Derating

Andrews deration An important titration for the estimation of reducing agents. The reducing agent is dissolved In concentrated hydrochloric acid and titrated with potassium iodale(V) solution. A drop of carbon tetrachloride is added to the solution and the end point is indicated by the disappearance of the iodine colour from this layer. The reducing agent is oxidized and the iodate reduced to ICl, i.e. a 4-eiectron change. 

For design, the slip velocity is derated to 70-80 percent of the calculated value to give some margin of safety this sets the design value of the continuous phase velocity (V ). The column cross sectional area (and therefore diameter) is set by QJVc- With the diameter set, the other dimensions can be set using the ratios given above. 

Gaseous fuels containing fractions whose ignition temperature is lower than that of methane may require the use of low-compression heads and a resulting derating of the gas engine. 

The performance of a motor is greatly influenced by a voltage unbalance in the supply system. It reduces its output and torque and results in a higher slip and rotor loss. This subject is covered in more detail in Section 12.2(v). For likely deratings, refer to Figure 12.1. Asystem with an unbalance of up to 1 % or so calls for no derating, whereas one having an unbalance of more than 5% is not recommended for an industrial application, because of a... 

To restrict the end temperature to less than the permissible limits, it is essential that the motor output be reduced, or for a required output, a higher-capacity motor be chosen. Table 1.7 gives the approximate derating factors for different ambient temperatures. Figure l.l I is based on these figures from which the derating factor even for intermediate temperatures can be quickly determined. 

Figure 1.11 Derating curve for higher ambient temperatures for insulation class E or B...

Figure 1.12 Derating curve for temperature rise restriction (drawn for class E insulation)...

Note If the manufacturer can ascertain that a 100 h.p. frame has a reserve capacity such that at full load the temperature rise will not go beyond 60°C, the derating as calculated below will not be necessary. 

If the machine is derated for a 60 C temperature rise above 40°C, then the total temperature the motor would attain will be 100°C. In this case, even if the ambient temperature rises to 50°C (the temperature rise remaining the same) the total temperature the motor will attain will be ... 

Therefore, derating the machine only for limiting the temperature rise to 60°C will be adequate. 

Derating from Table 1.7 for restricting the temperature rise to 60°C is 84%. 

Note A slightly higher derating for higher-speed motors, 1500 r.p.m. and above, is required in view of the cooling which in higher-speed motors will be affected more than in lower-speed motors. 

In the previous example, if altitude be taken as 2000 m, then a further derating by 94% will be essential. 

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. 

Torque and output varying as in the curves, in which case no derating is necessary,... 

Keeping torque constant throughout the speed range. At reduced speed the torque is low and therefore the motor rating should be derated accordingly, e.g. at 50% speed, the torque is 73%, To obtain 100% torque, the motor should be rated for 100/0.73, i,e, 137%,... 

Table 5.3 shows the derating values and Figure 5,9 the derating curve. The variation in the rotor current is also in the same proportion as the torque. 

For all the applications discussed above, which may require special starting and/or pull-out torques or speed variations, the use of static drives is more appropriate today. With the use of this technology, a standard motor can be made to perform any required duty, except the constructional features and the applicable deratings as discussed in Chapter 1. See also Example 7.1. 

Note This operating condition is, however, not specific for it does not stipulate the frequency of occurrence of such a contingency. It may be assumed that this condition will not occur more than once before thermal stability is reached. See also Section 3.7. In normal practice a motor meeting the other operating conditions noted above in all likelihood will satisfy this requirement also without needing yet another derating. See also Example 7,1. 

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