Document N2M6jmOkQbDanL56o9NY9k8jb

828 CHAPTER 39 1950 Guide Table 6. Locked-Rotob Current of Three-Phase, 60-Cycle Motors at 220 Volts*'1' HP Design B, C, AND D Design F HP D ESION B, C AND D Design F Amperes Amperes Amperes Amperes I or less 1H 2 3 6 7H 10 15 20 25 24 85 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 a 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 shall be inversely proportional to the voltage. 0 For 1 hp or less the value is 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 Motors-and Motor Controls 829, Fio. 6. Variation of 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 * SAME A3 STARTING Fig. 7. Comparative Performance of Squirrel-Cage Motors of 30 Hp and _______ Smaller Sizes Refer to Glossary at end of chapter.