Machinery's Handbook, 31st Edition
2650 Electric Motor Applications stokers, etc. The starting torque at 1800 rpm is 250 to 275 percent of full load torque for 3 hp (2.24 kW) and below; for 5–75 hp (3.73–56 kW) ratings the starting torque ranges from 185 to 150 percent of full load torque. They have low starting current requirements, usually no more than 5 to 6 times full load current and can be started at full voltage. Their slip (difference between synchronous speed and actual speed at rated load) is relatively low. Design C motors have high starting torque (up to 250 percent of full load torque) but low starting current. They can be started at full voltage. Slip at rated load is relatively low. They are used for compressors requiring a loaded start, heavy conveyors, reciprocating pumps and other applications requiring high starting torque. Design D motors have high slip at rated load, that is, the motor speed drops off apprecia bly as the load increases, permitting use of the stored energy of a flywheel. They provide heavy starting torque, up to 275 percent of full load torque, are quiet in operation and have relatively low starting current. Applications are for impact, shock and other high peak loads or flywheel drives such as trains, elevators, hoists, punch and drawing presses, shears, etc. Design F motors are no longer standard. They had low starting torque, about 125 percent of full-load torque, and low starting current. They were used to drive machines which required infrequent starting at no load or at very light load. Multiple-Speed Induction Motors.— This type has a number of windings in the stator so arranged and connected that the number of effective poles and hence the speed can be changed. These motors are for the same types of starting conditions as the conventional squirrel-cage induction motors and are available in designs that provide constant horse power at all rated speeds and in designs that provide constant torque at all rated speeds. Typical speed combinations obtainable in these motors are 600, 900, 1200 and 1800 rpm; 450, 600, 900 and 1200 rpm; and 600, 720, 900 and 1200 rpm. Where a gradual change in speed is called for, a wound rotor may be provided in addition to the multiple stator windings. Wound-Rotor Induction Motors.— These motors are designed for applications where extremely low starting current with high starting torque are called for, such as in blowers, conveyors, compressors, fans and pumps. They may be employed for adjustable-varying speed service where the speed range does not extend below 50 percent of synchronous speed, as for steel plate-forming rolls, printing presses, cranes, blowers, stokers, lathes and milling machines of certain types. The speed regulation of a wound rotor induc- tion motor ranges from 5 to 10 percent at maximum speed and from 18 to 30 percent at low speed. They are also employed for reversing service as in cranes, gates, hoists and elevators. High-Frequency Induction Motors.— This type is used in conjunction with frequency changers when very high speeds are desired, as on grinders, drills, routers, portable tools or woodworking machinery. These motors have an advantage over the series-wound or universal type of high speed motor in that they operate at a relatively constant speed over the entire load range. A motor-generator set, a two-unit frequency converter or a single unit inductor frequency converter may be used to supply three-phase power at the fre- quency required. The single unit frequency converter may be obtained for delivering any one of a number of frequencies ranging from 360 to 2160 cycles and it is self-driven and self-excited from the general polyphase power supply. Synchronous Motors.— These are widely used in electric timing devices; to drive ma- chines that must operate in synchronism; and also to operate compressors, rolling mills, crushers which are started without load, paper mill screens, shredders, vacuum pumps and motor-generator sets. Synchronous motors have an inherently high power factor and are often employed to make corrections for the low power factor of other types of motors on the same system.
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