Two Marks Questions with Answers

Two Marks Questions with Answers

Q. 1 State Fleming's left hand rule. AU: May-03

Ans. : 

The rule states that, 'Outstretch the three fingers of the left hand namely the first finger, middle finger and thumb such that they are mutually perpendicular to each other. Now point the first finger in the direction of magnetic field and the middle finger in the direction of the current then the thumb gives the direction of the force experienced by the conductor'.

Q. 2 How to reverse the direction of rotation of d.c. motor? AU: May-19

Ans. : To reverse direction of rotation of d.c. motor, either direction of main field produced by the field winding is reversed or direction of the current passing through the armature is reversed. The direction of the main field can be reversed by changing the direction of current passing through the field winding, which is possible by interchanging the polarities of supply which is given to the field winding.

Q. 3 What is back e.m.f.? AU: Dec.-11

Ans. 

After a motoring action, there exists a generating action. There is an induced e.m.f. in the rotating armature conductors according to Faraday's law of electromagnetic induction. According to the Lenz's law, this induced e.m.f. in the armature always acts in the opposite direction of the supply voltage. This e.m.f. tries to set up a current through the armature which is in the opposite direction to that, which supply voltage is forcing through the conductor. So as this e.m.f. always opposes the supply voltage, it is called back e.m.f. and denoted as E_b.

Q. 4 How does back e.m.f. in a d.c. motor makes the motor self regulating? or State the significance of back e.m.f. AU: May-06, 08, Dec.08,18, 19

Ans. : 

The back e.m.f. is proportional to speed, E_b ∝ N. When load is suddenly put onto the motor, motor tries to slow down. So speed of the motor reduces due to which back e.m.f. also decreases. So the net voltage across the armature (V-E_b) increases and motor draws more armature current hence the torque on the armature increases. The increase in the torque is just sufficient to satisfy increased load demand.

When load on the motor is decreased, the speed of the motor tries to increase. Hence back e.m.f. increases. This causes (V-E_b) to reduce which eventually reduces the current drawn by the armature. The motor speed stops increasing when the armature current is just enough to produce the less torque required by the new load. So back e.m.f. regulates the flow of armature current making themotor self regulating.

Q. 5 State the voltage equation of a d.c. motor.

Ans. 

In case of d.c. motor, supply voltage V has to overcome back e.m.f. E_b which is opposing V and also various drops as armature resistance drop I_a R_a brush drop etc. Infact the electrical work done in overcoming the back e.m.f. gets converted into the mechanical energy developed in the armature. Hence the voltage equation of a d.c. motor can be written as,

V = E_b +I_a R_a + Brush drop

Q. 6 Write the power balance equation of a motor. AU: Dec.-03

Ans. 

Multiplying both sides of the voltage equation by I, we get power equation as,

VI_a = E_b I_a + I_a² R_a

This equation is called power balance equation ofa d.c. motor.

V Ia = Net electrical power input to the armature measured in watts.

1_a² R = Power loss due to the resistance of the armature called armature copper loss.

So difference between V I_a and 1_a² Ra i.e. Input Losses gives the output of the armature.

So E_bl_a is called electrical equivalent of gross mechanical power develped by the armature. This is denotes as P_m.

Q. 7 State the condition for maximum power. Is this practically possible?

Ans. :

The condition for maximum power is,

E_b = V / 2 .......Condition for maximum power

This is practically impossible to achieve as for this Eb, current required is much more than its normal rated value. Large heat will be produced and efficiency of motor will be very low.

Q. 8 Write the torque equation of a d.c. motor.

Ans. :

The torque equation of d.c. motor is

T_a  = 1/2π ϕ × PZ / A where ϕ = Flux per pole, P = Number of poles, Z

=Number of conductors, A = Number of parallel paths and I_a is armature

current.

Q. 9 Why d.c. motor draws less current on no load?

Ans. On no load, the load torque requirement is zero and motor keeps on rotating at a speed of N₀ r.p.m. drawing an armature current of I_a0. This is just enough to produce a torque T_a0 which satisfies the friction, windage and iron losses of the motor. On no load, speed of the motor is large hence Ebo is also large hence (V – E_b0) is very small hence armature current I_a0 is also small. So motor draws less current on no load and takes more and more current as motor load increases.

Q. 10 Draw the circuit model of various types of motors. [Refer sections 4.8, 4.9 and 4.10] AU: May-11

Q. 11 Write the voltage and curent relationships for d.c. shunt and series motors.

[Refer sections 4.8.1 and 4.9.1]

Q. 12 Define speed regulation of d.c motor.

Ans. : The speed regulation for a d.c. motor is defined as the ratio of change in speed corresponding to no load and full load condition to speed corresponding to full load.

