Motors

MIL-HDBK-1003/6

decreases on decreased volume output and fan efficiency increases. Dampers are

closed during the startup of the boiler to reduce starting load on the motor.

b) Variable inlet vanes. These are used to change characteristic

curves of forced draft fans by providing swirl to the fan impeller. Variable

inlet vanes are slightly more effective in saving power than parallel blade

inlet box dampers. When furnishing variable inlet vanes for either

centrifugal of fixed-pitch axial fans, fan manufacturers provide a complete

performance envelope showing the effect of vane position on fan performance

and power. With fixed speed motors, power only slightly diminishes as air

volume is reduced. This system operates economically if air volume is 75

percent or above the design volume.

c) Two-speed motor drive. A centrifugal fan maintains its highest

level of efficiency at high loads. As the load drops the dampers close and

fan efficiency drops off rapidly. Two speed motors allow the fan to operate

at overall high efficiency with the dampers open at two running speeds. The

two speed motor is less expensive and easier to control than variable speed

drives.

Variable-speed motor drive. A variable speed gives an infinite

series of fan curves from which the points of highest system efficiency can be

chosen. A variable speed system significantly improves fan efficiency during

periods when the boiler is operating at less than its maximum load.

Additional Information. Fans and their usage are discussed in

8.3.8

Combustion Engineering Inc. Combustion/Fossil Power Systems, (1981),

Chapter 15.

Fan Drives. Fans are generally driven by either electric motor or

8.3.9

steam turbine. A steam turbine may be used when plant heat balance permits

and the exhaust steam is used for feedwater, space heating, or process. Dual

drive may be found necessary in the case of boiler startup.

8.4

Motors

8.4.1

Selection. Motors shall be selected for the maximum duty required

by the driven machine under most severe anticipated operating conditions.

Consider selection of high efficiency motors if service conditions can be met

and such motors are economically justified. For variable speed drive,

compatible controls must be available.

Power supply shall be designed to meet motor requirements under all

load conditions. Low voltage on A.C. induction motors results in the

following variations in design factors:

a)

Increased slip,

b)

Decreased torque,

152