Document em5zVB9qGBgNGKJ40p2e7o8ZE

HEATING VENTILATING AIR CONDITIONING CUIDE 1944 current, and (2) those motors which may operate on polyphase current. 1. Single phase motors are available in four common types: a. Capacitor motors. (1) Capacitor start, capacitor run. (2) Capacitor start, induction run. b. Repulsion induction motors. c. Repulsion start, induction run motors. d. Split phase motors. 2. Polyphase (2 or 3 phase) motors are available in three common types: . Squirrel-cage induction motor. . Wound rotor induction motor. c. 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 sminogtolersp.hasDe.omestic and semi-commercial installations will usually be Single Phase Motors Capacitor type motors are available in ratings up to 5 hp for general purposes. These motors are recommended for pumps, compressors and fan duty including housed centrifugal fans and propeller fans. The. general purpose motor is commonly known as a high torque capacitor motor having approximately 300 per cent starting torque with normal current and having a different value of capacitance for starting and running which is Automatically changed over by a mechanical or electrical means. Capacitor motors for fan duty are usually divided into, the open high torque type for belted fans and the totally inclosed non-ventilated low torque type for propeller fans mounted directly on the motor shaft. The open low torque capacitor motor may be used with sma|l centrifugal fans mounted on the motor shaft. - Although the motors for belted fans are called high torque, the available starting torque is somewhat less than the torque of the general purpose motor and the slip at full load is approximately 8 per cent. With this larger amount of slip, adjustable speed down to 60 or 70 per cent of rated speed may be obtained by line voltage variation. Motors for propeller fan drive may be supplied with sleeve bearings to obtain greater quietness in the smaller sizes where the fan thrust does not exceed approximately 25 lb. For larger fans, thrust ball bearing motors'should be used. Low torque capacitor motors have, approximately 50 to 60 per cent starting torque and do not change the value of capacitance from start to run. Two of the curves in Fig. 1 show the relation of torque and speed for the low and high torque capacitor motors. - Capacitor motors with high slip may have taps brought out from the main winding which, when, connected to the line, give.a second speed of '668 CHAPTER 36. MOTORS AND MOTOR CONTROLS from 65 to 70 per cent of the normal speed. This type of motor must be specially designed for the individual fan, otherwise the correct low speed will not be obtained. Care should be exercised in applying it to centrifugal fans where restriction to the air flow through the use of adjustable dampers changes the motor load and consequently the speed. This same effect is also found in transformer speed controllers, however, a series of trans former taps allows for a selection which partially overcomes the effect of change in motor load. Capacitor start-induction run motors are usually confined to the smaller- horsepower ratings and differ from the capacitor motors by having no running capacitor. The value of starting capacitance used may vary with the different types of applications involved. These motors may be used for practically any of the applications met in air conditioning. However, Fig. 1. Typical Speed-Torque Curves for Small Motors consideration should be given to the fact that they are not as quiet as a capacitor motor. Repulsion induction motors'start as repulsion motors and operate under full speed as combined repulsion and induction motors through the. in herent characteristics of the motor which has, in addition to the wire winding with commutator, a buried squirrel-cage winding. No additional switching devices are required to change over from start to run- This and the repulsion motor described later may be used for constant speed drives where high starting torque is required and where commutator and brush noise Are not factors. . .. The repulsion start-induction run motor starts as a repulsion motor, has a. switching: means: for transferring from start to run which short circuits the commutator and permits operation under full speed as a wound induction motor. This motor is suitable, for applicationssimilar to those for which the repulsion induction motor is used. The . split phase motor has a high resistance auxiliary winding In the 669