Carrier HVAC Handbook New Edition Part 8 Auxiliary Equipment

Part 8. Auxiliary Equipment | Chapter 2. Motors And Motor Controls

multi-speed operation by using an external resistor, reactor or auto transformer to vary the terminal voltage in fixed, predetermined steps. Other single-phase motors available (generally not in use with air conditioning equipment) are the repulsion-induction and repulsion-start, induction-run motors. These are the commutator type. This construction utilizes an arrangement in which the main field winding is connected in series with the compensating winding and the brushes are shortcircuited. These motors have been developed to produce a particularly high starting torque. A repulsion motor has the variable speed characteristics while the repulsion-start, induction-run induction-run motor has constant speed characteristics of the regular squirrel-cage induction motor. Fractional Fractional horsepower series motors that are adapted for use on either d-c or a-c circuits of a given voltage are called universal motors.

MOTOR MECHANICS AND ENVIRONMENT This section is devoted to the physical-mechanical aspects of motors in relation to power impressed, full load operation and the environmental conditions to which motors are subjected. INTERNAL OVERHEATING OF MOTOR

The motor rating is an arbitrarily specified safe operating limit for the machine determined in accordance with certain accepted standards. It is intended to represent the operating limit which the machine cannot ordinarily exceed for a considerable length of time without damage to itself. The motor may exceed its rated load by 10%, 25%, 50%, but at a risk of a rise in temperature that may permanently injure the winding and its insulation; in fact the motor will stall on reaching its maximum in torque rating regardless of  temperature. Motors designed for continuous service can carry specified loads for reasonably long periods of  time without exceeding the heating limits. limits. Windings and Insulation

From previous discussions it is quite evident that the motor windings are the heart of the motor. The power impressed impressed on them must be contained; con tained; therefore the winding winding must be electrically insulated from adjacent parts. An electric motor in operation is higher in temperature than the ambient; the various motor parts are actually at different temperatures as are also the sections of the winding. The section of the winding at

the highest temperature is termed “hot spot” and is usually on the axial center line of the core in one of the slots. Under normal operating conditions the temperature rise of the the motor is due to the natural process occurring during the conversion of electric energy into mechanical energy and the rotation of parts. There are three sources of energy losses appearing as heat that raise the motor temperature: 1. Winding – heat produced by a flow of current against resistance and equal to the product of  2 current squared and resistance (I R).  (With a motor design resulting in lower current and/or  lower resistance less heat is produced and the motor is more efficient.) 2. Iron core – heat produced by hysteresis* and eddy current losses set up by the magnetic field In the stator and rotor. 3. Mechanical parts – bearings, fans, brushes (when used). Proper control of the driven load or  number of motor starts required can also influence the winding losses. The losses occurring during full load operation of  the motor can be divided into two groups: (1) fixed 2 losses, running light losses (I R no-load current losses), iron losses, bearing friction and internal fan (when used), and (2) applied losses of the driven load. The major heat losses are in the windings. There is a definite maximum temperature which the windings can withstand under a given load and with a given insulation without undue deterioration either within themselves or in the insulation. In order that the maximum output of the motors may be secured without overheating, it is necessary to keep the heat losses to a minimum. Thus the insulation of the motor windings performs a dual function, that of an electric insulator  and also a controlled heat dissipator. NEMA has established six classes of insulation designed designed for various loads and for keeping the hot-spot temperatures within safe limits. Table 4 lists class designations, description of insulation materials, and the limiting safe hot-spot temperatures (C). The limiting hot-spot temperatures shown are de-termined de-termined by adding together: 1. Ambient environment temperature, normally 40 C (104 F). *Hysteresis is the conversion of electrical energy into heat  energy due to molecular friction opposing magnetic polarity  changes, friction that opposes the turning about of atoms.