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# Important Short Questions and Answers: Design of Electrical Machines - DC Machines

Design of Electrical Machines - DC Machines - Important Short Questions and Answers: Design of Electrical Machines - DC Machines

D.C. MACHINES

1.  Define gap expansion factor and give the equation for it.

The ratio of reluctance of flux path when armature with slot to reluctance of flux path when armature without slot.

Kgs = Ys / (Ys - Kcs Ws) > 1                slots

Kgd = L / (L-Kcd nd Wd) >1 ducts

2.     What is the advantage of large number of poles?

├ś                                          weight of iron parts decreases

├ś                                          weight of copper part decreases

├ś                                          length of commutator reduces

├ś                                          overall length of machine reduces

├ś                                          Distortion of field form becomes less at full load condition.

3.     Why the interlope is used in a dc machine.

├ś                                            To reduce the armature reaction.

├ś                                            To improve commutation.

4.     Why the brush is made up of carbon?

├ś                                                To reduce spark between brush and commutator.

├ś                                                To conduct electric current.

├ś                                                To avoid wear and tear due to rubbing.

5.     Define leakage coefficient and give the equation for it.

The ratio of total flux per pole to the useful flux per pole is called leakage coefficient or leakage factor.

C1 = ąż p/ąż=1.08 to 1.25

6.  Define iron stacking factor.

It is defined as the ratio of net length of armature to the gross length of the armature.

Ki = 0.9 to 0.96

7.  What is meant by peripheral speed of armature?

The distance travel by the armature per unit time is called as peripheral speed. n = speed in r.p.s.

D = diameter of armature in m

8.  Define armature reaction.

The flux produced due to current flow to the armature conductors opposes the main flux. This phenomenon is known as armature reaction.

9.     What are the effects of armature reaction?

├ś                                                    Reduction in emf

├ś                                                    Increase in iron loss

├ś                                                    Sparking and ring fire

├ś                                                    Delayed commutation

10.            What does staggering of brushes mean?

Brushes are provided in different planes instead of same plane at the surface of commutator to avoid the formation of ridges. This is called staggering.

11.            Mention the different modes of operation of a D.C. Machine.

Generator mode: In this mode, the machine is driven by a prime mover with mechanical power converted into electrical power.

├ś                                              Motor mode: The machine drives a mechanical load with the electrical power supplied converted into mechanical power.

├ś                                              Brake mode: The machine works as a generator and the electrical power developed is either pumped back to the supply as in regenerative braking.

12.            State use of a yoke in a D.C. machine.

The yoke serves as a path for flux in D.C. machine and it also serve as an enclosure for the machine.

13.            What purpose is served by the pole shoe in a D.C. machine?

The pole shoes serve the following purposes:

├ś                                                                    They spread out the flux in the air gap.

├ś                                                                    Since they are of larger cross section, the reluctance of the magnetic path is reduced.

├ś                                                                    They support the field coils.

14.            Mention the factors that affect the size of rotating machines.

The factors that affect the size of rotating machines are:

├ś                                                                    Speed and

├ś                                                                    Output co-efficient

15.            What is known as output equation?

The output of a machine can be expressed in terms of its main dimensions, specific magnetic and electric loadings and speed. The equation describing this relationship is known as output equation.

16. Derive the output equation of a D.C. machine.

Power developed by armature in KW,

Pa = Generated emf * armature current * 10-3

Pa = (ą¤ D L Bav ) (ą¤ D ac) n * 10-3

= (ą¤2 Bav ac * 10-3) D2 L n = C0 D2 L n

where C0 = ą¤2 Bav  ac * 10-3

D = armature diameter, m L = stator core length, m n = speed, rps

C0 is the output co-efficient

16.            How is specific magnetic loading determined?

The specific magnetic loading is determined by

├ś                                    Maximum flux density in iron parts of machine

├ś                                    Magnetizing current and core losses

17.            Calculate the output co-efficient of a dc shunt generator from the given data. Bg = 0.89 Wb/m2 ; ac = 3200 amp.cond/m ; ╬© = 0.66.

