Principles of Magnetic Circuit Design MCQs ( Design Of Electrical Machines ) MCQs – Design Of Electrical Machines MCQs
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Design Of Electrical Machines MCQs – Principles of Magnetic Circuit Design MCQs ( Design Of Electrical Machines ) MCQs
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Determination of Iron Losses & Slot Leakage
1. What is the other name for the iron loss?
a) field loss
b) armature loss
c) winding loss
d) core loss
Answer: d
Explanation: When ferromagnetic materials are subjected to a flux in a fixed direction in space and having a magnitude varying in time, losses are produced in the machine. This is called iron loss or core loss.
2. How many types of iron losses are present?
a) 2
b) 3
c) 4
d) 5
Answer: a
Explanation: There are basically 2 losses present under the core loss or iron loss. They are hysteresis loss and the eddy current loss.
3. What is the formula to obtain the hysteresis loss devised by Steinmetz?
a) hysteresis loss = hysteresis coefficient / frequency of magnetization * (maximum flux density)steinmetz coefficient
b) hysteresis loss = hysteresis coefficient * frequency of magnetization / (maximum flux density)steinmetz coefficient
c) hysteresis loss = hysteresis coefficient * frequency of magnetization * (maximum flux density)steinmetz coefficient
d) hysteresis loss = 1/hysteresis coefficient * frequency of magnetization * (maximum flux density)steinmetz coefficient
Answer: c
Explanation: The hysteresis coefficient along with the frequency of magnetization and maximum flux density is calculated first. Next the Steinmetz coefficient is calculated and on substitution it gives the hysteresis loss.
4. How are the eddy current losses in the machine reduced?
a) by using conducting materials
b) by using magnetic materials
c) by using insulating materials
d) by laminating the core
Answer: d
Explanation: Eddy current loss is one type of loss present under the iron or core loss. It can be reduced by laminating the core.
5. What is the relation between resistivity, magnetizing mmf and magnetizing current?
a) high resistivity, low magnetizing mmf, high magnetizing current
b) high resistivity, high magnetizing mmf, high magnetizing current
c) low resistivity, high magnetizing mmf, high magnetizing current
d) low resistivity, high magnetizing mmf, low magnetizing current
Answer: b
Explanation: The high resistivity materials are made use of to reduce the eddy current loss. The high resistivity materials have high magnetizing mmf which in turn leads to high magnetizing current.
6. What must be done in order to take additional iron losses into account in dc machines?
a) iron loss obtained from iron loss curve is multiplied by 1.2-1.4
b) iron loss obtained from iron loss curve is multiplied by 1.4-1.6
c) iron loss obtained from iron loss curve is multiplied by 1.3-1.5
d) iron loss obtained from iron loss curve is multiplied by 1.15-1.25
Answer: b
Explanation: In order to take additional losses into account the iron loss obtained from the iron loss curves is multiplied into 1.4-1.6 in dc machines and synchronous machines. The iron loss obtained from the iron loss curves is multiplied into 1.2-1.4 in induction motors.
7. What is the value of constant ‘a’ in the core part of the ac machines?
a) 4.7
b) 6.5
c) 6.7
d) 2.3
Answer: a
Explanation: The value of constant ‘a’ in the core part of the ac machines is 4.7 and that in the teeth part is 6.5. The value of constant ‘a’ in the core part of the dc machines is 6.5 and that in the teeth part is 2.3.
8. What are the equations that are to be satisfied by flux patterns?
a) Poisson equation
b) Laplace equation
c) Laplace and Poisson equation
d) Gauss equation
Answer: c
Explanation: The flux patterns must satisfy the Poisson equation within the section of the conductors. The flux patterns must satisfy the Laplace equation in the other parts.
9. How many assumptions can be used for the slot leakage calculation?
a) 2
b) 3
c) 4
d) 5
Answer: b
Explanation: There are 3 assumption made in the slot leakage calculation. They are the current in the slot conductors is uniformly distributed, the leakage path is straight across the slot and around the iron at the bottom and the permeance of air paths is only considered.
