Page 352 - Compression Machinery for Oil and Gas

Drivers Chapter 7 337

available but it is usually better to ensure adequate start-up unloading of the
compressor. Starting currents are high for full voltage across the line starting
and so a strong power supply is required. Power factor is quite low usually
in the range of 0.5–0.8 lagging. Lower speed motors and high-efficiency motors
tend to have lower power factors. This can result in higher power costs due to
demand charges unless separate power factor correction equipment is installed.
Induction motors are quite sensitive to pulsating torques especially electrically
stiff motors (low slip). The user is cautioned about the use of high-efficiency
motors with low slip (1% or lower), as compared to a standard motor with
1.5% or greater slip. Due to the negative effect of pulsating torque on current
pulsation and power factor the hope for improvement in efficiency may not be
achieved and much greater inertia is required in the compressor flywheel to
achieve acceptable current pulsation (ref API 618).

Mechanical Effects
Torque Pulsation
The compressor imposes a strong pulsating torque on the motor shaft at 1 and
all higher harmonics of the compressor speed, the strongest harmonic is a func-
tion of the number of throws and whether the compressor is at full load (all cyl-
inders double acting) or part load (some or all cylinders single acting or unequal
load head end to crank end). A two-throw compressor will have a very strong
2 harmonic. The strongest harmonic of the four-throw is the fourth harmonic.
The six-throw strongest is usually the sixth. The GMRC (gas machinery
research council) torsional guideline [2] notes that for a two-throw compressor
the motor should be designed for a torque of 100% of nameplate mean torque
plus a pulsation torque of 250%, for a 100% mean plus 200% pulsating, six-
throw compressor 100% mean and 150% pulsating. The pulsating torques
can be reduced by a compressor flywheel and a flywheel normally will be
required on a two- and four-throw compressor in order to meet the current pul-
sation requirement. The motor should be a heavy duty (also called severe duty).
For heavy duty applications, the motor manufacturer will typically provide a
large shaft, heavy duty fan, more internal bracing of windings, etc. Keyways
are best avoided, but if necessary they should include a radius fillet to mini-
mized torsional stresses [2]. It is best if the motor shaft at the drive end from
the core through shaft end (including bearing) be no less than the crankshaft
stub shaft [2].

Lateral Pulsation
The compressor imposes a strong lateral pulsation on the motor. This comes
from the rod loads of the compressor. They are transmitted to the motor in
two ways. Through the baseframe and foundation, and through the crankshaft
to the motor shaft. At each revolution, the two throws adjacent to the drive apply