Download Type Questions

UNIT – V
Objective Type
DC Machine
1. The induced emf in a DC M/C is proportional to:
(a) Speed of armature (b) field flux (c) armature current (d) both field
flux and speed (d) both field flux and armature current.
2. The commutator in DC M/C is used to (a) DC to AC (b) AC to DC (c) for
easy control
3. The electromagnetic torque developed in DC M/ C is dependent upon
(a) armature speed (b) armature current (c) field flux (d) armature current
and field flux both.
4. The speed of a DC Motor can be varied by (a) by varying supply voltage
(b) varying field current (c) either (a) or (b) (d) none of the above.
5. The direction of rotation of a DC series motor can be reversed by (a)
interchanging supply terminals (b) interchanging armature supply
terminals (c) interchanging field winding terminals (d) either (b) or (c)
6. The induced emf (back emf) has no relation with electromagnetic torque
(armature torque) in DC M/C (a) true (b) false (c) none
7. The polarity of induced emf in a generator can be reversed reversing the
direction of (rotation (b) field flux (c) rotation and field flux both (d)
either field flux or rotation
8. The speed of DC series motor is at no load is (a) 1500 rpm (b) 2000 rpm
(c) infinitely large (d) zero
Three Phase Induction Motor (IM)
1. The synchronous speed of a 6 pole IM at 50 Hz is (a) 1500 rpm (b)
750 rpm (c) 1000 rpm (d) non of these
2. The stator winding of an IM can be designed for (a) any number of
poles (b) odd No. of poles (c) even No. of poles
3. Slip of an IM at the time of starting is (a) Zero (b) unity (c) Infinity
(d) 0.05
4. The slip of an 4 pole, 50 Hz IM running at 1440 rpm is (a) 2% (b) 3%
(c) 4% (d) 6%
5. The direction of rotation of a 3 Phase IM can be reversed by
interchanging any two of the supply phases: (a) false (b) false
6. In a large IM generally the value of slip is (a) 0.2 % (b) 3 to 4% (c)
20% (d) 6 to 15%
7. There is no electrical connection between stator and rotor, still power
is transferred from stator to rotor through ( a) magnetic flux (b) air (c)
magnet (d) water
8. In an IM the rotor input is 600 W and slip is 4%. The rotor copper loss
is: (a) 600 W (b) 500 W (c) 24 W
9. The stator core of an IM is made of (a) laminated cast iron (b) mild
steel (c) silicon steel (d) wood
10.Slip rings of phase wound IM are made of: (a) wood (b) steel (c)
copper (d) cast iron
Single phase Induction Motor (SPIM)
1. The torque developed by a SPIM at synchronous speed is (a) positive
(b) negative (c) zero (d) non of these
2. SIPM is self starting motor: (a) true (b) false
3. Are capacitor start SPIM run motors s usually dual voltage motors:
(a) True (b) false.
4. Methods for speed control of SPIM is / are (a) high rotor resistance
(b) Reduced applied voltage to stator (c) both (a) and (b) (d) non of
these
5. Motor used in a ceiling fan is (a) split phase motor (b) capacitor run
motor (c) split phase capacitor type motor (d) non of these
Three phase Synchronous Machine
1. Synchronous speed of a 4 pole, 50 Hz motor is (a) 3000 rpm (b)
2500 rpm (c) 1500 rpm (d) 500 rpm
2. A synchronous motor can be run at any speed; (a) true (b) false
3. Synchronous motors are not self starting because (a) starters cannot
be used on these machines (b) starting winding is not provided on
these windings (c) the direction of rotation is not reversed (d) the
direction of instantaneous torque reverses after half cycle
4. A pony motor is basically a: (a) Dc series motor (b) DC shunt motor
(c) double winding ac /dc motor (d) small induction motor
5. A synchronous motor can be started by: providing damper winding
(b) pony motor (c) DC compound motor (d) any of these
6. The armature current in a synchronous motor will be least when PF:
(a) zero (b) unity (c) leading (d) lagging
7. Synchronous motor speed is controlled by varying (a) field
excitation (b) supply voltage (c) supply frequency only (d) both
supply voltage and frequency
8. What factors determine the number of poles of a synchronous motor;
(a) supply frequency (b) speed (c) both supply frequency and speed
Solution
DC Machine
3 Phase I/M
1 Phase I/M
3 Phase Syn M/c
1.d
1.c
1.c
1.c
2.b 3.c 4.c 5.d
2.c 3.b 4.c 5.b
2.b 3.a 4.c 5.c
2.b 3.d 4.d 5.d
6.b
6.b
7.d
7.a
8.c
8.c
6.b
7.c
8.c
9.a
10.c
Long / short Questions
DC Machine
1. Differentiate between (a) electric generator and motor (b) generated
and back emf.
2. Derive emf equation for DC M/c . A DC generator ha an armature emf
of 100V when the useful flux per pole is 20mWb and speed is
800rpm. Find the generated emf (a) with the same flux and speed of
1000rpm (b) with flux per pole of 24 mWb and speed of 900rpm.
3. Give symbolic representation of Different types of DC generators.
4. The armature of 6 pole, 600 rpm lap wound generator has 90 slots.
Each coil has 4 turns. Find the flux per pole.
5. What will be the change in emf induced if the flux is reduced by 20 5
and speed is increased by 20 %.
6. Give connection diagram of different types of DC motors indicating
the direction of various currents and voltages.
7. Derive the expression for torque in DC Series and Shunt motors .
8. Explain Speed vs Armature current, Torque vs Armature current, and
Torgue vs Speed characteristics of series and shunt motors.
9. A shunt generator delivers 50 Kw at 250 V when running at 400rpm.
Armature and field resistance are 0.02 and 50 ohms respectively.
Calculate the speed of the M/c when running as shunt motor and
taking 50 Kw input at 250V.
10. What are the various applications of DC series and shunt motors.
Three Phase Induction Motor (IM)
11.A 3 phase, 50 Hz, IM motor is running at 1460 rpm. Find the (a)
Synchronous speed (b) % Slip and rotor frequency.
12.A 3 phase, 50Hz, IM has a full load sped of 960 rpm, Find (a) % slip
(b) No. of poles (c) rotor emf frequency (d) speed of rotor field w.r.t.
rotor structure (e) speed of rotor field w.r.t. stator structure (f) speed
of rotor field w.r.t. stator field.
13.A 3 phase delta connected 440V, 50 Hz, IM has a rotor stand still emf
of 130V per phase. If the motor is running at 1440 rpm , Calculate (a)
% slip (b) rotor emf frequency (c) stator to rotor turn ratio
14.Draw typical slip-torque characteristics of (a) cage rotor type IM (b)
Wound rotor type with R1 and R2 when R1> R2
15.In a wound rotor type IM what is the effect of rotor resistance on
Starting torque, max torque and slip at FL
Single phase Induction Motor (SPIM)
1. SIPIM is not self starting. Explain it.
2. Explain why theoretical SPIM has no starting torque. Explain any
two methods used to produce starting torque in SPIM.
3. Compare 3 phase IM with SPIM on the basis of the following (a)
Starting torque (b) slip-torque characteristics (c) PF (d) Applications.
Three phase Synchronous Machine
9. Describe main constructional feature s of cylindrical and salient pole
alternator.
10.Explain principle of operation of a 3 phase alternator.
11.Why 3 phase syn motor is not self starting type. Explain any one
method to start syn motor.
12.Explain the following (a) Syn motor always run on synchronous
speed
(b Field winding is placed on rotor.
13.What is the effect of change in excitation on PF of syn motor.