ASSIGNMENT 1
DC Machines
Date of submission: 13th September, 2023
Venue: F105 during the Lecture class.
1) Design a long shunt compound-wound DC generator that delivers a rated voltage of 220V at full load
(100A). The series field winding has a resistance of 0.02 ohms, and the shunt field winding has a resistance
of 100 ohms. Determine the number of turns for both the series and shunt field windings.
2) A DC generator has the following data: Armature resistance, Ra= 0.2 ohms, Shunt field resistance, Rsh=
250 ohms, Rated voltage, V= 400V, Flux per pole, Φ= 0.03 Weber, Armature current, I = 80 A. Calculate
the voltage regulation and the percentage drop in terminal voltage when the generator is loaded from noload to full-load.
3) In a 4-pole DC shunt motor, the rated voltage is 220V, and the rated current is 40A. The field resistance
is 400 ohms, and the armature resistance is 0.2 ohms. Determine the speed of the motor when it is running
at full load, and the mechanical power developed.
4) A separately excited dc generator has OCC data at 1200 rmp as below.
VOC
10 54 160 196 248 284 300 360
If (A) 0 1 2 3 4 5 6 7
At If = 6 A when loaded at Ia = 50 A, it has a terminal voltage of 230 V at speed of 1200 rpm. Given: Ra
= 0.5 W. (a) Determine the armature voltage drop due to the demagnetizing effect of armature reaction.
(b) Assuming that this voltage drop in proportional to the square of armature current, determine the field
current for a terminal voltage of 230 V at armature current of Ia = 40 A
5) A series motor has an armature resistance of 0.7 ohms and field resistance of 0.3 ohms. It takes a current
of 15 A from a 200 V supply and runs at 800 rpm. Find the speed at which it will run when a 5 ohms
resistance is added in series to the motor and it takes the same current at the supply voltage.
6) A 480 V, 20 kW shunt motor took 2.5 A when running light. For an armature resistance to be 0.6 ohms,
field resistance of 800 ohms and brush drop of 2 V, find the full-load efficiency
7) The Hopkinson’s test on two machines gave the following results for full-load: Line voltage 250 V; line
current, excluding field current, 50A; motor armature current 380 A; field currents 5 A and 4.2A.
Calculate the efficiency of each machine. Armature resistance of each machine = 0.02 ohms. State the
assumptions made.
8) A starter is to be designed for a 10 kW, 250 V shunt motor. The armature resistance is 0.15 ohms. This
motor is to be started with a resistance in the armature circuit so that during the starting period the armature
current does not exceed 200% of the rated value or fall below the rated value. That is, the machine is to
start with 200% of armature current and, as soon as the current falls to rated value, sufficient series
resistance is to be cut out to restore current to 200% (or less in the last step). The process is to be repeated
till all the resistance is cut out. (a) Calculate the total resistance of the starter. (b) Also calculate the
resistance to be cut out in each step in the starting operation
9) A 200-V, 14.92 kW dc shunt motor when tested by the Swinburne method gave the following results:
Running light : armature current was 6.5 A and field current 2.2 A. With the armature locked, the current
was 70 A when a potential difference of 3 V was applied to the brushes. Estimate the efficiency of the
motor when working under full-load conditions.
10) A shunt-wound motor runs at 600 r.p.m. from a 230-V supply when taking a line current of 50 A. Its
armature and field resistances are 0.4 Ω and 104.5 Ω respectively. Neglecting the effects of armature
reaction and allowing 2 V brush drop, calculate (a) the no-load speed if the no-load line current is 5A (b)
the resistance to be placed in armature circuit in order to reduce the speed to 500 r.p.m. when motor is
taking a line current of 50 A (c) the percentage reduction in the flux per pole in order that the speed may
be 750 r.p.m. when the armature current is 30 A with no added resistance in the armature circuit.