Three Phase Induction Motors

3 Phase induction motors are most widely used in industries. It has the following advantages:

1. It is very simple and mechanically strong

2. It has sufficiently high efficiency

3. It is self-starting

4. It requires minimum of maintenance

However, it has some disadvantages also:

1. Its speed decreases with increase in load

2. Its starting torque is lower than that of an equivalent DC shunt motor

3. Its speed cannot be varied without sacrificing some of its efficiency Construction

An induction motor consists of two main parts:

1. Stator

2. Rotor.

1. The Stator:

   Stator has three parts

   1. Frame
   2. Core
   3. Winding

   1. Stator Frame:

      It is made of cast iron. It is used for supporting and protecting the armature core stampings and winding coils end. It has different shapes. A typical shape is shown in Figure. 1:

      Figure 1: Stator Frame of Three Phase Induction Motor

   2. Core:

      The core of an induction motor is buildup of silicon steel laminations. The inner part of the core has slots. The slots of large motors are open type but in smaller motors the slots are semi-closed. See Figure. 2

      Figure 1: Core of Three Phase Induction Motor

   3. Windings:

      The windings of each phase is made of multi-turn coils and are distributed in several slots per pole. Three phase supply is given to these windings.

2. Rotor:

   The rotor also has a core and windings. It rotates inside the stator. The core of rotor is made of silicon steel laminations. It has slots in the outer cylindrical surface. Rotors are of two type:

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

   1. Squirrel-cage rotor:

      It is the simplest type of induction motor and most generally used. It consists of a cylindrical laminated core with parallel slots for carrying rotor conductors. These conductors (also called bars) are permanently short circuited on themselves at the end. Hence no any extra resistance can be added to it. The slots of rotor are made of skew type (i.e. these are not quite parallel to the shaft) as shown in Figure. 3. This type of shape reduces the magnetic hum and locking tendency.

      Figure 3: Squirrel-Cage Rotor of Three Phase Induction Motor

   2. Wound Rotor:

      This type of rotor has windings. These windings are wound over the rotor core for same number of poles and phases as the stator has. The rotor is made star connected. The other three terminals of the rotor are brought out to connect with the three slip rings. To increase the starting torque, extra resistances can be added to this type of rotor. See Figure 4.

      Figure 4: Wound Rotor of Three Phase Induction Motor

Production Of Rotating Magnetic Field

When stationary coils (wound for two or three phases) are supplied by two or three phase supply respectively, a uniformly rotating magnetic field of constant magnitude is produced.

Two Phase Supply

Consider a 2-phase, 2-pole stator having identical windings 90 space degree apart as shown in Figure 5.

Figure 5: Two Phase Supply given to Stator

The flux waves produced due to 2-phase supply is shown in Figure 6. Let Ø1 and Ø2 are the instantaneous values of these fluxes. The resultant flux Ør at any instant is the Vector sum of these two fluxes (i.e., Ø1 and Ø2) at that instant.

Figure 6: Flux Wave Production in Stator

Now we consider the condition of flux at different intervals. Let Øm is the maximum value of the flux.

1. When Ø = 0° then Ø1= 0 and Ø2 = Øm and is negative. Now Ør = Øm and is negative. Value of Ør is represented by a vector pointing

2. When Ø = 45°, Ø1 = Øm /√2 and is positive, Ø2 = Øm /√2 and is negative.
   The resultant flux, Ør is downwards. See Figure 7(i)

   Again Ør = Øm but shifted through 45° clockwise as shown in Figure 7(ii)

3. When Ø = 90°, then Ø1 = Øm and Ø2 = 0 so Ør = Øm and is further shifted through an angle of 45o clockwise. Figure 7(iii)

4. When Ø = 135' then Ø1 = Øm/√2 and is positive, Ø2 = Øm/√2 is positive. Again Ør = fm and shifted further 45° clockwise. See Figure 7(iv)

5. When 0 = 180° Ø1 = 0° and Ø2 = Øm and Ør = Øm and shifted 45° clockwise. Figure 7(v)

Figure 7

Hence it concluded that:

1. the magnitude of the resultant vector is constant and is equal to Øm.
2. the resultant vector rotates at synchronous speed.

Three Phase Supply

Consider a 3-phase, two pole stator having three identical winding and are 120 space degrees apart as shown in Figure 8. The flux waves produce due to 3-phase supply is shown in Figure 8(c).

Figure 8: (a)(b)Three Phase Supply Given to Stator. (c) Three Phase Flux Produced in the Stator

Let Ø1, Ø2, Ø3 are the instantaneous values of the fluxes and Øm is maximum value of the flux. The resultant flux Ør is the vector sum of Ø1, Ø2, Ø3 Now we consider the condition of flux at different intervals.

1. When:

   θ = 0° then Ø1 = 0, Ø2 = - (√3/2) Øm
   Ø3 = (√3/2) Øm
   So Ør = 2 X (√3/2) Øm cos 60/2 = 1.5 Øm
   the vector Ør is shown downward in Figure 9(i)

2. When:

   θ = 60° then Ø1 = (√3/2) Øm, Ø2 = -(√3/2) Øm and Ø3 = 0
   So Ør = 2 X (√3/2) Øm cos 30o = 1.5 Øm
   The vector Ør rotated through an angle of 60° clockwise. Figure 9(ii).

3. when:

   θ = 120° then Ø1 = (√3/2) Øm, Ø2 = 0 and Ø3 = -(√3/2) Øm
   Again Ør = 1.5 Øm and is further rotated through an angle of 60° clockwise. Figure 9(iv)

4. when:

   0 = 180° then Ø1 = 0, Ø2 = (√3/2) Øm and Ø3 = -(√3/2) Øm
   Again Ør = 1.5 Øm and is rotated further through an angle of 60° clockwise Figure 9(iv).

Figure 9: Rotating of Magnetic Field

Hence, we conclude that:

1. The resultant flux is of constant magnitude i.e. 1.5 times the maximum value of flux.
2. The resultant flux rotates round the stator at synchronous speed.