THAPAR INSTITUTE OF ENGINEERING & TECHNOLOGY PATIALA
DEPARTMENT OF ELECTRICAL AND INSTRUMENATION ENGINEERING
UEE301 Direct Current Machines and Transformers
TUTORIAL 5: Speed Control and Starting of D.C. Motors
Course Instructors: Dr. Prasenjit Basak, Dr. Amit Kumar
Q1. A 230 V DC shunt motor has an armature circuit resistance of 0.4 Ω and field resistance
of 115 Ω. This motor drives a constant torque load and takes an armature current of 20 A at
800 rpm. If the motor speed is to be raised from 800 to 1000 rpm, find the resistance that
must be inserted in the shunt field circuit. Assume magnetization curve to be a straight line.
[29.93 Ω]
Q2. A DC shunt motor takes 50 A on full load from 250 V mains. Its speed is to be raised by
40% by weakening of the field flux. If the torque at raised speed is 20% more than that at the
initial speed, find the percntage change in filed field flux. The armature resistance (including
brushes) is 0.5 Ω. [35.3%]
Q3. (a) Explain what would happen if the DC motor is directly switched on to the supply
without any starter.
(b) Explain the functionof no-volt release in a three-point starter.
(c) Explain the advantages of four-point starter over the three-point starter.
(d) What is the difference between a starter and a controller?
(e) What would happen if the external resistance for starting of the DC motor is left in the
armature circuit?
