Tutorial 1: DCMachines Semester 1 2016
SECTION A
1. What is Electrical machine?
2. What is the advantage of DCmotorsover ACmotors?
3. What is the use of Commutator in DCmachine?
4. Mention at least ten partsof a DCmachine
5. Why field poles are laminated in DCmachine?
6. Do commutator convert ac to dcor dc to ac?
7. Explain what the function of the yoke isin dc machines
8. Mention the properties of inter-polesin order to identify in dc machines
9. Explain the two methodsof reducingspeed of a shunt motor
10.Draw and label the general arrangement of a dc machine
11.Sketch the magnetizing characteristics curve of a dc generator
12.Draw a circuit diagram of a series motor using a diverter for speed
control
13.With an aid of a circuit diagram, show how armature reaction occurs
14.Describe the following:
a) Frame/Stator/Yoke
b) Polesand pole coils
c) Armature
d) Commutator
e) Brushes
15.What are the functionsof the following:
a) Pole Tips
b) Armature Slots
c) Lamination Sheets
d) Commutator
e) Yoke
f) Eye Ball
g) Fan
h) Ball Bearings
i) Pole Winding
16.Mention two ways of controlling speed of a shunt motor
17.Mention four main losses in a dcmachine
18.Why are the losses in 8, important to know?
19.What are level, under-compounded and over-compounded generators?
20.What is the danger of running a series motor on no load?
21.Mention three ways of correcting armature reaction
22.111Why wouldn’
t a first time generator excite?
23.Why is the value of the field winding given in ohmsand not reactance?
24.Mention ten possible causes of sparkingin dcmachines
25.State the condition for maximum efficiency in dc machines
26.What is meant by pitting of ringsand how is it cured
27.What is the reason for brush vibration
28.Give reasonsfor possible short circuits between the turnsin the
armature of a dc machine
29.Explain the back emf asfound in series motors
30.Explain how magnetic flux is produced in dc machines
31.What advantages do shunt motorshave over series motors
32.Mention five possible causesof excessive temperature rise in a dc
machine
33.Mention three types of self-excited machines
34.How can the direction of a dc shunt machine be reversed
35.How can direction of a series motor be reversed
36.Assuming a shunt motor rotating clockwise, what happensif:
a) Field winding terminals are reversed
b) Armature windingsare reversed
c) Supply terminal is reversed
d) Both field and armature terminals reversed at the same time
37.Why is the shaft power always less than that developed in armature?
38.Why doesthe magnetic curve startsat a voltage higher than 0 Volts?
39.State the emf equation of the dc generator
Section B
1. A short shunt compound generator hasarmature, shunt field and series
field resistances of 0.4Ω, 45Ωand 0.3Ωrespectively. It delivers5kW at
240V calculate
a) Induced emf
b) Torque
c) Machine efficiency when driven at 1500rpm and PFEand PWINDare
4%of Electromagnetic power and windage losses are10%of
armature losses
2. A six-pole, lap wound, 220-V, shunt machine takes an armature current
of 2.5-A at no load and rotatesat 969-rpm. When loaded, it takes an
armature current of 54-A from and runsat 950-rpm. The resistance of
the armature circuit is0.18 Ωand there are 1044 armature conductors.
