Why the Shaded Pole Induction Motor designs for low power rating?

The power losses are very high in the shaded pole induction motor, and the power factor of the motor is low. The starting torque induces in the induction motor is also very low. Because of the following reasons the motor has poor efficiency. Thus, their designs are kept small, and the motor has low power ratings.

Construction of Shaded Pole Induction Motor

The shaded pole motor may have two or four poles. Here in this article, we use the two-pole motor for the sake of simplicity. The speed of the motor is inversely proportional to the number of poles used in the motor.

Stator – The stator of the shaded pole motor has a salient pole. The salient pole means the poles of the magnet are projected towards the armature of the motor. Each pole of the motor is excited by its exciting coil. The copper rings shade the loops. The loops are known as the shading coil.

The poles of the motor are laminated. Lamination means multiple layers of material are used for making the poles. So, that the strength of the pole increases.

The slot is constructed at some distance apart from the edge of the poles. The short-circuited copper coil is placed in this slot. The part which is covered with the copper ring is called the shaded part and which is not covered by the rings is called the unshaded part.

Rotor – The shaded pole motor uses the squirrel cage rotor. The bars of the rotor are skewed at an angle of 60º. The skew can be done for obtaining a better starting torque.

The construction of the motor is very simple because it does not contain any commutator, brushes, collector rings, etc., or any other part. The shaded pole induction motor does not have any centrifugal switch. Thus, the chances of failure of the motor are less.

The centrifugal switch is the type of electrical switch that starts operating by using the centrifugal force, generated by the rotating shaft. It is also used for controlling the speed of the shaft.

Shaded Pole Induction Motor Working

When the supply is connected to the windings of the rotor, the alternating flux induces in the core of the rotor. The small portion of the flux link with the shaded coil of the motor because it is short-circuited. The variation in the flux induces the voltage inside the ring because of which the circulating current induces in it.

The circulating current develops the flux in the ring which opposes the main flux of the motor. The flux induces in the shaded portion of the motor, i.e., a, and the unshaded portion of the motor, i.e., b have a phase difference. The main motor flux and the shaded ring flux are also having a space displacement by an angle of 90°.

The connection diagram of the Shaded Pole Motor is shown below:

As there is time and space displacement between the two fluxes, the rotating magnetic field induces in the coil. The rotating magnetic field develops the starting torque in the motor. The field rotates from the unshaded portion to the shaded portion of the motor.

Applications of the Shaded Pole Induction Motor

The various applications of the shaded poles motor are as follows:

• They are suitable for small devices like relays and fans because of their low cost and easy starting.
• Used in exhaust fans, hairdryers, and also table fans.
• Used in air conditioning and refrigeration equipment and cooling fans.
• Record players, tape recorders, projectors, photocopying machines.
• Used for starting electronic clocks and single-phase synchronous timing motors.

This type of motor is used to drive devices that require low starting torque.

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Considering the case when the rotor is stationary and only the main winding is excited. The motor behaves as a single-phase transformer with its secondary short circuit. The equivalent circuit diagram of the single phase motor with only its main winding energized is shown below:

Here,

• R_1m is the resistance of the main stator winding.
• X_1m is the leakage reactance of the main stator winding.
• X_M is the magnetizing reactance.
• R’₂ is the standstill rotor resistance referred to as the main stator winding.
• X’₂ is the standstill rotor leakage reactance referred to as the main stator winding.
• V_m is the applied voltage.
• I_m is the main winding current.

The core loss will be assumed to be lumped with the mechanical and stray losses as a part of the rotational losses of the rotor. The pulsating air gap flux in the motor at the standstill is resolved into two equal and opposite fluxes with the motor. The standstill impedance of each of the rotors referred to as the main stator winding is given as:

The equivalent circuit of a single-phase single winding induction motor with the standstill rotor is shown below. The forward and the backward flux induces a voltage E_mf and E_mb respectively in the main stator winding. E_m is the resultant induced voltage in the main winding.

At the standstill condition E_mf = E_mb

Now, with the help of an auxiliary winding, the motor is started. As the motor attains its normal speed, the auxiliary winding is removed. The effective rotor resistance of an induction motor depends on the slip of the rotor.

In the above circuit diagram, the air gap portion is split into two parts. The first part shows the effect of forward rotating flux and the second part shows the effect of the backward rotating flux. The effective rotor resistance with respect to the forward rotating flux is R_’2/2_S and with respect to the backward rotating flux is R’₂/2 (2-s).

When both forward and backward slips are taken into account, the equivalent circuit shown below is formed. In this condition, the motor is running on the main winding alone.

