# Model Questions on different topic of Electrical Machines.

Monday, February 3, 2020

**Edit****Question Bank**

Subject_ Electrical Machines

Subject_ Electrical Machines

1.Derive from first principle , the e.m.f. generated of DC
generator.

2. Obtain the torque equation of DC motor

**.****3.**Explain the construction of DC machine.

**4.**Show the power flow diagram of DC motor.

5. What is back e.m.f. ? Explain the significance of back
e.m.f.

6. Explain speed control of a D.C. series motor
and D.C. shunt motor.

7.Write short notes on : (

*i*) Interpole (*ii*) Compensating winding.
8. Draw and explain the Torque-Armature Current,
Speed-Current and Torque-Speed characteristics of (a)DC
series motor and (b) DC shunt motor.

9. What is the necessity of starter and explain three point
starter and four point starter ?

10.Compare lap winding and wave winding with neat diagram.

11. Explain construction and working of brushless DC motor.
State its advantages, disadvantages and applications.

12.Explain construction and working of stepper motor(permanent magnet type). Draw and explain briefly its performance
characteristics.

13.Explain construction and working of Universal motor. State
its advantages, disadvantages and applications.

14.Explain working of Servomotor motor.
State its advantages, disadvantages and applications.

15.
A 200 V, d.c shunt machine has an armature resistance of 0.5 Ω and field
resistance of 200 Ω. The machine is running at 1000 rpm as a motor drawing 31 A
from the availability mains. Calculate the speed at which the machine must be driven
to realize this as generator.

16.A
DC shunt motor runs at a speed of 1,000 rpm on no load taking a current of 6
Amp from supply, when connected to 220 V dc supply. Its full-load current is 50
Amp. Calculate its speed on full-load. Assume armature resistance

*= 0.5Ω and shunt winding resistance**= 110 Ω.*
17.
A DC series motor is run on a 220 V circuit with a regulating resistance of Rw for
speed adjustment. The armature and field coils have a total resistance of 0.5 Ω
on a certain load with R = zero, the current is 20 A and speed is 1200 rpm.
With another load and R = 4 Ω, the current is 19A. Find the new speed and also
the ratio of the two values of the power output of the motor. Assume the field
strength at 19 A to be 80% of that at 20 A.

18.
DC shunt motor runs at a speed of 1,000 rpm on no load taking a current of 6
Amp from supply, when connected to 220 V dc supply. Its full-load current is 50
Amp. Calculate its speed on full-load. Assume armature resistance

*= 0.5Ω and shunt winding resistance**= 110 Ω***19.**A 20-HP (14.92 kW), 230 V, 1150 rpm, 4 pole, d.c. shunt motor has a total conductors arranged in two parallel paths is 620 and yielding an armature circuit resistance of 0.2 Ω. When it supply rated power at rated speed it draws a line current of 74.8 A and a field current of 3 A. Calculate the :(

*i*)Flux per pole(

*ii*) Torque developed (

*iii*) Rotational losses(

*iv*)Total losses

**Question Bank**

**Subject_ Electrical Machines**

**(**

**THREE PHASE INDUCTION MOTORS**

**)**

1.Derive the condition for maximum torque under
running condition of three phase induction motor and hence proved (a)T

_{fl}/T_{max}= 2as/(a^{2}+s^{2}) (b) Tst/Tmax= 2a/(1+a^{2}) where a=Sm
2.Find the relation between rotor gross output power
,rotor copper loss and rotor input power.

3.With neat diagram explain the
construction and working of 1 phase split phase induction motor. State its
applications.

4.With neat diagram, explain
construction and working of shaded pole induction motor. Draw its torque-speed
characteristics. State applications of this motor.

5.Explain
the theory for production of rotating field by 3-phase current fed to
symmetrical 3-phase winding.

6. Distinguish between squirrel cage and phase wound induction motor.

6. Distinguish between squirrel cage and phase wound induction motor.

7.Explain
torque slip characteristics of three
phase induction motor.

8.Explain in detail rotor
resistance starter used for three phase
induction motor.

9.What
are different types of Induction motor starter ? Draw and explain Star-delta
starter.

10.Write short note: Power flow
diagram for three phase induction motor.

11.Why are starters necessary for starting
3-phase induction motor ?Also write a short note on auto-transformer starter.

12.Explain
the construction and working of Linear Induction Motor state its applications.

13.
A 3-phase, 6-pole, 50 Hz induction motor develops a shaft torque of150 N-m at
full load slip 4%. The rotating losses are 300W and that of the starter losses
are 1.5W calculate (

*i*) rotor copper loss (*ii*) rotor gross output (*iii*) efficiency at full load.
14.
A 3-phase, 6-pole, 50 Hz induction motor takes 60 A at full- load speed of 940
r.p.m. and develops a torque of 150 N-m. The starting current at rated voltage
is 150 A. What is the starting torque ? If a star/delta starter is employed,
determine the torque and current during starting

**.**

**15.**A 4 pole, 50 Hz, 3-phase induction motor has rotor resistance per phase of 0.03 Ω and reactance of 0.12 Ω per phase. Find speed at maximum torque ? Also find the value of external resistance per phase required to be inserted to obtain 75 percent of Tmax at start.

