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926 CHAPTER 40 1956 Guide Chabactebistics Full Voltage Starting Torque Starting Current HP Rang Application See Footnotes (o) to (e) Constant Speed Drives Squirrel-* purpose ] I Constant Normal 1-2.5 times* tunes (a) Fans and (c) centrifugal pumps and centrifugal g^uirrel-cage Design Constant Normal^l-2-5 Normal times 5-5 | Medium Small Squirrel-cage Design] Constant High 2-2.5 times' Normal 5-5 | Medium Small Squirrel-cage Design Constant Low 1.25 Low 4 times j Medium Large | Wound rotor | Constant ot variable High 1-25 Low 1-3 times times 1N (with secondary control; j Synchronous high 1 speed Synchronous low speed l Exactly con Normal stant 0.75-1.75 times \ Exactly con Low 0.3-0.4 stant times Normal 5-7 | Medium times Large Low 3--4 times j Medium Large (a) Fans and centri- - fugs! pumps and centrifu gal compres-. sors (5) Reciprocating pumps (e) and compressors started loaded Fans, centrifugal pump and compressors (o)Hoists * (b) reciprocating pumps and compressors (cl and frequent () or hard start (a) Fans and centri fugal pumps and centrifugal compressors (o) Reciprocating compressors starting un loaded Two value capacitor! Constant I Permanent split capacitor Capacitor start Constant Constant Repulsion Induction Constant Split phase 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 I Fractional (a) Fans and pumps (a) Fans (b) pumps and compressors (o) Fans (b) pumps and compressors (d) fans--direct ^ S in g le p ^ o Single phase a-c I Polypbaao ft-o Squirrel-cage high , Blip. Transformerj adjustment Squirrel-cage sepa rate winding or regrouped poles Wound rotor (a) Fans (b) pumps and ' ' compressors Fans centrifugal pumps and compressors (o) Fans, centrifugal Repulsion pumps (6) compressors (d) Fans, drect Capacitor low torque' tapped winding Capacitor low torque transformer ad justment Split phase re i g--r-o---u--,ped poles a. Drives having medium or low starting torque and inertia (WR*) such as fans and centrifugal pumps or rheciDproivceastinhgavpinugmpnsighansdtacrotimngprteosrsqouresss,tasurtcehd ausnrleoacidperod.cating pumps and compressors started loaded. ent or hard starting (large WR*) requires a higher starting and aocel- '1"Ce"Cd. d bp rating an/da. pFLanTs dsirSect'SconTne^ctdeda. rd MG1-4.10 on Motoraea. nSdtoGkeenr edrraivteosr*.. Motors and Motor Controls 927 time is prolonged by high WR? 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-Rotok 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 leas 1H 2 3 5 m 10 15 20 25 24 35 45 60 90 120 150 220 290 365 30 435 40 580 50 725 60 870 75 1085 270 360 450 540 675 100. 1450 900 125 1815 " v 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. 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 onequipment such as reciprocating compressors and pumps where other motors would draw high peak currents. Design F Motors have.low start-, ing 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.- Figs. 1 and 2 illustrate the characteristics of squirrel-cage motors. The: motor operates under load from near synchronous speed at light Toad to: * Refer to Glossary at end of chapter.