SSC JE DC Motor & DC Generator Solved Questions (2020)

Ques.1. For a dc machine shunt resistance and armature resistance values are (SSC-2018 Set-1)

1. High and High
2. High and Low
3. Low and Low
4. Low and High

Explanation:-

In DC machine the field resistance is low i.e the field resistance consists of fewer turns having the large cross-section area. While the resistance of shunt is high i.e the number of turns and smaller cross-section areas.

The resistance of the shunt field winding is large so that current in the winding is small compared to the rated armature current of the machine. To produce the necessary m m.f.. the winding consists of many turns.

The resistance of the series field winding is made as low as possible so that the voltage drop across the winding is minimum. To produce the same flux at rated conditions, the turns of a series winding are fewer than those of a shunt winding since the series winding carries the rated current of the machine.

Ques.2. If the field of a DC shunt motor gets opened while the motor is running, then the (SSC-2018 Set-1)

1. The motor will become slow
2. The motor will attain the dangerously high speed
3. Armature current will drop
4. The armature will oscillate about original speed as the mean speed

Answer.2. The motor will attain the dangerously high speed

Explanation:-

In the dc machine, the field coils or field winding is excited by the current in order to produce the magnetic flux. In a DC shunt machine, the speed is Directly proportional to the back EMF and Inversely proportional to the flux.

N ∝ Eb

Now if the field winding gets open than flux will become zero i.e φ = 0

N ∝ Eb/0

or

N =

Hence the speed of the DC Shunt Motor will attain the dangerous High Seed.

If the main field of a shunt motor or a compound motor is extremely weakened or if there is a complete loss of main field excitation, serious damage to the motor can occur under certain conditions of operation.

Since the speed of a dc motor is inversely proportional to flux, its speed tends to rise rapidly when the flux is decreased. If the field failure occurs on a motor that is coupled to a load, which can neither be removed nor be reduced to a very low value, the residual flux due to the open field will develop a torque that will not be able to sustain rotation. Thus the motor will stall, the heavy current will be drawn from the mains and the overload relay will trip.

On the other hand if field failure failure on an unloaded motor or if the application is such that it permits the motor to be unloaded or overhauled as in the ease of a hoist, the motor will not stall but instead its armature will accelerate quickly to a mechanically dangerous high speed or there will be destructive commutation. To prevent the above situation of over speeding, a field failure relay is used.

Ques.3. InterPoles winding is connected in (SSC-2018 Set-1, 2017)

1. Series with the armature
2. Series with the main pole
3. Parallel with the Armature
4. Parallel with the main Poles

Explanation:-

Interpoles In DC Machine

• In DC machine One way to reduce the effects of armature reaction is to place small auxiliary poles called “interpoles” between the main field poles. The interpoles have a few turns of large wire and are connected in series with the armature.
• One of the disadvantages of armature reaction is brush shifting, therefore a person is always required to adjust the brush position in the machine at the every load change. We observe that sparking in the brushes can be avoided if the voltage in the coils undergoing commutation is made zero.
• This method tries to do just the same. Small poles called commutating poles or interpoles are introduced in between the main poles along the geometrical neutral axis. Brushes are also set on this axis and kept fixed at this position for all the loads. The interpole winding has fewer turns of heavy copper conductor.

• Interpoles are connected in series with the armature winding so that they carry full armature current, as shown in Fig. As the load on the machine is increased, the current passing through the interpoles also increases, hence the flux produced by the interpoles is very large. Consequently, the large voltage is induced in the conductor that opposes the voltage due to the neutral plane shift and the net result is that they neutralize each other.
• Note that the interpoles can be used equally effectively in motors as well as in generators. When the mode of operation of the machine changes from the motor to generator, the currents in the armature and the interpoles are reverses in direction. Therefore, their voltage effects cancel each other out.

Thus, we can conclude that:

1. In a generator, interpoles must have the same polarity as the next upcoming pole.
2. In a motor, interpoles must have the same polarity as the previous main pole.

The MMF induced on the interpoles must be sufficient enough to neutralize the effect of armature reaction and to produce enough field in the interpole winding to overcome the reactance voltage due to commutation.

