Document KRL9Gjen7vabXoe39yjJEg9do

640 CHAPTER 45 1959 Guide Table 1.... Reasonable Horsepower Design Limits for Standard Motor Voltages PMf Soppfy Standard Motor Voltage Suggested Minimum Honepowtr SoppMtef Maximum Horsepower Alternating 1-phase 115 230 Alternating 3-phase 110 .220 440-550 2300 4000 4600 6600 None None None None 1 50 100 250 400 .T 15 15 200 1000 6000 7500 8000 None ventilation using an external blower; or (d) a small frame with Class B or H insulation permitting higher temperature rise. 2. Speed Range. Where more than one speed or a range of speeds is required, one of the motor types listed in Table 2 may be applicable, depending upon the power supply and the speed range required. 3. Horsepower Requirement. The horsepower required by the driven machine determines the motor rating. Where the load varies with time, a horsepower vs. time curve will permit determination of the peak horsepower required. The calcu lation of the root-mean-square (rms) horsepower indicates Table 2 .... Speed Ranges for Various Types of Motors Power Supply Trim Speed Range SinglePhase a-c * Brush-shifting repulsion motor * Capacitor motor with tapped winding Multi-speed capacitor motor 3:1 2:1 2 or 3 fixed speeds Polyphase a-c Multi-speed squirrel-cage * Wound-rotor motor * 2-speed wound-rotor motor Brush-shifting shunt motor * Brush-shifting series motor Squirrel-cage motors with variable frequency supply Motor-Generator Set--D-C Drive Motor Rectifiers--D-C Drive Motor 2, 3, or' 4 fixed speeds 2:1 4:1 20:1 3:1 Very wide range Very wide range Very wide range Shunt-wound standard con 2:1 in some cases stant-speed motor with field control d-c D-C motor with armature Wide control Adjustable-speed motor From 3:1 to 6:1 Shunt motor with adjustable- Very wide voltage supply * Speed regaletioa relatively wide. Unsuitable to some loads. the proper motor rating from a heating standpoint. In case of extremely large variations in load, or where shutdown, ac celerating, or decelerating periods constitute a large portion of the cycle, the rms horsepower may not give a true indica tion of the equivalent continuous load. The motor manu facturer should, therefore, be consulted. Where the load is maintained at a constant value for an extended period (varying from 15 min to 2 hr, depending on the size), the horsepower rating required will usually not be less than this constant value, regardless of other parts of the cycle. If the driven machine is to operate at more than one speed, the horsepower required at each speed must be determined. 4. Torque. The torque required to operate the driven ma chine at-every moment between initial breakaway and final shutdown is important in determining the type of motor. A torque-speed curve is desirable and sometimes essential. The starting torque or breakaway torque required by the driven machine may be as low as 10 percent, as in the case of medium-sized centrifugal pumps, or as high as 225 to 250 percent of full-load torque, as in the case of a loaded re-' ciprocating two-cylinder compressor. The breakaway.torque may vary greatly at different times because of frequency of start, temperature changes, type and amount of lubricant, etc. The motor torque available at the shaft must be well above the torque required by the driven machine, taking into consideration these variables as'well as the possibility of low voltage and the type of starter used. The torque required after breakaway for acceleration to full speed varies with different driven machines, remaining at a rather high value throughout acceleration for such ma chines as loaded compressors and plunger pumps. The torque delivered by the motor must at all points, up to full speed, be in excess of the torque required by the driven machine. The greater this excess torque, the faster will be the accelera tion. The approximate time required for acceleration from rest to full speed is: Time in seconds * (rpm) X WR* tITX 308) (1) where (rpm) = full-load speed in revolutions per minute. T ** average torque available for acceleration, foot pound. WR* = inertia of rotating parts, pound-foot square. If the time to accelerate on full voltage is greater than about 20 sec, special motors or starters may be required to avoid overheating. 5. Inertia of Driven Machine. The inertia or flywheel ef fect WR* of the rotating parts of the driven machine affects the accelerating time and, therefore, the heating of motors and control, particularly where reversing duty or frequent starting is involved. . Where synchronous motors are applied, the WR? must be known, since the pull-in torque required of this motor varies approximately as the square root of the total WR? of motor and load. The WR* of a rotating member of the driven machine which operates at a speed different from that of the motor may be converted to an equivalent value at the motor shaft by multiplying by ((rpm of rotating member) -5- (rpm of motor)]* (2) 6. Frequency of Starting. The frequency of starting the driven machine affects the motor and control by increasing Motors and Motor Controls 641 their heating, particularly where accele**tng time is pro longed by high WR? and high load torques. In general, driven machines starting more than 4 to 6 times per hour may re quire special motors and control. ALTERNATING-CURRENT MOTORS Alternating-current motors are divided into two main classifications: polyphase and single-phase (see Table 3), ac cording to the type of power supply. They are further sub divided 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 capacitorstart induction-run single-phase motor, for instance, will cost approximately twice as much as the corresponding three-. phase Design B squirrel-cage motor. In addition, the poly phase motor has the advantage of higher efficiency. Polyphase Motors The three types of polyphase motors are: squirrel-cage induction motors, wound-rotor induction motors, and syn chronous 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 motorsprovide 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 compresors and pumps where other motors would draw high peak currents. Design F motors have low starting 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 syn chronous speed at light load teraBout 95 percent synchronous speed at full load. Overloads decrease the speed further until the maximum torque point is reached. If the load torque exceeds this point the motor will stall. The motor torque varies as the square of the voltage. If the voltage drops 10 percent, the motor torque will be only 81 percent of rated value. Both power factor and efficiency are improved if the motor is operating as near rated load as possible. In addition, as shown in Fig. 2, power factor and efficiency are better for higher speed motors. Hermetically-enclosed polyphase a-c motors are now quite generally used with large centrifugal compressors. The motors are built into the compressors and are cooled by circulation of refrigerant through the motor windings or by circulation of condenser cooling water through the motor housing. In either case, the cooling is excellent and the motor can deliver more power than if it were air-cooled. Wound-rotor motors are used for applications requiring high starting torque at low starting current, because a wound- *Reto to Glossary at end ot chapter. Fig. 1 .... Speed Torque Characteristics of Squirrel-Cage Motors rotor motor with its controller and resistance can develop full load torque when starting with about full load current. For comparison, a squirrel-cage motor would require from 3 to 5 times as much current to develop full load torque at starting. The wound-rotor motor is also used for varying speed service to drive fans, blowers, and other continuous duty apparatus. The addition of resistance to the secondary winding of the wound-rotor motor changes the speed torque characteristics. The motor speed, with the resistance added, is dependent on load, and consequently, the motor has very poor speed regula tion when secondary resistance is added to reduce the speed Squirrel-Cage Induction Motors