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848 CHAPTER 39 1951 Guide Table 6. Locked-Rotob Cubbent op Three-Phase, 60-Cycle Motors at 220 VoLTB*'b HP Design B, C, AND D Amperes Design F Amperes HP Design B, C AND D Amperes Design F Amperes ltt 2 3 5 7Vi 10 15 20 25 24 35 45 60 90 120 150 220 290 365 30 40 50 60 75 100 125 ! 150 1 200 435 580 725 870 1085 1450 1815 2170 2900 270 360 450 540 675 900 1125 1350 1800 * The locked-rotor current of three-phase, 60-cyde, 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 shall be inversely proportional to the voltage. 9 For 1 hp or less the value ia given per hp. 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 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 6, 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 Fig. 5. Speed Torque Characteristics op Squirrel-Cage Motors Motors and Motor Controls 849 Fio. 6. Variation op Efficiency and Power Factor with Load fob a Typical 5 Hp, 4 Pole 60 Cycle, Design B, Squirrel-Cage Motor 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. 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 Specification C at end of chapter. Wound Rotor motors are used'for applications requiring high starting TYPE OF SQUIRREL CAOE MOTOR 190n0nSTARTING TORQUE* WU %OF FULL LOAD TORQUE 300 DESIGN A 0E5IGN 6 DESIGN C DESIGN 0 : .> - 2 3 S 3 J lJ 3 g,5 y* O o <3 e(V o* <3D aO u 3 o AC o fi * Xo oeo o- oN oPI 9 uz oJ y_ l1b11 l3b e n o L1J 1T1 ,v IT 1 oo V i o t- A< sss .V at < S3 5:3 -V1> l3b Bi t O rn (&h 1 LH 1 1 r1 i * SAME AS STARTING Fig. 7. Comparative Performance Of Squirrel-Cage Motors of 30 Hp and `______ Smaller Sizes Refer to Glossary at end of chapter.