# Design of Electrical Machines Questions and Answers – Field Form, Harmonic Analysis of Flux Distribution Curve

This set of Design of Electrical Machines online test focuses on “Field Form, Harmonic Analysis of Flux Distribution Curve”.

1. How is the flux distributed in the field form?
a) to reduce the high voltage
b) to reduce the high current
c) to reduce the harmonics
d) to keep the total reluctance low

Explanation: The flux in passing from poles into the armature, does not confine itself over the pole arc but spreads out over the entire pole pitch. The flux will distribute itself in the air gap in such a way that the total reluctance is minimum.

2. What does the flux distribution curve determine in the ac machine?
a) waveshape of voltage
b) waveshape of current
c) waveshape of power
d) commutation conditions

Explanation: The flux distribution curve in the ac machine determines the waveshape of the voltage. In the dc machine the flux distribution curve determines the commutation conditions.

3. How many techniques are used to plot the field form in salient pole machines?
a) 2
b) 3
c) 4
d) 5

Explanation: There are 2 techniques which are used to plot the field form in salient pole machines. They are carter’s fringe curves and the flux plotting by method of curvilinear squares.

4. What is the formula of the flux density in the gap at a distance ‘x’ from the centre of the pole?
a) flux density in the gap at a distance ‘x’ from the centre of the pole = length of air gap at the centre of pole * length of air gap at a distance ‘x’ from the centre of the pole * maximum flux density in air gap
b) flux density in the gap at a distance ‘x’ from the centre of the pole = length of air gap at the centre of pole / length of air gap at a distance ‘x’ from the centre of the pole * maximum flux density in air gap
c) flux density in the gap at a distance ‘x’ from the centre of the pole = length of air gap at the centre of pole * length of air gap at a distance ‘x’ from the centre of the pole / maximum flux density in air gap
d) flux density in the gap at a distance ‘x’ from the centre of the pole = 1/ length of air gap at the centre of pole * length of air gap at a distance ‘x’ from the centre of the pole * maximum flux density in air gap

Explanation: The length of air gap at the centre of pole, the length of air gap at a distance ‘x’ from the centre of the pole and the maximum flux density in air gap is calculated. On substitution the flux density in the gap at a distance ‘x’ from the centre of the pole.

5. The plot between carter’s coefficient and the relative flux density is the carter’s fringe curve.
a) true
b) false

Explanation: Carter’s fringe curves is one of the techniques used in the plotting of the field form in the salient pole machines. The plot between carter’s coefficient and the relative flux density is called as carter’s fringe curve.

6. What Is the formula of the permeance of the flux tube considering unit depth in the flux plotting technique?
a) permeance of the flux tube = permeability in the air * mean width of flux tube * mean length of flux tube
b) permeance of the flux tube = permeability in the air / mean width of flux tube * mean length of flux tube
c) permeance of the flux tube = permeability in the air * mean width of flux tube / mean length of flux tube
d) permeance of the flux tube =1/ permeability in the air * mean width of flux tube * mean length of flux tube

Explanation: The permeability in the air, mean width of the flux tube and the mean length of flux tube is calculated first. On substitution the permeance of the flux tube is obtained.

7. How many rules are to be followed while the flux plotting by method of curvilinear squares?
a) 2
b) 3
c) 4
d) 5

Explanation: The flux lines leave and enter from surfaces, bounding the gap, at right angles if it is assumed that iron has infinite permeability as compared with air and then the flux and equipotential lines intersect at right angles. The flux and the equipotential lines are so drawn that each flux is divided into equal number of curvilinear squares.

8. How many factors does the flux distribution in the rotating machines depend on?
a) 2
b) 3
c) 4
d) 5

Explanation: The flux distribution in the rotating machines depends upon 3 factors. They are shape of pole, the distribution of field windings and the load condition.

9. How should the flux distribution be in the case of ac machines?
a) sinusoidal
b) rectangular
c) square
d) circular

Explanation: The flux distribution in the case of ac machines should be sinusoidal. The flux distribution in the case of dc machines should be rectangular.

10. How should the air gap and the fringing effects be if the field form of a salient pole machine is rectangular?
a) air gap under the pole arc is not constant, fringing effects are considered
b) air gap under the pole arc is constant, fringing effects are considered
c) air gap under the pole arc is constant, fringing effects are not considered
d) air gap under the pole arc is not constant, fringing effects are not considered

Explanation: The field form of a salient pole machine is rectangular if the air gap under the pole arc is not constant. The field form of a salient pole machine is rectangular if the fringing effects are not considered.

11. What series is used to analyze the field form?
a) z-series
b) fourier series
c) fourier transform
d) z-transform

Explanation: The field form of a salient pole is rectangular is the air gap under pole arc is uniform and if fringing effects are neglected. The field form can be analyzed for its harmonic contents with the help of Fourier series.

12. What happens if the field form is symmetrical about the pole axis?
a) north and south pole of a machine are similar
b) no harmonics
c) no cosine terms
d) north and south pole of a machine are similar, no harmonics, no cosine terms

Explanation: If the field form is symmetrical about the pole axis the north and south pole of a machine are similar. As the poles are similar, no harmonics and no cosine terms and the constant term is zero.

13. What Is the formula for the amplitude of the fundamental curve?
a) amplitude of the fundamental curve = 1.27 * flux density in the air gap * cosine (phase angle/2)
b) amplitude of the fundamental curve = 1.27 / flux density in the air gap * cosine (phase angle/2)
c) amplitude of the fundamental curve = 1.27 * flux density in the air gap / cosine (phase angle/2)
d) amplitude of the fundamental curve = 1/1.27 * flux density in the air gap * cosine (phase angle/2)

Explanation: The flux density in the air gap is calculated along with the cosine of the phase angle divided by 2. On substitution, the amplitude of the fundamental curve is calculated.

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