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.
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.
