Farm Power And Machinery (1+1)

A.C. Motors

For most farm work where electric motors can be used, induction motors are generally preferred. Induction motors may be classified as :
Single phase motors : This type of motors are further classified as

(a) split phase type
(b) Capacitor start type
(c) Shaded pole type
(d) Wound rotor type
(e) Synchronous type
(f) Universal type

Poly phase motors are of two types :

(a) Two phase motors and
(b) Three phase motors.

Among the single phase motors, the first two types are very much in use and they are described in detail here.


Split-Phase Motor: This motor has four main parts, namely rotor, stator, end plates, and centrifugal switch. It is generally operated from a single phase lighting or power circuit. The rotor consists of three parts, the laminated core, the shaft and the squirrel cage winding. The third part consists of heavy copper bars which are placed in slots in iron core and which connected to each other by means of heavy copper rings placed on both ends of the core.
The stator is composed of a laminated iron core with semi closed slots, a heavy cast iron frame, and two windings of insulated copper wires that are wound into the slots and are called the running and starting windings. Both the winding are connected to the power line when the motor is started. As the current starts flowing through both the windings, it causes a magnetic field to be formed inside the motor. This magnetic field rotates and induces a current in the rotor winding which in term causes another magnetic field. These combine in such a manner as to causes rotation of the rotor. No sooner does the motor reach approximately 75 per cent of the full speed and then the starting windings is cut out of the circuit by means of centrifugal switch.
The end plates are fastened to the stator frame and mainly keep the rotor in position. End plates are fitted with either ball bearings or sleeve bearings in which the rotor shaft rotates.

Capacitor Motor: The construction of the capacitor motor is similar to that of the split phase motor. But an additional part known as the ‘Capacitor’ is also provided with these machines. The capacitor is usually mounted on the top of the motor. It is connected in series with the starting winding and, therefore, higher starting torque with lower starting current is developed. A capacitor has capacity to store electricity. It is formed when two conductors are separated by an insulator, such as waxed paper. Capacitors are rolled and placed in a metal container. They are rated in microfarads. Depending upon the size of the motor, its capacity may vary from 10 to 150 microfarads.
To produce a starting torque in a capacitor motor, a revolving magnetic field must be established inside the motor. This is accomplished by placing the starting winding 90 electrical degrees out of phase with the running winding. The capacitor helps the current in the starting winding to reach the maximum value before the current in the running winding becomes a maximum. This condition produces a revolving magnetic field in the stator which in turn induces current in the rotor winding. As a result the magnetic field acts in such a manner as to produce rotation of the motor.

Three Phase Induction Motor: Three Phase Induction Motor has three main parts: stator, rotor and end plates. Its construction is very similar to that of the split phase motor, but it is not provided with centrifugal switch. The stator consists of cast iron frame and laminated iron core. The coils in the slots of the iron core are connected to form three separate windings called phases. These windings are connected so that a magnetic field is foamed inside the stator and that causes the rotor to turn at a constant speed. The squirrel cage type rotor is mostly used. The end plates are meant for supporting the rotor shaft.

A.C.Motor Starters: If an A.C. motor is started on full line voltage, it will draw about four times its normal running current. On very large motors, it is always desirable to reduce the starting current, otherwise damage may be done to the motor and line disturbances may be created affecting operation of other motors, on the same line. For single phase small motors (below 1 kW), a hand operated switch is used to control the motor. In the case of medium size motors (2hp or more), it is necessary to insert in the line some device which will reduce the starting current. Push button type starters or direct on line starters and star-delta-starters are widely used for three phase up to 7.5 kW motors above 7.5 kW only star-delta starter is used.

Push Button Starter : This consists of two push buttons, one for starting (usually green) and other for stopping (usually red) the motor. Pressing the green button causes the contacts inside the switch to make and connect the motor across the line. Pressing the red or stop button causes the contacts to break apart and open the circuit to the motor. Generally the push button starter is equipped with an overload safety device. It opens the circuit to the motor if an over load current flows for a short period of time.

Star-delta-Starters : This starter uses the reduced voltage starting principle only to three phase-delta connected motors. While starting, the motor is connected to star and thus the line voltage is reduced to 1/?3 value which each phase of the motor received then (400 line voltage reduces to 230 volts). The reduced phase voltage thus reduces the phase current in the same proportion. To make use of this type of starter, it is necessary to bring 6 leads out of the motor so that they can be changed from star at starting time in delta in the running condition. In starting, as the starting lever is pressed down, the motor starts rotating as a star connected motor. After the motor accelerates the starting lever is raised up, thus the switch is closed in the running position. The motor now runs on full voltage.