Document jm6zxvnnEbr99OyqyLN1Ogbyy

684 CHAPTER 35 1949 Guide the constant speed motor with the exception that the starting resistor must be designed for speed regulating duty and must therefore be capable of carrying the motor current continuously. Speed controllers are avail able both for constant torque applications and varying torque drives (such as required by fans) in which the torque is reduced considerably at reduced speed. The Adjustable Voltage type of speed control is also often known as the variable voltage or Ward-Leonard system. For machines Requiring a wide range in speed control and a large number of steps of control this type of system is used most extensively. The drive consists of one or more d-c motors, the armatures of which are supplied with power from a d-c generator and the fields of all machines are excited from a constant 2000 1800 kr a isoo U 3 1400 Z 3 1200 fut 0- 1000 to . | 800 h 3 600 O bl 400 CL 200 5 \ \ `n &'\ l\ A a1 > A A* X > A \ \ 0 50 100 ISO PER CENT FULL LOAD TORQUE ADJUSTABLE VOLTAGE DRIVE--------------ADJUSTABLE VARYING SPEED DRIVE---------------- Fig. 3. Speed Torque Characteristics of Adjustable Varying Speed Drive and Adjustable Voltage Drive voltage exciter. A schematic diagram of connections for, an adjustable voltage drive is shown in Fig. 4. In most cases the d-c generator is a part of a three-unit set including a constant speed a-c driving motor, and a constant voltage exciter. As the voltage on the generator and consequently on the d-c motor or motors is adjusted by a rheostat in the generator field circuit, a great many steps are thus obtained in an efficient manner, - With constant field excitation on the motor the speed of the motor will vary approximately as the voltage on the generator. Extremely wide speed-ranges are possible with the adjustable voltage type of drive. Ranges as high as 10 to 1 are common and, by the addition of field, control on the motors, ranges as high as 40 to .l are permissible. This type of drive provides the advantage of good speed, regulation over the entire speed range, as shown in Fig. 3 in which this type' of drive is compared with the adjustable varying speed drive. Typical specifications for d-c motor control are shown in Table 3. Motors and Motor Controls 685 ALTERNATING CURRENT MOTORS Alternating current motors are divided into two main classifications: ;polyphase and single phase (see Table 1), according to the type of power supply used. They are further subdivided as to the type of motor wind ing. When polyphase power is available it is usually found more economical to apply polyphase motors in preference to single phase motors. A typical 5 hp, 1200 rpm capacitor start-induction run single phase motor, for-in stance, will cost approximately twice as much as the corresponding three phase Design B squirrel-cage motor. In addition, the polyphase motor has the advantages of higher power factor and higher efficiency. Polyphase Motors The three types of polyphase motors are: squirrel-cage induction motors, wound rotor induction motors, and synchronous motors. Squirrel-cage motors are specified by NEMA standards providing a variety of speed and torque characteristics. Design A motors provide TO A-C normal starting torque at starting current in excess of .Design B motors and are suitable for constant speed application, to equipment such as fans and blowers. Design B motors provide normal starting torque with NEMA starting current values which are acceptable by many power com panies for fulTvoltage starting.. They are used for the same type of appli cation as Design A. Design C motors provide high starting torque with starting current same as Design B and are used on compressors started without unloaders, and on reciprocating pumps. Design D motors have high slip* and are used with flywheels for widely pulsating loads on equip ment such as reciprocating compressors and pumps where other motors would draw high peak currents. Figs. 5, 6, 7, and 8 illustrate the characteristics of squirrel-cage motors. It will be noticed by inspection of Fig. 6 that both power factor and effi ciency are improved if the motors are operating as near rated load as pos sible. In addition, as shown in Fig. 8, power factor and efficiency are better for higher speed motors. Typical specifications for squirrel-cage motors are shown in Table 4. Wound Rotor motors are used for applications requiring high starting ' Refer to Glossary at end of chapter.