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

Design of Electrical Machines - Induction Motors - Important Short Questions and Answers: Design of Electrical Machines - Induction Motors

INDUCTION MOTORS

1. Define slot space factor.

The slot space factor is the ratio of conductor area per slot and slot area. It gives an indication of the space occupied by the conductors and the space available for insulation. The slot space factor for induction motor varies from 0.25 to

0.4.

2. Define distribution factor or breadth factor.

It is defined as the ratio of resultant emf when the winding is uniformly distributed to the resultant emf when the winding is bunched in the slot.

3. Define winding factor.

It is defined as the product of the pitch factor and the distribution factor.

Kw = Kp * Kd

4.                 Why the low voltage winding is placed nearer to the core and the high voltage winding in case of a core type transformer.

Ø                                          Insulation required will be less

Ø                                          Less possibility for fault occurrence

Ø                                          Easy to provide tapings

5.                 Why is it possible to design alternators to generate much higher voltage than dc generator?

In alternator the winding is provided in stator and hence maximum voltage can be

provided.

In dc generator the winding is provided in rotor and hence it is not possible to generate maximum voltage

6.      Why rotating machines with aluminum armature coils have increased leakage reactance?

Aluminum coils in armature require more space for accommodation of conductors. Large size slots are designed. Hence with large size slots the value of leakage reactance increases.

7.      Why the harmonic leakage flux in squirrel cage induction is motor is zero?

Since the rotor current balances the stator current at every point there is no harmonic leakage flux.

8.      Stepped core section is preferred to a square section for transformer, give reason?

Diameter of circumscribing circle can be reduced giving use of less copper

Due to increase in core area flux density can be reduced which results less iron loss.

9.                 Why choice of high specific loading in the design of synchronous generators loads to poor voltage regulation?

High value of specific electric loading will mean more number of turns per phase. This will cause high value of leakage reactance and poor voltage regulation.

10.            Define real flux density.

It is defined as the ratio of actual flux through the tooth to the tooth area.

11.            List the advantages and disadvantages of using closed type of rotor slot in squirrel cage induction motor.

Ø                                            Low reluctances

Ø                                            Less magnetizing current

Ø                                            Quitter operation

Ø                                            Large leakage reactance and so starting current is limited

12. Write the expression for rotor current. Where Ts = number of turns per phase for stator

Tr = number of turns per phase for rotor

Is  = Stator current

13. What are the ranges of efficiency and power factor in induction motor? 14.            The approximate efficiency of a three phase, 50 Hz, 4 pole induction motor running at 1350 rpm is ----------------------------------------

i)           90% ii) 40% iii) 65%  iv) None of the above.

Ans : i) 90%

15. What is the approximate efficiency of a 60 Hz, 6 pole, 3 phase induction motor running at 1050 rpm?

i)                         72% ii) 81.2% iii) 76.8% iv) 87.5%.

Ans : iv) 87.5%

16.            What is integral slot winding and fractional slot winding?

In integral slot winding, the total number of slots is chosen such that the slots per pole are an integer, which should be a multiple of number of phases. In fractional slot winding, the total number of slots is chosen such that the slots per pole are not an integer.

17.            Why fractional slot winding is not used for induction motor?

Windings with fractional number of slots per pole per phase create asymmetrical mmf distribution around the air gap and favour the creation of noise in the motor. Therefore, fractional windings are not used in induction motor starter.

18.            Write the expression for length of mean turn of stator winding?

Length of mean turn of stator, Lmts = 2L + 2.3 τ + 0.24

Where                  L = Stator core length

τ= pole pitch = П D / p

19. Name the methods used for reducing harmonic torques.

Ø                                    Chording

Ø                                    Integral slot winding

Ø                                    Skewing and

Ø                                    Increasing the length of air gap

20. What is Skewing?

Skewing is twisting either the stator or rotor core. The motor noise, vibrations, cogging and synchronous cusps can be reduced or even entirely eliminated by skewing either the stator or the rotor.

21. Give the expression for rotor current. Sr = Number of rotor slots

22.            What is full pitch and short pitch or chording?

When the coil span is equal to pole pitch (180 deg electrical), the winding is called full pitched winding. When the coil span is less than pole pitch (180 deg electrical), the winding is called short pitched or chorded.

23.            What are the different types of stator windings in induction motor?

Ø                                                        Mush winding

Ø                                                        Lap winding and

Ø                                                        Wave winding

24.            How the induction motor can be designed for best power factor?

For best power factor, the pole pitch τ is chosen such that τ = SQRT [(0.18 L)].

26.            What are the materials used for slip rings and brushes in induction motor?

The slip rings are made of brass or phosphor bronze. The brushes are made of metal graphite, which is an alloy of copper and carbon.

