Document xdxvbZvqDJaqGbnn6r0w6qQqm

858 Type CHAPTER 39 I' Table 3. Classification of Motors 8peed Character istics Full Voltage Starting Torque Starting Current HP -Range Constant Speed Drives 1952 Guide Application See Footnotes' (o) to () Polyphase a-o J -Squirrel-cage general Constant purpose Design A Normal 1-2.5 High 6-8 times times Squirrel-cage Design Constant Normal 1-2.5 Normal 5-6 times times Squirrel-cage -Design Constant High 2-2.5 Normal 5-6 . times times Wound rotor Constant or High 1-2.5 Low 1-3 variable .. times times (with second iry control) Synchronous high speed Synchronous low. . speed Exactly con Normal stant 0.75-1.75 times Normal 5-7 times Exactly con Low 0.3-0.4 Low 3-4 stant times times Two value .capacitor Constant Permanent split .capacitor Capacitor start Constant Constant Repulsion Induction Constant 0.<Q3 Split phase Constant - and adjustable High Low Moderate High Normal Normal Normal Normal Normal Normal All , Medium Small Medium . Small All Medium Large Medium Large (a) Fans and (e) centrifugal - pumps and centrifugal compressors (a) Fans and centri fugal pumps and centrifu gal compres sors (5) Reciprocating pumps (0) and compressors started loaded (a) Hoists (b) reciprocating pumps and ' compressors (c) and frequent (0) or hard start (a) Fans and centri fugal pumps and centrifugal compressors (a) Reciprocating compressors starting un loaded Small (5) Pumps and com- Fractional (a) Fans, Blowers Small Fraotional Medium . Small Fractional - (a) Fans and pumps (a) Fans (6) pumps and - compressors (a) Fans (6) pumps and compressors (d) fans--direct Adjustable Speed Drives Polyphase a-o | Squirrel-cage high Variable slip. Transformer adjustment 8quirrel-cage sepa Constant rate winding or multi regrouped poles speed Wound rotor Variable Normal Normal Normal or high Normal or low High Low (with second iry control) Repulsion ?a Variable High s Capacitor low torque Variable Low 3 tapped winding two speed Capacitor low torque Variable _ transformer ad- Low .S GO justment Split phase re grouped poles Constant Normal Normal Normal Low Normal Medium Small (a) Fans All (a) Fans (b) pumps and (e) compressors AU () Fans () centrifugal pumps and compressors Low and Fractional Fractional (a) Fans, centrifugal pumps (b) compressors (d) Fans, drect Fractional (d) Fans Fractional (d) Fans a. Drives having medium or low starting torque and inertia (WJ2*) such as fans and centrifugal pumps' or reciprocating pumps and compressors started unloaded. b. Drives having high starting torques, such as reciprocating pumps and compressors started loaded. c. Similar to (a) except where frequent or hard starting (large WR1) requires a higher starting and accel erating torque, d. Fans direct connected, e. Stoker drives. Motors and Motor Controls '859 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 advantages of higher power factor and 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 1H 2 3 5 7H 10 15 20 25 1 24 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-plrase, 60-cycle, constant-speed, induction motors, measured with rated voltage and frequency impressed aniTwith 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 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 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 on equipment such as reciprocating compressors and pumps where other motors would draw high peak currents. Figs. 1 and 2 illustrate the characteristics of squirrel-cage motors. Both power factor and efficiency are improved if the motors are operating Refer to Glossary at end of chapter.