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Department of Electrical & Electronics Engineering
EE 2251 Electrical Machine – I
Problems
DC Generator
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A 4 pole dc generator running at 1 500 rpm has an armature with 90 slots and 6 conductors per slot.
The flux per pole is 10 mWb. Determine the induced emf if the armature winding is (i) lap connected and
(ii) wave connected.
Determine the flux per pole for 6 pole dc machine having 240 wave connected conductors, which
generates an open circuit voltage of 500 V while running at 1 000 rpm.
A separately excited dc generator running at 1 000 rpm supplied 110 A at 220 V to a resistive load. If
the load resistance remains constant, what will be the load current if the speed is reduced at 800 rpm?
Armature resistance is 0.02 Ω. Field current is unaltered. Assume a voltage drop is 1 V per brush.
Ignore the effect of armature reaction.
4. A 4 pole separately excited dc generator has a lap connected armature with 480 conductors. The
armature resistance is 0.02 Ω.
With an output current of 400 A from the armature, the terminal
voltage is 230 V when the machine is driven at 900 rpm.
Determine the useful flux per pole.
A 4 pole, lap wound, dc shunt generator has a useful flux per pole of 0.06 Wb. The armature winding
consists of 220 turns each of 0.004 Ω resistance. Calculate the terminal voltage and load current when it
runs at 900 rpm, if the armature current is 50 A and field circuit resistance is 200 Ω.
A dc shunt motor takes 110 A from 440 V supply. The resistance of armature circuit is 0.2 Ω and that of
field circuit is 220 Ω. The machine has 6 poles and the armature is lap connected with 864 conductors.
The flux per pole is 0.06 Wb. Calculate the speed.
A 25 kW, 500 V dc series generator has armature and series field resistance of 0.05 Ω and 0.03 Ω
respectively. Calculate the generated emf and the armature current at full load.
A 4 pole, 500 V dc series motor has 944 wave connected armature conductors. The power input is 10
kW. Flux per pole = 0.036 Wb; armature resistance = 0.5 Ω and series field resistance = 0.7 Ω.
Determine the speed of operation.
A short-shunt compound generator has armature, shunt field and series field resistance of 0.8 Ω, 50 Ω
and 0.6 Ω respectively and supplies a load of 5 kW at 230 V. Calculate the emf generated in the
armature.
A long-shunt compound motor takes 100 A from 500 V supply. Its armature, shunt field and series field
resistance are 0.05 Ω, 250 Ω and 0.03 Ω respectively. The brush drop is 1 V per brush arm. Calculate
the induced emf.
An 8 pole dc shunt generator has 1 556 conductors, wave connected, in the armature and runs at 2 000
rpm. The terminal voltage is 250 V and the load is 10 Ω. The armature resistance is 0.5 Ω and the field
resistance is 500 Ω. Find the armature current and induced emf.
A 250 V, 5 kW, 6 pole lap-wound shunt generator has shunt field and armature resistance of 250 Ω and
0.5 Ω respectively. The generator supplies 100 lamps rated at 250 V, 40 W. Allowing a brush drop of 1 V
per brush, determine (i) the load current; (ii) the current in each parallel path of the armature and (iii) the
emf generated in the armature.
A 4 pole shut generator with lap wound armature supplied 2.4 kW at 120 V. The armature and field
copper losses are 69 W and 200 W respectively. Calculate the armature current and the generated emf.
14. A 25 kW compound generator works on full load with a terminal voltage of 250 V. The armature, series
and shunt field resistance are 0.1 Ω, 0.05 Ω and 125 Ω respectively. Calculate the generated emf when it
is (i) long-shunt and (ii) short-shunt.
15. A 4 pole, lap wound separately excited dc generator has an armature resistance of 0.4 Ω and is driven at
750 rom. The armature has 720 conductors and the flux per pole is 0.03 Wb. If the load resistance is 12
Ω, determine the terminal voltage of the machine.
16. A separately excited dc generator, when driven at 1 500 rpm, supplies a load current of 200 A at 250 V
to a circuit of constant resistance. What will be the current and the voltage if the speed is reduced to 1
250 rpm keeping field current unaltered? Armature resistance = 0.05 Ω; brush contact drop 2 V; neglect
the effect of armature reaction.
17. Estimate the percentage reduction in speed of a separately excited generator working with constant
excitation on 400 V constant voltage bus-bars to decrease its load from 600 to 400 kW. The machine
resistance is 0.02 Ω. Neglect armature reaction.
18. A 4 pole lap connected dc shunt generator having field and armature resistance of 50 Ω and 0.15 Ω
respectively supplies seventy five 200V, 60 W lamps. Calculate the armature current, the current through
the armature conductors and the induced emf. Allow brush contact drop as 2 V.
19. A 6 pole, wave wound, 500 rpm dc shunt generator has armature and field resistance of 0.5 Ω and 250 Ω
respectively. The armature has 250 conductors and the flux per pole is 40 mWb. If the load resistance is
15 Ω, determine the terminal voltage and load current.
20. A 4 pole dc shut generator has a wave connected armature. The armature and shunt field resistance are
0.2 Ω and 50 Ω respectively. If the generator supplies eighty, 200 V, 60 W lamps, find the generated emf,
the total armature current and the current through each armature conductor.
DC Motor
A 6 pole, 500 V dc motor has 720 wave connected conductors in its armature. The full load armature
current is 70 A and the flux per pole is 0.02 Wb. The armature resistance is 0.2 Ω and the contact drop is
1 V per brush. Calculate the full load speed.
2. A 230 V dc shunt motor takes a total current of 25 A from the supply lines. The resistance of the shunt
field winding is 200 Ω and that of the armature is 0.3 Ω. Find (i) the current in the armature and (ii) the
back emf.
3. A 110 kW, belt driven dc shunt generator driven at 375 rpm on 220 V bus-bars continues to run as a
motor when he belt breaks, then taken a power of .5 kW. Find its speed as a motor if the armature
resistance is 0.025 Ω and the field resistance is 110 Ω. Neglect armature reaction and take contact drop
under each brush as 1 V.
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(i) Derive an expression for the RMF value of EMF induced in a coil of N turns in the
presence
of
time
varying
flux.
(ii) Two coupled coils have self and mutual inductance of L 11 = 2 + 1 / (2x); L22 = 1 + 1 / (2x); L12 =
L21 = 1 / (2x). Over a certain range of linear displacement x. The first coil is excited by a constant
current of 20 A and the second by a constant current of – 10 A. Find (1) Mechanical work down if x
changes from 0.5 to 1 m. (2) Energy supplied by each electrical source in part (1) and (3) Change
in field energy in part (1)