Document 655VMo20DNbdJ47qjZzB9dn0E

874 CHAPTER 40 1953 Guide Type Table 3. Classification of Motors Speed Chabacteb- I8TIC8 Fun, Voltage Starting , Torque Starting Current HP Range. Constant Speed Drives Application See Footnotes (a) to (e) Squirrel-cage general Constantpurpose Design A Normal 1-2.5 Hi$h 6-8 times times Squirrel-cage Design Constant. Normal 1-2.5 Normal* 5-6 times . times 9 Squirrel-cage Design Constant. CS High 2-2.5 Normal 5-6 times times Wound rotor >> Constant or High 1-2.5 Low 1-3 variable times times (with secondary control) Synchronous high speed 'Synchronous low speed Exactly con- Normal . stant 0.75-1.75 times Normal 5-7 times Exactly con- Low 0.3-0.4 Low 3-4 stant times times Two value capacitor Constant > High . Permanent split S capacitor Capacitor start Constant Constant Repulsion Induction Constant .6s0 Split phase CO Constant and ad- justable Low Moderate High Normal Normal Normal Normal Normal Normal All (a) Fans and (c) centrifugal pumps and centrifugal Medium 'Small (o) Fans and centri fugal pumps and centrifu gal compres- Medium Small (5) Reciprocating pumps (e) and compressors All (b) reciprocating pumps and compressors . (c) and frequent ' Medium large fugal pumps , and centrifugal Medium Large (a) Reciprocating compressors, starting un- loaded Small (5) Pumps and com- Fractional (o).Fans, Blowers Small Frac- (a) Fans'and pumps Medium Small Fractional - (a) Fans (5) pumps and compressors (6) pumps and compressors (d ) fans--direct Adjustable Speed Drives | Polyphase a-o | Squirrel-cage high Variable slip. Transformer adjustment Squirrel-cage sepa Constant rate winding or multi regrouped poles speed Wound rotor Variable . Normal Normal Normal or high Normal or low .Hi$h Low (with second ary control) Repulsion Variable High <s .1 Capacitor low torque Variable Low * tapped winding ' two speed Capacitor low torque Variable Low transformer ad justment GO Split phase re grouped poles Constant Normal Normal Normal Low Normal Medium Small (a) Fans All (a) Fans (5) pumps and (c) compressors All (a) Fans (b) centrifugal pumps and compressors Low and Fractional Fractional (a) Fans, centrifugal pumps (b) compressors (<0 Fans, drect Fractional (d) Fans Fractional (d) Fans a. Drives having medium or low starting torque and inertia (IFIt*) such as fans and centrifugal pumps or reciprocating pumps and compressors started unloaded. b. -Drives having high starting torques, such as reciprocating pumps and compressors started loaded. c. Similar to (a) except where frequent or hard starting (large WR*) requires a higher starting and accel erating torque, d. Fans direct connected, t. Stoker drives. Motors and Motor Controls 875 time is prolonged by high WR1 and high load torques. In general, driven machines starting more than 4 to 6 times per hour may require'special motors and control. ALTERNATING CURRENT MOTORS Alternating current motors are divided into two> main classifications: polyphase and single phase (see Table. 3), according to the type of power supply. They are further subdivided by type of motor winding. . 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. Table 4. Locked-Rotor Current of Three-Phase, 60-Cycle Motors at 220 Volts*- b HP - Design B, C, AND D . Design. F HP Design B, C AND D Design F Amperes Amperes * Amperes Amperes 1 or less 1H 2 3 5 m 10 15 20 25 24 35 45 60 90 120 150 220 290 365 30 435 270 40 580 360 50 725 450 60 870 540 75 1085 675 100 1450 900 125 1815 1125 150. 2170 1350 200 2900 1800 *The locked-rotor current of three-phase, 60-cycle, constant-speed, induction motors, measured with rated voltage and frequency impressed and with rotor locked, shall not exceed the tabulated values. b Locked-rotor current at other voltages nhali be inversely proportional to the voltage. c For 1 hp or less the value is given per hp. Polyphase Motors The three types of polyphase motors are: squirrel-cage induction motors, wound rotor induction motors, and synchronous motors. Squirrel-cage motors are specific by NEMA standards providing a variety of speed and torque characteristics. Design A motors-provide 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 shown in Table 4, which are acceptable by many power companies for full voltage starting. They are used for. the same type of application 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 equipment such as reciprocating compressors and pumps where other motors would draw high peak currents. Figs. 1 and 2 illustrate the characteristics of squirrel-cage motors. Both power factor and efficiency are improved if the motors are operating Refer to Glossary at end of chapter..