The rotor impedance representing the effect of the forward field referred to the stator winding m is given by an impedance shown below:

The rotor impedance of a single phase induction motor representing the effect of the backward field referred to the stator winding m is given by an impedance shown below:

The simplified equivalent circuit of a single-phase induction motor with only its main winding energized is shown in the figure below:

Here,

The above equation (3) is the equation of the current in the stator winding.

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The connection diagram of a Permanent Split Capacitor Motor is shown below:

It is also called a Single Value Capacitor Motor. As the capacitor is always in the circuit and thus this type of motor does not contain any starting switch. The auxiliary winding is always there in the circuit. Therefore, the motor operates as the balanced two-phase motor. The motor produces a uniform torque and has a noise-free operation.

Advantages of Permanent Split Capacitor Motor

The single value capacitor motor has the following advantages:

• No centrifugal switch is required.
• Efficiency is high.
• As the capacitor is connected permanently in the circuit, the power factor is high.
• It has a higher pullout torque.

Limitations of Permanent Split Capacitor Motor

The limitations of the motor are as follows:

• The paper capacitor is used in the motor as an Electrolytic capacitor cannot be used for continuous running. The cost of the paper capacitor is higher, and the size is also large as compared to the electrolytic capacitor of the same ratings.
• It has low starting torque, less than full load torque.

Applications of Permanent Split Capacitor Motor

The various applications of the split motor are as follows:

• Used in fans and blowers in heaters and air conditioners.
• Used in refrigerator compressors.
• Used in office machinery.

This is all about a permanent split capacitor (PSC) motor.

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So this motor is named Capacitor Start Capacitor Run Motor and is sometimes known as Two Value Capacitor Motor. The connection diagram of the Two valve Capacitor Motor is shown below:

There are two capacitors in this motor represented by C_S and C_R. In the starting, the two capacitors are connected in parallel. The capacitor Cs is the Starting capacitor is short time rated. It is almost electrolytic. A large amount of current is required to obtain the starting torque. Therefore, the value of the capacitive reactance X should be low in the starting winding. Since, X_A = 1/2πfC_A, the value of the starting capacitor should be large.

The rated line current is smaller than the starting current at the normal operating condition of the motor. Hence, the value of the capacitive reactance should be large. Since, X_R = 1/2πfC_R, the value of the run capacitor should be small.

As the motor reaches the synchronous speed, the starting capacitor Cs is disconnected from the circuit by a centrifugal switch Sc. The capacitor C_R is connected permanently in the circuit and thus it is known as RUN Capacitor. The run capacitor is long time rated and is made of oil-filled paper.

The figure below shows the Phasor Diagram of the Capacitor Start Capacitor Run Motor.

Fig(a) shows the phasor diagram when at the starting both the capacitor are in the circuit and ϕ > 90⁰. Fig (b) shows the phasor when the starting capacitor is disconnected, and ϕ becomes equal to 90⁰.

The torque-speed characteristic of a two-value capacitor motor is shown below:

This type of motor is quiet and smooth running. They have higher efficiency than the motors that run on the main windings only. They are used for loads of higher inertia requiring frequent starts where the maximum pull-out torque and efficiency required are higher. The two value capacitor motors are used in pumping equipment, refrigeration, air compressors, etc.

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A resistor is connected in series with the auxiliary winding. The current in the two windings is not equal as a result, the rotating field is not uniform. Hence, the starting torque is small, of the order of 1.5 to 2 times the stated running torque. At the starting of the motor both the windings are connected in parallel.

As soon as the motor reaches the speed of about 70 to 80 % of the synchronous speed the starting winding is disconnected automatically from the supply mains. If the motors are rated about 100 Watt or more, a centrifugal switch is used to disconnect the starting winding and for the smaller rating motors relay is used for the disconnecting of the winding.

A relay is connected in series with the main winding. In the starting, the heavy current flows in the circuit, and the contact of the relay gets closed. Thus, the starting winding is in the circuit, and as the motor attains the predetermined speed, the current in the relay starts decreasing. Therefore, the relay opens and disconnects the auxiliary winding from the supply, making the motor runs on the main winding only.

The phasor diagram of the Split Phase Induction Motor is shown below:

The current in the main winding (I_M) lags behind the supply voltage V almost by the 90-degree angle. The current in the auxiliary winding I_A is approximately in phase with the line voltage. Thus, there exists a time difference between the currents of the two windings. The time phase difference ϕ is not 90 degrees, but of the order of 30 degrees. This phase difference is enough to produce a rotating magnetic field.

The Torque Speed Characteristic of the Split Phase motor is shown below:

Here, n₀ is the point at which the centrifugal switch operates. The starting torque of the resistance start motor is about 1.5 times the full load torque. The maximum torque is about 2.5 times the full load torque at about 75% of the synchronous speed. The starting current of the motor is high about 7 to 8 times the full load value.

