Different types of Motors
Single Phase Induction Motor
Single-phase power system is more widely used than three phase system for domestic purposes,
commercial purposes and to some extent in industrial uses.
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 a 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.
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 the 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 in three-phase winding in three phase induction motor.
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= frequency, P = No. of poles
The construction of the stator of the single-phase induction motor is similar to that of three phase
induction motor except that 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 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
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 more smooth and
quieter (i.e. less noisy).
To provide mechanical strength, these rotor conductors are braced to the end ring and hence form
a complete closed circuit resembling a cage and hence got its name as squirrel cage induction
motor.
As end rings permanently short the bars, the rotor electrical resistance is very small and it is not
possible to add external resistance as the bars get permanently shorted. The absence of slip ring
and brushes make the construction of single-phase induction motor very simple and robust.
Working Principle of Single-Phase Induction Motor
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 produce the required torque.
When we apply a single phase AC supply to the stator winding of single phase induction motor,
the alternating current starts flowing through the stator or main winding. This alternating current
produces an alternating flux called main flux. This main flux also links with the rotor conductors
and hence cut the rotor conductors.
According to the Faraday’s law of electromagnetic induction, emf gets induced in the rotor. As the
rotor circuit is closed, 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 the induction
principle, 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?
According to double field revolving theory, any alternating quantity is resolved 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, φm can be resolved into two components
Each of these components rotates in the opposite direction i.e., if one
φm/2
is rotating in a
clockwise direction then the other φm / 2 rotates in an 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 the starting condition is zero. So, the
single-phase induction motors are not self-starting motors.
Alternative explanation; 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 machine responds to each of the magnitude
fields separately. The net torque in the motor is equal to the sum of the torque due to each of the
two magnetic fields. Hence no resultant torque. Hence there is no starting torque in a single-phase
induction motor.
Methods for Making Single Phase Induction as Self-Starting Motor
Starting Methods of a Single-Phase Induction Motor
The Single-Phase Motor is not self-starting and hence needs an auxiliary means or equipment to
start the single-phase induction motor. Mechanical methods are impractical and, therefore the
motor is started temporarily by converting it into a two-phase motor.
Single-phase Induction motors are usually classified according to the auxiliary means used to start
the motor. They are classified according to the starting methods.
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.
1. Split Phase Induction Motor
The Split Phase Motor is also known as a Resistance Start Motor. It has a single cage rotor, and its
stator has two windings known as main winding and starting winding or auxiliary winding. Both
the windings are displaced 90 degrees in space. The main winding has very low resistance and a
high inductive reactance whereas the starting winding has high resistance and low inductive
reactance. The connection diagram of the motor is shown below:
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 (IM) lags behind the supply voltage V almost by the 90-degree
angle. The current in the auxiliary winding IA 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.
2. Capacitor Start Induction Motor
Capacitor Start Motors are single-phase Induction Motors that employ a capacitor in the auxiliary
winding circuit to produce a greater phase difference between the current in the main and the
auxiliary windings. The name capacitor starts itself shows that the motor uses a capacitor for the
purpose of starting. The figure below shows the connection diagram of a Capacitor Start Motor.
The capacitor start motor has a caged 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 CS is connected in series with the starting winding. A centrifugal switch SC is
also connected to the circuit. The value of the capacitor is so chosen that IA leads IM by about 900
so that the starting torque is maximum for a certain value of IA and IM. Again, the starting winding
is opened by the centrifugal switch when the motor attains about 75% of the synchronous speed.
The motor then operates as a single-phase induction motor and continues to accelerate till it reaches
the normal speed.
The Phasor Diagram of the Capacitor Start motor is shown below:
IM is the current in the main winding which is lagging the auxiliary current IA 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.
3. Capacitor Start Capacitor Run Motor
The Capacitor Start Capacitor Run Motor has a cage rotor, and its stator has two windings
known as Main and Auxiliary Windings. The two windings are displaced 90 degrees in space.
There are two capacitors in this method one is used at the time of the starting and is known as
starting capacitor. The other one is used for continuous running of the motor and is known as RUN
capacitor.
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 CS and CR. 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, XA = 1/2πfCA, 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, XR = 1/2πfCR, 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 CR 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.
4. Permanent Split Capacitor (PSC) Motor
The Permanent Split Capacitor motor also has a cage rotor and the two windings named as main
and auxiliary windings similar to that of a Capacitor Start and Capacitor Start Capacitor Run
Motor. It has only one capacitor connected in series with the starting winding. The capacitor C is
permanently connected in the circuit both at the starting and the running conditions. 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.
5. Shaded Pole Induction Motor
Definition: The shaded pole induction motor is simply a self-starting single-phase induction motor
whose one of the poles is shaded by the copper ring. The copper ring is also called the shaded ring.
This copper ring acts as a secondary winding for the motor. The shaded pole motor rotates only in
one particular direction, and the reverse movement of the motor is not possible.
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 induced 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 has 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.
Equivalent Circuit of a Single-Phase Induction Motor
The equivalent circuit of a Single-Phase Induction Motor can be obtained by two methods
named the Double Revolving Field Theory and Cross Field Theory. Firstly, the equivalent circuit
is developed on the basis of double revolving field theory when only its main winding is energized.
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,
•
R1m is the resistance of the main stator winding.
•
X1m is the leakage reactance of the main stator winding.
•
XM 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.
•
Vm is the applied voltage.
•
Im is the main winding current.
When the stator of single-phase induction motor is connected to single – phase supply, the stator
current produces a pulsating flux. According to the double – revolving field theory, the pulsating
air – gap flux in the motor at standstill can be resolved into two equal and opposite fluxes with the
motor. Since the magnitude of each rotating flux is one – half of the alternating flux, it is convenient
to assume that the two rotating fluxes are acting on two separate rotors. Thus, a single
– phase induction motor may be considered as consisting of two motors having a common stator
winding and two imaginary rotors, which rotate in opposite directions. The standstill impedance
of each rotor referred to the main stator winding is
The equivalent circuit of a single-phase single winding induction motor with the standstill rotor is
shown below.
The forward and the backward flux induce a voltage Emf and Emb respectively in the main stator
winding. Em is the resultant induced voltage in the main winding.
At the standstill condition Emf = Emb
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. The slip of
the rotor with respect to the forward rotating flux is S. The slip the rotor with respect to the
backward rotating flux is (2-S). When the forward and backward slips are taken in account, the
result is the equivalent circuit shown below which represent the motor 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:
The current in the stator winding is
The torque of the backward field is in opposite direction to that of the forward field, and therefore
the total air – gap power in a single-phase induction motor is
The power converted from electrical to mechanical from in a single phase induction motor is given
by
Example;
A 230 V, 50 Hz, 4 – pole single phase induction motor has the following equivalent circuit
impedances:
Friction, windage and core loss = 40 W
For a slip of 0.03pu, calculation (a) input current, (b) power factor, (c) developed power, (d) output
power, (e) efficiency.
Solution. Form the given data
For the forward field circuit
Determination of Equivalent Circuit Parameters
The parameter of the equivalent circuit of single – phase induction motor can be determined from
the blocked – rotor and no – load tests. These tests are performed with auxiliary winding kept open,
except for the capacitor – run motor.
Example
A 220 V, single – phase induction motor gave the following test results: Blocked – rotor test: 120V,
9.6A, 460W No – load test: 220V, 4.6A, 125W The stator winding resistance is 1.5 Ω, and during
the blocked – rotor test, the starting winding is open. Determine the equivalent circuit parameters.
Also, find the core, fraction and windage losses.