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

SYNCHRONOUS MACHINES

1.     Advantages of stationary armature and rotating field type machine.

^Ø                        Since armature winding is stationary the load circuit can be directly connected to it.

 ^Ø                              As the armature winding is fixed it is easy to provide insulation for high Voltages.

 ^Ø                        Weight of field system is less as compared to armature so that higher speed can be achieved.

^Ø                        Since the exciter supplies low voltage d.c. it requires less amount of insulation.

2.     Define critical speed?

The rotor of an alternator rotates with prime mover speed. The rotor core is structure which has certain mass and property of elasticity. The rotor core is designed corresponding to natural frequency is called critical speed.

3.     Give the importance of compensating winding in dc machine.

^Ø                        It is provided in pole shoe.

^Ø                        It is connected in series with armature winding.

^Ø                        It is used to reduce armature reaction.

^Ø                        Due to this winding full range of speed variation can be obtained.

4.     Mention superiority of hydrogen over air as coolant?

^Ø                                                      Heat transfer co-efficient of hydrogen is 1.5 times that of air.

^Ø                                                      Thermal conductivity of hydrogen is 7 times that of air.

^Ø                                                      Density of hydrogen is 0.07 times that of air.

5.     Why deep bar rotor construction is preferred in squirrel cage induction motor?

It is preferable when high starting torque is required. Because loose bars can be damaged quickly by mechanical vibration and thermal cycling.

6.  What is varnish impregnation?

The dipping of insulating material into varnish to improve the resistance to moisture and creeping discharge is called varnish impregnation.

7.     How to reduce the harmonic effects?

^Ø                                                            Short pitch winding

^Ø                                                            Distributed winding

^Ø                                                            Fractional slot winding

^Ø                                                            Large air gap length

8.     Define heating time constant of the machine.

The time taken by the machine to rise its temperature 63.2% of its final steady value.

9.     What are the types of stator winding?

^Ø                                                        Single layer winding

^Ø                                                        Double layer winding

10.            Why is it necessary to eliminate voids or air packets in high voltage multi lunch coils?

Since the voids carry air and air has poor thermal conductivity heat transfer will be poor. Hence voids should be eliminated.

11.            Classify synchronous machines.

^Ø                                Salient pole machine

^Ø                                Cylindrical rotor machine

12.            List the advantages of revolving field system.

The advantages are

^Ø                                                                      It permits the use of a stationary armature on which the windings can be easily braced and insulated for high voltage.

^Ø                                                                      The operation of slip rings on account of their sliding contact is under liable with large currents at high potential difference. The use of slip ring carrying large currents at high voltage is therefore avoided in the stationary armature construction.

13.              Write the output equation of synchronous machine.

The output equation of an synchronous machine is given by

KVA output Q = C₀ D² L n_s

Where C₀ = output co-efficient

= 1.11 Π² B_av ac K_ws 10^-3

Q = KVA output for alternator and KVA input for synchronous motor.

D = Diameter of stator core, m

L = Length of stator core, m n_s = Synchronous speed, rps

B_av =  Specific magnetic loading, wb/m²

ac = Specific electric loading, amp.cond/m K_ws= stator winding factor

14.              Mention the factors to be considered for the selection of number of armature slots?

^Ø                                                                    Balanced windings

^Ø                                                                    Cost

^Ø                                                                    Host spot temperature in winding

^Ø                                                                    Leakage reaction

^Ø                                                                    Tooth losses

^Ø                                                                    Tooth flux density

15.              What are the types of coils employed by the salient pole machines?

The armature windings of salient pole machines employ two types of coils:

^Ø                                                                        Single turn bar

^Ø                                                                        Multi turn

16.              How are iron and friction losses of an alternator measured?

Iron and friction losses of an alternator can be measured by coupling the alternator to a suitable calibrated d.c. motor and driving it at synchronous speed with normal excitation. Then,

Iron and Friction Losses = Output of motor in Watts.

