Machinery's Handbook, 31st Edition
ELECTRIC MOTOR STANDARDS 2647 Motor Types According to Variability of Speed.— Five types of motors classified accord - ing to variability of speed are: Constant-speed Motors: In this type of motor the normal operating speed is constant or practically constant; for example, a synchronous motor, an induction motor with small slip, or a direct-current shunt-wound motor. Varying-speed Motor: In this type of motor, the speed varies with the load, ordinarily decreasing when the load increases; such as a series-wound or repulsion motor. Adjustable-speed Motor: In this type of motor, the speed can be varied gradually over a considerable range, but when once adjusted remains practically unaffected by the load; such as a direct-current shunt-wound motor with field resistance control designed for a considerable range of speed adjustment. The base speed of an adjustable-speed motor is the lowest rated speed obtained at rated load and rated voltage at the temperature rise specified in the rating. Adjustable Varying-speed Motor: This type of motor is one in which the speed can be ad- justed gradually, but when once adjusted for a given load will vary in considerable degree with the change in load; such as a direct-current compound-wound motor adjusted by field control or a wound-rotor induction motor with rheostatic speed control. Multispeed Motor: This type of motor is one which can be operated at any one of two or more definite speeds, each being practically independent of the load; such as a direct-cur rent motor with two armature windings or an induction motor with windings capable of various pole groupings. In the case of multispeed permanent-split capacitor and shaded pole motors, the speeds are dependent upon the load. Pull-Up Torque.— NEMA Standard pull up torques for single-speed, polyphase, squir- rel-cage integral-horsepower motors, Designs A and B, with continuous ratings and with rated voltage and frequency applied are as follows: When the locked-rotor torque given in Table 2 is 110 percent or less, the pull-up torque is 90 percent of the locked-rotor torque; when the locked-rotor torque is greater than 110 percent but less than 145 percent, the pull-up torque is 100 percent of full-load torque; and when the locked-rotor torque is 145 percent or more, the pull-up torque is 70 percent of the locked-rotor torque. For Design C motors, with rated voltage and frequency applied, the pull-up torque is not less than 70 percent of the locked-rotor torque as given in Table 2. Types and Characteristics of Electric Motors Types of Direct-Current Motors.— Direct-current motors may be grouped into three general classes: series-wound; shunt-wound; and compound-wound. In the series-wound motor the field windings, which are fixed in the stator frame, and the armature windings, which are placed around the rotor, are connected in series so that all current passing through the armature also passes through the field. In the shunt-wound motor , both armature and field are connected across the main power supply so that the armature and field currents are separate. In the compound-wound motor , both series and shunt field windings are provided and these may be connected so that the currents in both are flowing in the same direction, called cumulative compounding , or so that the currents in each are flowing in opposite directions, called differential compounding . Characteristics of Series-Wound Direct-Current Motors.— In the series-wound motor, any increase in load results in more current passing through the armature and the field windings. As the field is strengthened by this increased current, the motor speed decreases. Conversely, as the load is decreased the field is weakened and the speed increases and at very light loads may become excessive. For this reason, series-wound direct-current mo- tors are usually directly connected or geared to the load to prevent “runaway.” (A series- wound motor, designated as series-shunt wound, is sometimes provided with a light shunt field winding to prevent dangerously high speeds at light loads.) The increase in armature current with increasing load produces increased torque, so that the series-wound motor is
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