The direction of the Resistance Start motor can be reversed by reversing the line connection of either the main winding or the starting winding. The reversal of the motor is possible at the standstill condition only.

Applications of Split Phase Induction Motor

This type of motor is cheap and is suitable for easily starting loads where the frequency of starting is limited. This type of motor is not used for drives that require more than 1 KW because of the low starting torque. The various applications are as follows:

• Used in the washing machine, and air conditioning fans.
• The motors are used in mixer grinders, floor polishers.
• Blowers, Centrifugal pumps.
• Drilling and lathe machine.

This is all about split phase induction motor.

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Contents:

• Phasor Diagram
• Characteristics of the Capacitor Start Motor
• Applications of the Capacitor Start Motor

The capacitor start motor has a cage rotor and has two windings on the stator. They are known as the main winding and the auxiliary or the starting winding. The two windings are placed 90 degrees apart. A capacitor C_S is connected in series with the starting winding. A centrifugal switch S_C is also connected to the circuit.

The Phasor Diagram of the Capacitor Start motor is shown below:

I_M is the current in the main winding which is lagging the auxiliary current I_A by 90 degrees as shown in the phasor diagram above. Thus, a single-phase supply current is split into two phases. The two windings are displaced apart by 90 degrees electrical, and their MMF’s are equal in magnitude but 90 degrees apart in the time phase.

The motor acts as a balanced two-phase motor. As the motor approaches its rated speed, the auxiliary winding and the starting capacitor are disconnected automatically by the centrifugal switch provided on the shaft of the motor.

Characteristics of the Capacitor Start Motor

The capacitor starts motor develops a much higher starting torque of about 3 to 4.5 times the full load torque. To obtain a high starting torque, the two conditions are essential. They are as follows:-

• The Starting capacitor value must be large.
• The valve of the starting winding resistance must be low.

The electrolytic capacitors of the order of the 250 µF are used because of the high Var rating of the capacitor requirement.

The Torque Speed Characteristic of the motor is shown below:

The characteristic shows that the starting torque is high. The cost of this motor is more as compared to the split-phase motor because of the additional cost of the capacitor. The Capacitor start motor can be reversed by first bringing the motor to rest condition and then reversing the connections of one of the windings.

Applications of the Capacitor Start Motor

The various applications of the motor are as follows:

• These motors are used for the loads of higher inertia where frequent starting is required.
• Used in pumps and compressors
• Used in the refrigerator and air conditioner compressors.
• They are also used for conveyors and machine tools.

This is all about the capacitor start motor.

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The various starting methods of a Single Phase Induction motor are as follows:

• Split Phase Motor
• Capacitor Start Motor
• Capacitor Start Capacitor Run Motor or Two value capacitor motor
• Permanent Split Capacitor (PSC) or Single value capacitor motor
• Shaded Pole Motor.

All these starting methods are explained individually in detail in the separate articles. These starting methods depend on the two alternating fields displaced in space and phase.

The rotating field is the resultant of the two individual fields. This rotating field reacts with the cage rotor to provide the starting torque. One field is produced by the field winding and the other by the auxiliary winding or the starting winding.

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The word Pulsating means that the field builds up in one direction falls to zero and then builds up in the opposite direction. Under these conditions, the rotor of an induction motor does not rotate. Hence, a single phase induction motor is not self-starting. It requires some special starting means.

If the 1 phase stator winding is excited and the rotor of the motor is rotated by an auxiliary means and the starting device is then removed, the motor continues to rotate in the direction in which it is started.

The performance of the single phase induction motor is analysed by the two theories. One is known as the Double Revolving Field Theory, and the other is Cross Field Theory. Both the theories are similar and explain the reason for the production of torque when the rotor is rotating.

Double Revolving Field Theory of Single Phase Induction Motor

The double revolving field theory of a single phase induction motor states that a pulsating magnetic field is resolved into two rotating magnetic fields. They are equal in magnitude but opposite in directions. The induction motor responds to each of the magnetic fields separately. The net torque in the motor is equal to the sum of the torque due to each of the two magnetic fields.

The equation for an alternating magnetic field is given as

Where βmax is the maximum value of the sinusoidally distributed air gap flux density produced by a properly distributed stator winding carrying an alternating current of the frequency ω, and α is the space displacement angle measured from the axis of the stator winding.

As we know,

So, the equation (1) can be written as

The first term of the right-hand side of the equation (2) represents the revolving field moving in the positive α direction. It is known as a Forward Rotating field. Similarly, the second term shows the revolving field moving in the negative α direction and is known as the Backward Rotating field.

The direction in which the single phase motor is started initially is known as the positive direction. Both the revolving field rotates at the synchronous speed. ω_s = 2πf in the opposite direction. Thus, the pulsating magnetic field is resolved into two rotating magnetic fields. Both are equal in magnitude and opposite in direction but at the same frequency.