17. Is the efficiency of an alternator determined by direct loading?

As with d.c. machines, the efficiency of an alternator is not determined by direct loading owing to the difficulty in finding a suitable load. The efficiency is generally determined from losses.

18. Draw a block representing the analysis method of design.

19. Mention the advantages of analysis method.

The advantages are

^Ø                                                                                          It is fairly easy to program, to use and to understand

^Ø                                                                                          Results in considerable time saving thereby giving quick returns of the investments made.

^Ø                                                                                          The programs based upon analysis methods are simple but they become the foundations for later day larger and sophisticated programs.

^Ø                                                                                          The results of analysis method are highly acceptable by designers.

20.        What is the length of mean turn of the armature?

The length of mean turn of the armature is

L_mt = 2 L + 2.5 τ + 0.06 KV +0.2 in metre

Where 2L is the length of turn embedded in the slots

L_mt  = 2.5 τ + 0.06 KV +0.2 in metre

Is the length in the overhang.

21. What is the limiting factor for the diameter of synchronous machine?

The limiting factor for the diameter of synchronous machine is the peripheral speed. The limiting value of peripheral speed is 175 m/sec for cylindrical rotor machines and 80 m/sec for salient pole machines.

22. Write the expression to calculate the height of field winding.

23. What is the total space required for field winding?

24. Give the expression to calculate the area of pole bodies.

25. How is the copper area of field winding calculated?

26. What are the advantages of synthesis method?

The greatest advantage of synthesis method is the savings in time in lapsed time and in engineering man hours on account of the decision making left to the computer itself.

27. What are the disadvantages of synthesis method?

The disadvantages are

^Ø                                                                    The synthesis method involves too much of logic since the logical decisions are taken by the computer. Now, the logical decisions have to be incorporated in the program and before they are incorporated in the program, the teams of engineers have to agree upon them. Firstly the logical decisions to arrive at a optimum design are too many and then there are too many people with too many ways to suggest to produce an optimum design and it becomes really hard to formulate a logic that really produces an optimum design.

^Ø                                                                    The formulation of a synthesis program taking into account the factor listed above would make it too complex. The complex program formulated at high cost would require the use of a large computer and also large running time involving huge expenditure.

28.        How is the efficiency of an alternator affected by load power factor?

The efficiency of an alternator depends not only on KVA output but also on power factor of the load. For a given load, efficiency is maximum at unity power factor and decreases as the power factor falls.

29. Name the two acceptable approaches to machine design.

The two commonly acceptable approaches to machines are

^Ø                                                                                    Analysis method

^Ø                                                                                    Synthesis method

30.        List few advantages of using a digital computer for the design of electrical machines.

The advantages are

^Ø                                                                                  It has capabilities to store amount of data, count integers, round off results down to integers and refers to tables, graphs and other data in advance.

^Ø                                                                                  It makes it possible to select an optimized design with a reduction in cost and improvement in performance.

31.        Give the purpose of providing damper windings in synchronous machines.

The purpose of damper winding is

^Ø                                                                    In synchronous generators, it is provided to suppress the negative sequence field and to damp the oscillations when the machine starts hunting.

^Ø                                                                    In synchronous motor, its function is to provide starting torque and to develop damping power when the machine starts hunting.

32.        What is the range of rotor current density?

Rotor current density ranges from about 2.5 A/mm² for conventionally cooled machines. However, in modern direct cooled generators, the rotor current density may be as high as 9.5– 14 A/mm².

33. Write the expression for air gap length in cylindrical rotor machine.

34. Mention the factors that govern the design of field system of alternator.

The following factors to be considered for the design of field system in alternator:

^Ø                                                                    Number of poles and voltage across each field coil

^Ø                                                                    Amp-turn per pole

^Ø                                                                    Copper loss in field coil

^Ø                                                                    Dissipating surface of field coil

Ø                                                                 Specific loss dissipation and allowable temperature rise

35. What is runway speed?

The runway speed is defined as the speed which the prime mover would have, if it is suddenly unloaded, when working at its rated load.

