EE8401 - Electrical Machines - II - Notes - Unit-3 Two marks - EEE 4th Sem

Subject Name: Electrical Machines - II

Subject Code: EE8401

B.E. EEE Dept - semester 04

Anna University Regulation 2017 B.E. EEE Dept 4th semester EE8401(Electrical Machines - II) 2 marks for 3rd unit is provided below. EE8401 all units 2 marks, Electrical Machines - II all units notes & syllabus are listed down for students to utilize it for scoring maximum marks with our study materials.

1.Under what condition, the slip in an induction motor is a) Negative b) Greater than one.

Answer

a) The slip of induction motor is negative when it runs more than the synchronous speed and acts as a generator. b) When the slip of induction motor is greater than one, it runs in a braking mode and it is rotated in opposite direction to the rotating magnetic field.

2.What are the two fundamental characteristics of a rotating magnetic field ?

Answer

The two characteristics of r.m.f. are, 1. The resultant magnitude of r.m.f. is always constant equal to 1.5 ϕₘ where ϕₘ is maximum amplitude of an individual flux due to any phase. 2. It always rotates at a speed equal to synchronous speed in space, given by Nₛ = 120f/P.

3.What measure can be taken for minimizing the effect of crawling in a 3-phase induction motor ?

Answer

The effect of crawling can be minimized by, 1. Selecting proper coil span and distribution of coils. 2. Using skewed rotor. 3. Chosing proper combination of stator and rotor slots.

4.What is the function of slip-ring in 3-phase induction motor ?

Answer

The slip rings are used in the 3-phase induction motor for connecting external resistance in the rotor winding to control the various parameters of the motor.

5.What is an induction generator ?

Answer

When slip of an induction motor is negative i.e. when the induction motor is made to run faster than the synchronous speed then it runs as a generator. It is called as induction generator.

6.What is crawling and cogging in induction motor ?

Answer

Due to variation of air gap reluctance there are slot harmonics present in the induction motor which produce unwanted torques. Out of all the harmonics, 7th harmonic is dominant and resultant torque can be considered to be the sum of fundamental torque and 7th harmonic torque. The 7th harmonic component set up a forward rotating field with a synchronous speed equal to 1/7th of the synchronous speed of the fundamental component. The 7th harmonic torque reaches its maximum positive value just before 1/7th synchronous speed Nₛ. Due to this, resultant torque characteristics shows a dip. And a stable operation is achieved near subsynchronous speed Nₛ/7. Thus motor does not accelerate upto its normal speed and will remain running at a speed nearly Nₛ/7. This is called crawling due to which stator draws high current with noise and vibrations. If number of stator slots S₁ are equal to number of rotor slots S₂ or integral multiple of rotor slots S₂ then variation of reluctance as a function of space will have pronounced effect producing strong forces than the accelerating torque. Due to this motor fails to start. This phenomenon is called cogging. This is observed in squirrel cage induction motor. The cogging can be avoided by selecting proper combination of stator and rotor poles at the time of design.

7.What are the purposes that could be served by external resistors connected in the rotor circuit of phase wound induction motor ?

Answer

1. To increase the starting torque. 2. To limit the high starting stator current. 3. To obtain the speed control.

8.How can the reversal of rotation of polyphase induction motor be attained ?

Answer

The direction of rotation of polyphase induction motor can be reversed by reversing the direction of rotation of rotating magnetic field produced by the stator. This is achieved by interchanging any two supply terminals while connecting to the stator.

9.Write the advantages of slip ring induction motor.

Answer

1. External resistance can be added in the rotor circuit. 2. Starting torque can be controlled. 3. Starting current can be limited. 4. Speed control from the rotor side is possible. 5. Due to the external resistance, the maximum torque at low speeds can be achieved.

10.What are the merits of inner and outer cage of double cage induction motor ?

Answer

Merits of inner cage : 1) The leakage reactance is high. 2) The resistance is small. 3) It operates in running condition. Merits of outer cage : 1) It provides high starting torque. 2) Its resistance is large limiting starting current at improved power p-f.

11.Name the type of rotors of induction motors.

Answer

1. Squirrel cage rotor 2. Wound or slip ring rotor

12.Write down the advantages of double cage rotor induction motor over single squirrel cage motor.

Answer

1. High starting torque 2. Excellent running performance 3. Low starting current. 4. With the proper choice of inner and outer cage parameters, wide range of torque-slip characteristics can be obtained.

13.What is locked rotor torque ?

Answer

The minimum amount of torque which an electric motor develops at all angular positions of rotor, when rated voltage at rated frequency is applied to it at start is called locked rotor torque.

14.Why slots on the rotor of induction motor are skewed ?

Answer

1. A magnetic hum i.e. noise gets reduced due to skewing hence skewing makes the motor operation quiter. 2. It makes the motor operation smooth. 3. The stator and rotor teeth may get magnetically locked. Such a tendency of magnetic locking gets reduced due to skewing. 4. It increases the effective transformation ratio between stator and rotor.

