Single phase induction motors

Introduction

The most common type of electric motor is the single face type, which finds why domestic, commercial and industrial applications. Single phase motors are small size motor affectional kilowatt ratings. Domestic appliances like fans, hair dryers, washing machine, vacuum cleaners, mixture, refrigerators, food processors and other kitchen equipment employee this motors. This motor also find applications in air conditioner fans blowers office machinery, small Power tools, dairy machine, small farming equipment.

Single phase motor are classified as follows:-

Induction motors
Commutator motors
Synchronous motors

Single phase induction motor principle

A single phase induction motor consists of a single phase winding mounted on stator and cage winding on the rotor. When a single phase supply is connected to the stator winding a pulsating magnetic field is produced. By pulsating field we mean that the field builds up in one direction, falls to 0, and then builds up in the opposite direction. under this condition, the rotor does not rotate due to inertia. Therefore, a single phase induction motor is inherently not self starting and acquired some special starting means. if, however, the single phase stator winding is excited and the rotor of the motor is started by an auxiliary means, and the starting device is then removed, the motor continues to rotate in the direction in which it is started.

two theories have been suggested to analyse the performance of a single phase induction motor, namely the double revolving field theory and the cross field theory full stop both the theories are fairly complicated, and neither has any advantage over the other in numerical calculations. Almost similar result are obtained with both the theories. This two theories explain why torque is produced in the rotor once it is turning.

Why single phase motor is not self starting

single phase induction motor has distributed stator winding and squirrel cage rotor. Now an alternating operating flux acting on a stationary squirrel cage rotor cannot produce rotation only a revolving flux can produce rotation that is by a single phase motor is not self starting. This is the reason is best for this type of question.

Starting method and types of single phase induction motor

we have seen that some means should be used to start the single phase induction motor. Mechanical methods are impractical and, therefore, the motor is is started temporally converting it into a two phase motor.

single phase induction motor are usually classified according to the auxiliary means used to start the motors.

The types of single phase induction motor / they are also classified as follows:;

Split phase motor
Capacitor start motor
Capacitor start capacitor run motor (or two value capacitor motor)
Permanent split capacitor motor (a single value capacitor motor)
Shaded pole motor

all this is starting method depend on to alternating fields displaced in space and phase. the resultant of the two field is rotating field. This rotating field reacts with the cage rotor to provide the starting torque. One field is produced by the main winding and the other by the auxiliary winding. The auxiliary winding is also called starting winding.

1.Split phase induction motor

in this figure I split phase induction motor. It is also called a resistance starter motor. It has a single cage rotor and its state has two winding ameen winding and starting auxiliary winding. The main field winding and the starting winding are displaced 90 degree in space like the winding in the two phase induction motor. The main winding has very low resistance and high inductive reactance.

Thus, the current I am in the main winding lags behind the supply voltage v why nearly 90 degree Indian next figure. The auxiliary winding has a resistor connected in series with it. It has the high resistance and low inductive reactance so that the current I in the auxiliary winding is nearly in face with the line voltage, there is a time phase difference between the current in the two windings. That time phase difference is not 90 degree but usually of the order of 30 degree. This phase difference is enough to produce a rotating magnetic field. Since the current in the to two winding are not equal, the rotating field is not uniform, and the starting torque is small of the order of 1.52 to 2 times the rated running torque. The main and auxiliary winding are connected in parallel during starting. The starting winding is disconnect from the supply automatically when the motor reaches speed about 70 to 80% of synchronous speed. for motor rated about hundred watt or more, centrifugally operated switch is used to disconnect the starting winding.

Four smaller motors relay is often used. The relay is connected in series with the main winding. at the time of starting, heavy current flow in the relay coil causing its contacts to close. This brings the starting winding into the circuit. as the motor reaches its prey determined speed of the order of 72 to 80% of synchronous speed, the current through the coil decreases.

Applications:-

split phase motor are cheap and they are most suitable for easily started loads where frequency of starting is limited. the common applications are washing machine, air conditioning fans, food mixer grinders, floor polishers, blowers, centrifugal pumps, small drills, office machinery, dairy machinery.because of law starting torque, they are seldom used for brides requiring more than 1 kilowatt.

2. Capacitors start motors

Figure 8.10 show the connection of capacitor start motor. It has a cage rotor and its starter has two winding namely, the main winding and the auxiliary winding starting winding. the two winding are displaced 90 degree in space. A capacitor is connected in series with the starting winding.

A centrifugal switch as Sc is also connected as shown in figure.

By choosing a capacitor of the proper batting the current Im in the main winding may be made to lag the current in the auxiliary winding by 90 degree. Thus a single phase supply current is split into two phases to be applied to the stator windings. Thus the winding are displays 90 degree electrical and their MMF are equal in magnitude but 90 degrees apart in time phase.

Therefore the motor act like a balance two phase motor. As the motor approaches its rated Speed, the auxiliary winding and the starting capacitor c are disconnected automatically by the centrifugal switch Sc mounted on the shaft. the motor is so named because it use the capacitor only for the purpose of starting.

