QUESTION BANK 17EE44
MODULE 1
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Derive the power equation and condition for maximum power for a DC Motor.
Derive an expression for the torque of a DC motor.
Explain the characteristics of i) DC Shunt motor ii) DC Series Motor iii) DC Compound Motor.
Explain the methods of Speed control for DC Shunt Motor.
Explain Ward Leonard System of Speed Control.
What is Starter? Explain the necessity of starter for a DC motor.
Explain the operation of three-point starter.
Explain the operation of four-point starter.
Enumerate the various losses in a DC machine. Which of these losses are i) Constant ii)
Proportional to Current iii) Proportional to current square.
With the help of block diagram, explain different stages of power in a DC Machine. How the
losses are varied?
Explain the power flow diagram in DC Machine as generator and as motor.
Define Electrical Efficiency of Generator and Derive the condition for maximum efficiency for DC
Generator.
Define Electrical Efficiency of Motor and Derive the condition for maximum efficiency for DC
Motor.
Explain the action of no-volt coil and over load release coil.
A 230 V DC Shunt motor with an armature resistance of 0.4Ω is excited to give constant field.
The motor runs at 500 rpm at full load and takes armature current of 30A. If resistance 1.1Ω is
placed in the armature circuit, find the speed of the motor at i) Full Load Torque ii) 1.5 Times
Full load torque.
A 230 V DC Shunt motor runs at 800 rpm and takes current of 50A. Find resistance to be added to
the field circuit to increase speed from 800 rpm to 1000rpm at an armature current of 80A.
Assume flux proportional to field current. Armature resistance = 0.15Ω and field resistance = 250Ω
A 200V DC Shunt motor takes 22 amps at rated voltage and runs at 1000rpm. Its field resistance
is 100Ω and armature circuit resistance including brushes is 0.1Ω. Compute the value of additional
resistance required in the armature circuit to reduce the speed to 800 rpm when i) The torque is
proportional to the speed ii) The load torque varies as the square of the speed.
A 4 pole DC series motor has an armature resistance of 0.1Ω Its field winding is divided in to two
groups, each having resistance of 0.01Ω. When the two groups are in series and supply voltage is
110V, the current drawn by the motor is 50A, running at 700 rpm. Find the speed if its field groups
are connected in parallel. Assume load torque constant and the flux proportional to the field
current.
A DC shunt machine when run as a motor on no load takes 440W and runs at 1000rpm. The field
current and armature resistance are 1 A and 0.5 Ω respectively. Calculate the efficiency of the
machine when (i) running as generator delivering 40A t 220V and (ii) as a motor taking 40A from
a 220V supply.
A 4 pole 250 V series motor has a wave connected armature with 1254 conductors. The flux per
pole is 22 mWb when the motor is taking 50A. Iron and Friction losses amount to be 1KW.
Armature resistance is 0.2Ω and series field resistance is 0.2Ω. Calculate (i) the Speed (ii)the b.h.p.
(iii) the shaft Torque. (iv) The efficiency at this load.
QUESTION BANK 17EE44
REVATI C M
QUESTION BANK 17EE44
MODULE 2
1. Explain how to obtain efficiency of a DC Machine by conducting brake test. State its advantages
and disadvantages.
2. Explain the retardation test on DC Machines.
3. Explain how the moment of inertia of a DC Machine can be estimated.
4. Describe the method of Hopkinson’s test for two identical shunt motors indicating how the
efficiency of each machine on full load is obtained.
5. Explain how will you obtain the efficiency of a DC Series machine by conducting the field test.
6. With a neat diagram, explain the importance and procedure of conduction Swinburne’s test on
DC Motor.
7. A 500V shunt motor takes 4 amps on no load. The armature resistance including that of brushes
is 0.2 Ω and field current is 1 Amps. Estimate the output and the efficiency when the input
current is i) 20Amp ii) 100 Amps.
8. In brake test conducted on a DC shunt motor the full load reading are observed as, Tension on
tight side = 9.1 Kg, Tension on Slack side = 0.8 Kg, Total current = 10 A. Supply voltage = 110V,
Speed =1320 rpm, the radius of brake drum is 7.5 cm. Calculate the efficiency at full load.
9. List the advantages of this test. Show the efficiency as i) Motor and ii) Generator can be
predetermined.
10. Define the term slip of an induction motor. Explain its significance.
11. A 4 pole, 3 phase 50 HZ star connected induction motor has a full load slip of 4%. Calculate full
load speed of the motor.
12. Explain the effect of slip on the following rotor parameters:
(i)frequency (ii)Induced emf (iii) Current (iv) Power factor v) reactance vi) impedance.
QUESTION BANK 17EE44
REVATI C M