16.
A 750 kW,50 Hz, 8 pole, 3-phase induction motor has a rotor impedance of
(0.2 +

*j*0.15) Ω at standstill. Full torque is obtained at 360 r.p.m. determine : (*a*) The ratio of maximum to full-load torque (*b*) The speed of maximum torque and (*c*) The rotor resistance to be added to get Tmax at starting.
17.The
power input to a 400 V, 60 Hz, 6 pole, 3-phase induction motor running at 1140
rpm is 40 kW at 0.8 p.f. lag. Stator loss and friction and
windage loss are 1kW and 2 kW respectively. Find-:(

*a*) rotor copper loss (*b*) input current (c) efficiency

**Question Bank**

**Subject_ Electrical Machines**

1) Explain Operation of 3-phase
Synchronous motor with constant excitation and variable load.

2) Explain Operation with 3-phase
Synchronous motor constant load and
variable excitation with suitable phasor
diagram and graph.

3) Draw and
explain Equivalent Circuit and Phasor Diagram of a Synchronous Motor.Write
Applications of 3-ph synchronous motors.

4) A 400 V,
7.46 KW 3 phase synchronous motor has negligible armature resistance and has synchronous
reactance of 10 Ω /phase.
Determine the minimum armature current and compounding induced emf for full
load conditions. Assume efficiency at full load = 85%.

5) Compare 3 phase synchronous motor with 3 phase
induction motor.

6) Explain any two methods of starting 3 phase synchronous
motor. State applications of 3 phase synchronous motor.

7)The efficiency of 3-phase , 400V star
connected synchronous motor is 95% and it takes 24A at full load unity power
factor. what will be induced emf and total mechanical power developed at full
load and 0.9 power factor leading? The synchronous impedance of the motor is
(0.2+2j) ohm/phase.

8) A 50 HP 410 V 50 Hz 3 phase star connected
synchronous motor has full load efficiency of 85%. The synchronous impedance of
the motor is (0.2 + j 2.0) Ω per phase. If the excitation of the motor is adjusted
to give leading p.f. of 0.85. Calculate : I) Full load induced emf line value.

II) Mechanical power developed.

9) Describe the phenomenon of hunting in synchronous
motor. Explain how it can be reduced.

10)Explain torque angle characteristics of synchronous
motor.

11)Explain role played by damper winding incase of
synchronous motor during a) starting
condition b)Running condition c)Load changes under running condition

12) Explain 'V' curves and inverted 'V' curves in case
of three phase synchronous motor.

13)A 1492 kW ,upf,3-phase
,star-connected ,2300 V, 50 Hz, synchronous motor has a synchronous reactance
of 1.95 ohm/phase. Find the Tmax in N-m which this motor can deliver
if it is supplied from a constant frequency source and if the field excitation
is constant at the value which would end in unity power factor at rated
load. Assume that the synchronous motor is of cylindrical rotor type. Neglect all losses.

**Question Bank**

**Subject_ Electrical Machines**

1) Explain construction detail and V-I characteristics of SCR.

2) Explain switching action and power loss of SCR during ON & OFF
condition.

3) Define following term in SCR (a) Latching current and (b) Holding
current

4) Draw two transistor analogy of SCR and derive anode current equation.

5) Explain forward blocking, conduction and reverse blocking mode of
operation of SCR.

6) What are different Turn ON method of SCR. Explain any two method.

7) What is commutation of SCR. Explain any two method of
commutation.

8) Explain construction detail and working of DIAC.

9)Explain in detail V-I characteristics of DIAC. Also write its
application

10) Explain construction detail and working of TRIAC.

11) Explain V-I characteristics of TRIAC. Also write its application.

12) Explain construction detail and different characteristics (Output and
Transfer) of MOSFET

13) Explain different region of operation of MOSFET. Also write its
different application.

14)Explain construction and working of E-channel MOSFET and P
-channel MOSFET in detail. Also write its different application.

15) Explain construction detail, working and different
characteristics (Output and Transfer) of IGBT

16) Explain construction detail, Working and V-I characteristics of GTO.

17)Compare MOSFET, IGBT and Power transistor with neat symbols.

18) Compare SCR,TRIAC and GTO with neat symbols.

**Question Bank**

**Subject_ Electrical Machines**

**(DRIVES AND SPEED CONTROLS)**

1) What is drives? Explain with suitable block diagram.

2) State advantages of Electrical drives.

3) What are different applications of electric drive.

4) Compare AC drive and DC drive.

5) Compare individual drive and group drive.

6) Explain different selection criteria of electrical drives

7) Explain speed control of 1 phase phase fully controlled
converter fed separately excited DC motor.

8) Explain the operation of three phase full controlled
converter fed separately excited DC motor.

9) Explain operation of Single-phase full controlled converter drive with
suitable diagram and waveform?

10) What is chopper drive? Explain Two Quadrant Chopper Drive with
suitable diagram?

11) Explain operation of three-phase full converter drive with suitable
diagram and waveform?

12) What is chopper drive? Explain four Quadrant Chopper Drive with
suitable diagram?

Explain different modes of operation of 4 quadrant drive.

13) Explain following speed control methods of 3 phase induction motor :

a) Stator voltage control method b) V/f control method c)
frequency control method

14) Explain V/f method of speed control of 3 phase induction motor. Why
the ratio V/f is to be kept constant?