ENT323 Semester 2 2017/2018
TUTORIAL 3 (ANSWER)
Synchronous Machines
1. A motor generator set used for providing variable frequency a.c. supply consists of a three-phase,
10-pole synchronous motor and a 24-pole, three- phase synchronous generator. The motorgenerator set is fed from a 25 Hz, three-phase a.c. supply. A 6-pole, three phase induction motor is
electrically connected to the terminals of the synchronous generator and runs at a slip of 5%.
Determine :
(i) the frequency of the generated voltage of the synchronous generator.
(ii) the speed at which the induction motor is running.
(iii) frequency of the rotor currents
2. A 500-V, 1-phase synchronous motor gives a net output mechanical power of 7.46 kW and
operates at 0.9 p.f. lagging. Its effective resistance is 0.8 Ω. If the iron and friction losses are 500 W
and excitation losses are 800 W, estimate the armature current. Calculate the commercial efficiency.
3. A synchronous motor having 40% reactance and a negligible resistance is to be operated at rated
load at (i) u.p.f. (ii) 0.8 p.f. lag (iii) 0.8 p.f. lead. What are the values of induced e.m.f. ? Indicate
assumptions made, if any.
4. A single phase alternator has armature impedance of 0.5 + j 0.866. When running as a
synchronous motor on 200 V supply, it provides a net output of 6 kW. The iron and friction losses
amount to 500 W. If current drawn by the motor is 50 A, evaluate the two possible phase angles of
current and two possible induced EMF.
5. A 1−φ, synchronous motor has a back e.m.f. of 250 V, leading by 150 electrical degrees over the
applied voltage of 200 volts. The synchronous reactance of the armature is 2.5 times its resistance.
Find the power factor at which the motor is operating and state whether the current drawn by the
motor is leading or lagging.
6. A 1500 kW, 4600 V, 600 rpm, 60 Hz synchronous motor possesses a synchronous reactance of 16
and a stator reactance of 0.2 per phase. The excitation voltage Eo is 2400 V and the moment of
intertia of the motor and its load is 275 kgm2. We wish to stop the motor by short circuiting the
armature while keeping the dc rotor current fixed.
Calculate:
a. The power dissipated in the armature at 600 rpm
b. The power dissipated in the armature at 150 rpm
c. The kinetic energy at 600 rpm
d. The kinetic energy at 150 rpm
e. The time required for the speed to fall from 600 rpm to 150 rpm
Special Machines
1. A stepper motor advances 2.5° per step. How many pulses are needed to complete 8 revolutions?
2. The two armatures on a hybrid stepper motor each have 50 salient poles (teeth). Calculate the
following:
(i) The angle between two successive teeth on an armature
(ii) The angle between one tooth on one armature and the next tooth on the other armature
(iii) The angle of advance per pulse
3. A stepper motor advances 1.8° per impulse and its slew rate is limited to 1200 pulses per second.
We wish to drive a machine tool at a speed of 500 rpm. Can this objective be achieved by coupling
the directly to the motor? If not, can you suggest a solution?
Electrical Drives and Control
1. A half-wave rectifier circuit employing an SCR is adjusted to have a gate current of 1mA. The
forward breakdown voltage of SCR is 100 V for Ig = 1mA. Assume load resistance = 100Ω and the
holding current to be zero. If a sinusoidal voltage of 200 V peak is applied, find :
(i) firing angle (ii) conduction angle (iii) average current.
2. An SCR full-wave rectifier supplies to a load of 100 Ω. If the peak a.c. voltage between centre tap
and one end of secondary is 200V, find (i) d.c. output voltage and (ii) load current for a firing angle of
60°.