Test: Generated EMF


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15 Questions MCQ Test Electrical Machines | Test: Generated EMF

Test: Generated EMF for Electrical Engineering (EE) 2022 is part of Electrical Machines preparation. The Test: Generated EMF questions and answers have been prepared according to the Electrical Engineering (EE) exam syllabus.The Test: Generated EMF MCQs are made for Electrical Engineering (EE) 2022 Exam. Find important definitions, questions, notes, meanings, examples, exercises, MCQs and online tests for Test: Generated EMF below.
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Test: Generated EMF - Question 1

Which of the following statements is/are correct regarding the generation of EMF in rotating electrical machines in the armature winding? EMF is generated ____________

Detailed Solution for Test: Generated EMF - Question 1

Any of the above mentioned methods will produce EMF in the armature windings.

Test: Generated EMF - Question 2

 The EMF equation e = NωrΦsinωrt is applicable to ____________

Detailed Solution for Test: Generated EMF - Question 2

e = NωrΦsinωrt-NdΦ/dtcosωrt, for time invariant field flux dΦ/dt = 0 ⇒ e = NωrΦsinωrt, is the general equation, and is applicable to both AC and DC systems.

Test: Generated EMF - Question 3

In the equation for RMS value of the generated EMF in a full pitched coil of an AC machine,E = Emax/√2 = √2πfrNΦ, fr depends on

Detailed Solution for Test: Generated EMF - Question 3

fr is called the rotational or speed frequency, since its value depends upon the relative velocity between the flux density wave and the armature coil.

Test: Generated EMF - Question 4

 In AC rotating machines, the generated or speed EMF

Detailed Solution for Test: Generated EMF - Question 4

e = Emaxcos(ωrt-π/2)
ψ = NΦcosωrt
It reveals that the speed or generated EMF lags by 90° the flux that generates it, and is true when flux is time invariant and is sine distributed in space.

Test: Generated EMF - Question 5

In a short pitched coil, the coil pitch factor kp, is given by ____________

Detailed Solution for Test: Generated EMF - Question 5

The effect of short pitched coil is to reduce the generated EMF, and this reduction factor is cosε/2, and is referred to as coil pitch factor.

Test: Generated EMF - Question 6

 Which of the following equations represents the RMS value of the generated EMF in a short-pitched N-turn armature coil of an AC machine ____________

Detailed Solution for Test: Generated EMF - Question 6

EMF induced = NΦωrkpsinωrt,
Emax = NΦωrkp when sinωrt = 1
RMS value, E = Emax/√2 = √2πfrkp

Test: Generated EMF - Question 7

The effect of short pitched coil on the generated EMF is _____________

Detailed Solution for Test: Generated EMF - Question 7

The effect of short pitched coil is to reduce the generated EMF and this reduction factor is cosε/2 and is referred to as coil pitch factor.

Test: Generated EMF - Question 8

A winding is distributed in the slots along the air gap periphery
(i) to add mechanical strength to the winding
(ii) to reduce the amount of conductor material required
(iii) to reduce the harmonics in generated EMF
(iv) to reduce the size of the machine
(v) for full utilization of iron and conductor materials
From these, the correct answer is

Detailed Solution for Test: Generated EMF - Question 8

In rotating electrical machines, the armature turns are usually distributed in slots rather than concentrated in single slot. This is essential from the view point of utilizing the armature periphery completely, and add mechanical strength.

Test: Generated EMF - Question 9

In an AC machine, the effect of distributing the turns in different slots, results in a further reduction of generated EMF by the factor kd. This factor is called ____________

Detailed Solution for Test: Generated EMF - Question 9

E = √2πkpkdfrNΦ, kd is the distribution factor.

Test: Generated EMF - Question 10

A polyphase induction motor of the slip ring or wound rotor type can be used ____________

Detailed Solution for Test: Generated EMF - Question 10

A polyphase induction motor can be used as a frequency converter (or changer) for changing the supply frequency f to other frequencies sf and (2-s)f at the slip ring terminals.

Test: Generated EMF - Question 11

The equation for slip speed is ____________

Detailed Solution for Test: Generated EMF - Question 11

The relative speed between rotor and rotating flux wave, i.e, (ω-ωr) is referred to as the slip speed in rad/sec
slip speed = (ω-ωr) rad/sec

Test: Generated EMF - Question 12

If the rotor of an induction motor is made to revolve in a direction opposite to the rotating flux wave, then RMS value of EMF induced in one phase of rotor E is proportional to ____________

Detailed Solution for Test: Generated EMF - Question 12

Relative velocity between rotor winding and rotating flux wave becomes
(ω+ωr) = ω+ω(1-s) = ω(2-s) and
E = 2πf(2-s)NphrkwΦ

Test: Generated EMF - Question 13

In an alternator, frequency per revolution is equal to

Detailed Solution for Test: Generated EMF - Question 13

For a P-pole machine, in n rev/sec, P/2n cycles/second are generated and cycles per second is referred to as frequency f of the EMF wave. Here in this question n=1
⇒ frequency per revolution = P/2
⇒ number of pole pairs.

Test: Generated EMF - Question 14

A 6 pole machine is rotating at a speed of 1200rpm. This speed in mechanical rad/sec and electrical radians per second is respectively

Detailed Solution for Test: Generated EMF - Question 14

N = 1200rpm,P = 6 and f = PN/120 = 60 cycles per sec.
1cycle = 360° electrical degrees = 2π electrical radians
f in electrical radians per sec = 60∗2π electrical radians/sec = 120π elect radians/sec
speed in mechanical radians/sec = 2/P(speed in electrical rad/sec) = 2/P(120π)=40π mechanical rad/sec.

Test: Generated EMF - Question 15

The DC machines are designed with flat topped flux density waves because

Detailed Solution for Test: Generated EMF - Question 15

 For the same value of peak flux density Bp, it is easy to see that average value of brush voltage could be more for a flat topped B-wave than for a sinusoidal flux density wave.

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