UNIT 6

UNIT 6

 

ALTERNATING CURRENT (AC) MACHINERY (PART II)

 

 

                            OBJECTIVES

 

 

General Objective

 

To understand and compare the three phase induction motor

 

 

Specific Objectives

 

By the end of this unit, you would be able to:

 

• explain the construction of the three phase induction motor
• differentiate the construction of the three phase squirrel cage induction motor and the three phase slip ring induction motor
• compare the advantages and disadvantages between squirrel cage induction motor and the slip ring induction motor

 

 

 

                                          INPUT

 

 

 

1. CONSTRUCTION OF THE THREE PHASE INDUCTION MOTOR

 

 

nduction motor has the same physical stator as a synchronous machine. It consists of  two major parts which are stator and rotor. These parts will be discussed in this unit. The rotor of the induction motor has a different rotor construction. There are two different types of induction motor rotor which can be placed inside the stator. One is called a squirrel cage rotor, while the other is called a wound rotor or slip rings rotor. Both of these types will be discussed in this sub-unit. Before that, we should know the terms of the stator and the rotor.

 

 

Figure 6.1: The stator of a typical induction motor

(Source: Electrical Machines, Drives and Power System 5th edition; Wildi Theodore)

 

 

Stator

 

The stator (Figure 6.1) consists of

steel frame that supports a hollow,

cylindrical core made up of stacked

laminations. A number of evenly

spaced slots, punched out of the

internal circumference of the

laminations, provide the space for

the stator winding.

 

2. Rotor

 

The rotor (Figure 6.2) is also composed of punched laminations. These are carefully stacked to create a series rotor slot to provide space for the rotor winding. We used two types of rotor windings: (1) conventional three phase windings made of insulated wire, and (2) squirrel cage windings. The type of winding differentiates the to two main classes of motors: squirrel cage induction motors and slip rings rotor induction motor (also called wound rotor induction motor)             

 

(a) Squirrel cage rotor

(b) Slip rings rotor

 

Figure 6.2: The rotor of a typical induction motor

(Source: Electrical Machines, Drives and Power System 5th edition; Wildi Theodore)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 6 A
 

 

 

 

 

 

1. Circle the best answer

 

1. What are the major parts inside the induction motor

=A=               squirrel cage and wound

=B=               stator and rotor

=C=               steel frame and cylindrical core

=D=               hollow and steel frame

 

2. Choose the best statement about stator

I              steel frame used to support a hollow

II              cylindrical core made up of stacked laminations

III              it gives classes of induction motors

 

=A=               I and II

=B=              II and III

=C=               I and III

=D=              I, II, and III

 

 

 

(iii)              Choose the best statement about stator

I              the rotor is composed of punched lamination

II              punched lamination provide space for the rotor winding

III              it gives classes of induction motors

=A=               I and II

=B=              II and III

=C=               I and III

=D=              I, II, and III

 

2. From these figures given, name each figure.

	(i) …………………………………………..
	(ii) ………………………………………….
	(iii) …………………………………………..

FEEDBACK TO ACTIVITY 6 A
 

 

6.1.              (i) B                            (ii)  A                            (iii) D

 

6.2.              (i) Slip rings rotor                            (ii) stator                            (iii) Squirrel cage rotor

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

E3106/06/10

ELECTRICAL MACHINERY & CONTROL

INPUT

6.2 THE TYPES OF THE THREE PHASE INDUCTION MOTOR

Figure 6.3: Squirrel cage rotor induction motor

There are two types of induction motor which are based on its rotor construction. One is called a squirrel cage rotor, while the other is called a slip rings rotor.

1. Squirrel Cage Rotor Induction Motor

Figure 6.3 shows a squirrel cage rotor induction

motor rotors. A squirrel cage induction motor

rotor consists of a series of conducting bars laid

into slots carved in the face of the rotor and

shorted at either end by large shorting rings.

This design is referred to as a squirrel cage

rotor because the conductors, if examined by

them, would be like one of the exercise wheels that squirrel or hamsters run on.

