Motors, alternating current types

Polyphase Motors. Alternating-current polyphase motors are of the squirrel-cage, wound-rotor, or synchronous types. 

Within these basic principles there are many types of electric motors. Each has its own individual operating characteristics peculiarly suited to specific drive applications. Equations (29-1) through (29-9), presented in Table 29-1, describe the general operating characteristics of alternating-current motors. When several types are suitable, selection is based on initial installed cost and operating costs (including maintenance and consideration of rehability). 

Alternating-current motors are of three general types, induction, synchronous, and series, and are defined as follows ... 

Induction Motors. An induction motor is an alternating-current motor in which a primary winding on one member (usually the stator) is connected to the power source and a polyphase secondary winding or a squirrel-cage secondary winding on the other member (usually the rotor) carries induced current. There are two types ... 

Universal Motors. A universal motor is a series-wound motor designed to operate at approximately the same speed and output on either direct current or single-phase alternating current of a frequency not greater than 60 cycles/s and approximately the same rms voltage. There are two types ... 

Use of wound-rotor induction motors has been largely in continuous-duty constant-speed supplications where particularly high starting torques and low starting currents are required simultaneously, such as in reciprocating pumps and compressors. These motors are also used where only alternating current is available to drive machines that require speed adjustment, such as types of fans and conveyors. 

Most electronic equipment shares the television set s need for a number of differing voltages for the operation of individual components. This alone may be sufficient justification for the inclusion of a direct current to alternating current converter in fuel cell power systems. In addition, alternating current electric motors are more suitable in most applications. They tend to operate at a rotational speed controlled by the frequency of the current. If completely unloaded they speed up to this fixed velocity and accelerate no further. Many types of direct current motors, if operated unloaded, will continue to accelerate until they fail. A belt driven fan operated by an alternating current motor is undamaged by the failure of the belt. A direct current motor will require a special safety circuit to shut it down in case of belt failure. If the belt and the safety circuit both fail, the motor will speed up until it destroys itself. 

Extruder motors are usually electric, but some systems utilize hydraulic motors. For example, injection molding machines use hydraulics to develop clamp tonnage. Electric motors may be of the direct current (DC) or alternating current (AC) types. Traditionally, DC motors, which regulate speed through voltage control, have been more popular because they could provide the necessary power at a lower cost. However, recent advances in frequency control - the technique used to regulate speed in AC motors - have caused this type of motor to become more widely used. 

The three basic types of drives are alternating current (ac), direct current (dc), and hydraulic. While a number of drives have been used in extruders, the most common are dc silicon control rectified (SCR) and ac adjustable frequency drives. A dc SCR drive is a sohd-state dc rectifier connected to a dc motor. The base speed is about 1 percent, but reduces to 0.1 percent when a tachometer is added to the drive. These drives are very reliable, can handle high starting torques, can maintain a constant torque through a speed range of 20 1, and are relatively easy to maintain (that is, replace brushes). However, since the drives have brushes, they are limited to noncorrosive polymers. 

Electrical energy is furnished to the plant at 5000 volts, 50 cycles, three phase. Two 1300-kva. transformers step the voltage down to 115 volts. The 50-cycle power is converted to 500 cycles by alternators with a capacity of 36 kw. A single motor drives two alternators, and a self-induction coil is used to regulate the power factor and ensure operational stability. The 500-cycle current is stepped up to 10,000 volts for the tubular ozonizers and 18,000 to 20,000 volts for the plate type. 

An electric generator can be just like a motor, but a steam engine or water turbine causes the rotation and electricity is taken out, as most readers know. A dc generator requires a commutator, which tends to wear out quickly when extremely high currents are involved. An ac generator, often called an "alternator," does not need a commutator, so that type is used in modem automobiles. 

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