Document dkpnjxXyaa77RXwwjeg3rE56

tion is ended. The second type, called low voltage protection, causes theSjtf motor line contactor to drop; out on low voltage, but prevents restartihgff? when the voltage returns to normal except by the action of an OperatorfSS This latter type of protection is desirable where it is necessary for the-S operator to make initial starting adjustments on the machine. s#' Manual control for an alternating or a direct current motor is usually located near the motor. When so located an operator must be present'/' to start and stop or change the speed of the motor by operating the control mechanism. Manual control is sometimes employed only as a device to; ' give overload protection, and another device is employed to start and stop1 the motor. Manual control is used particularly on small motors which operate unit heaters, small blowers, and room coolers in an air conditioning system. In other cases manual control in the form of drums, when used" with multi-speed motors, is used only as a speed setting device, while the: ,--iNUAL STARTER Reduced voltage} MOTOR Arrangements 2, 3 and 4 are optional for motors up to IVi hp, 220 volts. Fig. 9. Recommended Controls for Single Phase Motors starting and stopping functions operate automatically through thermostats and pressure switches. Because of the increasing complexity of air conditioning systems, the equipment is operated preferably by automatic control, and less dependence is placed on manual operation and regulation. Automatic control of motor starters may be accomplished by the use of remote push button stations, by a thermostat, float switch, pressure regulator, or other similar pilot devices. An added advantage of auto; matic control is that the main wiring for the starter may be installed near the motor, while the starter may be operated by a remote control device. GLOSSARY General Definitions NEMA is the abbreviation for the National Electrical Manufacturers Association. AEIC is the abbreviation for the Association of Edison Illuminating Companies. EEI is the abbreviation for the Edison Electric Institute. Speed Regulation (d-c motors) is the change in speed between no-load and fullload, expressed in percent of full-load speed; for example, a motor having a no-1oaa speed of 1200 rpm and a full-load speed of 1140 rpm would have a speed regulation ot 5.26 percent. Slip (a-c induction motors) is the difference between the motor speed and syn chronous speed expressed in percent of synchronous speed, e.g., a 1200 rpm motor operating at 1140 rpm would have a slip of 5 percent. Motors and Motor Controls 929 Torque is an expression of the. turning effort developed by the motor at the shaft, and is usually expressed in ounce-feet for fractional horsepower motors, and in poundfeet for motors of larger ratings. Primary is the term usually applied to the high voltage or line side of a transformer or motor. In the case of the wound rotor motor the primary is the stator winding. Secondary is the term usually applied to the low voltage or load side of a trans former or motor. In the case of the wound rotor motor the secondary is the rotor winding. NEMA Classification of Motor Enclosures Open motors (40 C rise, rated load, 50 C rise, service factor load) are self-ventilated machines having no restriction to ventilation other than that necessitated by me chanical construction. Guarded motors (50 C Rise) have all ventilating openings limited in size by either design of structural parts, perforated covers or screens preventing direct access to live or rotating parts. Semi-Guarded motors (50 C rise) have the ventilating openings in the top half of the frame only protected by perforated covers. Drip Proof motors (50 C rise) are so constructed that drops of liquid or solid par ticles falling on the machine at any angle not greater than 15 deg from the vertical, cannot enter the machine either directly or by striking and running along a hori zontal or inclined surface. Splash Proof motors (50 C rise) are so constructed that drops of liquid or solid particles falling on the machine or coining toward it in a straight line at any angle not greater than 100 deg from the vertical, cannot enter the machine either directly or by striking and running along the surface. Totally Enclosed Non-Venlilated motors (55 C rise) are so constructed as to pre vent exchange of air between inside and outside of the case, but are not air tight and are not equipped with external cooling means. Totally Enclosed Fan-Cooled motors (55 C rise) are similar to totally enclosed, nonventilated machines, except that exterior cooling is provided by means of a fan or fans integral with the machine. Explosion Proof motors (55 C rise) have an enclosing case designed to withstand an explosion of a specified gas or vapor which may occur within it, and to prevent Wtion of the gas or vapor surrounding the motor by sparks, flashes, or explosions of th gag or vapor which may occur within the machine casing. Dust Explosion Proof motors (55 C rise) have an enclosing case designed and con structed so as not to cause the ignition or explosion of an atmosphere of the specific jtnst, or to cause ignition of dust on or around the machine. (Proper overload pro tection and cleanliness are required for successful operation). , *l*er Proof motors (55 C rise) are so constructed as to exclude water applied in the form of a stream from a hose. Motor Speed Classifications A Constant Speed motor is one in which the speed remains practically constant with E|i-g- lr- load; e.g., a d-c shunt wound motor or a-c squirrel-cage motor with low j '* Varying Speed motor is one in which the speed varies with the load, usually pressing when the load increases; e.g., a d-c series motor or an induction motor "ith large slip. Adjustable Varying Speed motor is one in which the speed can be adjusted vvith H when once adjusted for a given load will vary in considerable degree Contr^nge in load; e.g., a shunt wound d-c motor adjusted by armature resistance a co* Speed motor is one in which the speed can be varied gradually over IoadnSldera^e ran&e> hut when once adjusted remains practically unaffected by the open i ^ y a ^ c shun^ motor with field resistance control. The standard ratings for in adjustable speed motors, having a speed range of 3 to 1 and greater are n acct>rdance with the following: ^ ^*SLan^ar^ continuous horsepower rating at 150 percent of minimum speed a temperature rise of 40 C. ) The next higher standard continuous horsepower rating at 3 times minimum speed with a temperature rise of 40 C.