THAPAR INSTITUTE OF ENGINEERING & TECHNOLOGY PATIALA
DEPARTMENT OF ELECTRICAL AND INSTRUMENATION ENGINEERING
UEE306 Electro-Mechanical Energy Conversion
TUTORIAL 4
(D.C. Motor Characteristics & Speed Control)
Course Instructors: Dr. Prasenjit Basak & Dr. Amit Kumar
Q1. A 230V, DC shunt motor, takes an armature current of 3.33 A at rated voltage and at a
no-load speed of 1000 rpm. The resistances of the armature circuit and field circuit are
respectively 0.3 Ω and 160 Ω. The line current at full load and rated voltage is 40 A.
Calculate at full load, the speed and the developed torque in case the armature reaction
weakens the no-load flux by 4%. [993.6 rpm, 80.95 Nm]
Q2. A 15 kW, 230V, 80A, 1000 rpm DC series motor has the following full load losses
expressed in percentage of motor input:
Armature circuit ohmic loss (including brush loss) = 2.8%
Field ohmic loss= 2.6%.
Rotational loss= 2.2%
Neglect the armature reaction and magnetic saturation and assume the rotational loss to
remain constant.
If the motor draws half the rated current at rated voltage, determine
(a) Speed in rpm and
(b) Shaft power output. [2055 rpm, 8547 W]
Q3. A 220 V DC shunt motor takes 22 A at rated voltage and runs at 1000 rpm. Its field
resistance is 100 Ω and armature circuit resistance (including brushes) is 0.1 Ω. Compute the
value of additional resistance required in the armature circuit to reduce that speed to 800 rpm;
when
(a) the load torque is independent of speed.
(b) The load torque is proportional to speed.
(c) The load torque varies as the square of the speed
(d) The load torque increases as the cube of the speed. [1.98 Ω, 2.5 Ω, 3.15 Ω, 3.96Ω]
Q4. A 4-pole DC series fan motor takes an armature current of 60 A, when running steadily
at 2000 rpm on a 220 V supply. The four field coils are now connected in two parallel groups
of two in series. Assuming that the flux produced is directly proportional to the exciting
current and the load torque increases as the square of the speed, find the new speed and the
armature current. Neglect losses and assume constant supply voltage. [2378.4 rpm, 100.9 A]
