coils. We can easily adjust the number of turns per coil can with the help of concentric coils. The mmf distribution is almost sinusoidal.
    2. Except for shaded pole motor, the asynchronous motor has two stator windings namely the main winding and the auxiliary winding. These two windings are placed in space quadrature to each other.

Rotor of Single Phase Induction Motor

    1. The construction of the rotor of the single-phase induction motor is similar to the squirrel cage three-phase induction motor. The rotor is cylindrical and has slots all over its periphery. The slots are not made parallel to each other but are a little bit skewed as the skewing prevents magnetic locking of stator and rotor teeth and makes the

working of induction motor

    1. more smooth and quieter, i.e. less noise. The squirrel cage rotor consists of aluminium, brass or copper bars. These aluminium or copper bars are called rotor conductors and placed in the slots on the periphery of the rotor. The copper or aluminium rings permanently short the rotor conductors called the end rings. To provide mechanical strength, these rotor conductors are braced to the end ring and hence form a complete closed circuit resembling like a cage and hence got its name as squirrel cage induction motor. As end rings permanently short the bars, the rotor

electrical resistance

    1. is very small and it is not possible to add external

resistance

    1. as the bars get permanently shorted. The absence of slip ring and brushes make the

construction of single phase induction motor

    1. very simple and robust.

Working Principle of Single Phase Induction Motor

NOTE: We know that for the working of any electrical motor whether its AC or DC motor, we require two fluxes as the interaction of these two fluxes produced the required torque.

    1. When we apply a single phase AC supply to the stator winding of single phase induction motor, the alternating

current

    1. starts flowing through the stator or main winding. This alternating current produces an alternating flux called main flux. This main

flux

    1. also links with the rotor conductors and hence cut the rotor conductors. According to the

Faraday’s law of electromagnetic induction

    1. , emf gets induced in the rotor. As the rotor circuit is closed one so, the current starts flowing in the rotor. This current is called the rotor current. This rotor current produces its flux called rotor flux. Since this flux is produced due to induction principle so, the motor working on this principle got its name as an induction motor. Now there are two fluxes one is main flux, and another is called rotor flux. These two fluxes produce the desired torque which is required by the motor to rotate.

Why Single Phase Induction Motor is not Self Starting?

    1. According to double field revolving theory, we can resolve any alternating quantity into two components. Each component has a magnitude equal to the half of the maximum magnitude of the alternating quantity, and both these components rotate in the opposite direction to each other. For example – a flux, φ can be resolved into two components

    1. Each of these components rotates in the opposite direction i. e if one φ

m

    1. / 2 is rotating in clockwise direction then the other φ

m

    1. / 2 rotates in anticlockwise direction.

When we apply a single phase AC supply to the stator winding of single phase induction motor, it produces its flux of magnitude, φm. According to the double field revolving theory, this alternating flux, φm is divided into two components of magnitude φm /2. Each of these components will rotate in the opposite direction, with the synchronous speed, Ns. Let us call these two components of flux as forwarding component of flux, φf and the backward component of flux, φb. The resultant of these two components of flux at any instant of time gives the value of instantaneous stator flux at that particular instant.

 Now at starting condition, both the forward and backward components of flux are exactly opposite to each other. Also, both of these components of flux are equal in magnitude. So, they cancel each other and hence the net torque experienced by the rotor at starting condition is zero. So, the single phase induction motors are not self-starting motors.

Methods for Making Single Phase Induction as Self Starting Motor

    1. From the above topic, we can easily conclude that the single-phase induction motors are not self-starting because the produced stator flux is alternating in nature and at the starting, the two components of this flux cancel each other and hence there is no net torque. The solution to this problem is that if we make the stator flux rotating type, rather than alternating type, which rotates in one particular direction only. Then the induction motor will become self-starting. Now for producing this rotating magnetic field, we require two alternating flux, having some phase difference angle between them. When these two fluxes interact with each other, they will produce a resultant flux. This resultant flux is rotating in nature and rotates in space in one particular direction only. Once the motor starts running, we can remove the additional flux. The motor will continue to run under the influence of the main flux only. Depending upon the methods for making asynchronous motor as Self Starting Motor, there are mainly four types of single phase induction motor namely,

      1. Split phase induction motor,
      2. Capacitor start inductor motor,
      3. Capacitor start capacitor run induction motor,
      4. Shaded pole induction motor.
      5. Permanent split capacitor motor or single value capacitor motor.