2. Slip Rings Rotor Induction Motor

Slip rings rotor induction motor is also known as a wound rotor induction motor. A

wound rotor has a complete set of three-phase windings that is a mirror image of

Figure 6.4: Slip rings rotor induction motor

(Source: Electrical Machinery Fundamentals (3rd edition) Stephen J.Chapman)

windings on the stator. The three phases of the rotor windings are usually Y-connected, and the ends of the three rotor wires are tied to slip rings on the rotor’s shaft. The rotor windings are shortened through brushes riding on the slip rings.

Therefore, the wound rotor induction motors have

their rotor currents accessible at the stator brushes,

where they can be examined and where extra

resistance can be inserted into the rotor circuit.

It is possible to take advantage of this feature to

modify the torque-speed characteristic of the motor.

a complete wound rotor induction motor is shown in Figure 6.4.

The evolution of the induction motor? The motors shown in this figure are all rated at 220V and 15hp. There has been a dramatic decrease in motor size and material requirements in induction motors since the first practical ones were produced in the 1890s.

(Source: Electrical Machinery Fundamentals (3rd edition) Stephen J. Chapman)

1903

1910

1920

1940

1954

1974

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 6 B

6.3

I’m consists of a series of conducting bars laid into slots carved in the face of the rotor and shortened at either end by large shorting rings. WHO AM I?

FEEDBACK TO ACTIVITY 6 B

2. Squirrel cage rotor induction motor

INPUT

6.3 ADVANTAGES OF THE TYPES OF INDUCTION MOTOR.

After you have understood the previous sub unit, you should also know the advantages for both of types of induction motor. Table 6.1 present the advantages of both squirrel cage and slip ring rotor induction motor.

Table 6.1: Advantages of The Squirrel Cage and The Slip Ring Rotor

The Squirrel Cage	The Slip Ring Rotor
Cheap and robust Slightly higher efficiency and power factor Explosion-proof since the absence of slip-rings and brushes eliminates the risk of sparking	The starting torque is much higher and the starting current much lower The speed can be varied especially by means of solid-state switching

Test your UNDERSTANDING before you continue to the next input

ACTIVITY 6 C

3. Listed in the table below are the advantages of squirrel cage and slip rings rotor. List out the advantages given in the right columns.

Cheap	Low starting current	robust
Slightly higher efficiency	Speed varied depends on solid-state switching	Slightly power factor
High starting torque

The Squirrel Cage	The Slip Ring Rotor
1. …………………………………………… 2. …………………………………………… 3. …………………………………………… 4. ……………………………………………	……………………………………………. ……………………………………………. ……………………………………………. …………………………………………….

FEEDBACK TO ACTIVITY 6 C

6.4

The Squirrel Cage	The Slip Ring Rotor
Cheap Robust Slightly higher efficiency Slightly power factor	High starting torque Low starting current The speed varied depends on solid-state switching

SELF-ASSESMENT

If you face any problem, discuss it with your lecturer

You are approaching success. TRY all the questions in this self-assessment section and check your answers with those given in the feedback on Self-Assessment given on the next page.

Question 6-1

A. Define stator and rotor?

Question 6-2

A. Describe the slip ring squirrel cage rotor induction motor and the slip ring rotor induction motor.

Question 6-3

A. What are the advantages of the slip ring squirrel cage rotor induction motor and the slip ring rotor induction motor?

FEEDBACK TO SELF-ASSESMENT

Question 6-1

1. The stator consists of steel frame that supports a hollow, cylindrical core made up of stacked laminations. A number of evenly spaced slots, punched out of the internal circumference of the laminations, provide the space for the stator winding. The rotor is also composed of punched laminations. These are carefully stacked to create a series rotor slot to provide space for the rotor winding.

Question 6-2

A. see notes 6.1.1 and 6.1.2

Question 6-3

A. Table 6.1: Advantages of The Squirrel Cage and The Slip Ring Rotor

The Squirrel Cage	The Slip Ring Rotor
Cheapest and more robust Slightly higher efficiency and power factor Explosion-proof since the absence of slip-rings and brushes eliminates the risk of sparking	The starting torque is much higher and the starting current much lower The speed can be varied especially by means of solid-state switching