Related Posts Plugin for WordPress, Blogger...

Saturday 15 October 2011

SINGLE PHASE INDUCTION MOTORS INTERVIEW QUESTIONS SET 1

Please Name some Single-phase induction motors?
(i) split-phase type
(ii) capacitor type
(iii) shaded-pole type
Please name two types of Repulsion motors?
(i) Repulsion-start induction-run motor
(ii) Repulsion-induction motor
What are different types of single phase Synchronous motors?
(i) Reluctance motor
(ii) Hysteresis motor
How a single phase motor is similar to 3 phase motor?
A single phase induction motor is very similar to a 3-phase squirrel cage induction motor. It has
(i) a squirrel-cage rotor identical to a 3-phase motor
(ii) a single-phase winding on the stator
What is major difference between a 3-phase induction motor and a single-phase induction motor?
Unlike a 3-phase induction motor, a single-phase induction motor is not self- starting but requires some starting means.
Why single phase induction motor does not self starts?
The single-phase stator winding produces a magnetic field that pulsates in strength in a sinusoidal manner. The field polarity reverses after each half cycle but the field does not rotate. Consequently, the alternating flux cannot produce rotation in a stationary squirrel-cage rotor.
What is basic method to start single phase induction motor and why it is not preferred?
If the rotor of a single-phase motor is rotated in one direction by some mechanical means, it will continue to run in the direction of rotation. As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below the synchronous speed. Once the motor is running at this speed, it will continue to rotate even though single-phase current is flowing through the stator winding. This method of starting is generally not convenient for large motors. Nor can it be employed fur a motor located at some inaccessible spot.
Explain why single phase induction motor does not start in the light of Double-Field Revolving Theory?
The double-field revolving theory is proposed to explain this dilemma of no torque at start and yet torque once rotated. This theory is based on the fact that an alternating sinusoidal flux can be represented by two revolving fluxes, each equal to one-half of the maximum value of alternating flux (i.e., m/2) and each rotating at synchronous speed in opposite directions.
Therefore, an alternating field can be replaced by two relating fields of half its amplitude rotating in opposite directions at synchronous speed. Note that the resultant vector of two revolving flux vectors is a stationary vector that oscillates in length with time along X-axis.
What is the position of rotor at standstill?
Consider the case that the rotor is stationary and the stator winding is connected to a single-phase supply. The alternating flux produced by the stator winding can be presented as the sum of two rotating fluxes 1 and 2, each equal to one half of the maximum value of alternating flux and each rotating at synchronous speed (Ns = 120 f/P) in opposite directions.
At standstill, these two torques are equal and opposite and the net torque developed is zero. Therefore, single-phase induction motor is not self-starting.
What is the value of slip at standstill?
Note that each rotating field tends to drive the rotor in the direction in which the field rotates. Thus the point of zero slip for one field corresponds to 200% slip for the other . The value of 100% slip (standstill condition) is the same for both the fields.
Explain motor action from standstill to running condition?
At standstill, s = 1 so that impedances of the two circuits are equal. Therefore, rotor currents are equal i.e., I2f = I2b. However, when the rotor rotates, the impedances of the two rotor circuits are unequal and the rotor current I2b is higher (and also at a lower power factor) than the rotor current I2f. Their m.m.f.s, which oppose the stator m.m.f.s, will result in a reduction of the backward rotating flux. Consequently, as speed increases, the forward flux increases, increasing the driving torque while the backward flux decreases, reducing the opposing torque. The motor-quickly accelerates to the final speed.
clip_image002
How to make Single-Phase Induction Motor Self-Starting?
The single-phase induction motor is not self- starting and it is undesirable to resort to mechanical spinning of the shaft or pulling a belt to start it. To make a single-phase induction motor self-starting, we should somehow produce a revolving stator magnetic field. This may be achieved by converting a single-phase supply into two-phase supply through the use of an additional winding. When the motor attains sufficient speed, the starting means (i.e., additional winding) may be removed depending upon the type of the motor.
Name the types of single phase motors with respect to the method employed to make them self-starting?
(i) Split-phase motors-started by two phase motor action through the use of an auxiliary or starting winding.
(ii) Capacitor motors-started by two-phase motor action through the use of an auxiliary winding and a capacitor.
(iii) Shaded-pole motors-started by the motion of the magnetic field produced by means of a shading coil around a portion of the pole structure.
How to produce Rotating Magnetic Field From 2-Phase Supply?
As with a 3-phase supply, a 2-phase balanced supply also produces a rotating magnetic field of constant magnitude. With the exception of the shaded-pole motor, all single-phase induction motors are started as 2-phase machine. Once so started, the motor will continue to run on single-phase supply.
What is the major cause of noisy operation of the single phase induction motor?
If the two windings arc displaced 90° electrical but produce fields that are not equal and that are not 90° apart in time, the resultant field is still rotating but is not constant in magnitude. One effect of this non uniform rotating field is the production of a torque that is non-uniform and that, therefore, causes noisy operation of the motor. Since 2-phase operation ceases once the motor is started, the operation of the motor then becomes smooth.
Describe the principal of operation of Split-Phase Induction Motor?
The stator of a split-phase induction motor is provided with an auxiliary or starting winding S in addition to the main or running winding M. The starting winding is located 90° electrical from the main winding and operates only during the brief period when the motor starts up. The two windings are so resigned that the starting winding S has a high resistance and relatively small reactance while the main winding M has relatively low resistance and large reactance as shown in the schematic connections in Fig. Consequently, the currents flowing in the two windings have reasonable phase difference c (25° to 30°) as shown in the phasor diagram in Fig.
clip_image002[6]
 
