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ME1331
Problem Sheet 4
Dr. M. K. Darwish
1)
A 230 V motor has an armature circuit resistance of 0.6
Ω
.
If the fullload
armature current is 30 A and the noload armature current is 4 A, find the
change in the back e.m.f. from noload to fullload.
(15.6 V)
2)
A 440 V shunt motor has an armature resistance of 0.8
Ω
and field
resistance of 200
Ω
.
Determine the back e.m.f. when giving an output of
10 h.p. at 85% efficiency.
(425.8 V)
3)
The armature resistance of a 200 V shunt motor is 0.4
Ω
and the noload
armature current is 2 A.
When loaded and taking an armature current of
50 A, the speed is 1200 r.p.m.
Find approximately the noload speed.
(1328 r.p.m.)
4)
A 25 kW, 250 V, d.c. shunt generator has an armature and field resistances
of 0.06
Ω
and 100
Ω
respectively.
Determine the total armature power
developed when working (i) as a generator delivering 25 kW output and (ii)
as a motor taking 25 kW input.
(26.25 kW, 23.8 kW)
5)
A 20 kW compound generator works on full load with a terminal voltage of
250 V.
The armature, series, and shunt windings have a resistance of
0.05
Ω
, 0.025
Ω
, and 100
Ω
respectively.
Calculate the total e.m.f. generated
in the armature when the machine is connected shortshunt.
(256.126 V)
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 Spring '10
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