Document DgNXZMwd9g1gxEDv76zoomZQ

618 CHAPTER 34 . 1948 Guide "allow for variations in internal load, an inside thermostat should be used as a high limit to reduce further the heatinput if necessary. If a thermally light radiant surface is used, controls may be applied in the same manner as for typical convection heating. The terms thermally heavy and thermally light, referring to capacity for heat storage, are comparative and descriptive rather than exact. For example, a concrete floor panel in a frame structure without insulation would represent a heavy panel in a light structure. A frame type (metal lath and plaster) panel in a concrete structure would represent a light panel in a heavy structure. As indicated previously a heavy panel in a heavy "structure provides comfortable conditions if outside controls are used in addition to the inside thermostat. But if a heavy panel is used in a light structure, rapid changes in outdoor conditions may cause dis comfort in spite of outdoor controls, because the structure reacts so much more rapidly than the radiant heating surface. Compensation for Increase of MRT In order to maintain comfort, the air temperature in a panel heated space should be lowered as the heating load increases. Usually the required reduction in air temperature is not great and a conventional fixed control point room thermostat may be used unless a rather large infiltration load exists, or if untempered mechanical ventilation is used. Because of the relationships existing between MRT (mean radiant temperature) and air temperature in the space (and the variable MRT from point to point in the space), a conventional type of room thermostat (either fixed or variable control point as previously determined) may provide simple and satisfactory control. Lowered Night Temperature In general, lowered night temperature control is not recommended with heavy, panels though it may be satisfactory with light panels. Best practice indicates continuous circulation of the heat in a medium with control provided to vary the temperature of the medium. INDICATING AND RECORDING EQUIPMENT _ In addition to the automatic control of temperature and humidity con ditions, visual indication and permanent chart records of the variables involved are desirable. They provide an accurate check on the per formance of the system both from the standpoint of conditions main tained and cost of operation. Instruments are available to provide accurate records of these variables, such as pressure, temperature, humidity, flow and COj, which go to make up a.complete heating or air conditioning system. In some cases the control equipment is provided with indicating or recording mechanisms by means of which the perform ance of the controls may be observed or recorded and in other cases sepa rate instruments are used for the purpose. Chapter 35 MOTORS AND MOTOR CONTROLS Motor Rating; Function* of Motor Control Equipment; Direct Current Motors, Types, Control Equipment and Specifications; Alternating Current Motors, Types, Control Equipment and Specifications; Gear Motors; Glossary of Motor Terms, Enclosures, Speed Classification and Mounting THE electric motor, available in many different types suitable for various services, is now the most widely used form of prime mover. The equipment for starting, controlling and protecting these motors varies with the type and with the functions it is desired to attain. Motors are divided into two general classifications, alternating-current or directcurrent, depending on the power source to be used. In selecting a motor for a particular application consideration must first be given to the type of power supply available. All machinery has certain load characteristics which may vary with speed. Some types may have a constant torque over wide ranges of speedt while others may have changing torques with changing speed. Consideration should be given to selecting the motor and the motor control which best suit the requirements of the drive. MOTOR "RATING The rating of an electric motor depends upon the total temperature which the motor attains under operating conditions. This total tem perature depends on both the ambient temperature and the temperature rise of the motor. As motor temperature rise is in turn determined by the ability of the motor to dissipate heat, circulation to the motor should not be restricted. Improper selection of motors with regard to tem perature ratings may result in high motor operating temperatures with accompanying reduction in motor life. In general, the electrical insulation is the portion of the motor most susceptible to injury from high operating temperatures. Of the several types of insulation which are available, the most common type, specified as Class A by the National Electrical Manufacturers Association, consists of cotton, felt, paper or similar organic materials and permits a 55 C rise in temperature over a 40 C ambient temperature. Class B insulation consists of mica, asbestos, fiber glass, or similar inorganic materials and permits a 75 C rise in temperature over the 40 C ambient temperature. Other types of insulation such as silicone resin are available and permit extremely high operating temperatures. The mechanical construction of the different types of motor enclosures and the rise in temperature with Class A insulation for each type are enumerated in the glossary at the end of this chapter. Since the dif ference in temperature between the hottest spot and the nominal tem perature, as measured by a thermometer, is greater for a completely unprotected machine than it is for an enclosed machine, the permissible temperature rise is smaller for an open motor. FUNCTIONS OF CONTROL EQUIPMENT FOR MOTORS In general, control equipment for all types of motors should provide: (1) means of disconnecting the motor from the power supply; (2) means 619