15.Why the stator core of induction motor made of silicon content steel stamping ?

Answer

For reducing the hysteresis loss the material the core must be high grade silicon content steel while the laminated construction using the stampings is used to reduce the eddy current loss.

16.A 3 phase induction motor does not run at synchronous speed. Why ?

Answer

If the rotor of an induction motor catches the speed of the rotating magnetic field, the relative motion between rotor and the rotating magnetic field will vanish (Nₛ - N = 0). So induced e.m.f. will vanish and hence there cannot be rotor current and the rotor flux which is essential to produce the torque on the rotor. Eventually motor will stop. But immediately there will exist a relative motion between rotor and rotating magnetic field and it will start. But due to inertia of rotor, this does not happen in practice and rotor continues to rotate with a speed slightly less than the synchronous speed of the rotating magnetic field in the steady state. Hence a three phase induction motor never rotates at synchronous speed.

17.What is rotating magnetic field ? At what speed it rotates ?

Answer

The rotating magnetic field can be defined as the field or flux having constant amplitude but whose axis is continuously rotating in a plane with a certain speed. In three phase induction motors such a rotating magnetic field is produced by supplying currents to a set of three phase stationary windings, with the help of three phase a.c. supply. It rotates at a speed called synchronous speed given by, Nₛ = 120f/P = speed of R.M.F. where f = Supply frequency in Hz and P = Number of poles

18.On which factors the speed of rotating magnetic field depends ?

Answer

The speed of rotating magnetic field depends on the supply frequency and the number of pole for which winding is wound.

19.For a machine having 4 poles, find the speed of rotating magnetic field if it is excited by 50 Hz supply.

Answer

P=4, f=50 Hz Nₛ = 120f/P = (120*50)/4 = 1500 r.p.m. .. speed of rotating magnetic field

20.What is slip of an induction motor ? State its expression.

Answer

The slip of the induction motor is defined as the difference between the synchronous speed (Nₛ) and actual speed of rotor i.e. motor (N) expressed as a fraction of the synchronous speed (Nₛ). The percentage slip is expressed as, %s = ( (Nₛ - N) / N )* 100

21.What is the slip at start ? How slip affects the rotor frequency ?

Answer

At start the speed of the induction motor is zero hence the slip is unity. Due to the slip the rotor frequency is much less than the supply frequency. The relation between the two is given by, fᵣ = sf Thus frequency of rotor induced e.m.f. in running condition (fᵣ) is slip times the supply frequency (f).

22.State the various losses occurring in an induction motor.

Answer

The various power losses in an induction motor can be classified as, i) Constant losses : These are further divided as core losses in stator and rotor cores and mechanical losses. The core losses are hysteresis and eddy current losses while the mechanical losses are friction and windage losses at the bearing. ii) Variable losses : These include the copper losses in stator and rotor winding due to current flowing in the winding. These are called I²R losses.

23.State the applications of various types of induction motors.

Answer

i) Squirrel cage type of motors having moderate starting torque and constant speed characteristics preferred for driving fans, blowers, water pumps, grinders, lathe machines, printing machines, drilling machine. ii) Slip ring induction motors can have high starting torque as high as maximum torque. Hence they are preferred for lifts, hoists, elevators, cranes, compressors.

24.Why the reactance of three phase induction motor greatly varies between starting and running condition ?

Answer

At start the reactance of induction motor is high. In running condition the frequency of rotor becomes 'sf' which is very small as slip s is very small. The reactance is directly proportional to the frequency. Thus it also reduces according to the frequency. Hence there is considerable variation between reactance of induction motor at start and in running condition.

25.Why rotor core losses are negligible in three phase induction motor ?

Answer

The core losses depend on the voltage and the frequency. In the running condition the induced e.m.f. in the rotor is slip times the induced e.m.f. in the standstill condition while the rotor frequency is slip times the supply frequency. The slip of three phase induction motor is very small hence in running condition rotor induced e.m.f. and rotor frequency both are very small. Hence the rotor core losses are negligible in three phase induction motor.

26.State the principle of 3 phase induction motor.

Answer

When a 3 phase supply is given to the stator winding rotating magnetic field is developed which rotates at synchronous speed. The rotor at rest cuts this rotating magnetic field and e.m.f. is induced in the rotor conductors. As rotor is short circuited, the rotor current is established. Due to this, rotor produces its own flux called rotor flux. This rotor flux interacts with stator rotating magnetic field to produce necessary torque due to which the induction motor rotates. The initial rotor induced e.m.f. is due to induction principle hence the motor is called induction motor.

27.Mention the advantages of squirrel cage induction motor.

Answer

1. Simple construction 2. Slip ring and brushes are absent hence construction is robust and maintenance free. 3. Rotor automatically adjusts itself for same number of poles as that of stator. 4. Rotor resistance is small hence less copper losses and high efficiency.

28.State the effect of rotor resistance on starting torque.