Mathematically it is expressed as,

% Speed regulation = N_{no load} -N_{full load} / N_{full load} × 100

Q. 13 State the important d.c. motor characteristics.

Ans. :

i) Torque-Armature current characteristics (T Vs I_a): The graph showing the relationship between the torque and the armature current is called a torque-armature current characteristic. These are also called electrical characteristics.

ii) Speed-Armature current characteristics (N Vs I_a): The graph showing the relationship between the speed and armature current characteristics.

iii) Speed-Torque characteristics (N Vs T): The graph showing the relationship between the speed and the torque of the motor is called speed-torque characteristics of the motor. These are also called mechanical characteristics.

Q. 14 Why is a d.c. series motor used to start heavy loads? or Why d.c. series motor is never started on no load ? AU: Dec.-06, 10, 18, May-09

Ans. : 

In case of a d.c. series motor, ϕ ∝ I_a on no load as I_a is small hence flux produced is also very small. According to speed equation,

N ∞ 1 / ϕ  as E_b is almost constant.

So on very light load or no load as flux is very small, the motor tries to run at dangerously high speed which may damage the motor mechanically. This can be seen from the speed-armature current and the speed-torque characteristics that on low armature current and low torque condition motor shows a tendency to rotate with dangerously high speed. Hence a d.c. series motor is used to start heavy loads and never used on no load. Many times a flywheel is used with d.c. series motor to avoid no load starting condition.

Q. 15 State the various applications of d.c. motors. [Refer section 4.16] AU: Dec.-08, 09

Q. 16 Draw the speed-current and torque-current characteristics of a d.c. series motor. [Refer section 4.14] AU: Dec, -03,04

Q. 17 Draw the speed-current and torque-current characteristics of a d.c. shunt motor. [Refer section 4.13] AU: Dec, -05

Q. 18 What is the need for starter in a d.c. motor?

AU: Dec.-06,18,19, May-17

Ans. : 

At the starting instant the speed of the motor is zero, (N= 0). As speed is zero, there cannot be any back e.m.f. as E_b ∝ N and N is zero at start.

E_b at start = 0

From voltage equation, at start V = I_a R_a

i.e. I_a = V /R_a ........... As E_b =0

Generally motor is switched on with normal voltage and as armature resistance is very small, the armature current at start is very high. Such high current drawn by the armature at start is highly objectionable hence the starter is required to limit the high starting current in case of d.c. motors.

Q. 19 Draw the neat diagram of three point starter. [Refer section 4.17.1]

Q. 20 State the function of no volt coil of the starter. [Refer section 4.17.1.1]

AU: Dec.-04, May-11

Q. 21Explain the action of overload release in three point starter. [Refer section 4.17.1.2]

Q. 22 Draw the neat diagram of four point starter. [Refer section 4.17.2]

Q. 23 Mention the factor affecting the speed of d.c. motor. AU : May-19

Ans. :

The factors affecting the speed of a d.c. motor are,

1. The flux ϕ 2. The voltage across the armature 3. The applied voltage V

Q. 24 List the different methods of speed control of d.c. shunt motor.

AU: May-04, 08, 09, 10

Ans : 

1. Changing the flux o by controlling the current through the field winding called flux control methods.

2. Changing the armature path resistance which grin turn changes the voltage applied across the armature called rheostatic control.

3. Changing the applied voltage called voltage control method.

Q. 25 State the advantages and disadvantages of flux control method used for controlling speed of d.c.shunt motor.

[Refer sections 4.20.1.1 and 4.20.1.2]

Q. 26 State the advantages and disadvantages of rheostatic control method used for controlling speed of d.c. shunt motor.

[Refer sections 4.20.2.2 and 4.20.2.3]

Q. 27 State the advantages and disadvantages of applied voltage control method used for controlling speed of d.c. shunt motor.

[Refer sections 4.20.3.1 and 4.20.3.2]

Q. 28 State the methods of speed control in d.c. series motor.AU: Dec.-12

Ans. : 

1. Rheostatic control: Controlling the armature voltage.

2. Applied voltage control: Controlling the voltage applied.

3. Flux control: In this, the various ways of controlling flux are, i) Flux divertor

ii) Armature divertor iii) Tapped field iv) Series-Parallel grouping of field coils.

