What is the Three Phase induction motor Working Principle?
Like any electric motor, a 3-phase induction motor has a stator and rotor. The stator carries a 3-phase winding (called stator winding) while the rotor carries a short circuited winding (called rotor winding). Only the stator winding is fed from 3-phase supply. The rotor winding derives its voltage and power from the externally energized stator winding through electromagnetic induction and hence the name acquired. The induction motor may be considered to be a transformer with a rotating secondary and it can, therefore, be described as a "transformer-type" a.c. machine in which electrical energy is converted into mechanical energy.
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What are the advantages and disadvantages of three phase induction motor?
Advantages are:
(i) It has simple and rugged construction.
(ii) It is relatively cheap.
(iii) It requires little maintenance.
(iv) It has high efficiency and reasonably good power factor.
(v) It has self-starting torque.
Disadvantages are:
(i) It is essentially a constant speed motor and its speed cannot be changed easily.
(ii) Its starting torque is inferior to d.c. shunt motor.
Describe the construction of a three phase motor?
A 3-phase induction motor has two main parts (i) stator and (ii) rotor. The rotor is separated from the stator by a small air-gap which ranges from 0.4 mm to 4 mm, depending on the power of the motor.
With what material the Stator of three phase induction motor is made?
It consists of a steel frame which encloses a hollow, cylindrical core made up of thin laminations of silicon steel to reduce hysteresis and eddy current loss. A number of evenly spaced slots are provided on the inner periphery of the laminations. The insulated conductors are placed
in the stator slots and are suitably connected to form a balanced 3-phase star or delta connected circuit. The 3-phase WINDING stator winding is wound for a definite number of poles as per requirement of speed.
What is the effect of no. of poles on speed of motor?
Greater the number of poles, lesser is the speed of the motor and vice-versa. When 3-phase supply is given to the stator winding, a rotating magnetic field of constant magnitude is produced. This rotating field induces currents in the rotor by electromagnetic induction.
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With what material the rotor of 3 phase motor is made?
Rotor. The rotor, mounted on a shaft, is a hollow laminated core having slots on its outer periphery.
In which motor both stator and rotor winding are connected to voltage source?
In d.c. motor where both stator winding (i.e., field winding) and the rotor winding (i.e., armature winding) are connected to a voltage source.
What are the major types of Rotor Winding placed in 3 phase induction motor?
The winding placed in these slots (called rotor winding) may be one of the following two type :
(1) Squirrel cage type
(ii) Wound type
Differentiate between Squirrel cage type rotor and Wound type rotor?
Squirrel cage rotor consists of a laminated cylindrical core having parallel slots on its outer periphery. One copper or aluminum bar is placed in each slot. All these bars are joined at each end by metal rings called end rings. This forms a permanently short-circuited winding which is indestructible. The entire construction (bars and end rings) resembles a. squirrel cage and hence the name. The rotor is not connected electrically to the supply but has current induced in it by transformer action from the stator.
While wound rotor consists of a laminated cylindrical core and carries a 3-phase winding, similar to the one on the stator.
How rotor windings are connected to supply and why wound rotor type is preferred to squirrel cage type induction motor?
The rotor winding is uniformly distributed in the slots and is usually star-connected. The open ends of the rotor winding are brought out and joined to three insulated slip rings mounted on the rotor shaft with one brush resting on each slip ring. The three brushes are connected to a 3-phase star-connected rheostat. At starting, the external resistances are included in the rotor circuit to give a large starting torque. These resistances are gradually reduced to zero as the motor runs up to speed. The external resistances are used during starting period only. When the motor attains normal speed, the three brushes are short-circuited so that wound rotor runs like a squirrel cage rotor.
What do you mean by squirrel cage induction motors?
Those induction motors which employ squirrel cage rotor are called squirrel cage induction motors. Most of 3-phase induction motors use squirrel cage rotor as it has a remarkably simple and robust construction enabling it to operate in the most adverse circumstances.
Why the starting torque of Squirrel cage induction motor is LOW?
It suffers from the disadvantage of a low starting torque. It is because the rotor bars are permanently short-circuited and it is not possible to add any external resistance to the rotor circuit to have a large starting torque.
How Rotating Magnetic Field is produced in three phase induction motor?
When a 3-phase winding is energized from a 3-phase supply, a rotating magnetic field is produced. This field is such that its poles do not remain in a fixed position on the stator but go on shifting their positions around the stator. For this reason, it is called a rotating field.
What is Synchronous speed?
speed at which the revolving flux rotates is called synchronous speed (Ns). its value depends upon the number of poles and the supply frequency.
Since revolutions per second is equal to the revolutions per minute (Ns) divided
by 60 and the number of cycles per second is the frequency f,
NS=(120F)/P.
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Magnetic flux is said to rotate at synchronous speed why?
The speed of the rotating magnetic field is the same as the speed of the alternator that is supplying power to the motor if the two have the same number
of poles. Hence the magnetic flux is said to rotate at synchronous speed.
Please draw the Equivalent Circuit of Induction Motor
in case of a transformer, the approximate equivalent circuit of an induction motor is obtained by shifting the shunt branch (Rc Xm) to the input terminals . This step has been taken on the assumption that voltage drop in R1 and X1 is small and the terminal voltage V1 does not appreciably differ from the induced voltage E1. Fig shows the approximate equivalent circuit per phase of an induction motor where all values have been referred to primary (i.e., stator).
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