Specifications Temperature rise limits

The dB/d/is limited to 6 T/s out of concern that larger values could cause nerve stimulation. The r-f exposure is limited to a specific absorption rate (SAR) of 0.4 W/kg for the whole body, 0.32 W/kg averaged over the head, and less than 8.0 W/kg spatial peakia any one gram of tissue. These numbers are designed to limit the temperature rise to less than 1°C and localized temperature of no greater than 38°C head, 39°C tmnk, and 40°C ia the extremities. 

Very high temperature-rise permissible limits of resistance units render them unsuitable for installations which are fire-prone, such its pulp and paper industries, chemical industries, refineries, textile mills, etc. For specific iipphcations and surroundings, however, resistance design can be altered (derated) to restrict the temperature rise to within desirable limits. 

Many of the major motor manufacturers discovered on comparison of their own specifications to the IEEE Standard 841-1994 that their designs already exceeded the requirements of the new standard. Others made a few modifications, and their units satisfied the new standard. The standard required, among other items, (a) a no-load vibration limit of 0.08 in./sec and (b) a temperature rise of 80°C maximum with Class B insulation at rated load. The life of the motor is essentially controlled by the life of its internal insulation and is represented by Figure 14-11. ... 

The current-carrying capacity of the wire is not directly related to the dielectric. This is determined by the conductor resistance and the heating effect that it produces in the wire. The required current-carrying capacity determines the size of the wire and thus the size of the insulator. The temperature rise caused by the current flow determines the type of insulation to be used. If the wire is limited to 140°F (60°C) service, the insulation can be one of those discussed above. If the wire is to operate at 300°F (150° C), another specification for plastic wire with better heat resistance such as TP polyester or PTFE is used. 

IEC60085 and IEC60034 part 1 describe the limitations placed on materials used inside motors (and other electrical equipment). Most electrical machines with air or gas as the cooling medium use Class B or F solid insulation material. Where the environment is harsh, and high ambient temperatures occur, then it is advisable to specify Class F insulation materials but with a restriction of Class B temperature rise. Such a specification will inherently increase the mean time to failure of the materials since they will be less stressed. 

To assess the thermal controllability of the process, the value obtained for the heat of reaction by using one of the methods cited above has first to be multiplied by the mass firaction of the limiting component A of the process and divided by its molecular weight in order to obtain the specific heat of reaction. Dividing this result by the specific heat capacity of the reaction system yields the so-called adiabatic temperature rise ATad ... 

Heating should be instantaneous to prevent drawn out transfer of the pyrolyzates through the injection port. Heated filament and Curie-point pyrolysis result in less secondary pyrolysis products compared to furnace pyrolysis. Curie-point pyrolyzers accurately reproduce pyrolysis conditions with a rapid temperature rise time, yet the choice of different pyrolysis temperatures is limited. Very little sample preparation or pretreatment is required for laser pyrolysis however, a specific laser wavelength may not be appropriate for aU types of samples. 

A definite-purpose motor, on the other hand, is designed for specific service conditions and appUcations. It differs from the general-pnrpose motor with respect to rating, service factor, and temperature rise, one or all of which have limits much narrower than those of general-purpose motors. Definite-purpose motors conform to established NEMA standards, are produced in high volume, and are often low in cost compared with general-purpose motors of the same ratings. However, use of a definite-purpose motor for a duty other than that for which it was intended must be carefully considered. 

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