Output co-efficient , C0 =  ą¤ 2 ╬© Bg  ac * 10-3

=                                                                                                                               ą¤2 * 0.66 * 0.89 * 3200 * 10-3

=                                                                                                                             185.5 KW / m3 ŌĆō rps.

18.            What is the range of specific magnetic loading in D.C. Machine?

The usual range of specific magnetic loading in dc machine is 0.4 to 0.8 Wb/m2.

19. What are the factors to be considered for the selection of number of poles in dc machine?

The factors to be considered for the selection of number of poles in dc machine are:

├ś                                                              Frequency

├ś                                                              Weight of iron parts

├ś                                                              Weight of copper

├ś                                                           Length of commutator

├ś                                                              Lab our charges

├ś                                                              Flash over and distortion of field mmf

20.            What are the quantities that are affected by the number of poles?

Weight of iron and copper, length of commutator and dimension of brushes are the quantities affected by the number of poles.

21. List the disadvantages of large number of poles.

The large number of poles results in increases of the following:

├ś                                            Frequency of flux reversals

├ś                                            Labour charges

├ś                                            Possibility of flash over between brush arms

22.            Mention guiding factors for the selection of number of poles.

├ś                                            The frequency should lie between 25 to 50 Hz.

├ś                                            The value of current per parallel path is limited to 200A, thus the current per brush arm should not be more than 400A.

├ś                                            The armature mmf should not be too large. The mmf per pole should be in the range 5000 to 12,500 AT.

├ś                                            Choose the largest value of poles which satisfies the above three conditions.

23.            What are the losses at the commutator surface?

The losses at the commutator surface are the brush contact losses and brush friction losses.

24. Write down the expression for brush friction losses.

The brush friction loss is given as Pbf = ┬Ą Pb AB Vc

Where                ┬Ą = co-efficient of friction

Pb = brush contact pressure

AB = total contact area of all brushes, m2

Vc = Peripheral speed of commutator, m/s

25.            What are the advantages of carbon brushes?

├ś                                            They lubricate and polish the commutator

├ś                                            If sparking occurs, they damage the commutator less than with the copper brushes.

├ś                                            They provide good commutation.

26.            What is the height occupied by series field coil in a field pole?

In a field pole of compound machine, approximately 80% of the height is occupied by shunt field coil and 20% by the series field coil.

27. How the Ampere turns of the series field coil is estimated?

In compound machines, the ampere turns to be developed by the series field coil are estimated as 15 to 25% of full load armature mmf. In series machines, the ampere turns to be developed by the series field coil are estimated as 1.15 to 1.25 times of full load armature mmf.

28.            Mention the factors to be considered for the design of shunt field coils.

├ś                                            Mmf per pole and flux density

├ś                                            Loss dissipated from the surface of field coil

├ś                                            Resistance of the field coil

├ś                                            Current density in the field conductors

29.            State the use of interpoles.

The interpoles are used in D.C. machines to neutralize the cross magnetizing armature mmf at the interpolar axis and to neutralize the reactance voltage in the coil undergoing commutation.

30.            State the relation between the armature and the commutator diameter for various ratings of D.C. machines. The diameter of the commutator is chosen as 60 to 80% of armature diameter. The limiting factor is the peripheral speed. The typical choice of commutator diameter for various voltage ratings are listed here: Where D is the armature diameter.

31. Why is the value of magnetizing current not a series design consideration in D.C.machines?

The value of magnetizing current is not a series design consideration in D.C.machines as there is sample space on salient poles to accommodate the required number of field turns.

32. What should be the peripheral speed of the commutator?

The commutator peripheral speed is generally kept below 15 m/s. Higher peripheral speeds upto 30m/s are used but should be avoided wherever possible. The higher commutator peripheral speeds generally lead to commutation difficulties.

33.            How is the length of commutator designed?

The length of the commutator is designed based upon the space required by the brushes and upon the surface required to dissipate the heat generated by the commutator losses.

