Q1: A single-phase full-controlled thyristor converter bridge is used for regenerative braking of a separately excited DC motor with the following specifications:
Assume that the motor is running at 600rpm and the armature terminals of the motor are suitably reversed for regenerative braking. If the armature current of the motor is to be maintained at the rated value, the triggering angle of the converter bridge in degrees should be _____ (rounded off to 2 decimal places). (2024)
(a) 114.58
(b) 152.36
(c) 108.25
(d) 145.35
Ans: (a)
Sol: Rating of DC machine
210 V, 10 A, 1200 rpm,
Armature terminals reversed
∴ Eb → Negative
Inv for regenerative braking
Q2: A shunt-connected DC motor operates at its rated terminal voltage. Its no-load speed is 200 radian/second. At its rated torque of 500 Nm, its speed is 180 radian/second. The motor is used to directly drive a load whose load torque TL depends on its rotational speed ωr (in radian/second), such that TL = 2.78 × ωr. Neglecting rotational losses, the steady-state speed (in radian/second) of the motor, when it drives this load, is _______. (SET-2 (2015))
(a) 120
(b) 180
(c) 240
(d) 285
Ans: (b)
Sol: After neglecting rotational losses,
Q3: The separately excited dc motor in the figure below has a rated armature current of 20 A and a rated armature voltage of 150 V. An ideal chopper switching at 5 kHz is used to control the armature voltage. If La = 0.1mH, Ra = 1Ω, neglecting armature reaction, the duty ratio of the chopper to obtain 50% of the rated torque at the rated speed and the rated field current is (2013)
(a) 0.4
(b) 0.5
(c) 0.6
(d) 0.7
Ans: (d)
Sol: For half the rated torque,
Q4: A 3-phase squirrel cage induction motor supplied from a balanced 3-phase source drives a mechanical load. The torque-speed characteristics of the motor(solid curve) and of the load(dotted curve) are shown. Of the two equilibrium points A and B, which of the following options correctly describes the stability of A and B ? (2009)
(a) A is stable, B is unstable
(b) A is unstable, B is stable
(c) Both are stable
(d) Both are unstable
Ans: (a)
Sol: Let torque developed by the motor = Tm
Load torque =TL
Accelerating torque, Ta = Tm − TL
At point A,
So, no accelerating , hence the motor rums at speed NA.
When a smal disturbance decreases speed to Nc
At point C,
So, motor will accelerate and comes to point A. When the disturbance increases speed to ND.
At point D,
Motor will decelerate and comes at point A
So, motor is stable at point A.
When the motor is operating at point B.,
If a small disturbance decreases speed to NE.
Motor will decelerate and motor speed will keep on decreasing.
If the disturbance increases speed to NF.
At point F,
Motor will accelerate and motor speed will keep on increasing.
So, motor is unstable at point B.
Q5: A single phase fully controlled converter bridge is used for electrical braking of a separately excited dc motor. The dc motor load is represented by an equivalent circuit as shown in the figure.
Assume that the load inductance is sufficient to ensure continuous and ripple free load current. The firing angle of the bridge for a load current of I0 = 10A will be (2008)
(a) 44°
(b) 51°
(c) 129°
(d) 136°
Ans: (c)
Sol: Average output voltage of the converter,
Applying KVL,
Q6: A three-phase, 440 V, 50 Hz ac mains fed thyristor bridge is feeding a 440 V dc, 15 kW, 1500 rpm separately excited dc motor with a ripple free continuos current in the dc link under all operating conditions, Neglecting the losses, the power factor of the ac mains at half the rated speed is (2007)
(a) 0.354
(b) 0.372
(c) 0.9
(d) 0.955
Ans: (a)
Sol: For a separately excited dc motor
back emf = Ea = V0 − IaRa
Since, losses are neglected Ra can be neglected
So,
At rated voltage V0 = 440V and N = 1500rpm, so, at half the rated speed (N/2) = 750 rpm output voltage of the bridge (V0) is 220V.
if Ia is the average value of armature current rms value of supply current will be
Power delivered to the motor
P0 = V0Ia
Input VA to the thyristor bridge
Q7: A solar cell of 350 V is feeding power to an ac supply of 440 V, 50 Hz through a 3-phase fully controlled bridge converter. A large inductance is connected in the dc circuit to maintain the dc current at 20 A. If the solar cell resistance is 0.5 Ω, then each thyristor will be reverse biased for a period of (2006)
(a) 125°
(b) 120°
(c) 60°
(d) 55°
Ans: (d)
Sol: Solar cell emf = 350V
DC current Idc = 20A
Solar cell resistance
The bridge acts as inverter,
Output voltage of 3 − ϕ fully controlled bridge
Therefore, each thyristor will be reverse biased for a period of 55°.
