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9. Explain the operation of a GFCI.
10. At what current value are people-protector GFCIs set for?
Polyphase Induction Motors
Basically, there are three general types of AC motors, namely
1. Synchronous motors
2. Polyphase induction motors
3. Single-phase motors
This chapter will deal with polyphase induction motors, as this type of motor is the most used of the three types and the most trouble-free. Also, the theory that will be learned here will fit in very well with that of the other two types of motors.
The polyphase induction type of motor depends upon the principle of a rotary magnetic field. Polyphase motors using a rotating magnetic field were invented by Nikola Tesla in 1898.
Rotating Magnetic Field
To create a rotary magnetic field of force, it is necessary to have two or more magnetomotive forces acting to produce a flux in the same motor field area, but at a phase angle displacement of both time and space with respect to each other. The two or more magnetic fields combine to create a resultant magnetic field that may be rotated in both directions, and the speed of rotation depends upon the frequency of the AC supply.
Figure 30-1 illustrates the basic principle of a two-phase rotating field, with a phase relationship of 90°. The two-phase field is used for simplification. Three-phase rotating fields produce similar results, except there are three fields of force 120° apart, instead of 90° apart. (In Chapter 23, covering polyphase systems, it was shown that with two-phase systems, the displacement between phases is 90°.)
In Figure 30-1, phase 1 is at a maximum position, 1'; thus, pole A is north and pole A' is south, and phase 2 is zero. Thus, the compass needle points north (N). The poles covered by phase 1 are A and A', and the poles covered by phase 2 are B and B'. For position 2', phase 1 and phase 2 are both in a partial positive direction; thus, poles A and B' are both north and poles B and A' are both south, and the compass needle has moved 45° clockwise. In position 3', phase 1 is zero and phase 2 is maximum positive; thus, we
1' 2' 31 41 5' 61 V 81 1‘
Figure 30-1 Two AC currents 90° apart producing a rotating field.
314 Chapter 30
Polyphase Induction Motors 315
have pole B’ north, and pole B is south, and the compass needle has moved another 90° clockwise.
By following the two sine waves, magnetic poles, and positioning of the compass needle, it will be seen that as phases 1 and 2 move 360°, the compass has made one 360° movement from 1’ back to 1’. Thus, the magnetic field has rotated 360° clockwise in one cycle.
Since most motors are three-phase, they will have three separate windings per pole on the stator (outside stationary portion of the motor). These three pole windings and their currents are 120° apart, and as they rotate around the stator winding, a rotating field is created. The direction of rotation depends upon the phase connections to the stator winding. If a three-phase motor runs the opposite direction to what is wanted, you merely have to reverse the connections of two phase conductors of any of the three conductors. This will reverse the direction of field rotation and the direction of the rotor rotation.
The speed of the rotating magnetic field depends upon the number of poles for which the motor is wound and the frequency and number of phases of the applied AC current. The speed referred to here is the synchronous speed, and the following formula is used to determine the synchronous speed:
Synchronous Speed in rpm
120 X Frequency in Hertz Number of Poles
S = synchronous speed in revolutions per minute f = frequency in hertz P = number of poles
As an example, a three-phase four-pole motor at 60 Hz has a synchronous speed of 1800 rpm, which is calculated as follows:
s = 120^0 = 1800
A three-phase six-pole motor at 60 Hz has a synchronous speed of 1200 rpm:
S = = 1200
316 Chapter 30
The stator or stationary part of the motor has the winding to which the three-phase voltage is applied. The rotor or moving portion of the motor has a much different type of winding from the stator, and the rotor winding is in no manner connected to the applied voltage. It receives its voltage by induction, as does a transformer.
Squirrel Cage Rotor Winding
Figure 30-2 illustrates the squirrel cage winding on a rotor. It is composed of metal bars or rods through holes in the rotor laminations, and these rods or bars are welded, brazed, die-cast, etc., to metal rings around the outside of the motor rotor. This in reality constitutes metal bars through the laminations, and these bars are short-circuited by the metal and rings.
Figure 30-2 Squirrel cage winding.
In the comparison just made to transformers, the stator winding is the primary winding, and the rotor squirrel cage is the secondary winding, but it is a short-circuited winding.
The theory of the manner in which current is induced in the rotor may be illustrated by Figure 30-3. Suppose that a rotating field of force, due to currents in the stator windings, rotates from polar projections A to B to C to D, in the direction of the arrow, K. The lines of force due to this field are in direction D-B. As the field rotates, it cuts squarely across conductors E-F and G-H of the rotor, which for the moment are stationary. This direction of cutting will induce currents that will flow toward the observer in the conductors E-F and away from the observer in conductors G-H. The direction of the resultant flux of these induced currents will evidently be A-C. This produces a north pole on the rotor at N,