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DC Machines & Synchronous Machines
10EE54
Question Bank Solutions
UNIT-2: DC Motor:
1.Classify DC motors according to their field winding connections. Draw the circuit diagrams
and write the voltage and current relationships in all the types.
Dec 2013
Sol: Depends upon the field winding connected to the armature
(1) DC Shunt Motor
(2) DC Series Motor
(3) DC Compound Motor
(i) Cumulative Compound Motor
(a) Long shunt
(b) Short shunt
(ii) Differential Compound Motor
(a) Long shunt
(b) Short shunt
DC Shunt Motor: Ish = V/Rsh and
Ia = I– Ish.
Where I is the line current
Eb= V - IaRa– B.C.D – A.R.D, where B.C.D is brush contact drop(1 V/brush)
A.R.D is the armature reaction drop
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DC Machines & Synchronous Machines
2.
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DC Series Motor: Ia = I = Ise
Eb= V - Ia(Ra +Rse) – B.C.D – A.R.D.
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DC Machines & Synchronous Machines
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2.With the help of relevant characteristics explain why a series motor should never be started at
no load.Dec 2013, June 2015
In any DC. Motor the speed depends inversely on the flux. Consider when stating a series motor
the current flowing will be high, but the flux that the motor produces will be moderate since the
series winding has less number of turns. If you start the motor without load the speed will
increase as speed increases the back emf increases. This decreases the current through the series
winding and hence flux decreases and the speed father increases. Theoretically speaking the s
peed becomes infinite But practically the mot will accelerate to very high speed. „which will
damage the motor. But if you start the motor with the load, you are actually reducing the
starting speed hence the motor runs safely.
3.
A 200 V DC shunt motor takes 22 amp at rated voltage and runs at 1000 rpm. Its field
resistance is 100 ohm and armature resistance is 0.1 ohm. Compute the value of
additional resistance required in the armature circuit to reduce the. Speed to 800 rpm
when i) The load torque is proportional to the speed ii) The load torque proportional to
the square of the speed.
Dec 2013
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4.
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What is meant by back emf? Explain its significance. Dec 2013,June 2013
Significance of Back E.M.F. It is seen in the generating action, that when a conductor cuts the
lines of flux, e.m.f. gets induced in the conductor. The question is obvious that in a d.c. motor,
after a motoring action, armature starts rotating and armature conductors cut the main flux. So is
there a generating action existing in a motor ? The answer to this question is 'Yes'. After a
motoring action, there exists a generating action. There is an induced e.m.f. in the rotating
armature conductors according to Faraday's law of electromagnetic induction. This induced e.m.f.
in the armature always acts in the opposite direction of the supply voltage. This is according to the
Lenz's law which states that the direction of the induced e.m.f. is always so as to oppose the cause
producing it. In a d c. motor, electrical input i.e. the supply voltage is the cause and hence this
induced e.m.f. opposes the supply voltage. This e.m.f. tries' to set up a current through the
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DC Machines & Synchronous Machines
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armature which is in the opposite direction to that, which supply voltage is forcing through the
conductor. So as this e.m.f. always opposes the supply voltage, it is called back e.m.f. and denoted
as Eb. Though it is denoted as Eb, basically it gets generated by the generating action which we
have seen earlier in case of generators. So its magnitude can be determined by the e.m.f. equation
5.
Derive torque equation of a DC motor.
June 2013, Dec 2014,June 2015
Torque Equation of a D.C. Motor
It is seen that the tuning or twisting force about an axis is called torque. Consider a wheel of
radius R meters acted upon by a circumferential force F newton as shown in the Fig. The wheel
is rotating at a speed of N r.p.m.
Then angular speed of the wheel is w = 2πN/60 rad/sec
So work done in one revolution is, W = F x distance travelled in one revolution
= F x 2 π R Joules
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6.
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A 440 V, DC shunt motor has a no load speed of 2000 rpm. It is running at 1000 rpm at full
load torque, reduced armature voltage and full field. If the load torque is reduced to 50% of
rated value, with armature voltage and filed voltage held constant at previous voltages the O
speed increases to 1050 rpm. Find the armature voltage drop at full load. Neglect the effect of
armature reaction.
June 2013
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DC Machines & Synchronous Machines
7.
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What are the different methods of speed control of D.C. shunt motor, explain them in
brief.
Dec 2013, Dec 2014
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8.
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Explain the torque-Armature Current characteristics of shunt and series motors.June
2013, Dec 2014, 08 Marks
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9.
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Explain in brief the Ward-Leonard speed control of DC motors.
June 2013
10. A series motor whit an unsaturated magnetic circuit and 0.5 0hm total resistance when
running at a certain speed takes 60 A at 500V. If the load torque varies as the cube of the
speed. Calculate the resistance required to reduce the speed by 25%.
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June 2013,June 2014
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11. Write a short notes on Permanent Magnet DC Motors.
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Dec 2014