Answer

As the rotor resistance at start increases, the starting torque increases. The starting torque can be maximum when R₂ =X₂.

29.Is it possible to add an external resistance in the rotor circuit of a cage induction motor ? Justify your answer.

Answer

It is not possible to add an external resistance in the rotor circuit of a cage induction motor. In a cage induction motor, the rotor is in the form of bars which are permanently short circuited. Hence its self resistance is almost zero. For such a short circuited rotor, an external resistance can not be added.

30.Why three phase induction motor called singly excited motor ?

Answer

Generally in an electric motor, electric supply is given to both stator and rotor. But in induction motor, only stator is excited by external three phase supply and e.m.f. gets induced in the rotor by induction principle. Such a machine which is excited only from one side is called singly excited hence the three phase induction motor is called singly excited motor.

31.Why are most of the 3-phase induction motors constructed with delta connected stator winding ?

Answer

For the same power rating, the current drawn by the delta connected stator winding, from the supply is much less than the star connected stator winding, in running condition. Due to this, the stator copper losses are less and performance of induction motor is bettter. Hence most of the three phase induction motors are constructed with delta connected stator winding.

32.What is meant by single phasing.

Answer

For proper working of 3 phase induction motor, it must be connected to 3 phase a.c. supply. Once the 3 phase induction motor is started, it continues to run even if one of the three phase supply lines get disconnected. The loss of current through one of the supply lines is called single phasing.

33.What are the effects of single phasing of an induction motor ?

Answer

1. Speed of the motor reduces. 2. A huming noise and vibrations are developed in the motor due to uneven torque production. 3. The current in the remaining two lines increases which is approxiemately 2-4 times the normal values. 4. The motor gets over heated and may burn if single phase protection is not provided. 5. If single phasing occurs when motor is at rest, it cannot start.

34.The starting torque of a squirrel cage induction motor cannot be altered when the applied voltage is constant . Why ?

Answer

The starting torque can be altered by changing rotor resistance for constant voltage. In squirrel cage induction motor, the rotor is short circuited and it is not possible to add external resistance in the rotor circuit. Hence starting torque of squirrel cage induction motor can not be altered for constant voltage.

35.How do change in supply voltage and frequency affect the performance of a 3 phase induction motor ?

Answer

The torque T in the induction motor is directly proportional to sV² where s is slip and V is supply voltage under constant frequency. Thus as voltage changes, the torque changes as V². The slip at which Tₘₐₓ occurs remains same but value of Tₘₐₓ changes as voltage changes. If frequency is changed keeping voltage constant, then air gap flux changes which affects the induction motor performance badly. It may cause core saturation leading to sharp increase in the current. Practically (V/f) ratio is maintained constant to keep air gap flux constant though voltage and frequency are changed.

36.What are the merits and demerits of double squirrel cage induction motors ?

Answer

Merits are, i) Low starting current ii) High starting torque iii) Wide range of torque-slip characteristics can be obtained. iv) Gives excellent running performance. Demerits are, i) Maximum torque is small. ii) Power factor is low. iii) For same rating, cost is higher than single cage motor. iv) Effective rotor resistance is high hence at start rotor heating is large.

37.A 3 phase, 4 pole, induction motor operates from a supply whose frequency is 50 Hz. Calculate the frequency of the rotor current at standstill and the speed at which the magnetic field of the stator is rotating.

Answer

P=4, f=50Hz The frequency of the rotor current at standstill is same as that of supply frequency, which is 50 Hz. The speed of stator magnetic field is Nₛ, given by Nₛ = 120f/p = 120*50/4 = 1500 r.p.m.

38.Explain why an induction motor, at no load, operates at very low power factor.

Answer

On no load, the magnetizing component of input current is the large part while load part is least. This is inductive component of current. As load increases, the in phase current supplied increases while the magnetizing component remains same for all the loads. The higher magnetization current at no load is required to overcome air gap reluctance. Due to this large lagging current component, the p.f. of induction motor is low on no load.

39.What is the use of circle diagram of an induction motor ?

Answer

The circle diagram is useful in predicting the performance parameters of an induction motor such as motor input, losses, power factor, slip, efficiency at any load condition and also the maximum quantities as maximum torque, maximum power factor, maximum output etc.

40.Which two tests are conducted to obtain the data for the circle diagram ?

Answer

1. No load test or open circuit test 2. Blocked rotor test or short circuit test.

41.How to conduct no load test on induction motor ?

Answer

In the no load test, the motor is made to run without any load i.e. no load condition. The speed of the motor is very close to the synchronous speed but less than the synchronous speed. The rated voltage is applied to the stator. The input line current and total input power is measured. The two wattmeter method is used to measure the total input power.

42.State the effect of increase in load on the power factor, efficiency, input current and slip of an induction motor.

Answer

As the load on an induction motor increases the power factor increases, the efficiency increases upto certain load and then decreases, the input current increases and slip also increases.

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