SSC JE Electrical Previous Year Question Paper 2018-SET1|SSC JE 2018

The hysteresis motor can be considered a self-starting synchronous motor. From the time of starting until it reaches synchronous speed, the motor produces a synchronizing torque and an ideally flat speed/torque characteristic. Due to the absence of slots and teeth on the rotor, the operation of this motor is smooth and quiet.

 

The motor behavior is similar to that of a conventional synchronous motor but there is no excitation winding on the rotor or permanent magnets, giving the motor a high efficiency and power factor. These characteristics make it ideal for many industrial applications, such as in electronic and medical equipment, tape and video recorder drives, computer drives, clocks, gyroscope rotors for inertial navigation, robotics, and other special applications where precision and reliability are required.

The rotor of the Hysteresis Motor is the smooth cylindrical type that is made up of hard magnetic material like chrome steel or alnico for high retentivity (it is the capacity of an object to retain magnetism after the action of the magnetizing force has ceased. This requires selecting a material with a high hysteresis loop area. The rotor does not carry any winding. The stator construction is either split phase type or shaded pole type. 

 

The motor starts rotating due to eddy current and hysteresis torque developed on the rotor At synchronous speed, there is no induced emf in the rotor, as the stator synchronously rotating field and the rotor is stationary with respect to each other.

In the absence of induced eddy current, the torque due to eddy current is zero. At synchronous speed, the rotor torque is only due to the hysteresis effect.

When the rotor rotates at synchronous speed, the stator revolving field flux induces Poles on the rotor. Due to the hysteresis effect, the rotor polarities linger an instant after the stators

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 is 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 of 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 the 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.

During the no-load condition, the current flowing is only charging current due to line capacitance. It increases the capacitive var in the system. Since the line is under no load the line inductance will be less. Therefore, the capacitive var becomes greater than inductive var during no load or light load condition.

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 a 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.

Let a three-phase A.C system is used for the transmission. The various parameters are,

P = Power transmitted in kW

V = Line voltage in volts

cosφ = Power Factor of load

I = Length of line in meters

A = Area of the cross-section of the conductor in square metros

p = Resistivity of conductor material

R = Resistance per conductor in Ω

The resistance per conductor is given as

R = Ρl ⁄ A

Power in a three-phase AC circuit is 

P = √3 VI cosφ

The load current I can be calculated as

$I = \dfrac{P}{{\sqrt 3 V\cos \Phi }}$

The total copper Loss in a conductor is given as

W = 3I²R

$\begin{array}{l}\therefore W = 3 \times \dfrac{{{P^2}}}{{3{V^2}Co{s^2}\Phi }} \times \dfrac{{\rho l}}{A}\\\\A = \dfrac{{{P^2}}}{{W{V^2}Co{s^2}\Phi }} \times \rho l\end{array}$

The volume of copper used is

Volume = Area × Length

V = 3AL

$Vol = 3 \times \dfrac{{{P^2}}}{{W{V^2}Co{s^2}\Phi }} \times \rho {l^2}$

It can be seen from the equation that the volume of copper required is inversely proportional to the square of the transmission voltage and the power factor, for the given P. W. p and l.

Thus greater is the transmission voltage level, lesser is the volume of copper required i.e. the weight of copper used for the conductors. The conductor material required is less, for higher transmission voltage. 

An AC or DC generator requires a direct current to energize its magnetic field. The DC field current is obtained from a separate source called an exciter. Either rotating or static-type exciters are used for AC power generation systems. There are two types of rotating exciters: brush and brushless. Rectifier is a static exciter.

The primary difference between brush and brushless exciters is the method used to transfer DC exciting current to the generator fields. Static excitation for the generator fields is provided in several forms including field-flash voltage from storage batteries and voltage from a system of solid-state components. DC generators are either separately excited or self-excited.

EXCITATION SYSTEMS in current use include direct-connected or gear-connected shaft-driven DC generators, belt-driven or separate prime mover or motor-driven DC generators, and DC supplied through static rectifiers.

Static Excitation System

As the name indicates, all the components in this type of excitation system are static. A set of rectifiers is fed from a transformer that steps down the main generator or auxiliary bus voltage. The rectifiers supply the main generator excitation current directly through slip rings and they may be controlled or uncontrolled.

 

An external source of DC is necessary for the initial excitation of the field windings. On engine-driven generators, the initial excitation may be obtained from the storage batteries used to start the engine or from the control voltage at the switchgear.

The main advantage of the static exciter is improved response as the field current is controlled directly by the thyristor rectifier but, of course, if the generator terminal voltage is depressed too low then excitation power will be lost. It is again possible to provide the power supply from both voltage and current transformers at the generator terminals but it is doubtful whether the improved response of the static exciter over a permanent magnet brushless scheme can be justified for many small embedded generators.

For domestic wiring, the most extensively used conductor material is copper or aluminum. To prevent any leakage of current from the conductor and also to provide mechanical strength, it is surrounded by ar insulation and sheath. Normally, the cables are classified according to the insulation used over the conductor The selection of suitable cable for installation depends upon the following considerations:

(I) The nature of conditions under which the cable is to be used (for example, underwound, banging is air, in damp conditions, etc.). 

(ii) The operating voltage.

(iii) The current capacity of the installation.

The operating conditions decide the type of insulation and other protection needed around the conductor of the cable. The operating voltage determines the thickness of the insulation. The current capacity of the installation determines the cross-sectional area or size of the cable conductor. The insulation should have high resistance, high dielectric strength, good mechanical strength and high temperature withstand capability.