 
When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting winding carries current Is.
Since main winding is made highly inductive while the starting winding
highly resistive, the currents Im and Is have a reasonable phase angle a (25° to 30°) between them as shown in Fig. Consequently, a weak revolving field approximating to that of a 2-phase machine is produced which starts the motor. where k is a constant whose magnitude depends upon the design of the motor.
When the motor reaches about 75% of synchronous speed, the centrifugal switch opens the circuit of the starting winding. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. The normal speed of the motor is below the synchronous speed and depends upon the load on the motor.
What is starting torque and starting current of split phase induction motor?
The spinning torque is 15 to 2 times the full-loud torque mid (lie starting current is 6 to 8 times the full-load current.
Why split-phase induction motors are most popular single- phase motors in the market?
Due to their low cost, split-phase induction motors are most popular single- phase motors in the market.
Why built-in-thermal relay is necessary in split phase induction motor and what is its purpose?
Since the starting winding is made of fine wire, the current density is high and the winding heats up quickly. If the starting period exceeds 5 seconds, the winding may burn out unless the motor is protected by built-in-thermal relay. This motor is, therefore, suitable where starting periods are not frequent.
What is speed variation of split phase induction motor from no load to full load?
An important characteristic of these motors is that they are essentially constant-speed motors. The speed variation is 2-5% from no-load to full- load.
Usually where split phase induction motors are used?
These motors are suitable where a moderate starting torque is required and where starting periods are infrequent e.g., to drive:
(a) fans
(b) washing machines
(c) oil burners
(d) small machine tools etc.
The power rating of such motors generally lies between 60 W and 250 W.
 

































