Another important function of the interpole is to neutralize the cross-magnetizing effect of the armature reaction, as shown in Fig. Here, vector FM represents the MMF due to main poles, FA represents the cross-magnetizing MMF due to the armature reaction and FC represents the interpole MMF which is directly opposite to the FA so that they cancel each other out.

It is important to note here that the interpoles do not affect the flux distribution under the pole faces. So, even by using the interpoles in the machine, the flux weakening problem is not completely eliminated. Most medium-size general-purpose motors correct the sparking problems with the interpoles and just live with the flux weakening problems.

Main Functions of the Interpole

1. The interpole neutralizes the reactance voltage and gives spark-free commutation.
2. It neutralizes the cross-magnetizing effect of armature reaction so that the brushes are not required to be shifted from its original position for any load.

Ques.4. In the large machine, the flat copper strips known as (SSC-2018 Set-1)

1. Windings
2. Bushes
3. Risers
4. Either of these

Explanation:-

In large machines, flat copper strips known as risers are used forming clip connections to armature bar conductors as shown in Figure.

Ques.5. Lap winding is preferred for (SSC-2018 Set-1)

1. Low current and Low Voltage
2. High current and High Voltage
3. High current and Low Voltage
4. Low current and High Voltage

Answer.3. High current and Low Voltage

Explanation:-

Armature Winding

•  An armature is that part of the DC machine where EMF is induced.
• Armature coils are wound on the armature core and placed inside the armature slots.

Two methods can be used to wound armature coil:

1. Lap winding
2. Wave Winding

Lap winding: Here, the end of one coil is connected to the beginning of the next coil. If connections are made this way the coils look as if they are superimposed on each other and then given a push in one direction as shown in Fig.

Lap wound armatures are constructed with relatively few turns of large wire. They are commonly used in machines that are intended to operate on low voltage and high currents, such as starter motors in automobiles, streetcars, and trolleys. Lap wound armatures have their windings connected in parallel with each other.

Wave winding is another type of armature winding. In this winding, the end of one coil is connected to the starting of another coil of the same polarity as that of the first coil.

Wave wound armatures are intended for used in high voltage, low current machines, such as high voltage generators. The armature windings are connected in series . in a generator, the voltage produced in each winding combines, to increase the total output voltage. In a motor, the voltage applied to the circuit is divided across each winding.

Ques.6. The motor used on a small lathe is usually (SSC-2018 Set-1)

1. Universal Motor
2. D.C shunt Motor
3. Single-phase capacitor run Motor
4. 3-Phase Synchronous Motor

Explanation:-

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.

Ques.7. Which of the following is a static exciter? (SSC-2018 Set-1)

1. DC Separately excited generator
2. Amplidyne
4. Rectifier

Explanation:-

An AC or DC generator requires 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. 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 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.

Ques.7. Dynamic braking can be used for which of the following? (SSC-2018 Set-2)

1. Shunt Motors
2. Series Motors
3. Compound Motors
4. All option are correct

Explanation:-

Dynamic braking can be used to slow both direct and alternating current motors. Dynamic braking is sometimes referred to as magnetic braking because in both instances it employs the use of magnetic fields to slow the rotation of a motor. The advantage of dynamic braking is that there is no mechanical brake shoe to wear out. The disadvantage is that the dynamic brake cannot hold a suspended load. Although dynamic braking can be used for both direct and alternating current motors but the principles and methods used for each motor are very different.

Dynamic Braking for Direct Current Motors

A direct current machine can be used as either a mote or generator. When used as a motor, the electrical energy converted into mechanical energy. When used as a generator, mechanical energy is converted into electric energy. The principle of dynamic braking for a direct current motor is to change the motor into a generator When a generator produces electrical power, its product counter torques making the armature hard to turn. The amount of counter-torque produced by the generator is proportional to the armature current.

Shunt Motors The connections for dynamic braking are shown in Fig.

The motor operates at its rated voltage. When braking is required, the armature is switched on to an external resistance Re. The field remains connected to( the supply with full excitation and the induced voltage in the armature has the same polarity. The armature current reverses and flows in a direction opposite to the current during motoring, developing a braking torque. Even though the motor is braked by generator action the method is not similar to regenerative braking. The braking is effective and the motor stops very fast if the field is available at its full value, for which reason it is separately excited. If the field is shunt excited, the field current falls with speed leading to very poor braking below the critical speed.

