THREE-PHASE ALTERNATING CURRENT MOTORS - Free Online Tutorial Part 1
Most of the power-generating systems produce AC. For this reason, a majority of the motors used operate on AC. There are other advantages to using AC. In general, AC motors are less expensive and easier to maintain than DC machines.
An AC motor is particularly well suited for constant speed operations. This is because its speed is determined by the frequency of the power source and the number of poles constructed in the motor.
Alternating current motors are built in different sizes, shapes, and ratings for many different applications. It is impossible to address all forms of AC motors in this text. This article will address only the squirrel cage induction motor.
INDUCTION MOTOR PRINCIPLE
The principle of the revolving magnetic field is the key to the operation of the AC motor. Induction motors rely on revolving magnetic fields in their stators (stationary windings) to cause their rotors to turn. Stators themselves do not turn. Stators are permanently attached to the inside of the motor housing in the same manner that the stationary windings in the generator are connected to the main frame. The revolving magnetic fields created in the stator windings provide the necessary torque to move the rotor.
The idea is simple. A magnetic field in a stator can be made to appear to rotate electrically, around the inside periphery of the motor housing. This is done by overlapping several different stator windings. A magnetic field is developed in each different stator winding at a different time. Just before the magnetic field of one winding decays, the winding overlapping it develops the same magnetic polarity. As this second magnetic field decays in the second winding, another overlapping winding develops a magnetic field of the same polarity, and the sequence repeats itself. Successive stator windings develop magnetic fields in an orderly procession and appear to progressively move around the inside of the motor housing.
These individual magnetic fields are the property of current flow in the motor stator. This current flow comes from the three individual phase currents of the three-phase generator output. The figure shows the three single-phase voltages/currents that develop in the generator main armature completing individual circuits. Circuit A-B in the generator armature has a like A-B winding in the motor's stator. Each of the three circuit combinations (A-B, B-C, and C-A) are developed independently in the generator over a short period of time. The generator circuits are then completed through the motor's stator windings in a similar manner. As long as the current and magnetic field develops and decays in an orderly, progressive manner around the periphery of the motor frame, a revolving magnetic field exists.
A revolving magnetic field in the stator is only part of the operation. Another magnetic field needs to be created in the rotor so that the torque and rotation can develop using the principles of magnetic attraction and repulsion. The magnetic field developed in the rotor is a product of induction. As soon as the stator and the rotor windings develop their magnetic affiliation, torque will develop, and the rotor will turn.
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.
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.
The table shows the speed of the revolving field (or synchronous speed) for a 60-hertz generated power supply.
DIRECTION OF ROTATION
The direction of rotation of three-phase machines are determined by the phase sequence. Normal phase sequence on board Army vessels is A-B-C. This can be verified from the switchboard. A set of lights indicates the phase sequence from the power source.
As the generator rotates, current flow is induced in the armature. Each phase in the armature becomes electrically active. The order in which the phases become electrically active determines the order in which the motor's stator receives the current. The motor that receives current A-B-C-A-B-C will rotate in a given direction. If any two leads change places, then the two affected phases change their sequence of arrival. If phases B and C are exchanged, then phase C will follow phase A. This reverses the direction of the revolving magnetic field in the stator. Current arrives at the motor C-BA-C-B-A. When the revolving field in the motor's stator changes direction, the motor's rotor changes direction. Reversing the generator's output will turn the motor's rotor in the opposite direction as well. If the generator's output is reversed, then it is known as C-B-A phase sequence.
By reversing any two phase wires, either at the generator's armature or the motor's terminals, the phase sequence will change at that point. Reversing any two leads, at the same point, will restore normal phase sequence. Industry standard dictates configuration control by identifying the conductors to be exchanged: the A and C phase for generators, P1 and P3 for feeders, L1 and L3 for branch circuits, or T1 and T3 for motor terminals.
CONSTRUCTION AND OPERATION
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.
The motor frame, among other considerations, is a determining factor in the placement of the motor. Each motor frame enclosure has certain characteristics and specific vessel applications. There are seven basic types of enclosures:
In an open-type enclosure, the end bells are open and provide for maximum motor ventilation. This is the lowest cost motor enclosure.
In a semiguarded enclosure, the end bells are open, but screens are provided to prevent objects from falling into the motor. There is no protection against water or liquids.
In a guarded enclosure, screens and guards exist over any opening in the motor housing. Limited openings are provided to limit access to live and rotating components within the motor enclosure. Generally, the holes must prevent a 1/2-inch diameter rod from entering the enclosure.
In a drip-proof enclosure, the end bells are covered to prevent liquid from entering the enclosure at an angle not greater than 15 degrees from the vertical.
In a splash-proof enclosure, the motor openings are constructed to prevent liquid drops or solid particles from entering the motor at any angle not greater than 100 degrees from the vertical.
A waterproof enclosure prevents any moisture or water leakage from entering the motor and interfering with its successful operation.
A watertight enclosure prevents a stream of water from a hose (not less than 1 inch in diameter, under a head of 35 feet, from a distance of 10 feet) from any direction from entering the motor for a period of at least 15 minutes.
Electric equipment exposed to the weather or in a space where it is exposed to seas, splashing, or similar conditions must be watertight or in a watertight enclosure. Electric motors, however, must be either watertight or waterproof (Code of Federal Regulations, Title 46, Subpart 111.01-9).