Q8: The speed of a 3-phase, 440 V, 50 Hz induction motor is to be controlled over a wide range from zero speed to 1.5 time the rated speed using a 3-phase voltage source inverter. It is desired to keep the flux in the machine constant in the constant torque region by controlling the terminal voltage as the frequency changes. The inverter output voltage vs frequency characteristic should be (2006)
(a) (b) (c) (d) Ans: (d)
Sol: For a 3 − ϕ induction motor, stator voltage per phase is given by
To keep the ϕ constant, constant V/f control is applied and voltage (V1) varies linearly with frequency (f1). For frequency above 50 Hz, such control is not possible because voltage can not be increased above rated voltage. Therefore, for frequency above 50 Hz, voltage is kept constant, During this control induction motor is said to be working in field weakling mode.
Q9: An electric motor, developing a starting torque of 15 Nm, starts with a load torque of 7 Nm on its shaft. If the acceleration at start is 2 rad/sec2 , the moment of inertia of the system must be (neglecting viscous and coulomb friction) (2005)
(a) 0.25 kgm2
(b) 0.25 Nm2
(c) 4 kgm2
(d) 4 Nm2
Ans: (c)
Sol: Ts = Starting torque developed by the motor =15 N-m
TL= Load torque = 7 N-m
Ta = Accelerating torque =Ts − Tl = 15 - 7 = 8 N-m
α = Acceleration = 2rad/sec2
Ta = Iα
I = Moment of inertia
Q10: A variable speed drive rated for 1500 rpm, 40 Nm is reversing under no load. Figure shows the reversing torque and the speed during the transient. The moment of inertia of the drive is (2004)
(a) 0.048 kg m2
(b) 0.064 kg m2
(c) 0.096 kg m2
(d) 0.128 kg m2
Ans: (a)
Sol: Speed changes from -1500 rpm to 500 rpm in 0.5 sec
So angular acceleration,
Q11: A single-phase half-controlled rectifier is driving a separately excited dc motor. The dc motor has a back emf constant of 0.5 V/rpm. The armature current is 5A without any ripple. The armature resistance is 2 Ω. The converter is working from a 230V, single-phase ac source with a firing angle of 30°. Under this operating condition, the speed of the motor will be (2004)
(a) 339 rpm
(b) 359 rpm
(c) 366 rpm
(d) 386 rpm
Ans: (a)
Sol: where,
kb = Back-emf constant = 0.25 V/rpm
Average output voltage of 1 − ϕ half controlled rectifier = V
Assume that the motor is running at 600rpm and the armature terminals of the motor are suitably reversed for regenerative braking. If the armature current of the motor is to be maintained at the rated value, the triggering angle of the converter bridge in degrees should be _____ (rounded off to 2 decimal places). (2024)
Rating of DC machine
Armature terminals reversed
After neglecting rotational losses,
(a) 0.4
For half the rated torque,
(a) A is stable, B is unstable
Let torque developed by the motor = Tm
So, no accelerating , hence the motor rums at speed NA.
So, motor will accelerate and comes to point A. When the disturbance increases speed to ND.
Motor will decelerate and comes at point A
If a small disturbance decreases speed to NE.
Motor will decelerate and motor speed will keep on decreasing.
Motor will accelerate and motor speed will keep on increasing.
Assume that the load inductance is sufficient to ensure continuous and ripple free load current. The firing angle of the bridge for a load current of I0 = 10A will be (2008)
Applying KVL,
For a separately excited dc motor
At rated voltage V0 = 440V and N = 1500rpm, so, at half the rated speed (N/2) = 750 rpm output voltage of the bridge (V0) is 220V.
Power delivered to the motor
The bridge acts as inverter,
Therefore, each thyristor will be reverse biased for a period of 55°. 
(b) 
(c) 
(d) 
Ans: (d)
To keep the ϕ constant, constant V/f control is applied and voltage (V1) varies linearly with frequency (f1). For frequency above 50 Hz, such control is not possible because voltage can not be increased above rated voltage. Therefore, for frequency above 50 Hz, voltage is kept constant, During this control induction motor is said to be working in field weakling mode.
(a) 0.048 kg m2
where,