Applications:-

capacitor start motor are used for lots of higher inertia where frequent start are required. this motor are most suitable for pumps and compressors, and therefore they are widely used in refrigerators and in air conditioner compressor. They are also used for conveyors and some machine tools.

2. Two value capacitor motors

Figure show the systematic diagram of two value capacitor motor. It has a cage rotor and its stator has two winding namely the main winding and the auxiliary winding full stop the two windings are displaced 90 degree in space. The motors use two capacitor Cs and Cr.the two capacitor are connected in parallel at starting.

The capacitor is called the starting capacitor. In order to obtain high starting torque, a large current is required. For the purpose the capacitive reactance in the starting winding should be low. the value of starting capacitor should be large. The starting capacitor is short time rated at the almost always electrolytic.

during normal operations, the rated line current is smaller than the starting current. Handscomb are the capacitive reactance should be large. Since the value of rotor capacitors should be small. As the motor approaches synchronous speed, the capacitor starting capacitor is disconnected by a centrifugal switch. The capacitor rota capacitor is permanently connected in the circuit. It is called the run capacitor. It is long time rated for continuous running full stop it is value of oil field paper construction. Since one capacitor starting capacitor is used only at starting and the rotor capacitor for continuous running, this motor is also called capacitor start capacitor run motor.

Figure ab show the phasor diagram of for two value capacitor motor. At starting both the capacitor are in the starting. When the capacitor starting capacitor is disconnected face become 90 degree as shown in figure b.

the torque speed characteristics of two value capacitor motor in sun in figure two value capacitor motor are quit and smooth running. They have a higher efficiency than motor that run on the main winding alone.

Applications:-

two value capacitor motor are used for lots of higher inertia requiring frequent start where the maximum pull out torque and efficiency required are higher. They are used in pumping equipment, refrigeration, air compressor etc.

3.Permanent split capacitor motor

a permanent split capacitor motor is shown in figure. It has a cage rotor and its stator has two winding, namely, the main winding and auxiliary winding. this single phase induction motor has only one capacitor c which is connected in series with the starting winding. the capacitor c is permanently connected in the circuit both at starting and running conditions. A permanent split capacitor motor is also called the single value capacitors motor.

Since the capacitor c is always in the circuit this type of motor has no starting switch. the auxiliary winding is also in the circuit, and therefore this motor operate in the same way as a balanced two phase motor. Consequently, it produce a uniform torque. The motor is there for less noisy during operations.

Applications:-

permanent split capacitor motor are used for fans and blowers in heaters and air conditioner and to drive refrigerator compressor. They are also used to drive office machinery.

3. Shaded pole motor

a shaded pole motor is a simple type of self starting single phase induction motor. It consists of a stator and a case type rotor. The stator is made up of silent Paul's. Each pole is located on side and a copper ring is fitted on the smaller part a as shown in figure. This part is called the shaded pole. The ring is usually a single turn coil and is known as shading coil.

when alternating current flow in the field winding, and alternating flux is produced in the field core. a portion of this flex links with the shading coil, which behaves as a short circuited secondary of a transformer. a voltage is induced in the shading coil, and this voltage circulates the current in it. The induced current produce the flux called the induced flux which oppose the main coflex. The shading coil, this,causes the flux in the shaded portion a to lag behind the flux in the shaded portion b of the pole.at the same time, the main flux and the shaded pole flux are displaced in space. This is space displacement is less than 90 degree. since there is time and space displacement between the two fluxes, the condition for setting up rotating magnetic field are produced under the action of rotating flux is starting torque is developed on the cage rotor. the direction of the rotating field is from the unshaded to the shaded portion of the pole in figure above the direction of rotating is clockwise. In a shaded pole motor the river cell of direction of rotation is not possible.

Applications:-

Shaded pole motor are very cheap. The starting torque developed by a shaded pole motor is very low. the losses are higher and the power factor is low. Consequently, deficiency is also very low. For this reason, the shaded pole motor are built only in small size of the power rating of the order of 40 watt or less.they are used to drive device which require low starting torque. They are most suitable for small device like release, fans of all kinds etc. Because of their low initial cost and easy starting. the most common application are table fans, exhaust fans, hair dryers, fans for refrigeration and air conditioner equipments, electronic equipment cooling fans etc. they are also used in record players, tape recorder, slide projectors, photocopy machine, in starting electric clocks and other single phase asynchronous timing motors.

Comparison between single phase and three phase induction motor

Most single phase induction motor are constructed in fractional kilowatt capacity and are used in places where 3 phase supply is not readily available. single phase motor when compared with three phase induction motor have the following disadvantage

single phase motor develop about 50% of the output of that of three phase motor for the same size and temperature rise.
Single phase motor have lower power factor.
The starting torque is low in single phase motors.
Single phase motor have low efficiency.
Single phase motor are costlier than three phase motor of the same rating.

however, single phase induction motor are simple robust, reliable and less expensive for small ratings. They are used in low power drives in small industries and domestic and commercial applications. They are generally available up to 1 kilowatt rating.

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