If the induced emf is210.28-V, calculate the flux per pole.(12.72mWb)
3. A long shunt generator isrunning at 1000-rpm and supplies22-kW at
terminal voltage of 220-V. the resistance of the armature, shunt field
and series field is 0.05 Ω, 110 Ωand 0.06 Ωrespectively. The overall
efficiency at the above load is88 %. Determine
The copper losses
The core and mechanical losses
Torque exerted by the prime mover (238.73Nm)
4. A 240 V shunt motor runson no load at 800 rpm with no external
resistor in the field circuit, armature current is 2 A. Calculate the value of
the resistor required in series with the shunt winding so that the motor
may run at 950 rpm when taking armature current of 30 A. The field
resistance and the armature resistances are 160 Ωand 0.4 Ω,
respectively (39.34Ω)
5. A shunt machine hasarmature and field resistances of 0.04 Ωand 100 Ω
respectively. When driven at 600 rpm, the machine delivers50 kW at
460 V. Calculate
a) the machine efficiency when running asa generator (93.84%)
b) Efficiency and speed when running asa motor taking 50 kW from the
same supply of 460 Vassuming rotational losses of 400 W(93.6%)
6. A 600 V shunt motor runsat 650 rpm without the excitation resistor in
the field and draws an armature current of 6 A. Calculate the value of
resistor required in series with the shunt winding so that the motor may
run at 800 rpm when taking armature current of 50 A. The shunt
winding resistance is200 Ω and that of the armature is 0.4 Ω(53.37Ω)
7. A six pole, 500-V wave connected shunt motor has1200 armature
conductorsand useful flux per pole of 20mWb. The armature and field
resistances are 0.5 Ωand 250 Ωrespectively. What will be the speed and
torquedeveloped by the motor when it draws20-A from the same
supply mains? Assuming magnetic and mechanical losses of 900-W,
determine
Developed torque
Output power
Efficiency at thisload (79.38%)
8. A self-excited LongShunt Machine has Armature, Field, and Seriesand
Inter-pole resistance of 0.1Ω, 250 Ω, 0.3 Ωand 0.15 Ωrespectively.
When connected to 400 V and driven as a generator at 600 rpm it
delivers20 kW. Calculate the Generator Torque, Efficiency and its speed
when running asa motor and taking 20 kW from same supply. Assume
PMECH = 2%of Generator rated power (351.803Nm, 88.9%, 522.965rpm)
9. A short compound generator hasarmature, field and series field
resistances of 0.8 Ω, 45 Ωand 0.6 Ωrespectively and supply 5 kW load
at 250 V. Calculate the electromagnetic power of the machine
(7298.85W)
10.A four pole compound generator hasarmature, shunt field and series
field resistances of 0.1 Ω, 200 Ωand 0.3 Ωrespectively. It delivers5 kW
at 200 V and allowing 1 V drop per brush contact resistance, calculate for
long and short shunt
a)
Induced Emf (212.4 V& 212.1V)
b)
Total power (5722.4W & 5722.4W)
c)
Machine efficiency (87.4%& 87.62 %)
d)
Flux per pole if the armature has 100 lap-connected conductors
running at 750 rpm. Assume rotational losses of 200 W (169.92mWb)
11.A 10 kW, 250 V, Self-excited DCShunt Generator hastotal rotational
losses of 400 W. The armature circuit including brush contact resistance
of 0.5 Ω, field resistance of 250 Ω. Calculate
a) Shaft input power (11.5 kW)
b) Machine efficiency (87.03%)
c) Armature current for maximum efficiency (36.06 A)
d) Machine output power for maximum efficiency (8763.88W)
12.A 250 V dc shunt motor takes a no load current of 4 A and hasarmature
resistance of 0.5 Ωand a field resistance of 250 Ω. Calculate the
efficiency and percentage change in speed when the motor takes full
load current of 50 A(82.43 %& 9.255 %)
13.A six pole armature is wound with 240 conductors. What isthe number
of conductorsper path if the machine is
a) Lap wound, (40)
b) Wave wound and (120)
c) Coil span is machine has24 slots. (4)
14.A short shunt compound generator hasarmature, shunt, series field
resistances of 0.8 , 45 and 0.6 and supply 5 kW load at 250 V.
Calculate
a) Generated Emf in the armature and (282.66 V)
b) Electromagnetic power when driven at 1500 rpm and PFE & PWIND, are
4 %of electromagnetic power and windage losses is 10 %of the
armature losses. (7298.85 W)
c) Torque required to drive the load (46.466 Nm)
d) Efficiency (65.41 %)
15.A DCshunt, self-excited generator delivers 20 A at 250 V when driven at
1500 rpm. The field current is 1 A and armature resistance is 0.4 . If
core, friction and windage losses are 250 W, determine the power
required to drive the machine at the above load. (5674 W)
16. In a brake test on a DCmotor the effective load on a brake pulley was
625 N, the effective diameter of a pulley 650 mm and speed of 720 rpm.
The motor draws65 A at 240 V. Calculate:
a) Pout (15.3 kW)
b) Efficiency at this load (98.2 %)
16.A four pole dc generator gives 410V on open-circuit when driven at 900rpm.
