This document contains multiple questions related to DC motors and generators. It includes questions about calculating speed given a change in series resistance for a shunt motor, calculating series resistance to achieve a given speed for a series motor, calculating new speed when adding series resistance to a shunt motor where torque varies with speed squared, calculating efficiency and optimal output power for a shunt motor, and questions calculating flux, pole arc length, and air gap flux density for DC generators.

1. a4. ( a ) The armature and shuntfieldresistance of a230V shuntmotor are 0.1Ωand 230Ω
respectively.Ittakesacurrentof 61A at 1000rpm. If the currenttakenremainsunalteredfind
the resistance tobe includedinserieswiththe armature circuittoreduce the speedto750rpm.
(10 marks)
a4.( b ) A 200V seriesmotorhas a total resistance of 0.5Ω. It runsat 800 rpm takingan inputcurrentof
10A. Findthe seriesresistance required toreduce the speedto600 rpm, the inputcurrentbeingkept
constant.Assume the magnetizationcharacteristictobe a straightline.
(5 marks)

2. 5. A 500V shuntmotor drivesaload the torque of whichvariesas the square of the speed.When
runningat certainspeedittakesa currentof 30A. If a resistance of 10Ωisincludedinserieswiththe
armature determine the newspeedasa percentage of the original speed.Shuntfieldresistance of the
motor = 250Ω. Neglectarmature resistance.(20marks)

3. aExample 29.39. A d.c. shuntmachine while runningasgeneratordevelopsavoltage of 250 V
at 1000 r.p.m.on no-load.Ithas armature resistance of 0.5 Ω and fieldresistance of 250 Ω. When
the machine runsas motor,inputto it at no-loadis4 A at 250 V. Calculate the speedandefficiency
of the machine whenitrunsas a motor taking40 A at 250 V.Armature reactionweakensthe fieldby
4 %.
Example 29.42. A 7.46 kW,250-V shuntmotortakesa line currentof 5 A whenrunninglight.
Calculate the efficiencyasamotor whendeliveringfull loadoutput,if the armature andfieldresis-tance
are 0.5 Ω and250 Ω respectively.Atwhatoutputpowerwill the efficiencybe maximum?Is
it possible toobtainthisoutputfromthe machine ?

4. aExample 26.21(a). A 6-pole dcgeneratorrunsat 1200 r.p.m.on no-loadandhas a generatede.m.f.of
250 V.Its arma-ture diameteris350 mm and the radial air-gapbetweenthe fieldpolesandthe
armature is 3 mm. The axial lengthof the fieldpolesis260 mmand the fieldpole effective coverageis
80% includingfringing.If the armature has96 coilshaving3 turnsper coil andis woundduplex lap,
calculate (a)flux perpole(b)effectivepole arclengthand(c)average air-gapfluxdensity.

5. Example 26.33. The armature of a four-pole d.c.shuntgeneratorislap-woundandgenerates
216 V whenrunningat 600 r.p.m.Armature has 144 slots,with6 conductorsper slot.If thisarmature is
rewound,wave-connected,findthe e.m.f.generatedwiththe same fluxper pole butrunningat500
r.p.m.Commentonthe armature output powerinthe twocases.Assumptionisthatfieldside issuitably
modifiedinthe twocases.