Thursday 6 October 2011

THREE PHASE INDUCTION MOTOR INTERVIEW QUESTION SET 4

What is the importance of slip in a 3-phase induction motor ?
The rate at which the flux cuts the rotor conductors is directly proportional to the difference between the speed of rotating field (N s) and that of the rotor (N). If the rotor speed were to become equal to the speed of the rotating field, there would be no generated e.m.f. (and hence current) in the rotor conductors. Consequently, there would be no motor action. Thus it is the slip of the rotor (Ns – N r.p.m.) which causes e.m.f.s to be generated and currents to flow in the rotor conductors. This is precisely the principle of operation of 3-phase induction motor.
What is the difference between squirrel cage rotor and wound rotor ?
Essentially, there is little difference between the squirrel cage and wound rotors. The object of the latter is merely to bring out the ends of polyphase rotor winding to slip rings in order that additional external resistance may be connected in series to improve the starting torque.
What are the advantages of wound rotor motors over squirrel cage motors ?
Ans. The wound-rotor motors have the following advantages over the squirrel cage motors :
(i) High starting torque and low starting current.
(ii) Smooth acceleration under heavy load.
(iii) No abnormal heating during starting.
(iv) Good running characteristics after rotor resistance is cut out.
(v) Adjusting speed.
What are the disadvantages of wound rotor motors compared to squirrel cage motors?
The wound-rotor motors have the following disadvantages as compared to squirrel cage motors:
(i) The initial and maintenance costs are greater than that of squirrel cage motors.
(ii) Poor speed regulation when run with resistance in the rotor circuit.
What is the origin of the name squirrel cage rotor ?
When this type of rotor first appeared, the common squirrel was a frequent house pet. The usual cage that housed him contained a rotating wheel that the animal could enter. This wheel afforded exercise and amusement to the pet. Since the rotor resembled squirrel's exercise cage. it was named squirrel cage rotor.
Why is the power factor of a 3-phase induction motor low at no-toad?
Because of air-gap, the reluctance of the magnetic circuit of 3-phase induction motor is very high. Consequently, the current drawn by the motor on no load is largely magnetizing cur­rent; the no load current lagging the applied voltage by a large angle. For this reason, the p.f. of a lightly loaded 3-phase induction motor is very low.
Why is the power factor of a fully loaded 3-phase induction motor not very high?
A 3-phase induction motor draws a large magnetizing current because of high reluctance of the magnetic circuit; air-gap being the main reason. As load is added, the active component of current increases, resulting in higher power factor. However,because of the large value of magnetizing current, which is present regardless of load, the p.f. of 3-phase induction motor even at full-load seldom exceeds 0.85.
What are the advantages of skewed slots in the rotor of a squirrel cage motor ?
It is a usual practice to use the rotor of a squirrel cage motor with skewed slots i.e. slots that are not parallel to the shaft axis. This arrangement offers the following advantages :
(i) It reduces motor noise and vibration.
(ii) It increases the starting torque and reduces the starting current.
(iii) It increases the rotor resistance due Ix increased length of rotor bars.
How will you design the rotor of a squirrel cage motor to have high starting torque ?
When a high starting torque is required, a squirrel cage machine with specially designed rotor can be used without much sacrifice of efficiency but with some reduction of rated output. This can be achieved by either of the following two ways
(i) The rotor bars can be made very deep so that owing to skin effect, their resistance will be high at starting when the rotor frequency is equal to the supply frequency.
(ii) By using double squirrel cage rotor
Why is the maximum torque of a squirrel cage induction motor called pull-out torque ?
The maximum torque of a squirrel cage induction motor is also called pull-out torque because of the way the motor reacts to an overload. After the maximum torque point (which is three to four times the full-load torque), the decrease in rotor* p.f. is greater than the increase in rotor current resulting in a decreasing torque and the motor quickly comes to stop.
It is generally experienced that a lathe will stall on a heavy cut. The machine will slow down as its cutting load is increased until suddenly it will stall and hum or growl loudly. This condition will persist until the load is removed or a fuse blows.
When will rotor resistance dominate rotor reactance and vice-versa?
 (i) When a 3-phase induction motor is in normal operation, the rotor frequency f '= s f where f is the supply frequency) is low and so is the rotor reactance. The current that flows is then largely limited by rotor resistance rather than reactance.
(ii) When the rotor is stationary (i.e. at standstill), s = 1. This means that rotor sees the full‑
line frequency (i.e. f' = f) and its reactance is dominant compared to its resistance.
Why is the sum of rotor core loss and friction and windage losses of a 3-phase induction motor nearly constant at all loads?
This is explained as under :
(i) At no load, the rotor speed is maximum. The frequency and hence the rotor core loss is practically zero. However, the friction and windage losses are maximum.
(ii) When the load is increased, the rotor speed decreases and hence rotor frequency in­creases. Consequently, the rotor core loss increases while friction and Windage losses decrease.
It is found that at all loads, the rotor core loss plus friction and windage losses remain nearly constant.
Explain the statement that induction motor is fundamentally a transformer ?
Ans. The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is a short-circuited secondary. This is evident, particularly when the rotor is stationary. The rotor current establishes a flux which opposes and, therefore, tends to weaken the stator flux. This causes more current to flow in the stator winding just as as increase in secondary current in a transformer causes a corresponding increase in primary current. Very often the analysis of an induction motor is made on the same lines as the transformer with the modifica­tion that short-circuited secondary is considered rotating.
What are the essential differences between 3-phase induction motor and a transformer ?
The essential differences between a 3-phase induction motor and a power transformer are as under :
(1) Unlike that of a transformer, the magnetic circuit of a 3-phase induction motor has an air-gap. This increases the reluctance of the magnetic circuit of the motor to a great extent. Consequently, the magnetizing current drawn by an induction motor is much more than that of a power transformer.
(ii) The windings of a power transformer are cylindrical whereas those of induction motor are distributed. This affects the turn ratio.
(iii) In a 3-phase induction motor, electrical energy is converted into mechanical energy. However, in a transformer, electrical energy is transferred from one circuit to another, usually with a change in voltage level.
(iv) A transformer is a static device and, therefore, friction and windage losses are absent. Howev- r, 3-phase induction motor is a rotating machine and is attended by friction and windage losses. For this reason, the efficiency of a transformer is more than that of an induction motor.
What is the advantage of a double squirrel cage induction motor ?
The advantage of a double-squirrel cage motor is that it provides high starting torque and low starting current.
How does a double-squirrel cage induction motor operate (i) at starting (ii) under running conditions ?
A double-squirrel cage motor has two rotor windings, one inside the other.
impedance of the outer winding is less than that of the inner winding, resulting in a large proportion of current flowing in the outer high-resistance winding. This provides good starting torque.
As the motor accelerates, the rotor frequency decreases, thereby lowering the reactance of inner winding, allowing it to carry a largy proportion of the total current. At the normal operating speed, the rotor frequency is so low that nearly all the rotor current flows in the low-resistance inner cage, resulting in high efficiency and good speed regu­lation.
How is the speed of a squirrel cage induction motor changed?
Ans. The formula for the synchronous speed of a 3-phase induction motor is given by :
Ns =120f /P
It is clear that supply frequency and the number of poles are the only variable factors deter­mining the synchronous speed. A variation of frequency is impossible because motor is connected to commercial supply which has a fixed frequency. Therefore, speed of a squirrel cage induction motor can be changed by varying the number of poles. Multispeed squirrel cage motors are provided with stator windings that can be reconnected to form different number of poles.

























