10. What are the factors the slot leakage permeance will depend upon?
a) shape of the slot
b) arrangement of winding in the slot
c) number of windings
d) shape of the slot and arrangement of winding in the slot
Answer: d
Explanation: The slot leakage permeance depends upon the shape of the slots. The slot leakage permeance also depends upon the arrangement of winding in the slot.
11. What is the formula of the permeance of the strip in the conductor portion?
a) permeance of the strip = permeability in air * area of the flux path * length of flux path
b) permeance of the strip = permeability in air / area of the flux path * length of flux path
c) permeance of the strip = permeability in air * area of the flux path / length of flux path
d) permeance of the strip =1 / permeability in air * area of the flux path * length of flux path
Answer: c
Explanation: The permeability in air, area of the flux path and the length of flux path is first calculated. On substitution, the permeance of the strip is obtained.
12. What is the formula of the effective permeance of conductor portion?
a) effective permeance of conductor portion = 1/total flux linkages * total turns * total mmf
b) effective permeance of conductor portion = total flux linkages / total turns * total mmf
c) effective permeance of conductor portion = total flux linkages * total turns * total mmf
d) effective permeance of conductor portion = total flux linkages * total turns / total mmf
Answer: b
Explanation: The total flux linkages, total turns and total mmf is first calculated. On substitution the effective permeance of conductor portion is obtained.
13. Which machine incorporates the usage of the closed slots?
a) dc motors
b) synchronous motors
c) induction motors
d) special motors
Answer: c
Explanation: The closed slots are being made use often in the small induction motors. The closed slots are used to increase the leakage reactance which in turn decreases the starting current.
14. The leakage flux on the top of the slot is through iron, which is called ‘bridge’?
a) true
b) false
Answer: a
Explanation: The leakage flux on the top of the slot is through iron, which is called ‘bridge’. Therefore the specific permeance of this path depends upon the permeance of iron which in turn depends upon the degree of saturation and hence upon the relative permeability of iron.
Pulsation Losses & Types of Leakages
1. What happens in the rotating electrical machine?
a) armatures are slotted
b) armatures are slotted and results in the movement of rotor
c) the rotor remains stationary
d) the rotor slots are rotating
Answer: b
Explanation: In the rotating electrical machine, the armatures are slotted and there is movement of rotor. Due to the movement of the rotor there are rapid changes of local gap reluctance.
2. What is pulsation losses?
a) flux pulsations are caused due to the slotted armature
b) flux pulsations are caused due to the rotation of machine
c) flux pulsation occurs due to rotor slots are rotating
d) flux pulsations occur due to the change in reluctance
Answer: d
Explanation: In the rotating electrical machine, the armatures are slotted and there is movement of rotor which leads to changes of local gap reluctance. This change leads to flux pulsations which causes additional losses called pulsating losses.
3. In which machine part/parts does the pulsation loss occurs?
a) teeth
b) pole face
c) conductors
d) teeth and pole faces
Answer: d
Explanation: The pulsation losses occurs in the teeth of the machine. The pulsation losses also occur in the pole faces.
4. How are the pulsation losses aggravated?
a) if the air gap is small compared with slot openings
b) if the air gap is reduced
c) if the air gap is increased
d) if the air gap is made larger than the slot openings
Answer: a
Explanation: The pulsation losses occurs in the pole faces and teeth. The pulsation losses are aggravated if the air gap is small compared with slot openings.
5. The slotting produces harmonic fields which cause high frequency losses near the gap surface.
a) true
b) false
Answer: a
Explanation: The pulsation losses are aggravated if the length of air gap is small than the slot openings. The slotting produces harmonic fields which cause high frequency losses near the gap surface.
6. In which machine is the pulsation losses considerable?
a) synchronous motors
b) induction motors
c) dc shunt motors
d) dc series motors
Answer: b
Explanation: The pulsation losses are aggravated if the length of air gap is small than the slot openings. The pulsation losses although are considerable in the induction motors.