At the standstill condition, the induced voltages are equal and opposite as a result; the two torques are also equal and opposite. Thus, the net torque is zero and, therefore, a single phase induction motor has no starting torque.

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Contents:

• Stator Circuit Model
• Rotor Circuit Model
• Approximate Equivalent Circuit of an Induction Motor

Stator Circuit Model

The stator circuit model of an induction motor consists of a stator phase winding resistance R₁, stator phase winding leakage reactance X₁ as shown in the circuit diagram below:

The no-load current I₀ is simulated by a pure inductive reactor X₀ taking the magnetizing component I_µ and a non-inductive resistor R₀ carrying the core loss current I_ω. Thus,

The total magnetizing current I₀ is considerably larger in the case of the induction motor as compared to that of a transformer. This is because of the higher reluctance caused by the air gap of the induction motor. As we know that, in a transformer, the no-load current varies from 2 to 5% of the rated current, whereas in an induction motor the no-load current is about 25 to 40% of the rated current depending upon the size of the motor. The value of the magnetizing reactance X₀ is also very small in an induction motor.

Rotor Circuit Model

When a three phase supply is applied to the stator windings, a voltage is induced in the rotor windings of the machine. The greater will be the relative motion of the rotor and the stator magnetic fields, the greater will be the resulting rotor voltage. The largest relative motion occurs at the standstill condition. This condition is also known as the locked rotor or blocked rotor condition. If the induced rotor voltage at this condition is E₂₀ then the induced voltage at any slip is given by the equation shown below:

The rotor resistance is constant and is independent of the slip. The reactance of the induction motor depends upon the inductance of the rotor and the frequency of the voltage and current in the rotor.

If L₂ is the inductance of the rotor, the rotor reactance is given by the equation shown below:

But, as we know,

Therefore,

Where X₂₀ is the standstill reactance of the rotor.

The rotor circuit is shown below:

The rotor impedance is given by the equation below:

The rotor current per phase is given by the equation shown below:

Here, I₂ is the slip frequency current produced by a slip frequency induced voltage sE₂₀ acting in the rotor circuit having an impedance per phase of (R₂ + jsX₂₀).

Now, dividing the equation (5) by slip s we get the following equation:

The R₂ is a constant resistance and a variable leakage reactance sX₂₀. Similarly, the rotor circuit shown below has a constant leakage reactance X₂₀ and a variable resistance R₂/s.

Equation (6) above explains the secondary circuit of an imaginary transformer, with a constant voltage ratio and with the same frequency of both sides. This imaginary stationary rotor carries the same current as of the actual rotating rotor. This makes it possible to transfer the secondary rotor impedance to the primary stator side.

Approximate Equivalent Circuit of an Induction Motor

The equivalent circuit is further simplified by shifting the shunt impedance branches R₀ and X₀ to the input terminals as shown in the circuit diagram below:

The approximate circuit is based on the assumption that V₁ = E₁ = E’₂. In the above circuit, the only component that depends on the slip is the resistance. All the other quantities are constant. The following equations can be written at any given slip s is as follows:

Impedance beyond AA’ is given as:

Putting the value of ZAA’ from the equation (7) in equation (8) we get,

Therefore,

No-load current I₀ is

The total stator current is given by the equation shown below:

Total core losses are given by the equation shown below:

The air gap power per phase is given as: The developed torque is given by the equation shown below:

The above equation is the torque equation of an induction motor. The approximate equivalent circuit model is the standard for all performance calculations of an induction motor.

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The residual flux present in the machine provides the initial excitation. In the absence of the residual flux, the machine is momentarily run as an induction motor to create the residual flux. The motor is running slightly above the synchronous speed at no load by a prime mover. A small EMF is induced in the stator at a frequency proportional to the rotor speed.

The voltage appearing across the three-phase capacitor bank gives rise to a leading current drawn by the capacitor bank. This current is almost equal to the lagging current supplied back to the generator.

The flux set up by the current helps the initial residual flux causing an increase in the total flux. As a result, the voltage is increased. This increase in voltage causes an increase in the exciting current and a further increase in the terminal voltage.

This increase in voltage continues till the point where the magnetization characteristic of the machine and the voltage-current characteristic of the capacitor bank intersect each other. The graph below shows the magnetization curve and the V-I_C Characteristic.

At this point, the reactive volt-amperes required by the generator are equal to the reactive volt-amperes supplied by the capacitor bank. The operating frequency depends upon the rotor speed, and the change in load affects the speed of the rotor. The voltage is mainly controlled by the capacitive reactance at the operating frequency. The major disadvantage of an Isolated Induction Generator is that for a lagging power factor load, the voltage collapse very rapidly.

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