Assignment-6(D.C. Motors)
1. (A).Explain the principle
of operation of DC motors.
(B). the armature of a 6 – pole, 6 circuit DC shunt
motor takes 300 A at speed of 400 rpm. The flux per pole 75 mWb. The number of
armature turns is 500. The torque lost in windage, friction and iron losses can
be assumed as 2.5 %. Calculate
i) The torque developed by the armature
ii) The shaft torque.
2. (A) Derive
the torque equation of a DC motor.
(B) A 200V DC shunt motor takes a
total current of 100 A and runs at 750 rpm. The resistance of the armature
winding and shunt field winding is 0.1 ohms and 40 ohms respectively. Find the
torque developed by the armature.
3. (A) what is the significance of back emf generated in a DC machine
hence explain the principle of operation of a DC motor. (or)
Explain how back emf makes d.c motor self
regulating.
( B) A
100V series motor takes 45 A when running at 750 rpm. Its armature resistance
is 0.22 ohms, while the series field resistance is 0.13 ohms. Iron and
frictional losses amounts to 750 W.Find the shaft power.
4. (A) Draw the power flow diagram of a DC shunt
motor and also explain the following terms
i. Electrical Efficiency,
ii. Mechanical Efficiency and
iii.
Commercial Efficiency.
(B) A 6 pole DC motor has a wave connected
armature with 87 slots, each slot containing 6 conductors. The flux per pole is
20mwb and the armature has a resistance of 0.13ohm when the motor is connected
to 240V supply and the armature draws a current of 80A driving a load
of15KW.caluculate (i)speed (ii)Armature
torque and (iii)shaft torque
5.(A) Write the applications of
Dc motors.
(B) A 460V D.C series motor runs at 1000RPM
taking a current of 40A.caluculate the speed and percentage change in torque if
the load is reduced so that the motor is taking 30A.total resistance of the
armature and field circuits is 0.8Ω.Assume ‘Ø’ is proportional to filed
current.
6. (A) Explain the losses in D.C. Motor (b) What happens if D.C series motor is
started on no load?(C) why series motors are applicable for traction
applications