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976 CHAPTER 39 1957 Guide" Type Table 3. Classification op Motors Speed Characteb- ISTICS Full Voltage Starting Torque StarringCurrent HP Range Constant Speed Drives Application (a) to (e) Squirrel-cage general purpose Design A 1 Normal 1-2.51 High 6-8 times'' I times All Squirrel-cage Design| Normal 1-2.5| Normal 5-6 Medium times' times Small Squirrel-cage Design C High 2-2.5 times' Normal 5-6 times Medium Small Squirrel-cage Design Constant F Low 1.25 Low 4 times Wound rotor Constant or High 1-2.5 variable times Low 1-3 times (with secondary control) Medium Large All Synchronous high speed Synchronous low speed Exactly con- Normal stant 0.75-1.75 times Exactly con Low 0.3-0.4 stant times Normal 5-7 times Low 3-4 times Medium Large Medium Large (a) Fans and (c) centrifugal pumps and centnfug&l compressors (a) Fans and centri fugal pumps and centrifu gal compres sors (5) Reciprocating pumps (e) and compressors started loaded Fans, centrifugal pumps and compressors (a) Hoists (b) reciprocating pumps and compressors c) and frequent 'e) or hard start o) Fans and centri fugal pumps and centrifugal compressors (a) Reciprocating compressors starting un loaded Two value capacitor| Constant Permanent split capacitor Capacitor start Constant Constant Repulsion Induction Constant Split phase Constant and ad justable High Low Moderate High Normal Normal Normal Normal Normal Normal Small Fractional (b) Pumps and com pressors (a) Fans, Blowers Small Frac tional Medium Small Fractional (a) Fans and pumps (c) Fans (5) pumps and compressors (a) Fans (b) pumps and compressors (d ) fans--direct Adjustable Speed Drives 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 Hish Low (mth secondary control) Medium fima.11 All All (a) Fans *1 Fans pumps and (c) compressors \a) Fans ib) centrifugal pumps and compressors Repulsion Variable High Capacitor low torque! Variable tapped winding two speed Capacitor low torque] Variable transformer ad justment Split phase re grouped poles Low Low Normal Normal Normal Low Normal Low and Fractional Fractional (o) Fans, centrifugal pumps (b) compressors (d) Fans, drect Fractional (d) Fans Fractional (d) Fans -a. Drives having medium or low starring torque and inertia (WR*) such as fans and centrifugal pumps or ri>e.ciDprroivceastinhgavpinugmhpisghansdtacrotimngprteosrsqouresss,tsaurtcehdausnrleocaidperodc. ating pumps and compressors started loaded,- e. Similar to (a) except where frequent or hard starring (large Wfi*) requires a higher starting and erating torque. d. Fans direct connected. e. Stoker drive*- /. Torque depends on hp raring and speed. See NEMA Standard MGl-4.10 on Motors and Getter*to*3- Motors and Motor Controls 977 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 advantage of higher efficiency. Table 4. Locked-Rotor Current op 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 m 2 3 5 7H 10 15 20 25 24c 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 shall 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 G 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. Design F Motors have low startmg current and low torques. They are used on power systems of limited capacity. Careful application must be made since the low breakdown torque provides only limited safety margin for overload or low voltage conditions. A drop in voltage, for instance, could cause the motor to stall. Pigs. 1 and 2 illustrate the characteristics of squirrel-cage motors. The fcotor operates under load from near synchronous speed at light load to Refer to Glossary at end of chapter.