Document emgzKk02J8zYXRaJZ0KdGJG09

.682 CHAPTER 35 '" * 1949 Guide Table 2. Typical Specifications for d-c Motors Constant 1 fShunt 1 1 ' ' ` f1151 Hp, .... ........RPM, Adjustable Speed, (Compound Wound,'{230 Volts. Varying . (Series (600 d-c Motors For Driving (Application) Motor Shall Be Arranged For Mounting And Shall Be Provided With A........................______________ lverucai j (Open, Splashproof, etc.) Type Of Enclosure, NEMA jq}^ Insulation, And Bearings. Table 3. Typical Specifications for d-c Motor Control Constant Control For Adjustable 1 Speed d-c Motors Shall Be {^""ea`ic} And Shall ^Adjustable Varying, Consist of a (circuit ^Breaker} 811 Enclosed Controller Providing Overload Protection [ And Low Voltage n}j Table 4. Typical Specifications for Squirrel-Cage Motors " 12081 {B fjj B Hp,............................. RPM, Cycles, Volts, Phase, (550j Squirrel-Cage Induction Motors of the NEMA {Design A--Normal Starting Torque, Starting Current above NEMA value) Design B--Normal Starting Torque, NEMA Starting Current I Design C--High Starting Torque, NEMA Starting Current ' Design D--High Slip High Starting Torque [ j Driving.. ............................. Motor Shall Be Arranged For Mounting i . , (Application) lvertical J And Shall Be Provided With A.........................................Type of Enclosure, NEMA (Open, Splashproof, etc.) {clSs b}Insulation, And {lafTM} Bearings. Refer to following section on Alternating Current Motors for explanation of types. . . Table 5. Typical Specifications for Wound Rotor Motors [208]/ ;; Bjj BHp,..................RPM, Cycles,Voits, Phase, Wound Rotor (55J ' Induction Motors For Driving.............. ...... ........ Motor Shall Be Arranged - for. (Application) .y ; . (u0I?f--rtal) Mounting And Shall Be Provided With. A:................... ... V ertical / ' . . (Open, Splashprbof. etc.) Type of Enclosure, NEMA gjlnsulalion and |gi^gve| Bearings. Motorsand'Motor~Controls 683 the'use of: a fused safety switch or circuit breaker and a face-plate type starter- For. automatic, push button starting a safety switch or circuit breaker, and an:automatic starter are recommended .Adjustable Speed Motors are normally shunt wound and are operated at various speeds by varying a resistance connected in series with the motor field. A maximum range of speed of about 5 to 1 can be obtained by this means. Rated, speed regulation is 22 per cent for motors of this type from 2 to 5 hp; 15 per cent is standard in larger sizes. The NEMA practice on. rating adjustable speed d-c motors is defined in the Glossary at the end of the chapter. The control for the adjustable speed motor consists of the addition of UNDER k2 HP "5 SMALL CIRCUIT BREAKER !c HP & LARGER RHEOSTAT MOTOR ADJUSTABLE SPEEO 3 JL| PI SAFETY SWITCH - CONSTANT SPEEO MANUAL STARTING 4 J_, OIJ SAFETY SWITCH CONSTANT SPEED PUSHBUTTON STARTING STARTER & SPEED REGULATOR rtl STARTING IZJ RHEOSTAT HqJMIjm, =MOTOR rfso ^JUiy [Tmotor MOTOR Fia. 2. Recommended Controls for d-c Motors a field rheostat for speed control to the equipment specified for the constant speed motor.- ^ . Adjustable Varying Speed* motors are d-c motors in which the speed is varied'by-the`addition of resistance in series with the'armature. The speed of the motor by this means is always less than the rated full field speed and varies'widely with a change in load, especially with high series resistance; Fig. 3 illustrates typical speed characteristics of this type of motor for different values of armature resistance. The addition oif series, resistance in the armature circuit reduces the motor speed by lowering the voltage on the armature. At one-half speed the,voltage is approximately one-half of line voltage. Consequently, with rated full load current the power delivered by the motor will be only orieffialf of the maximum, e.g., it will be 5 hp from a 10 hp motor, because the other 5 hp will be lost- in the resistance. It is, therefore, evident that the'efficiency' of the motor is reduced at reduced speeds due to the loss in the resistor. > Control for,the adjustable varying speed motor is similar to that for * Refer to Glossary at end of chapter.