Comparison between Single Phase and Three Phase Induction Motors

      1. Single phase induction motors are simple in construction, reliable and economical for small power rating as compared to three phase induction motors.
      2. The electrical power factor of single phase induction motors is low as compared to three phase induction motors.
      3. For the same size, the single-phase induction motors develop about 50% of the output as that of three phase induction motors.
      4. The starting torque is also low for asynchronous motors / single phase induction motor.
      5. The efficiency of single phase induction motors is less compared to that of three-phase induction motors.

Single phase induction motors

    are simple, robust, reliable and cheaper for small ratings. They are available up to 1 KW rating.

The single phase induction motors are made self starting by providing an additional flux by some additional means. Now depending upon these additional means the single phase induction motors are classified as:

        1. Split phase induction motor.
        2. Capacitor start inductor motor.
        3. Capacitor start capacitor run induction motor (two value capacitor method).
        4. Permanent split capacitor (PSC) motor .
        5. Shaded pole induction motor.

Split Phase Induction Motor

In addition to the main winding or running winding, the stator of single phase induction motor carries another winding called auxiliary winding or starting winding. A centrifugal switch is connected in series with auxiliary winding. The purpose of this switch is to disconnect the auxiliary winding from the main circuit when the motor attains a speed up to 75 to 80% of the synchronous speed. We know that the running winding is inductive in nature. Our aim is to create the phase difference between the two winding and this is possible if the starting winding carries high resistance. Let us say

Irun is the current flowing through the main or running winding, Istart is the current flowing in starting winding, and VT is the supply voltage. split phase induction motor

We know that for highly resistive winding the current is almost in phase with the voltage and for highly inductive winding the current lag behind the voltage by large angle. The starting winding is highly resistive so, the current flowing in the starting winding lags behind the applied voltage by very small angle and the running winding is highly inductive in nature so, the current flowing in running winding lags behind applied voltage by large angle. The resultant of these two current is IT. The resultant of these two current produce rotating magnetic field which rotates in one direction. In split phase induction motor the starting and main current get splitted from each other by some angle so this motor got its name as split phase induction motor.

Applications of Split Phase Induction Motor

Split phase induction motors have low starting current and moderate starting torque. So these motors are used in fans, blowers, centrifugal pumps, washing machine, grinder, lathes, air conditioning fans, etc. These motors are available in the size ranging from 1 / 20 to 1 / 2 KW.

Capacitor Start IM and Capacitor Start Capacitor Run IM

capacitor start run induction motor The working principle and construction of Capacitor start inductor motors and capacitor start capacitor run induction motors are almost the same. We already know that single phase induction motor is not self starting because the magnetic field produced is not rotating type. In order to produce rotating magnetic field there must be some phase difference. In case of split phase induction motor we use resistance for creating phase difference but here we use capacitor for this purpose. We are familiar with this fact that the current flowing through the capacitor leads the voltage. So, in capacitor start inductor motor and capacitor start capacitor run induction motor we are using two winding, the main winding and the starting winding. With starting winding we connect a capacitor so the current flowing in the capacitor i.e Ist leads the applied voltage by some angle, φst.

The running winding is inductive in nature so, the current flowing in running winding lags behind applied voltage by an angle, φm. Now there occur large phase angle differences between these two currents which produce an resultant current, I and this will produce a rotating magnetic field. Since the torque produced by these motors depends upon the phase angle difference, which is almost 90o. So, these motors produce very high starting torque. In case of capacitor start induction motor, the centrifugal switch is provided so as to disconnect the starting winding when the motor attains a speed up to 75 to 80% of the synchronous speed but in case of capacitor start capacitors run induction motor there is no centrifugal switch so, the >capacitor remains in the circuit and helps to improve the power factor and the running conditions of single phase induction motor.