27.            Write the expression for output equation and output co-efficient of induction motor.

The equation for input KVA is considered as output equation in induction motor.

The input KVA, Q = C0 D2 L ns  in KVA

Output co-efficient C0 = 11 Bav ac Kws *10-3 in KVA/ m3 rps.

28.            List the advantages of using open slots.

Ø                                  The winding coils can be formed and fully insulated before installing and also it is easier to replace the individual coils.

Ø                                  It avoids excessive slot leakage thereby reducing the leakage reactance.

29.            Give the advantages of using semi-enclosed stator slots.

The advantages are less air gap contraction factor giving a small value of magnetizing current, low tooth pulsation loss and mush quiter operation(less noise). Semi enclosed slots are mostly preferred for induction motor.

30.            What is the maximum value of flux density in stator teeth?

The maximum value of flux density in stator tooth should not exceed 1.7 Wb/m2.

A high value of flux density leads to a higher iron loss and a greater magnetizing mmf.

28.            What are the problems that occur in induction motor due to certain combinations of stator and rotor slots?

The problems in induction motor due to certain combinations of stator and rotor slots are

Ø                                The motor may refuse to start

Ø                                The motor may crawl at some sub-synchronous speed

Ø                                Severe vibrations are developed and so the noise will be excessive

32. . List the rules for selecting rotor slots.

       Number of stator slots should not be equal to rotor slots satisfactory results are obtained when Sr is 15 to 30% larger or smaller than Ss.

       The difference (Ss - Sr) should not be equal to + or - p, + or – 2p or + or – 5 p to avoid synchronous cusps.

       The difference (Ss - Sr) should not be equal to + or - 1, + or – 2 , + or – (p+1) or + or – (p+2) to avoid noise and vibrations.

32.            What are the main dimensions of induction motor?

Ø                                            Stator core internal diameter

Ø                                            Stator core length

33.            Why induction motor is called as rotating transformer?

The principle of operation of induction motor is similar to that of a transformer. The stator winding is equivalent to primary of a transformer. The rotor winding is equivalent to short circuited secondary of a transformer. In transformer, the secondary is fixed but in induction motor it is allowed to rotate.

35. How slip ring motor is started?

The slip ring motor is started by using rotor resistance starter. The starter consists of star connected variable resistances and protection circuits. The resistances are connected to slip rings. While starting, full resistance is included in the rotor circuit to get high starting torque. Once the rotor starts rotating, the resistances are gradually reduced in steps. At running condition, the slip rings are shorted and so it is equivalent to squirrel cage rotor.

36.            What are the special features of the cage rotor of induction machine?

Ø                                                            The cage rotor can adopt itself for any number of phases and poles

Ø                                                            It is suitable for any type of starting method except using rotor resistance starter

Ø                                                            It is cheaper and rugged

Ø                                                            Rotor over hang leakage reactance is lesser which results in better power factor, greater pull out torque and over load capacity.

37.            Name the materials used to insulate the laminations of the core of induction motor.

The materials used to insulate the laminations of the core of induction motor are kaolin and varnish.

38. Where mush winding is used?

The mush winding is used in small induction motors of ratings less than 5HP.

39. What is the minimum value of slot pitch of a 3 phase induction motor?

The minimum value of slot pitch of a 3 phase induction motor is 15 mm.

40.            Write the formula for air-gap in case of three phase induction motor in terms of length and diameter.

The length of air-gap, lg = 0.2 + 2 SQRT[(D L)] in mm

Where D and L are expressed in meters.

40.            What is crawling and cogging?

Crawling is a phenomenon in which the induction motor runs at a speed lesser than sub synchronous speed.

Cogging is a phenomenon in which the induction motor refuses to start.

42. What is harmonic induction torque and harmonic synchronous torque?

Harmonic induction torques are torques produced by harmonic fields due to stator winding and slots.

Harmonic synchronous torques are torques produced by the combined effect of same order of stator and rotor harmonic fields.

43.            What is the condition for obtaining the maximum torque in case of 3-phase induction motor?

The maximum torque occurs in induction motor when rotor reactance is equal to rotor resistance.

44.            What is runaway speed?

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

45.            State three important features of turbo-alternator rotors.

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.

46.            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 slow speed operations.

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

Number of poles and voltage across each field coil

Amp-turn per pole

Copper loss in field coil

Dissipating surface of field coil

Specific loss dissipation and allowable temperature rise

48. Mention the different tests that conducted in an induction motor.

 No load test or open circuit test

 Short circuit test or load test

49. Give the different runaway speeds for various turbines. 50.     What are the factors that are affected due to SCR.

 Voltage regulation

 Stability

 Short circuit current

 Parallel operation

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