Length of commutator,  Lc = nb ( Wb + Cb )+ C1 + C2

Where nb = number of brushes per spindle

Wb = width of each brush

Cb = clearance between the brushes

C1  = clearance allowed for staggering the brushes

C2  = clearance for allowing the end play

34. What is the purpose of slot insulation?

The conductors are placed on the slots in the armature. When the armature rotates, the insulation of the conductors may damage due to vibrations. This may lead to a short circuit with armature core if the slots are not insulated.

35.            State any three conditions in deciding the choice of number of slots for a large D.C.machine.

├ś                                              The slot loading should be less than 1500 ampere conductors.

├ś                                              The number of slots per pole should be greater than or equal to 9 to avoid sparking.

├ś                                              The slot pitch should lie between 25 to 35 mm.

36.            What are the factors that influence the choice of commutator diameter?

├ś                                            Peripheral speed

├ś                                            The peripheral voltage gradient should be limited to 3V/mm

├ś                                            Number of coils in the armature

37.            What type of copper is used for commutator segment?

The commutator segments are made of hard drawn copper or silver copper (0.05% silver)

38.            What are the materials used for brushes in D.C.machine?

├ś                                            Natural graphite

├ś                                            Hard carbon

├ś                                            Electro graphite

├ś                                            Metal graphite

39.            What are the points to be considered while fixing up the dimensions of the slot?

├ś                                            Excessive flux density

├ś                                            Flux pulsations

├ś                                            Eddy current loss in conductors

├ś                                            Reactance voltage

├ś                                            Mechanical difficulties

40.            Mention the factors that govern the choice of number of armature slots in a d.C.machine.

├ś                                         Slot pitch

├ś                                            Commutation

├ś                                            Suitability for winding

├ś                                            Flux pulsations

41.            What is back pitch?

The distance between top and bottom coil sides of a coil measured around the back of the armature is called the back pitch. The back pitch is measured in terms of coil sides.

42.            When are the pulsations and oscillations of air gap flux reduced to minimum?

The pulsations and oscillations of air gap flux reduced to minimum when,

├ś                                                                    The number of slots under the pole shoe is equal an integer plus ┬Į.

├ś                                                                    The number of slots per pole is equal to an integer plus ┬Į.

43.            What factor decides the minimum number of armature coils?

The maximum voltage between adjacent commutator segments decides the minimum number of coils.

44. Explain how depth of armature core for a D.C. machine is determined.

Let ŪŠ = Flux/pole ; Li = Net iron length of armature;

ŪŠc = Flux in armature core ; dc = depth of armature core ;

Bc = Flux density in the armature core ; Ac = Area of cross-section of armature core.

Now ŪŠc = ŪŠ/2 and Ac = ŪŠc / Bc

Also Ac = Li dc             dc = ŪŠ / 2 Li Bc

45.            List the characteristics of wave winding.

├ś                                            The number of parallel paths is two.

├ś                                            The current through a conductor is Ia / 2 , where Ia is the armature current.

├ś                                            The winding will have less number of conductors with larger area of cross-section

├ś                                            The emf induced in both the parallel paths will be always equal

46.            What are the applications of D.C. special motors?

The D.C. special motors are used in closed loop control system as power actuators and to provide linear motions. They are also used as clutches, couplings, eddy current brakes, very high speed drives, etc.,.

47. Why square pole is preferred?

If the cross-section of the pole body is square then the length of the mean turn of field winding is minimum. Hence to reduce the copper requirement a square cross-section is preferred for the poles of D.C.machine.

48. Distinguish between lap and wave windings used in D.C. machine.

The lap and wave windings primarily differ from each other in the following two factors:

├ś                                                                    The number of circuits between the positive and negative brushes, i.e., number of parallel paths.

├ś                                                                    The manner in which the coil ends are connected to the commutator Segments.

49.            What are dummy coils?

The coils which are placed in armature slot for mechanical balance but not connected electrically to the armature winding are called dummy coils.

50. What are the different types of commutation?

The different types of commutation are:

├ś                                                  Resistance commutation

├ś                                                  Retarded commutation

├ś                                                  Accelerated commutation

├ś                                               Sinusoidal commutation

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Design of Electrical Machines : DC Machines : Important Short Questions and Answers: Design of Electrical Machines - DC Machines |