SSC JE ELECTRICAL 2018 WITH SOLUTION SET-1

SPLIT-PHASE MOTORS
Split-phase motors also referred to as induction-star induction-run (ISIR) motors, have a relatively low starting torque compared with the other single-phase motors but more torque than the shaded-pole motor. They range in size from 1/20 horsepower to abort 1/3 horsepower. Split-phase motors get their name from the fact that a single power supply is split between two individual windings — the run and the start — to produce the necessary torque to start the Motor.
A split phase induction motor is provided with the main winding and auxiliary or start winding placed in space quadrature and are connected in parallel to a single-phase supply. The main winding has a low resistance and high reactance whereas the auxiliary winding has a high resistance and low reactance. The auxiliary winding is effective during the starting of the motor and is disconnected from the supply when the motor attains 75 percent of its synchronous speed. A centrifugal switch, S is used to disconnect the auxiliary winding from the source as shown in Fig. Under the normal running condition, only the main winding is effective.
The run winding is energized whenever the motor is energized. This winding has a lower resistance than the start winding, which is only in the circuit long enough to help the motor start. For the motor to start, both the start and run windings must be energized. When both windings are energized, the current flows through each of the windings at a different rate, creating a phase shift.
The phase shift is measured in electrical degrees and is often referred to as the phase angle. The larger the phase angle, the more starting torque a motor has. For a point of future reference, a split-phase motor, as just described, has a phase angle of about 30° degrees. If the run and start windings were constructed and configured exactly the same, with the same size wire and the same number of turns, the phase angle would be zero, the magnetic field would have no imbalance, and the motor would not start.
Once the split-phase motor has started, the start winding must be removed from the electric circuit. The start winding is designed to be energized for only a short time and can become damaged if it is not de-energized. One commonly used device to remove the start winding from the circuit is the centrifugal switch (Fig.), which opens and closes its contacts depending on the speed of the motor. The electrical contacts on the switch are connected in series with the start winding and are normally closed.
When the voltage is initially applied to the motor, both the start and run windings are energized and the motor begins to turn. Once the motor has reached a speed equal to about 70 percent of its rated speed, the contacts oi the centrifugal switch open, de-energizing the start winding. The run winding, or main winding, is now the only winding energized, and the motor continues to run in this fashion since the turning rotor now creates the imbalance needed to keep the motor running. When the motor is de-energized, it begins to slow down and the centrifugal switch closes in preparation for the next startup. Other components can be added to the split-phase motor to increase its torque, as well as its range of applications such as the capacitor, potential relay etc.

The function of the starting relay provided in the refrigerator is to start the split-phase induction motor by connecting the auxiliary winding or starting winding across the main supply in addition to the main winding at the time of starting. This helps to make the split-phase inflection motor as the self-induction motor is unable to start.
The first method of disconnecting the starting winding of a split-phase motor is already discussed in the above Question i.e Question Number 63.
Another method of disconnecting the auxiliary winding when the motor has picked up speed is by using an electromagnetic relay as has been shown in Fig
The torque required to start the motor is significantly more than needed in the running condition At the starting time of the motor, electrical power is given to start relay and winding of the motor. It also provides current to the starting winding of the motor. The starting winding provides sufficient torque so that the motor starts running. As the motor speed increases, the torque requirement decreases and thereby the current required by the motor also decreases. The current in starting relay is not able to hold the relay and it gets released which opens the starting winding contacts. Therefore, the starting winding gets disconnected.

The dc shunt motor has a speed regulation of 5-10%, and hence, it is used for constant-speed drives like pumps, blowers, fans, etc. Though induction motors are preferable in these applications, the dc shunt motor is cheaper for low-speed drives. When the driven load requires a wide range of speed control, both below and above-rated speed, a dc shunt motor is employed, e.g. in lathes and other machining Tools. These motors are widely used in steel and aluminum rolling mills and the Ward Leonard speed control system.

All split-phase motors have a start and run winding. The start windings must be disconnected from the circuit within a very short period of time or they will overheat. Several methods are used to disconnect the start windings. The centrifugal switch is the most common method in open motors; however, electronic start switches are sometimes used.
The centrifugal switch is used to disconnect the start winding from the circuit when the motor reaches approximately 75% of the rated speed. 
A centrifugal switch is a mechanical device attached to the end of the shaft with weights that will sling outward when the motor reaches approximately 75% speed. For example, if the motor has a rated speed of 1725 rpm, the centrifugal weights will change position at 1294 r.p.m (1725 x 0.75) and open a switch to remove the start winding from the circuit. This switch is under a fairly large current load, so a spark will occur.
If the switch fails to open its contacts and remove the start winding, the motor will draw too much current, and the overload device will cause it to stop. When the motor is de-energized, it will slow down and the centrifugal switch will close its contacts in preparation for the next motor starting attempt. The more the switch is used, the more its contacts will burn from the arc. If this type of motor is started many times, the first thing that will likely fail is the centrifugal switch. This switch makes an audible sound when the motor starts and stops.
Hence if the starting switch fails to open when needed, the starting winding mill almost always overheats and burns out.
 

A long transmission line has a large capacitance. When a long line is operating under the no-load condition, the receiving-end voltage is greater than the sending end voltage. This is known as the Ferranti effect. This phenomenon can be explained by the following reasoning. It was first noticed by Ferranti on overhead lines supplying a lightly loaded network. The Ferranti effect is due to the charging current of the line. The value of current at the sending end at no-load and normal operating voltage applied at the sending end is called the charging current.
A simple explanation of the Ferranti effect can be given by approximating the distributed parameters of the line by lumped impedance as shown in Figure. Since usually, the capacitive reactance of the line is quite large as compared to the inductive reactance, under the no-load or lightly loaded condition, the line current is of leading pf. The phasor diagram is given below for this operating condition. The charging current produces the drop in the reactance of the line which is in phase opposition to the receiving-end voltage and hence the sending-end voltage becomes smaller than the receiving-end voltage.
Note:-
The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied. The relative voltage rise is proportional to the square of the line length.
The Ferranti effect is much more pronounced in underground cables, even in short lengths, because of their high capacitance.