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A.C Motor

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Rural Technology

AC Motor

The motor which operates with AC supply is called an AC Motor. It transfers the electrical energy into mechanical energy.

The Induction motor is the most commonly used AC Motor, because of its simplicity its rugged construction & its low cost. It has a rotor & a stator. The induction motor, rotor is made up of a high grade steel laminated cylindrical with slots in its surface. The squirrel cage winding is made up of heavy copper bars connected together at each end by a metal ring. The air gap between the rotor & stator is very small to obtain maximum field strength.

The stator is made of a laminated slot in it. The two windings are placed in the slot, that winding s known as running winding & starting winding. Both windings are placed at 90 electrical degrees in the slot. Principle of AC Motor - When a single phase AC motor is connected to the AC supply the rotating magnetic field generated in the stator induces a magnetic field in the rotor, the two fields interact & cause the rotor term.

Types of single phase motor -

1. Split Phase Induction Motor 2. Capacitor Induction Motor 3. Universal Induction Motor 4. Shaded Pole Induction Motor

1) Split phase induction motor -

Split phase motor is an AC motor of fractional horse power & is used to operate washing machine & small pumps. The motor consists of a rotating part called rotor. A stationery part called stator & plates & centrifugal switch. Fig. 3 Split phase induction motor

Working of AC Motor -

At the start the current flowing through both the running & starting windings causes a magnetic field to be formed insight the motor. This magnetic field rotates & induces a current in the rotor winding which in turn causes another magnetic field.

Fig. 4 Working of A C Motor
This magnetic combine in such manner as to cause rotation of the rotor. A starting winding is necessary at the start in order to produce the rotating field after the motor is running the starting winding is no longer needed & is cut out of the circuit by means of the centrifugal switch.

2) Capacitor induction motor -

The capacitor motor operates on alternating current & it is available from 1/20 hp to 3 hp. It is widely used to operate refrigerator, compressors, washing machines, pumps & air conditioners. Operation of capacitor induction motor - The capacitor start motor is connected to the supply. Current start to flow through the running & starting torque, which in turn to produce rotation of the motor. When the motor reaches approximately 75% of full speed the centrifugal switch opens. This action cut the both the winding & the capacitor from the supply. The motor runs on the running winding only.

Fig 5 Operation of capacitor induction motor

3) Universal induction motor -

A universal motor is one which can be operated on either direct current or single phase alternating current. This motor is used in vacuum cleaner, food mixer, drills,& sewing machine.

Fig. 6 Construction of Universal Induction Motor

Construction of universal induction motor -

Universal motor consists of frame which is a rolled steel or aluminum. The field polls are generally held in frame by means of bolts. Field core is constructed of lamination which are tightly pressed together & held by reverts.

The armature is similar to that of small DC motor. It consists essentially of a laminated core having slots & a commutator to which the leads of armature winding are connected. The end plates are located on the ends of frame & held in place by screw. The plates house the ball bearings or sleeve bearings in which the armature shaft revolves.

Operation of universal motor -

The universal motor is so constructed that when the armature & field coils are connected in series & the current applied the magnetic lines of force created by the fields will react with the lines of force created by the armature & cause rotation. This is true regardless of whether the current is alternating or direct.

4) Shaded pole induction motor -

The shaded pole motor is single phase AC motor varying from approximately 1/100 to 1/20 hp. It is used for applications required very low starting torque such as small fans & blowers.

Construction of shaded pole motor -

It consist of stator or field frame, a rotor & end plates stator is usually of the concentrated field type & has a laminated core consisting of salient field poles on which a coil of wire is placed. The poles are provided with a slot near one end in which a solid copper coil of one turn is placed . This coil is called the shading coil, the shaded pole motor have the squirrel cage rotor.

Fig. 7 Construction of shaded pole motor

Operation of shaded pole motor -

On starting a current is induced into the shaded poles from the main poles. The shading coils establish a magnetic field which is out of phase with that established by the main fields & a shifting field is produced sufficient to give the desired starting torque. When the motor reaches speed, the effect of the shading coils are negligible.

3 - Phase Motor -

Three phase motors AC motors that are designed for three - phase operation. Three phase motors vary from fractional horsepower size to several thousand horsepower. These motors have a fairly constant speed characteristic & are made in designs giving a variety of torque others a low starting torque. Some are designed to draw a normal starting current. They are made for practically every standard voltage & frequency & are very often dual voltage motors. Three-phase motors are used to drive machine tools pumps, elevators, fans, cranes, hoists, blowers & may other machines.

Construction of 3 - Phase Motor -

3-Phase motor consists of stator, windings, rotor & end plates. The stator is consist of a frame & laminated steel core & a winding form of individual coils placed in slots. The rotor may be a dicast aluminum, squirrel cage type. In these types a laminated core pressed on a shaft. Its construction of rotor is same as split phase rotor motor. The end plates are bolted to each side of the stator frame & contain the bearings in which the shaft revolves.

Operation of 3 - Phase Motor -

When 3-phase winding is connected to the supply it produces the rotating magnetic field this magnetic field induces a current in the rotor winding which in turn causes another magnetic field. These magnetic field combine in such manner as to cause rotation of the rotor.

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