Q. 29 Draw the setup of Ward-Leonard speed control method. [Refer section 4.22]

Q. 30 How does 4-point starter differ from 3-point starter? AU: May-07

Ans. : 

In three point, NVC is in series with the field winding while in four point starter NVC is connected independently across the supply through the fourth terminal called 'N' in addition to the 'L', 'F' and 'A'. Hence any change in the field current does not affect the performance of the NVC. Thus it is ensured that NVC always produce a force which is enough to hold the handle in 'RUN' position, against force of the spring, under all the operating conditions.

Q. 31 When you will say the motor is running at base speed? AU: Dec.-07

Ans. : 

The speed with which motor runs when its armature is excited at a rated voltage and the field winding carries maximum rated current, is called base speed of the motor. It is the rated speed of the motor which is mentioned on the name plate of the machine.

Q. 32 Draw the characteristics of d.c. compund motor. [Refer section 4.15]

AU: Dec.-09

Q. 33 Why d.c. series motor is not suitable for belt driven loads? AU: May-12

Ans. : 

For belt driven loads, there is possibility of breaking of a belt causing no load condition for the series motor. But on no load, d.c. series motor tries to run at dangerously high speed and may get damaged. To avoid such situation, d.c. series motor is not suitable for belt driven loads.

Q. 34 Write down the applications of d.c. series motor. AU: May-13

Ans. : 

The applications of d.c. series motor are, cranes, hoists, elevators, trolleys, conveyors and electrical locomotives.

Q. 35 Specify the techniques used to control the speed of DC shunt motor for below and above the rated speed? AU: May-15, Dec.-18

Ans. : 

For speed control of d.c. shunt motor below rated speed, rheostatic control method is used in which voltage across the armature is controlled. While for the speed control above rated speed, the flux control method is used in which current through field winding is controlled.

Q. 36 Why DC series motor is suited for traction applications? AU: May-15

Ans. : 

The series motor has characteristics that it produces high starting torque and it can not be started on no load. It has capability of handling overloads. Its speed control is easy. All these features are well suited for the electric traction hence series motor is used for traction applications.

Q. 37 Why DC series motor is called as variable speed motor?AU: Dec.-15

Ans. : In d.c series motor, the flux is proportional to I_a. Hence the speed N is inversely proportional to I_a as E_b is almost constant due to small voltage drop I_a (R_a + R_se). Thus N ∞ 1 / I_a Hence as load changes, I_a changes which causes large change in speed. Hence d.c. series motor is called variable speed motor.

Q. 38 Draw speed-torque characteristics of DC series motor. AU : May-16

Ans. :

For d.c. Series motor, T ∝I_a² and N ∝ I / I_a

Hence N ∞ 1 / √T

Thus the speed-torque characteristics are as shown in the Fig. 4.23.1

Q. 39 What is meant by plugging? AU : May-16

Ans. : Plugging is the method of braking the motors electrically. In this method, the direction of motor is suddenly reversed by reversing the armature connections of the motor. Thus motor tries to accelerate in other direction after coming to rest. An auxiliary device is used to cut off the supply as soon as motor comes to rest.

Q. 40 What are the applications of DC motor? (Refer section 4.16)  AU : May-17

Q. 41 List various method of starting D.C motor. AU: Dec. 16

Ans. : The methods of starting d.c. motor are,

i) Three point starter ii) Four point starter.

Q. 42 What is meant by dynamic braking in D.C motor?

AU: Dec.-16

Ans. : 

This is the method used to brake the motor and stop. In this method, the armature is simply disconnected from the supply and connected to a resistance, keeping field connected to the supply. All the kinetic energy of the moving armature gets converted to electrical and dissipated in the resistance connected. By varying value of this resistance, braking torque can be controlled.

Q. 43 What will happen to the speed of a D.C. motor when its flux approaches zero? AU: Dec.-17

Ans. :

For a d.c. motor N ∞ E_b / ϕ ∝ 1 /  ϕ as change in E_b is very small due to small values of armature resistance and series field resistance. Thus when flux approaches to zero, then speed approaches to infinity. Practically motor tries to run at dangerously high speed and may get damaged.

Q. 44 Mention the effects of differential compounding and cumulatively compound on the performance of D.C. compound motor. AU: Dec.-17

Ans: 

In cumulative compound motor, shunt field provides the definite flux and series flux helps shunt field flux to increase flux level as motor is loaded. Thus this motor is capable of producing large torque at low speeds and without any risk of high speeds on no load.

In differential compound type, as load increases, series flux increases and opposes the shunt flux which reduces the overall flux. Thus motor runs at higher speed as N ∝ (1 / ϕ). Thus speed increases as load increases and this property is dangerous. Hence differential compound motor is generally not used in practice.