With the exception of the reciprocating and rotary compressors, the equipment considered is of the centrifugal type (fans, refrigeration compressors, pumps) operating at starting within a system of high and low sides that are equalized. Ordinarily no specific requirement of starting torque other than normal is needed.
At times with large centrifugal compressors or fans there exists a pull-up torque problem because of the large rotational inertia (Wk2 )* of the massive impellers or wheels. The pull-up minimum torque must exceed Wk2 . A possible problem may exist when small equipment is driven by an oversized motor with overpowering torque; this equipment may be damaged because of excessive acceleration or torque applied.
The duration of acceleration to full speed at full voltage is usually 1 to 3 seconds. With open centrifugal compressors using a standard integral oil pump, it should be at least 8 seconds, permitting oil to reach all the lubricated surfaces before high speed is developed.
When an auxiliary oil pump is used, it should be at least 5 seconds to prevent excessive stress con-centration on the keyway of the compressor shaft. The present methods of starting the larger reciprocating compressors with cylinders either fully or partially unloaded permit the use of normal starting torque motors.
The smaller, hermetically driven compressors are started fully loaded with a requirement of high starting torque. The application of rotary compressors as low pressure differential boosters in a refrigeration system does not present any unusual requirement of starting torque; therefore, a normal starting torque motor is applicable.
Centrifugal, propeller or axial fans may be either belt or direct connected to electric motor drives. In all cases the torque requirements are normal. The fans should start smoothly and without undue noise. The fan cfm is directly proportional to speed; the system resistance varies as the square of speed; and the horsepower required varies as the cube of speed.
Because the gas density at the suction of a centrifugal compressor usually increases during shutdown, the torque requirements of centrifugal compressors do not necessarily follow the fan laws. Normal torque motors are usually used for driving centrifugal compressors; however, they must be started either with an almost completely closed suction damper or pre-rotation guide vanes to prevent the increased gas density from imposing an excessive overload at full speed.
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