7. What are the factors the permeance depends upon in the zigzag leakage?
a) relative position of stator
b) relative position of rotor
c) relative position of stator and rotor
d) stored energy at any position
Answer: c
Explanation: The permeance for the path of zigzag leakage will depend upon the relative position of stator. The permeance for the path of zigzag leakage will depend upon the relative position of rotor.
8. What is the formula for the stored energy at any position?
a) stored energy at any position = mmf per slot2 * permeance in a particular position
b) stored energy at any position = 2 * mmf per slot2 * permeance in a particular position
c) stored energy at any position = 1/2 * mmf per slot2 * permeance in a particular position
d) stored energy at any position = 1/3 * mmf per slot2 * permeance in a particular position
Answer: c
Explanation: First the mmf per slot along with the permeance in a particular position is calculated. On substitution, the stored energy at any position is calculated.
9. What is the formula for the zigzag permeance?
a) zigzag permeance = average width of the rotor tooth / (1/2 * mmf per slot2)
b) zigzag permeance = average width of the rotor tooth * (1/2 * mmf per slot2)
c) zigzag permeance = 1/average width of the rotor tooth *(1/2 * mmf per slot2)
d) zigzag permeance = average width of the rotor tooth *(1/2 / mmf per slot2)
Answer: a
Explanation: The average width of the rotor tooth is first calculated along with the mmf per slot. On substitution, the zigzag permeance is found out.
10. What is the formula of the zigzag specific permeance?
a) zigzag specific permeance = average width of the rotor tooth * length / (1/2 * mmf per slot2)
b) zigzag specific permeance = average width of the rotor tooth / length * (1/2 * mmf per slot2)
c) zigzag specific permeance = average width of the rotor tooth * length * (1/2 * mmf per slot2)
d) zigzag specific permeance =1/ average width of the rotor tooth * length * (1/2 * mmf per slot2)
Answer: b
Explanation: The average width of the rotor tooth along with the length and mmf per slot is calculated. On substitution the zigzag specific permeance is obtained.
11. What are the factors the overhang leakage reactance is obtained?
a) length of the overhang
b) diameter of the overhang
c) shape of the overhang
d) length of the overhang along with the shape of the overhang
Answer: d
Explanation: The overhang leakage reactance depends upon the length of the overhang. It also depends upon the shape of the overhang.
Design Of Electromagnetism MCQs
12. The overhang leakage reactance depends on the degree of saturation in the ferromagnetic parts.
a) true
b) false
Answer: a
Explanation: The overhang leakage reactance depends on the length of the overhang and the shape of the overhang. The overhang leakage reactance also depends upon the degree of saturation in the ferromagnetic parts.
13. What is the relation between the overhang specific permeance and the slot pitch?
a) overhang specific permeance is directly proportional to the slot pitch
b) overhang specific permeance is indirectly proportional to the slot pitch
c) overhang specific permeance is directly proportional to the square of the slot pitch
d) overhang specific permeance is indirectly proportional to the square of the slot pitch
Answer: b
Explanation: The overhang specific permeance relation obtained is an empirical relation. In the empirical relation, the overhang specific permeance is indirectly proportional to the slot pitch.
Specific Permeance & Calculation of Magnetizing Current
1. What is specific permeance?
a) specific permeance is product of permeance of unit length and depth of field
b) specific permeance is ratio of permeance of unit length and depth of field
c) specific permeance is the permeance per unit length
d) specific permeance is the permeance per unit pole
Answer: c
Explanation: Specific permeance is defined as permeance per unit length. It is also known as the depth of field.
2. What is the formula of the specific permeance?
a) specific permeance = permeability in air * ∫small change in width + length
b) specific permeance = permeability in air * ∫small change in width/length
c) specific permeance = permeability in air * ∫small change in width * length
d) specific permeance = 1/permeability in air * ∫small change in width * length
Answer: b
Explanation: The permeability in air, small change in width and length is calculated first. On substitution the specific permeance is calculated.
3. What is the assumption made in the calculation of the specific permeance?
a) voltage is kept constant
b) current is kept constant
c) mmf is kept constant
d) speed is kept constant
Answer: c
Explanation: The mmf is kept constant over all the flux tubes. The mmf should be kept constant when the integration is carried out during the calculation of specific permeance.
