Document zoDkO91QMxEjvG8Do6k9jDe2a

Heating Ventilating Air Conditioning Guide 1938 (3) Manual contactor or reduced voltage compensator with overload and low voltage release. (4) Drum switch equipped with latch coil to give low voltage release. 2. Automatic Control: a. To start on full voltage. (1) Without overload device. (2) With overload device. (3) With combination overload device and knife switch. b. Reduced voltage starting. (1) Primary resistance type starter. (2) Auto compensator type. (3) Reactance type. PILOT CONTROLS In selecting pilot control devices to operate in conjunction with either manual or automatic motor control, it is necessary that they be classified as follows: 1. Two Wire Control. Most thermostats, float switches, and pressure regulators, provide two wire control which gives low voltage release. A three position pilot switch can be used in connection with this method and thus provide manual control. With a low voltage (12 or 20 volt) control circuit it is desirable to use a low voltage thermostat. When this type of thermostat is used it will be found that a saving in the wiring cost results. When using the low voltage thermostat on a control circuit a relay and trans former panel should be used instead of the low voltage coil on the starter. 2. Three Wire Control. Momentary contact start and stop push button stations are usually furnished as standard accessories with automatic starters, which gives low voltage protection. This control cannot be used in combination with two wire pilot devices. In selecting manual control for an alternating or a direct current motor, the common practice is to locate the control near the motor. When the control is installed at the motor, an operator must be present to start and stop or change the speed of the motor by operating the control mechanism. Frequently manual control is employed only as a device to give overload protection and another device is employed to start and stop 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 multispeed motors, is only used as a speed setting device with the starting and stopping functions operated automatically through thermostats, and pressure switches. Because of the increasing complexity, of air conditioning systems, heating, ventilating and air conditioning equipment is being operated on automatic control with less dependence on manual operation and regu lation. Automatic control of motor starters may be accomplished by the use of remote push button stations, by a thermostat, float switch, pressure regu lator or other similar* pilot devices. An added advantage of automatic control is that the main wiring for the starter may be installed near the motor, while the starter may be operated by a control device located else where. In the majority of air conditioning installations, requiring motors 1 hp and larger, two or three phase alternating current is usually supplied. 712 Chapter 38. Motors and Controls DIRECT CURRENT MOTOR CONTROLS Air conditioning installations using direct current power are now only' used where alternating current is not available. Direct current motors are always started through starters, which are devices using a resistance to be put in series with the armature circuit during starting only, the resistance being gradually cut out as the motor comes up to speed: The starting current is held within safe limits by the use of the resistance. The speed of a direct current motor may be regulated by the following methods: 1. Speed regulation by field control--by using a device with resistance to be put in series with the field winding. After the motor has been started to be used to increase the speed of the motor above full field speed. 2. Speed regulation by armature control--by using devices with resistance to be put in series with the armature circuit to be used to reduce the speed of the motor below full field or normal speed. 3. Combinations of field and armature control, so that the starting, field control, or armature control may be combined in a single unit. Field control is usually preferred, depending on the size of the instal lation. For example, if a direct current motor were required with speed regulation between 1200 and 600 rpm, a choice of supplying a 1200 rpm motor with armature control or a 600 rpm motor with field control, both giving the same speed variation would be possible. While the 1200 rpm motor with armature control is lower in first cost than the 600 rpm motor with field control, the cost of operating the 600 rpm motor with field control is less and will save the difference in first cost over a period of time depending on the size of installation. A wide speed variation can be easily obtained in a direct current motor by using a combination of field and armature control. SQUIRREL CAGE MOTOR CONTROL To meet the requirements of various drives of an air conditioning system, three-types of squirrel cage, two or three phase motors may be used : 1. Normal torque, normal starting current. 2. Normal torque, low starting current. 3. High torque, low starting current. Because of the large current inrush of the normal torque, normal starting current motor, central stations usually require current limiting starting equipment on such motors above 5 hp. To meet the starting current requirements, manual or automatic current limiting starting com pensators are used. These compensators are equipped with'50, 65 and 80 per cent voltage taps, the 65 per cent tap being regularly furnished when the compensator leaves the factory. Motors 5 hp and smaller have starting currents within the requirements of central stations and manual or magnetic, full voltage control may be used. The normal torque, low starting current motor has a starting current which is approximately 20 per cent less than the normal current motor on 713