The motor stator is the stationary winding bolted to the inside of the motor housing. The stator windings have a very low resistance. The three-phase AC generator armature is built very similar to the three-phase AC motor stator. Each machine has the stationary conductor winding insulated its entire length to prevent turn-to-turn shorts. The winding is also insulated from the frame. The motor stator winding is identical to a generator armature that has a like amount of poles. Each winding is overlapped and is electrically and mechanically 120 degrees out of phase.
The figure above shows an end view of the stationary windings. Each of the three-phase windings are divided into many additional coils uniformly distributed throughout the stator. This even distribution allows more effective use out of the magnetic fields that will be developed within the stator windings when current is present. This also produces a more even torque (pulling and pushing by magnetic forces) for the rotor.
The rotor looks like a solid cylinder supported at each end by bearings (Figure 16-6). Upon closer examination, you may see thin bars embedded in the laminated cylinder at an angle almost parallel to the rotor shaft. At each end of the cylindrical rotor core, there are shorting rings. Each end of a bar is connected to the shorting rings. These rotor windings are similar in construction to the amortisseur or damper windings found in the generator.
These short-circuited rotor bars become a transformer secondary. The magnetic field established in the stator induces an EMF in the rotor bars. The rotor bars and the shorting rings complete a circuit, and a current flow is then established in these rotor bars. Remember, whenever a current flow is established so is a magnetic field. Since this magnetic field is the property of induction and induction opposes that which creates it, the magnetic field pole in the rotor is of the opposite polarity of the stator field pole that generated it. Magnetism principles apply, and the rotor's polarity is attracted to the stator's opposite polarity. The revolving field of the stator, in effect the revolving magnetic polarity, pulls and pushes the initially established rotor field in the rotor. The pulling and pushing produces torque, and the motor rotor turns.
Short-Circuited Rotor Bars
Words often used to describe the solid bar windings found in the induction motor rotor are "short-circuited bars." A short circuit is a very low resistance situation that has very little restraint in reducing current flow. A short circuit condition can have devastating effects on the entire electrical environment. The rotor bars are designed for very low resistance to obtain certain motor operating characteristics. The rotor bars themselves are not entirely the cause for the short circuit condition. The great inrush of motor current is initiated because of the relative motion between the stationary rotor winding and the revolving stator field. This is part of the maximum current the motor will draw initially from the distribution system. Through transformer-like action, the great difference in relative motion induces a large EMF and resulting current flow in the rotor.
The inrush will be dramatically reduced as the rotor speed increases. The closer the rotor RPM is in relation to the speed of the revolving stator magnetic field, the less relative motion exists. Less relative motion means less induced EMF and a reduction in rotor and stator winding currents. Shortly after power is applied to the motor, the current is reduced to as little as 10 percent. Once the motor is operating at normal speed, the full-load current (FLC), stipulated on the data plate, is maintained. Large motors installed on Army watercraft can have an increase in current 6 to 12 times greater than the data plate FLC rating. Mechanically overloading a motor slows the rotor and increases current. It is the increase in current, no matter how little, that results in heating sufficient to destroy motors.
If the rotor could turn at synchronous speed, then there would be no relative motion between the magnetic field of the stator and the rotor conductor bars. This would end the induction process in the rotor, and the rotor would lose its magnetic field.
This is not possible with an induction motor. If rotor speed equaled synchronous speed, the rotor would stop. However, as soon as the rotor slowed, even slightly, induced EMF and current would again flow in the rotor winding. Rotor speed would be maintained somewhere below synchronous speed. Slip is the difference between the synchronous speed and the actual speed of the rotor. Slip is more often expressed as a percentage:
Percent slip = (synchronous speed - rotor speed) x 100 synchronous speed
Percent slip = (1,800 RPM - 1,785 RPM) x 100 1,800 RPM
Percent slip = 15 x 100 1,800
Percent slip = 0.8 percent
An induction motor will always have a difference in speed between the rotor and the stator field. Without this difference, there would be no relative motion between the field and rotor and no induction or magnetic field in the rotor.
Rotor and therefore motor speed is determined by the number of poles, the frequency, and the percentage of slip.
Induction motor rotors are designed to have a specific amount of resistance. The resistance in the rotor determines the comparative ease with which the magnetic field in the rotor becomes established. The motor starting current, slip, and torque are modified by the rotor resistance. By developing a motor with a high rotor resistance, a larger slip is developed because the magnetic field of the rotor cannot develop very quickly. A step-by-step sequence of events portrays the actions between the stator and rotor in a relatively high rotor resistance induction motor:
- Alternating current in the revolving stator field induces an EMF in the rotor bars.
- The high resistance in the rotor prevents the rapid building of the rotor's magnetic field.
- The inability of the rotor to rapidly build a magnetic field fails to allow the rotor to increase in speed rapidly.
- Because the rotor does not increase in speed rapidly, there is a greater relative motion between the revolving stator field and the slow-moving rotor.
- The greater relative motion, from a slow-moving rotor, increases the EMF into the rotor bars.
- The increased rotor EMF generates an increased current flow in the short-circuited rotor bars.
- The increased current increases the rotor's magnetic field.
- The increased magnetic field increases the magnetic attraction of the rotor to the stator's revolving field.
- The rotor develops a greater torque to operate heavier loads.
However, extra torque does not come without some complications. Increased torque means an increased current demand on the distribution system. There is also an increase in slip at full load. Higher resistance rotors are not acceptable for all applications. This is the reason for the many rotor designs.
The rotor resistances are identified by the National Electrical Manufacturers Association (NEMA) and designated by design.