Wednesday 5 October 2011

THREE PHASE INDUCTION MOTOR INTERVIEW Questions 3

What are the different Ratings used for Induction Motor?
The nameplate of a 3-phase induction motor provides the following information:
(i) Horsepower
(ii) Line voltage
(iii) Line current
(iv) Speed
(v) Frequency
(vi) Temperature rise
What do you mean by horsepower rating and does it shows motor synchronous speed?
The horsepower rating is the mechanical output of the motor when it is operated at rated line voltage, rated frequency and rated speed. Under these conditions, the line current is that specified on the nameplate and the temperature rise does not exceed that specified.
The speed given on the nameplate is the actual speed of the motor at rated full- load; it is not the synchronous speed.
Why double-cage construction is used in Motors?
In order to provide high starting torque at low starting current, double-cage construction is used.
As the name suggests, the rotor of this motor has two squirrel-cage windings located one above the other.
What is the function of outer winding of double cage construction?
The outer winding consists of bars of smaller cross-section short-circuited by end rings. Therefore, the resistance of this winding is high. Since the outer winding has relatively open slots and a poorer flux path around its bars so it has a low inductance. Thus the resistance of the outer squirrel-cage winding is high and its inductance is low.
What is the function of inner winding of double cage construction?
The inner winding consists of bars of greater cross-section short-circuited by end rings. Therefore, the resistance of this winding is low. Since the bars of the inner winding are thoroughly buried in iron, it has a high inductance [See Fig. (8.35 (ii))]. Thus the resistance of the inner squirrel- cage winding is low and its inductance is high.
clip_image002
 
What do you mean by synchronous speed of a 3-phase induction motor ?
The speed at which the flux produced by 3-phase stator windings of induction motor rotates is called the synchronous speed of the motor. It is given by :
Ns = 120 f/p
where Ns= synchronous speed in r.p.m.
f = supply frequency in Hz
P = number of poles
Why is the field producing winding of a 3-phase induction motor made stationary ?
A 3-phase induction motor has two windings viz stator winding supported by the stationary part of the machine and the rotor winding placed on the rotor. So far as the basic operation of the motor is concerned, it makes no difference which winding is located on the stator. The machine will function equally well with field producing winding as either the stationary or rotating element. Making the field winding stationary element eliminates the use of slip rings and brushes and, therefore, will result in a very trouble-free construction.
why does the rotor of a 3-phase induction motor rotate in the same direction as the rotating field ?
When three-phase stator winding is fed from 3-phase supply, a rotating magnetic field is set up which cuts the rotor conductors. Since the rotor circuit is closed, currents start flowing in the rotor conductors. Now, rotor conductors are carrying currents and are in the magnetic field. Therefore, mechanical force acts on the rotor, tending to move it in the same direction as the stator field. The fact that the rotor is urged to follow the stator field (i.e. rotor moves in the direction of stator field) can be explained by Lenz's law. According to Lenz's law, the direction of rotor currents will be such so as to oppose the cause producing them. Now, the cause producing the rotor currents is the relative speed between the rotating field and station­ary rotor. Hence to reduce this relative speed, the rotor starts running in the same direction as the stator field and tries to catch it.
Why cannot 3-phase induction motor run at synchronous 'speed ?
The rotor follows the stator field. In actual practice, rotor, can never reach the speed of stator field (i.e. synchronous speed). If it did, there would be no relative movement between the stator field and rotor conductors and, therefore, no torque to drive the motor. Hence 3-phase induction motor can never run at synchronous speed.
Why is the air gap between the rotor and stator of a 3-phase induction motor kept as short as possible ?
The air gap between the rotor and stator of a 3-phase induction motor is made as small as possible in order that :
(i) mutual flux may be produced with a minimum exciting current.
(it) leakage reactance are as small as possible.
How is magnetizing current kept small in a 3-phase induction motor ?
The magnetizing current drawn by a 3-phase induction motor is very large (30-50% of full-load stator winding current) owing to the presence of air-gap between the stator and rotor of the