Page 353 - Compression Machinery for Oil and Gas

338 SECTION II Types of Equipment

the rod load to the crankshaft at the instantaneous angle of the connecting rod.
As the crankshaft rotates, it roughly orbits and because the two drive end throws
are in phase but opposed, the crankshaft also angles. The motion of the crank-
shaft is limited by the bearing clearance which will typically not exceed 0.1% of
the shaft diameter as a diametral clearance. The motion at the crankshaft is pri-
marily at 1 but there is some excitation at all higher harmonics. For a two
bearing motor connected via a flexible coupling the crank lateral motion will
cause vibration of the flywheel (0.2mm p.p. for a 200mm crankshaft assuming
nonresonant motion), but only a limited amount is transmitted through the cou-
pling to the motor. However, for rigidly connect motors, for example, a single-
bearing synchronous motor, the entire lateral motion of the crankshaft is trans-
mitted directly. The outboard bearing and motor shaft needs to be designed for
the crankshaft orbital motion while limiting the vibration at the bearing to
acceptable limits. Because of the compressor pulsating loads the vibration at
the drive motor will exceed what is normally considered acceptable. For exam-
ple, ISO 10816-3 list an acceptable vibration at the motor of 4.5mm/srms, how-
ever, this standard specifically excludes motors driving reciprocating
compressors. A more realistic limit would be the acceptable value listed for
the compressor frame which is 8mm/srms as a typical vibration limit for the
motor frame and the bearings.
Reciprocating compressors impose a cyclic torque and lateral vibration
transmitted through the crankshaft to the motor shaft and lateral vibration trans-
mitted to the motor through the foundation. This means that driving a recipro-
cating compressor needs to be considered severe duty and appropriate
considerations are required.

Variable Frequency Drives
Induction and synchronous motors are designed for a specific voltage per fre-
quency ratio (V/Hz). Voltage is the supply voltage to the motor, and frequency is
the supply frequency. The V/Hz ratio is directly proportional to the amount of
magnetic flux in the motor magnetic material (stator and rotor core lamina-
tions). The torque developed on motor shaft is proportional to the strength of
the rotating flux. The type and the amount of magnetic material used in motor
construction are factors to define motor power rating.
With constant supply power frequency, higher voltage causes higher V/Hz
ratio and higher flux. With constant supply voltage, lower supply frequency
would cause higher V/Hz ratio and higher flux. Higher flux increases the motor
torque capability. When motor operates at higher V/Hz than rated, the overflux-
ing occurs, which may cause saturation of the stator and rotor magnetic core.
Saturation causes overheating and can lead to motor failure. When motor oper-
ates at lower V/Hz than rated, the magnetic flux is reduced. Reduced flux
reduces the torque capability and affects the motor ability to handle the load.