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Application of Capacitor Start IM and Capacitor Start Capacitor Run IM

These motors have high starting torque hence they are used in conveyors, grinder, air conditioners, compressor, etc. They are available up to 6 KW.

Permanent Split Capacitor (PSC) Motor

It has a cage rotor and stator. Stator has two windings – main and auxiliary winding. It has only one capacitor in series with starting winding. It has no starting switch. Advantages and Applications No centrifugal switch is needed. It has higher efficiency and pull out torque. It finds applications in fans and blowers in heaters and air conditioners. It is also used to drive office machinery.

Shaded Pole Single Phase Induction Motors

shaded pole single phase induction motor The stator of the shaded pole single phase induction motor has salient or projected poles. These poles are shaded by copper band or ring which is inductive in nature. The poles are divided into two unequal halves. The smaller portion carries the copper band and is called as shaded portion of the pole.

ACTION: When a single phase supply is given to the stator of shaded pole induction motor an alternating flux is produced. This change of flux induces emf in the shaded coil. Since this shaded portion is short circuited, the current is produced in it in such a direction to oppose the main flux. The flux in shaded pole lags behind the flux in the unshaded pole. The phase difference between these two fluxes produces resultant rotating flux. We know that the stator winding current is alternating in nature and so is the flux produced by the stator current. In order to clearly understand the working of shaded pole induction motor consider three regions-

      1. When the flux changes its value from zero to nearly maximum positive value.
      2. When the flux remains almost constant at its maximum value.
      3. When the flux decreases from maximum positive value to zero.

REGION 1: When the flux changes its value from zero to nearly maximum positive value – In this region the rate of rise of flux and hence current is very high. According to Faraday’s law whenever there is change in flux emf gets induced. Since the copper band is short circuit the current starts flowing in the copper band due to this induced emf. This current in copper band produces its own flux. Now according to Lenz’s law the direction of this current in copper band is such that it opposes its own cause i.e rise in current. So the shaded ring flux opposes the main flux, which leads to the crowding of flux in non shaded part of stator and the flux weaken in shaded part. This non uniform distribution of flux causes magnetic axis to shift in the middle of the non shaded part.

REGION 2: When the flux remains almost constant at its maximum value- In this region the rate of rise of current and hence flux remains almost constant. Hence there is very little induced emf in the shaded portion. The flux produced by this induced emf has no effect on the main flux and hence distribution of flux remains uniform and the magnetic axis lies at the center of the pole.

REGION 3: When the flux decreases from maximum positive value to zero – In this region the rate of decrease in the flux and hence current is very high. According to Faraday’s law whenever there is change in flux emf gets induced. Since the copper band is short circuit the current starts flowing in the copper band due to this induced emf. This current in copper band produces its own flux. Now according to Lenz’s law the direction of the current in copper band is such that it opposes its own cause i.e decrease in current. So the shaded ring flux aids the main flux, which leads to the crowding of flux in shaded part of stator and the flux weaken in non shaded part. This non uniform distribution of flux causes magnetic axis to shift in the middle of the shaded part of the pole. This shifting of magnetic axis continues for negative cycle also and leads to the production of rotating magnetic field. The direction of this field is from non shaded part of the pole to the shaded part of the pole.

Advantages and Disadvantages of Shaded Pole Motor

The advantages of shaded pole induction motor are

  1. Very economical and reliable.
  2. Construction is simple and robust because there is no centrifugal switch.

The disadvantages of shaded pole induction motor are

  1. Low power factor.
  2. The starting torque is very poor.
  3. The efficiency is very low as, the copper losses are high due to presence of copper band.
  4. The speed reversal is also difficult and expensive as it requires another set of copper rings.

Applications of Shaded Pole Motor

Applications of Shaded pole motors induction motor are- Due to their low starting torques and reasonable cost these motors are mostly employed in small instruments, hair dryers, toys, record players, small fans, electric clocks etc. These motors are usually available in a range of 1/300 to 1/20 KW.


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