er without a change of frequency. This energy transfer usually takes place with a voltage change, although the latter is not always necessary or even desirable. The electric circuits being insulated from each other, as they are in most transformers, they are conventional and are generally referred to as transformers. In some special cases, the electric circuits
Сare joined together, in which case the device is referred to as an autotransformer. The electric transformer winding being connected to the source of supply is called the primary, the winding that feeds the load being known as the secondary. Some transformers are designed to raise theиprimary voltage to a higher value, in which case they are known as step-up transformers; others are constructed to reduce the primary voltage to a lower value, in which case they are called step-down transformers. In step-up transformers the current on the secondary side is lowered in бАthe same ratio as the voltage is raised, in step-down transformers the current on the secondary side being raised in the same ratio as the voltage is lowered. Transformers have many applications in a-c circuits that require both the raising and lowering of the primary voltage as well as the lowering and raising of the primary current. When used in groups in poly phase circuits, they are especially valuable in performing many im portant functions, one of which, apart from its volt- age-changing use, is to change the number of phases from two to three, three to two, three to six, or several other combinations.
TYPES AND CHARACTERISTICSДOF
ALTERNATING CURRENT MOTORS
Only three general types of d-c motor are found in practice, a comparatively large number of different constructions being available for use in a-c systems. The reasons for this situationИis that each type of a-c motor is confined to narrower operating characteristics, especially with regard to such important matters as torque, overload capacity, speed variation, speed control, and starting procedures. Furthermore, a-c motors must be constructed for operation on single-phase service or polyphase (either twoor three-phase) service; in one type of construction they must perform satisfactorily on d-c service as well as on singlephase alternating current.
Classification of Single-Phase Motors. Single-phase motors generally have low horsepower ratings and are used to operate mechanical devices and machines requiring a comparatively small amount of power. Their greatest fields of application are in the fractional-horsepower
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range, that is, below 1 hp. Motors larger than the latter, up to perhaps 10 hp, are sometimes used on farms and in small shops and factories where polyphase power is not available. Polyphase motors generally have better operating characteristics than single-phase machines and cost less per horsepower, so that it is usually true that single-phase mo-
Сtors are used in the larger sizes only because of twoor three-phase service not being available.
In the single-phase classification may be listed the following types of motors: shaded-pole, reluctance, split-phase (with or without capacitorиstarting), repulsion, repulsion-start, repulsion-induction, series (a-c only or universal), and synchronous.
Shaded-pole and reluctance motors are built in very small sizes from about 1/500 to 1/6 hp; they are cheap to construct, have low starting torque,бАlittle overload capacity, and low efficiency and may be speed-controlled.
Standard split-phase motors are manufactured in sizes up to 3/4 hp; they are comparatively low in cost, have fair starting torque, not much overload capacity, and fair efficiency, and operate at nearly constant speed. Split-phase motors equipped with capacitors have high starting torque and may or may not be arranged to continue to run with a capacitor. Their capacitor being used only during the starting period, they are called capacitor-start split-phase motors; two values of capacitor being provided, one for startingДand another for running, they are referred to as two-value capacitor motors. However, whether or not these motors are provided with capacitors, they are all, nevertheless, split-phase motors.
Series motors are usually constructed for service on direct or alternating current up to 60 cycles, in which caseИthey are called universal motors. When properly designed, they will operate with complete satisfaction on direct or alternating current, developing high starting torque, having excellent overload capacity and good efficiency, and permitting the speed to be controlled over very wide limits. Such motors are not as trouble-free as those described above (shaded-pole, reluctance, and split-phase types), because they have the usual commutator and brushes and their, accompanying commutation problems.
Synchronous motors, as the name implies, operate at synchronous speed, that is, a definite, constant speed determined only by the frequency of the supply and the number of poles on the machine. They have very little starting torque, practically no overload capacity, and are quite in-
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efficient; they have, however, the one important characteristic possessed by none of the motors previously discussed, that is, absolute constancy of speed, a requirement that is very important for timing devices.
Classification of Polyphase Motors. Polyphase motors, that is, machines served with twoor three-phase power, may be classified as Сfollows: induction (squirrel-cage or wound-rotor types), commutator, or
synchronous.
Squirrel-cage induction motors are widely used because of their having, generally speaking, desirable all-purpose characteristics. They are comparatively low in cost per horsepower, have good starting torque and overload capacity, are highly efficient, and are particularly rugged and trouble-free. These motors will operate in an atmosphere containing dirt, moisture, or corrosive or explosive fumes and can even
и be constructedбАto perform submerged in oil or water. They are, practi-
cally speaking, constant-speed motors in the sense that change in load does not affect the speed by more than about 5 per cent. Such motors are, however, of a disadvantage when it becomes necessary to control the speed, because it is usually difficult or expensive, from the standpoint of additional equipment, to do so. When speed control becomes a necessary requirement of an application, the squirrel-cage rotor is often replaced by a wound rotor, its winding ends being connected to slip rings. Speed control is then accomplished by connecting a resistor con-
considerably more starting torque.ДIt does, however, have a lower fullload efficiency and a greater speed variation with load changes than does the squirrel-cage type of motor.
