LABORATORY WORK № 9

DEVELOPING A ROTATING MAGNETIC FIELD

1.1.         Objectives

Get acquainted with starting transients of the current and velocity of the squirrel-cage induction motor.

1.2.         Tasks

1.       Experimentally obtain starting transients of motor phase cur-rent at no load.

2.       Experimentally obtain starting transients of motor speed at no load.

1.3.         Developing a rotating magnetic field

A rotating magnetic field must be developed in the stator of an AC motor in order to produce mechanical rotation of the rotor. Wire is coiled into loops and placed in slots in the motor housing. These loops of wire are referred to as the stator windings. The following drawing illustrates a three-phase stator. Phase windings (A, B, and C) are placed 120° apart. In this example, a second set of three-phase windings is installed. The number of poles is determined by how many times a phase winding appears. In this example each phase winding appears two times. This is a two-pole stator. If each phase winding appeared four times it would be a four-pole stator.

 

 

Fig. 9.1. Two-pole stator winding

 

When AC voltage is applied to the stator, the current flows through the windings. The magnetic field developed in a phase winding depends on the direction of current flow through that winding. The following chart is used here for explanation only. It assumes that a positive current flow in A1, B1 and C1 windings result in a north pole.

Table 9.1

Two-pole stator winding

Winding	Current Flow Direction
	Positive	Negative
A1	North	South
A2	South	North
B1	North	South
B2	South	North
C1	North	South
C2	South	North

It is easier to visualize a magnetic field if time is picked when no current is flowing through one phase. In the following illustration, for example, time has been selected during phase A which has no current flow, phase B has current flow in a negative direction and phase C has current flow in a positive direction.

 

Fig. 9.2. Illustration of rotating magnetic field

 

Based on the above chart, B1 and C2 are south poles and B2 and C1 are north poles. Magnetic lines of flux leave B2 North Pole and enter the nearest South Pole, C2. Magnetic lines of flux also leave C1 North Pole and enter the nearest South Pole B1. Magnetic field results are indicated by the arrow.

The amount of flux lines (F) the magnetic field produces is proportional to the voltage (E) divided by the frequency (f).

Increasing the supply of voltage increases the flux of the magnetic field. Decreasing the frequency increases the flux:

 

                                                               (9.1)

 

Rotor construction

The most common type of rotor is «squirrel cage» rotor. The construction of squirrel cage rotor is reminiscent of rotating exercise wheels found in cages of pet rodents. The rotor consists of a stack of steel laminations with evenly spaced conductor bars around the circumference. The conductor bars are mechanically and electrically connected with end rings. A slight skewing of the bars helps to reduce audible hum. The rotor and shaft are an integral part.

 

Slip

There must be a relative difference in speed between the rotor and the rotating magnetic field. The difference in speed of the rotating magnetic field, expressed in RPM, and the rotor, expressed in RPM, is known as slip.

Slip is expressed as a percentage or as a fraction of the synchronous speed :

 

                                                     (9.2)

 

as related by slip s, defined as:

 

                                                        (9.3)

 

where P is number of poles.

 

1.4.         Method of testing

1.       Measurement of current transient .

2.       Connect the circuit shown in Fig. 9.3.

 

 

 

Fig. 9.3. Electrical circuit for measurement of current and speed

Transients

 

3.       Switch on and adjust the oscilloscope.

4.       Switch on the induction motor and get the curve of current transient in the screen.

5.       Measurement of speed transient .

6.       Connect the oscilloscope to the terminals of tachogenerator BR load resistor.

7.       Switch on and adjust the oscilloscope.

8.       Switch on the induction motor and get the curve, proportional to rotational speed of motor.

9.       Calculate electromechanical time constant from the obtained curve.

 

1.5.         Content of report

1.       Task of the work and experimental circuit.

2.       Experimental curve of current starting transients.

3.       Experimental curve of speed starting transients.

4.       Calculation of electro-mechanical time constant from speed starting transient curve.

5.       Conclusions.

 

1.6.         Control questions

1.       Explain what elements are denoted as QF, PA, PV, TA, BR in the electrical circuit.

2.       What is called electromechanical time constant?

3.       How can you find value of electromechanical speed constant from the speed transient curve?

4.       What current is measure red by ammeter PA?

5.       What will happen if resistance TA will be turned off?

6.       For what purpose is tachogenerator BR used in the circuit?