Series Motors:-  When dynamic braking, is employed the armature current would reverse. Obviously, the field MMF also reverses, causing demagnetization. To avoid this, the field connections are reverse connected before the series combination of armature and field is switched on to the braking resistance. The machine is then able to self-excited in this case. The connections of a series motor during braking is shown in Fig.

At the instant of initiating the braking, the current is more and hence the flux builds up. The torque developed is approximately proportional to the square of the armature current. At this instant, the braking effect is more and there may be a jump in the torque developed, causing an objectionable shock to the load.

Ques.8. The parts of the armature electric circuit which take an active part in E.M.F generation are_____  (SSC-2018 Set-2, 2016)

1. The coil sides inside the slots
2. The overhangs
3. Both the coil sides inside the slots and the overhangs
4. The commutator segments

Answer.1. The coil sides inside the slots

Explanation:-

The parts of the armature electric circuit which take an active part in E.M.F generation are the coil sides inside the slots.

Turn: Two conductors Iying in a magnetic field connected in series at the back,  so that emf induced in them is additive is known as a turn.

Coil. A coil may be a single turn coil having only two conductors, as shown in fig. it may be a multi-turn coil having more than two conductors as shown in fig. Multi-turn coils are used to develop higher voltages.

Short-Pitched coil & Full Pitched Coil

When the coils are full pitched, the induced emf in a coil is the arithmetic sum of the emf induced in two sides of the same coil since the coil sides are displaced by 180° electrical. In this case, the two sides of the same coil are placed at a similar position of two adjacent poles (North and South).

In the case of short-pitched and overpitched coils, the resultant induced emf is reduced because the two sides would fall under the influence of the same pole at some instant. In this instance, the induced emf in the two sides will oppose each other causing a reduction in resultant emf (phase difference).

The advantage of short-pitched winding is that in this case the copper used for end connection is reduced substantially which reduces the cost of the machine. It also improves the commutation (reduction of sparking at brushes) because the inductance of overhang connections is reduced Moreover, it reduces the copper losses and improves efficiency to some extent. Hence, many times short pitch winding is used.

Ques.9. The Interpoles in the DC Machine has a tapering shape in order to (SSC-2018 Set-2)

1. Reduce the overall weight
2. Reduce the saturation in the interpole
3. Economise of the material required for the interpoles
4. Increase the acceleration of commutation

Answer.2. Reduce the saturation in the interpole

Explanation:-

Interpoles In DC Machine

• In DC machine One way to reduce the effects of armature reaction is to place small auxiliary poles called “interpoles” between the main field poles. The interpoles have a few turns of large wire and are connected in series with the armature.
• One of the disadvantages of armature reaction is brush shifting, therefore a person is always required to adjust the brush position in the machine at the every load change. We observe that sparking in the brushes can be avoided if the voltage in the coils undergoing commutation is made zero.
• This method tries to do just the same. Small poles called commutating poles or interpoles are introduced in between the main poles along the geometrical neutral axis. Brushes are also set on this axis and kept fixed at this position for all the loads. The interpole winding has fewer turns of heavy copper conductor.

• Interpoles are connected in series with the armature winding so that they carry full armature current, as shown in Fig. As the load on the machine is increased, the current passing through the interpoles also increases, hence the flux produced by the interpoles is very large. Consequently, the large voltage is induced in the conductor that opposes the voltage due to the neutral plane shift and the net result is that they neutralize each other.
• Interpoles are tapered in size with comparatively more air gap in order to avoid easy saturation of flux in the interpolar windings as.armature current flows through it.

Ques.10. A DC generator can be termed as____ (SSC-2018 Set-2)

1. Rotating amplifier
2. Prime Mover
3. Power Pump
4. None of these

Explanation:-

Cross-field machines are also called rotating amplifier which are DC machines with an additional brush set.

The output power is controlled by varying the field current of the generator. If P2 is the output power and P1 is the input power to the field winding, the power amplification becomes P2/P1. The input power is usually 1 percent of output power and hence P2/P1= 100. There is a power amplification in the DC generator. Since it is rotating, the rotating amplifier is Derived. Depending on the degree of compensation, the cross-Geld generators can be classified as (i) metadyne and (ii) amplidyne.