Document dEEj5ar8z2nrZ5bOrO6Ly4GB

American Society of Heating and Ventilating Engineers Guide, 1936 Direct current motors are available with speed characteristics of four types: 1. Constant speed. 2. Adjustable speed. 3. Adjustable varying speed. 4. Varying speed. Constant Speed motors may be shunt wound or compound wound. Shunt wound motors have a nearly flat speed-load characteristic, with a regulation of 12 per cent for 5 hp and smaller and 10 per cent for 7)4 hp and larger, based on full load speed. Compound wound motors have a speed regulation over the range from full load to no load of not more than 25 per cent, based on full load speed. Adjustable Speed motors are usually shunt wound since it is impractical to maintain the proper relation between the shunt and series fields of compound wound motors when wide variations of the field strength are required to obtain the speed adjustment. Adjustment of the speed of. shunt wound motors is obtained by field control, with the minimum or base speed at full field strength and higher speeds at reduced field strength (obtained by adding resistance in the field circuit). The speed regulation from no load to full load will not exceed 22 per cent for 2 to 5 hp; nor 15 per cent for 7)4 hp and larger. Below 2 hp, the regulation may exceed 22 per cent. If closer speed regulation is required, specifically wound motors must be obtained. Practically constant horsepower output is obtained at all speeds up to a ratio of 2 to 1. For higher speed ratios, the horsepower rating at the minimum speed is less than at the maximum speed, this difference varying with the speed ratio. High efficiency is maintained over the entire speed range. Most listed constant speed motors are suitable for operation up to a speed ratio of 2 to 1 by the use of proper control equipment. Adjustable Varying Speed motors may be either shunt or compound wound and speed adjustment is obtained by adding resistance in series with the armature. The speed thus obtained is always below the rated speed. Any standard shunt or compound wound constant speed motor may be used in conjunction with the proper armature resistor. The usual range of speed reduction is 50 per cent. The speed obtained for any setting of the resistor depends on the load of the motor and will vary with this load. \ The speed regulation at high speed is comparable to a constant speed motor, but becomes poorer as the speed is decreased. When operating at reduced speed, an~ increased torque requirement which the motor could easily handle at rated speed is easily-sufficient to stall the motor; for example, a motor operating at two-thirds speed would be stalled by a torque about 50 per cent in excess of the normal requirement. The efficiency of the motor is reduced as the speed.is reduced, since the loss in the resistor is greater at lower speeds. Speed reductioji by armature control is usually selected where: 1. A wide speed range is not required. 2. Close speed regulation is not necessary. 748 Chapter 42--Motors and Their Control 3. Operating time at reduced speed is short. 4. Operating load at reduced speed is small so that the reduced efficiency can be ignored. Varying Speed motors are series wound and the speed varies with the load on the motor. They should be used where: 1. The load is practically constant or increases with speed. 2. The motor can easily be controlled by hand. They should not be used where there is a possibility of operation without load or at a reduced load, as the speed of the motor may become dangerously high. For shunt wound motors with full field strength, the starting torque varies almost directly with the starting current, which is dependent on the resistance in the armature circuit. With varying positions of the starting rheostat, it is possible to obtain a wide range of starting torque, within the limits of starting current permitted by the power company. A compound wound motor requires somewhat iess current for the same starting torque. The maximum torque is limited by commutation. ALTERNATING CURRENT MOTORS Alternating current motors may be divided into two main groups, namely, (1) those operating on single phase current, and (2) those oper ating on polyphase current. 1. Single phase motors are available in three common types: a. Repulsion start, induction run. b. Capacitor type. c. Split phase. 2. Polyphase (2 or 3 phase) motors are available in four common types: o. Squirrel cage induction motor. b. Automatic start induction motor. c. Slip ring, wound rotor induction motor. d. Synchronous motor. Where the public utility supplying the current determines that a particular installation should be served with polyphase current, it is generally understood that the major portion of the motors will be for polyphase current, although it . is commonly acceptable for the smaller motors to be single phase. This will in general, limit the use of single phase current to the smaller motor ratings and the polyphase to the larger motors. Single Phase Motors Repulsion start, induction run motors are suitable for use with fans, centrifugal pumps, reciprocating pumps and compressors (started loaded or unloaded) since these motors have relatively high starting torques and low starting currents. This type of. motor has a constant speed characterristic. The motor has a commutator and brushes to obtain starting and an integral automatic governor for changing to an induction motor when nearly up to speed.