36.                    State the important features of turbo-alternators.

^Ø                                            The rotors of turbo-alternators have large axial length and small diameters.

^Ø                                            Damping torque is provided by the rotor itself and so there is no necessity for additional damper winding.

^Ø                                            They are suitable for high speed operations and so number of poles is usually 2 or 4.

37.                    What are the prime movers used for a) salient pole alternator b) Non-salient pole alternator?

The prime movers used for salient pole alternators are water wheels like Kaplan turbine, Francis turbine, pelton wheel, etc., and diesel or petrol engines.

The prime movers used for Non-salient pole alternators are steam turbines and gas turbines.

38.                    Distinguish between cylindrical pole and salient pole construction.

In cylindrical pole construction the rotor is made of solid cylinder and slots are cut on the outer periphery of the cylinder to accommodate field conductors.

In salient pole construction, the circular or rectangular poles are mounted on the outer surface of a cylinder. The field coils are fixed on the pole.

The cylindrical pole construction is suitable for high speed operations, whereas the salient pole construction is suitable for the slow speed operations.

39. Salient pole alternators are not suitable for high speeds. Why?

The salient pole rotors cannot withstand the mechanical stresses developed at high speeds. The projecting poles may be damaged due to mechanical stresses.

40. State the factors for separation of D and L for cylindrical rotor machine.

The separation of D and L in cylindrical rotor machine depends on the following factors:

^Ø                                                                      Peripheral speed

^Ø                                                                      Number of poles

^Ø                                                                      Short circuit ratio (SCR)

41. Define pitch factor.

42. Define distribution factor.

43.                    Mention the advantages of fractional slot winding.

^Ø                                                                    In low speed machines with large number of poles, the fractional slot winding will reduce tooth harmonics.

^Ø                                                                    A range of machines with different speeds can be designed with a single lamination.

^Ø                                                                    The fractional slot winding reduces the harmonics in mf and the leakage reactance of the windings.

^Ø                                                                    The fractional slot winding allows only short chorded winding. Therefore the length of mean turn of a coil reduces which results in shorter end connections and so saving in copper.

44.                    What is short circuit ratio (SCR)?

The Short circuit ratio (SCR) is defined as the ratio of field current required to produce rated voltage on open circuit to field current required to circulate rated current at short circuit.

It is also given by the reciprocal of synchronous reactance, X_d in p.u.

^Ø                                                                                            For turbo-alternators SCR is between 0.5 to 0.7.

^Ø                                                                                            For salient pole alternator SCR varies from 1.0 to 1.5

45.                    List the factors to be considered for the choice of specific electric loading in synchronous machines.

^Ø                                                                                            Copper loss

^Ø                                                                                            Temperature rise

^Ø                                                                                            Synchronous reactance

^Ø                                                                                            Stray load losses

^Ø                                                                                            Voltage rating

46.                    Determine the total number of slots in the stator of an alternator having 4 poles, 3 phase, 6 slots per pole for each phase?

Total number of slots = slots/pole/phase * number of poles * Number of phases

=                                                                                                                       6 * 4* 3

=                                                                                                                       72 slots.

47.                    How the value of SCR affects the design of alternator.

For high stability and low regulation, the value of SCR should be high, which requires large air gap. When the length of air gap is large, the mmf requirement will be high and so the field system will be large. Hence the machine will be costlier.

48.                    What are the advantages of large air gap in synchronous machines?

The advantages of large air gap are:

^Ø                                      Reduction in armature reaction

^Ø                                      Small value of regulation

^Ø                                      Higher value of stability

^Ø                                      Better cooling

^Ø                                      Lower tooth pulsation losses

^Ø                                      Smaller unbalanced magnetic pull

49.                    Write the expression for the length of air gap in salient pole synchronous machine.

50. Why alternators are rated in KVA?

The KVA rating of ac machine depends on the power factor of the load. The power factor in turn depends on the operating conditions. The operating conditions differ from place to place. Therefore the KVA rating is specified for all ac machines.