THREE-PHASE AC MOTORS
Industry uses, in addition to single-phase AC, a power source called polyphase AC (“poly” meaning “many”).The most common form of polyphase AC is three-phase. Three-phase AC consists of three alternating currents of equal frequency and amplitude, but differing in phase from each other by one-third of a period.
The biggest advantage in using three-phase power is in the machines it supplies. Three-phase motors are much simpler in construction than other types and, hence, require less maintenance. A more powerful machine can be built into a smaller frame and it will operate at a higher efficiency. All AC motors then can be classified into single-phase and polyphase motors. Because polyphase motors are the most commonly used in industrial applications. Polyphase motors make up the largest single type in use today and usually are the first to be considered for the average industrial application. There are several types of polyphase motors. The most common type of motor in this group is the squirrel-cage polyphase induction motor so called because the rotor is constructed like a squirrel-cage
The next figure shows a cutaway view of a three-phase induction motor. There is very little difference between the AC motor and the AC generator. The rotor is supported by bearings at each end. The stator is freed in position to the inside of the motor frame. The frame encloses all the components of the motor.
REVOLVING FIELD OPERATION
The rotating field is set up by out-of-phase currents in the stator windings. The figure below shows the manner in which a rotating field is produced by stationary coils or windings when they are supplied by a three-phase current source. For the purpose of explanation, rotation of the field is developed in the figure by "stopping" it at six selected positions, or instants. These instants are marked off at 60-degree intervals on the sine waves representing currents in the three phases A, B, and C.
At instant 1, the current in phase B is maximum positive. (Assume plus 10 amperes in this example.) Current is considered to be positive when it is flowing out from a motor terminal. At the same time (instant 1), current flows into A and C terminals at half value (minus 5 amperes each in this case). These currents combine at the neutral (common connection) to supply plus 10 amperes out through the B phase.
The resulting field at instant 1 is established downward and to the right as shown by the arrow NS. The major part of this field is produced by the B phase (full strength at this time) and is aided by the adjacent phases A and C (half strength). The weaker parts of the field are indicated by the letters n ands. The field is a two-pole field extending across the space that would normally contain the rotor.
At instant 2, the current in phase B is reduced to half value (plus 5 amperes in this example). The current in phase C has reversed its flow from minus 5 amperes to plus 5 amperes, and the current in phase A has increased from minus 5 amperes to minus 10 amperes.
The resulting field at instant 2 is now established upward and to the right as shown by the arrow NS. The major part of the field is produced by phase A (full strength) and the weaker parts by phases B and C (half strength).
At instant 3, the current in phase C is plus 10 amperes, and the field extends vertically upward. At instant 4 the current in phase B becomes minus 10 amperes, and the field extends upward and to the left. At instant 5, the current in phase A becomes plus 10 amperes, and the field extends downward and to the left. At instant 6, the current in phase C is minus 10 amperes, and the field extends vertically downward. In instant 7 (not shown), the current corresponds to instant 1 when the field again extends downward and to the right.
Thus, a full rotation of the two-pole field has been done through one full cycle of 360 electrical degrees of the three-phase currents flowing through the stator windings.
SYNCHRONOUS SPEED
The number of poles in the motor will determine how many times the magnetic field in the stator revolves for any given generated frequency. The term "pole" should bring to mind the terms used in Chapter 2 on magnetism. The following definition of a motor pole gives it a practical application value: A motor pole is the completed circuit of a motor stator winding that, when energized by a current, will produce a magnetic field concentration, or polarity.
The speed of the revolving stator field is called synchronous speed. The synchronous speed depends on two factors:
- - The number of poles.
- - The frequency of the power source.
The synchronous speed, in turn, determines the speed of the motor rotor. Just as with the generator prime mover speed, the generated frequency and rotor speed are directly related. The number of poles in the motor determines how fast the revolving field will move around the inside periphery of the motor housing at a given frequency. The more poles a motor has, the longer it takes to energize all the sets of poles and the slower the motor field will revolve at 60 hertz.
Reference
- http://www.oddparts.com/acsi/motortut.htm
- http://www.pacontrol.com/3phasemotors1.html
No comments:
Post a Comment