Calculate the flux per pole for a wave-connected armature that has39 slots,
with 16 conductorsper slot. (0.0219Wb)
17.The resistance of the field circuit of a shunt-excited dc generator is200W.
When the output of the generator is 100kW, the terminal voltage is 500V
and the generated emf is 525V respectively. Calculate:
a. The armature resistance (0.123)
b. The value of the generated emf when the output is60kW, if the
terminal voltage is then 250V. (534.5V)
18.A short –shunt compound generator hasarmature, shunt field and series
field resistances of 0.8W, 45W, and 0.6W respectively and supply a load of
5kW at 250V. Calculate the emf generated in the armature. (282.7V)
19.A shunt generator is to be converted into a level-compounded generator by
addinga series field winding. From a test on the machine with a shunt
excitation only, it is found that the shunt current is 4.1A to give 440V on noload and 5.8A to give the same voltage when supplying a full-load current of
200A. The shunt winding has1200 turnsper pole. Determine the number of
series turnsper pole. (10)
20.Calculate the number of series turnsper pole required for a 100-kW, 460-V,
level compound generator. Without a series winding it isfound that the
shunt current hasto be 2A on no-load and 6.25A on full-load to maintain
460V. The number of turnsper pole for the shunt winding is 2000. If the
series coils were wound 8 turnsper pole and had a total resistance of 0.01W,
what value of diverter resistance would be required to give level
compounding?(6, 0.0296)
21.A 300-kW, 500-V, 8-pole dc generator has768 armature conductors, lapconnected. Calculate:
a. The demagnetising mmf
b. The crossmmf per pole if the brushesare given a lead of 5°e from
the GNPand
c. The number of turnsrequired for the compole if the mmf for the
compole is1.3 times the armature mmf. (200A(t), 3400A(t), 8)
22.A 6-pole armature is wound with 498 conductors. The fluxand the speed is
such that the average induced emf in each conductor is2V. The current in
each conductor is120A. Find the total current and generated emf of the
armature if the winding isconnected:
a. Wave and
b. Lap
(240A, 498V, 720A, 166V, 9.5kW)
23.A 4-pole armature is wound with 564 conductorsand is driven at 800rpm,
total current , the emf and the electrical power generated in the armature if
the conductorsare connected :
a. Wave and
b. Lap
(120A, 301V, 240A, 150V, 36.12kW)
24.An 8-pole, lap-connected armature has 96 slots with 6 conductorsper slot
and isdriven at 500rpm. The useful flux per pole is 0.09Wb. Calculate the
generated emf. (432V)
25.The wave-connected armature of a 4-pole dc generator is required to
generate an emf of 520V when driven at 660rpm. Calculate the flux per pole
required if the armature has144slots with two coil sides per slot, each coil
consisting of 3 turns. (0.0274Wb)
26.An 8-pole, lap-connected dc generator hasan armature 640 conductors.
The ratio of pole arcto pole pitch is 0.7. calculate the mmf per pole of a
compensating winding to give uniform air gap density when the total
armature current is 900A
27.The no-load speed of a 240 V, dc shunt motor is 820 rpm and the current is
2 A. calculate the resistance in series with the field to increase the speed of
the motor to 1000 rpm for the armature current of 30 A. The armature and
field resistances are 0.4 Ωand 160 Ωrespectively. (44.7Ω)
28.A 115 V, longshunt, compound generator supplies a load of 400 A, at rated
voltage. The IFis 10 A, RA is 0.05 Ωand RSERis0.02 Ωand rotational loss of
2 kW. Determine the machine efficiency. (77.5 %)
29.The armature of a dc motor hasa resistance of 0.1 Ωand is connected to
a230 V supply. It takes a no load armature current of 1.8 A at a speed of 998
rpm. If the full load armature current is 75 A at a rated voltage of 230 V,
Calculate
Back emf in the armature at full load conditions(222.5 V)
Electro-magnetic power at full load speed if the flux is reduced by 4%due to
armature reaction (1006 rpm)