4. What is the formula of the effective permeance?
a) effective permeance = effective flux/total mmf
b) effective permeance = effective flux/mmf of air gap
c) effective permeance = effective flux * total mmf
d) effective permeance = effective flux * mmf of air gap
Answer: a
Explanation: The effective flux and the total mmf is first calculated. On substitution of the values the effective permeance is calculated.
5. What is the formula of the flux dividing into infinitesimal parts?
a) flux dividing into infinitesimal parts = mmf producing the flux / permeance of infinitesimal part
b) flux dividing into infinitesimal parts = mmf producing the flux * permeance of infinitesimal part
c) flux dividing into infinitesimal parts = mmf producing the flux + permeance of infinitesimal part
d) flux dividing into infinitesimal parts = mmf producing the flux – permeance of infinitesimal part
Answer: b
Explanation: The mmf producing the flux and the permeance of infinitesimal part is calculated. On substitution the flux dividing into infinitesimal parts is calculated.
6. What is the relation between the specific permeance of a differential path and the length?
a) specific permeance of a differential path is directly proportional to the length
b) specific permeance of a differential path is indirectly proportional to the length
c) specific permeance of a differential path is directly proportional to the square of the length
d) specific permeance of a differential path is indirectly proportional to the square of the length
Answer: b
Explanation: The specific permeance is defined as the permeance per unit length. It is indirectly proportional to the length.
7. How many factors does the value of the magnetizing current depends upon?
a) 2
b) 3
c) 4
d) 5
Answer: b
Explanation: There are 3 factors upon which the magnetizing current depends upon. They are total mmf required, number of turns in the exciting winding and upon the way in which the winding is distributed.
8. What is the formula for the magnetizing current?
a) magnetizing current = total mmf * number of turns
b) magnetizing current = total mmf / number of turns
c) magnetizing current = total mmf + number of turns
d) magnetizing current = total mmf – number of turns
Answer: b
Explanation: First the total mmf is calculated along with the number of turns of the magnetizing winding. Then on substitution the magnetizing current is obtained.
9. What is the formula for the rms value of the magnetizing current?
a) rms value of the magnetizing current = maximum magnetizing current / peak factor
b) rms value of the magnetizing current = maximum magnetizing current * peak factor
c) rms value of the magnetizing current = maximum magnetizing current + peak factor
d) rms value of the magnetizing current = maximum magnetizing current – peak factor
Answer: a
Explanation: The maximum value of the magnetizing current and the peak factor are calculated first. On substitution, the rms value of the magnetizing current is obtained.
10. What is the relation of the type of winding with the flux linkage?
a) in distributed windings the flux does not link with all the turns
b) in distributed windings the flux links with all the turns
c) in concentrated windings the flux links with all the turns
d) in concentrated windings the flux does not link with all the turns
Answer: a
Explanation: The magnetizing current is actually calculated for the concentrated windings and the distributed windings. In the distributed windings, the flux does not link with all the turns.
11. What is the relation of the magnetizing current with the turns per phase?
a) magnetizing current is directly proportional to the turns per phase
b) magnetizing current is directly proportional to the square of the turns per phase
c) magnetizing current is indirectly proportional to the turns per phase
d) magnetizing current is indirectly proportional to the turns per phase
Answer: c
Explanation: The magnetizing current is actually calculated for the concentrated windings and the distributed windings. In the distributed windings, magnetizing current is indirectly proportional to the turns per phase.
12. The plot of the flux density distribution curve is between the interpolar axis consisting of the flux density and the angle difference between phases.
a) true
b) false
Answer: a
Explanation: The flux density distribution curve is used to calculate the magnetizing current in the non sinusoidal flux distribution of the distributed windings. The curve is between the flux density and the phase angle.
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
Answer: d
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
Answer: a
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
Answer: a
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
Answer: b
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
Answer: a
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
Answer: c
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
Answer: b
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
Answer: b
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
Answer: a
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
Answer: c
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
Answer: b
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
Answer: d
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)
Answer: a
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.