troller to the brushes riding on the slip rings; the greater the resistance inserted, the lower the speed, and vice versa. Wound-rotor motors, therefore, differ from squirrel-cage motors only by the construction of
the rotor, the stator of both types being exactly similar. In addition to its
speed-control feature, the wound-rotor induction motor also develops И
Synchronous motors for polyphase service are generally constructed with a stator core and winding similar to those used on in-
duction motors (squirrel-cage or wound-rotor), but with a rotor consisting of a set of salient poles. The latter must be excited with direct current from a small exciter, that is, a self-excited shunt generator, mounted on an extension of the motor shaft or coming from a separate d-c source. Direct current is fed to the rotor field through brushes and slip rings. Since synchronous motors, as such, have no starting torque, it is always necessary to provide the rotor poles with a
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complete squirrel cage built into the pole faces. The motor can then be started in much the same way as are squirrel-cage induction machines; when nearly synchronous speed is reached, the d-c rotor field is excited, after which the motor continues to run at exactly synchronous speed. The outstanding advantages of this type of motor are (1) absolutely
Сconstant speed, determined only by the frequency of the supply and the number of rotor poles, and (2) the possibility of adjusting the motor power factor to any desirable value. Synchronous motors, when property designed, have good starting torque, overload capacity, and efficiency.иThey are more expensive than induction machines in the smallest sizes, but cost about as much as squirrel-cage or wound-rotor motors in ratings of more than 100 hp. As a rule, synchronous motors are used in applications requiring infrequent starting, where the load is substantially constant,бАand where high power factor or power-factor correction is desirable and profitable.
Although the speed of a wound-rotor motor can be changed over a wide range by the insertion of resistance in the rotor circuit, the efficiency of operation is very low at reduced speed. To offset this disadvantage, particularly in large motors where energy cost is important, special types of machines have been developed. There is a special motor construction that has wide speed-control possibilities In this motor, the stator is of the usual construction found in induction machines, but the rotor differs greatly from any ofДthese already described. The latter has two windings, one on the top of the other, placed in deep slots. The primary winding, in the bottom of the slots, is connected to slip rings and is fed, through brushes, with polyphase alternating current. The other winding, on top of the primary and next to the rotor surface, is connected to a commutator. Finally, the statorИwinding, called the secondary, is connected to brushes riding on the commutator. Speed control is accomplished by shifting the brushes over the commutator, the method used to be an ingenious mechanical lever construction permitting the motion of all brushes simultaneously by the manual or motorcontrolled operation of a handle. Such motors are high in cost per horsepower but have good efficiency, starting torque, and overload capacity. They are used only when it is extremely important that the speed be varied over a wide range. An additional advantage of this motor is that power-factor adjustment is also possible.
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AC INDUCTION MOTORS
AC induction motors are ideal for most industrial and commercial applications because of their simple construction and low number of parts, which reduce maintenance cost. Induction motors are frequently used for both constant-speed and adjustable speed drive (ASD) applications.
СThe two basic parts of an induction motor are the stationary stator located in the motor frame and the rotor that is free to rotate with the motor shaft. Today's motor design and construction are highly refined. For example, stator and rotor laminations have been designed to achieve maximum magneticиdensity with minimum core losses and heating. The basic simplicity of this design ensures high efficiency and makes them easily adaptable to a variety of shapes and enclosures.
A three-phase induction motor can best be understood by examining the three-phaseбАvoltage source that powers the motor. Three-phase currents flowing in the motor leads establish a rotating magnetic field in the stator coils. This magnetic field continuously pulsates across the air gap and into the rotor. As magnetic flux cuts across the rotor bars, a voltage is induced in them, much as a voltage is induced in the secondary winding of a transformer. Because the rotor bars are part of a closed circuit (including the end rings), a current begins to circulate in them. The rotor current in turn produces a magnetic field that interacts with the magnetic field of the stator. Since this field is rotating and magnetically interlocked with the rotor, the rotor is dragged around with theДstator field.
When there is no mechanical load on the motor shaft (no-load condition), the rotor almost manages to keep up with the synchronous speed of the rotating magnetic field in the stator coils. Drag from bearing friction and air resistance prevents perfect synchronicity. As the load increases on the motor shaft, the actual speed of the rotor tendsИto fall further behind the speed of the rotating magnetic field in the stator. This difference in speed causes more magnetic lines to be cut, resulting in more torque being developed in the rotor and delivered to the shaft mechanical load. The rotor always turns at the exact speed necessary to produce the torque required to meet the load placed on the motor shaft at that moment in time. This is usually a dynamic situation, with the motor shaft speed constantly changing slightly to accommodate minor variations in load.
The rotor consists of copper or aluminum bars connected together at the ends with heavy rings. The construction is similar to that of a squirrel cage, a term often used to describe this type of ac induction motor.
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