Document gDGaRZVoQ3LMXwGkZD6E4e13Q

996 CHAPTER 39 1958 Guide Type Table 3. Classification of Motors Spbfd * Chahactbb- IST1C8 Full Voltage Starting Torque Starting Current HP Range Constant Speed Drives Application See Footnotes (o) TO () Squirrel-cage general Constant ' purpose Design A Normal 1-2.5 High 6-8 times-' times All Squirrel-cage Design Constant Normal 1-2.5 Normal 5-6 Medium times-' times Small Squirrel-cage Design Constant High 2-2.5 Normal 5-6 Medium times' times Small cO je Squirrel-cage Design Constant Low 1.25 Low 4 times Medium Large > Wound rotor P Constant or High 1-3,5 Low 1-3 variable times times (with secondary control) All speed .. 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 (e) centrifugal pumps and centrifugal compressors (a) Fans and centri fugal pumps and centrifugal compres- .. sore (6) Reciprocating., pumps (e) and compressore started loaded Fans, centrifugal pumps and compressors (a) Hoists (6) -reciprocating .. pumps and compressors (e) and frequent () or hard start (a) Fans and centri- fugal pumps and centrifugal compressore (a) Reciprocating compressors starting un- loaded Two value capacitor Constant Permanent split Constant A capacitor e Capacitor.start . Constant a 3a Repulsion Induction Constant High Low .Moderate High 3 Split phase Constant justable Normal Normal Normal Normal Normal Normal Small Fractional (6) Pumps and com pressors (o) Fans, Blowers Small Frac tional Medium Small . Fractional (o) Fans-and pumps () Fans () pumps and compressore (a) Fans (5) pumps and compressors (d) fans--direct ---__________________ _ Adjustable Speed Drives A Squirrel-cage high Variable slip. Transformer adjustment 8 Squirrel-cage sepa Constant M rate winding or multi a>> regrouped poles Wound rotor speed Variable f Normal Normal Normal or Normal or high low High Low (with second ary control) o Repulsion Variable High 8 Capacitor low torque Variable 3O. tapped winding two speed Capacitor low torque Variable transformer ad- justment Split phase re- Constant grouped poles Low Low Normal Normal Normal Low Normal Medium Small (a) Fans All (a) Fans (b) pumps and (c) compressors All (o) Fans (6) centrifugal pumps and compressors Low and Fractional Fractional () Fans, centrifugal pumps () compressors (d) Fans, drect Fractional - (rf) Fans Fractional (<f) Fans o. Drivra having medium or low atarting torque and inertia (WR') such as fans and centrifugal pumps or reciprocating pumpe and compressors started unloaded. . If^ded. 6. Drives having high starting torques, such ss reciprocating pumps and compressore started ica c. Similar to to) except where frequent or hard starting (large WR*) requires a higher starting ana era^tiTngortqoruqeued.epends on hp rating andds.pFeeadn.s dSiereecNt EcoMnnAecStetadn. dard MG 1-1.10 on M,otors and. pGaennantoiB> Motors and Motor Controls 997 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-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 m 2 3 5 m 10 15 20 25 24 35 45 60 00 120 150 220 290 365 30 435 40 580 50 725 60 870 75 1085 270 360 450 540 675 100 1450 900 125 1815 1125 150 2170 1350 200 2900 1800 `The locked-rotor current of three-phjuie, 60-cycle, constanLepeed, induction motors, measured with fated-voltage and frequency impressed and with rotor locked, shall not exceed the tabulated values. c 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 ""blowers. Design B motors provide normal starting torque with of EMA starting current values shown in Table 4, which are acceptable by many power companies for full voltage starting. They are used for the same ype of application as Design A. Design C motors provide high starting tr9ue with starting current same as Design B, and are used on compressors started without unloaders, and on reciprocating pumps. Design D motors nave 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 start- g current and low torques. They are used on power systems of limited pacity. Careful application must be made since the low breakdown condv prov'^es only limited safety margin for overload or low voltage ditaons. A drop in voltage, for instance, could cause the motor to stall. '8s' 1 nnd 2 illustrate the characteristics of squirrel-cage motors. The __lor operates under load from near synchronous speed at light load to mfer to Glossary at end of chapter.