Page 351 - Compression Machinery for Oil and Gas

336 SECTION II Types of Equipment

crank effort and the driven inertia (supplied by the packager). Most motor man-
ufacturers will calculate the current pulsation for both induct and synchronous
motors using a simple single degree of freedom model, electrical bus-magnetic
field stiffness inertia of motor and driven equipment lumped as one mass. This
is accurate for most systems where the torsional natural frequency is above four
times running speed. For applications where the first torsional natural frequency
is below four times running frequency, a more accurate result can be obtained
using a 2 degree of freedom model; electrical bus—magnetic field stiffness—
motor inertia—equivalent shaft stiffness—flywheel and compressor stiffness,
lumped as one mass. Not all motor manufacturers can do this calculation,
but it will show more accurate and lower current pulsation for torsionally soft
systems. The motor manufacturer will determine the minimum amount of sys-
tem inertia required to achieve 66% for synchronous and 40% for induction
motors. The required inertia can be added using a compressor mounted flywheel
if the motor inertia is not adequate. Note for a synchronous motor it is recom-
mended that the driven inertia always be less than the motor inertia. This is
because of the strong (up to 40% of nameplate torque) 2 slip frequency pul-
sating torque that is induced during acceleration. During acceleration this
2 slip frequency torque will inevitably coincide and excite torsional resonance
at some brief point during start-up assuming that torsional resonances occur
between 0 and 120Hz (for a 60Hz electrical frequency). Reciprocating com-
pressor drives are torsionally robust so if the driven inertia is less than the motor
inertia then the resulting torsional stresses are unlikely to exceed permissible
limits. See Chapter 5 for a further discussion of torsional analysis.

Summary Comparison between Induction and Synchronous Motors
Synchronous motor
Typically, 1%–2% better efficiency than induction and runs at a power factor of
1.0 or 0.8 leading. Therefore, synchronous motors can improve the overall
power factor of the plant resulting in reduced electrical demand charges and
are less affected by the pulsating compressor torques. Has low starting current
than an induction motor. Starting torque is lower than induction motor, 40/30/
150 is typical but 60/60/175 is available. Care is required for proper compressor
start up unloading especially at the 95%–100% speed point where the motor
pulls into synchronization. A synchronous motor is especially useful in slow
speed applications (under 400rpm) when it is comparable to the cost of an
induction motor and is normally used in a single-bearing configuration directly
and rigidly bolted to the crankshaft.

Induction motor
Lowest-cost driver provides good starting torque, usually 60% at breakaway
which continuously rises to a peak at about 95% speed. Higher torques are