SINGLE AND 3-PHASE INDUCTION MOTORS

 3-PHASE INDUCTION MOTORS: There are two important factors to be considered in starting of induction motors:

1. The starting current is drawn from the supply, and
2. The starting torque.

The starting current should be kept low to avoid overheating of the motor and excessive voltage drops in the supply network. The starting torque must be about 50 to 100% more than the expected load torque to ensure that the motor runs up in a reasonably short time. 

 At synchronous speed, s = 0, and therefore ,  R /s= ∞ .so I2' = 0. 

The stator current, therefore comprises only the magnetizingng current i.e. I1 = Iφ and is quite therefore quite small. 

At low speeds, 2 2 'R jX s + = ∞ is small, and therefore I2' is quite high and consequently, I1 is quite large.

  Actua the typical starting currents for an induction machine are ~ 5 to 8 times the normal running current. 

Hence the starting currents should be reduced. The most usual methods of starting 3-

phase induction motors are: 

For slip-ring motors 

      Rotor resistance starting 

For squirrel-cage motors 

      Direct-on -line starting 

    Star-delta starting 

    Autotransformer starting. 

1. Rotor resistance starting 

By adding eternal resistance to the rotor circuit any starting torque up to the maximum torque can be achieved; and by gradually cutting out the resistance a high torque can be maintained throughout the starting period. The added resistance also reduces the starting current, so that a starting torque in the range of 2 to 2.5 times the full load torque can be obtained at a starting current of 1 to 1.5 times the full load current. 

2. Direct-on-line starting 

This is the most simple and inexpensive method of starting a squirrel cage induction motor. The motor is switched on directly to full supply voltage. The initial starting current is large, normally about 5 to 7 times the rated current but the starting torque is likely to be 0.75 to 2 times the full load torque. To avoid excessive supply voltage drops because of large starting currents the method is restricted to small motors only.  

To decrease the starting current cage motors of medium and larger sizes are started at a reduced supply voltage. The reduced supply voltage starting is applied in the next two methods. 

3. Star-Delta starting 

This is applicable to motors designed for delta connection in normal running conditions Both ends of each phase of the stator winding are brought out and connected to a 3-phase change -over the switch.For starting, the stator windings are connected in star and when the machine is running the switch is thrown quickly to the running position, thus connecting the motor in delta for normal operation. The phase voltages & the phase currents of the motor in star connection are reduced to 1√3 of the direct -on -line values in delta. The line current is 1/3 of the value in delta.  A disadvantage of this method is that the starting torque (which is proportional to the square of the applied voltage) is also reduced to 1/3 of its delta value. 

4. Auto-transformer starting

This method also reduces the initial voltage applied to the motor and therefore the starting current and torque. The motor, which can be connected permanently in delta or in the tar, is switched first on the reduced voltage from a 3-phase tapped auto -transformer and when it has accelerated sufficiently, it is switched to the running (full voltage) position. 

The principle is similar to star/delta starting and has similar limitations. The advantage of the method is that the current and torque can be adjusted to the required value, by taking the correct tapping on the autotransformer. This method is more expensive because of the additional autotransformer.   

SINGLE-PHASE INDUCTION MOTORS : 

There are probably more single-phase ac induction motors in use today than the total of all the other types put together.  

It is logical that the least expensive, lowest maintenance type of ac motor should beused most often. The single-phase ac induction motor fits that description.  Unlike polyphase induction motors, the stator field in the single-phase motor does not rotate. Instead it simply alternates polarity between poles as the ac voltage changes polarity.  

Voltage is induced in the rotor as a result of magnetic induction, and a magnetic field is produced around the rotor. This field will always be in opposition to the stator field (Lenz's law applies). The interaction between the rotor and stator fields will not produce rotation, however. The interaction is shown by the double-ended arrow in figure below, view A. Because this force is across the rotor and through the pole pieces, there is no rotary motion, just a push and/or pull along this line.

Now, if the rotor is rotated by some outside force (a twist of your hand, or something), the push-pull along the line in figure 4-10, view A, is disturbed. Look at the fields as shown in figure, view B. At this instant the south pole on the rotor is being attracted by the left-hand pole. The north rotor pole is being attracted to the right-hand pole. All of this is a result of the rotor being rotated 90° by the outside force. The pull that now exists between the two fields becomes a rotary force, turning the rotor toward magnetic correspondence with the stator. Because of the two fields continuously alternate, they will never actually line up, and the rotor will continue to turn once started. It  remains for us to learn practical methods of getting the rotor to start.  

There are several types of single-phase induction motors in use today. Basically, they are identical except for the means of starting. In this chapter we will discuss the split-phase and shaded-pole motors; so named because of the methods employed to get them started. Once they are up to operating speed, all single-phase induction motors operate the same.