Page 186 - Compression Machinery for Oil and Gas

Reciprocating Compressors Chapter 5 175

Advantage and Disadvantage Comparison: Induction
vs. Synchronous Motors
Synchronous Motors Typically, it is 1%–2% better efficiency than induction.
It runs at a power factor of 1.0 or 0.8 leading so can improve the overall power
factor of the plant resulting in reduced electrical demand charges. It is less
impacted by the pulsating compressor torques. Starting current is lower in syn-
chronous motor than in induction motor. Starting torque is lower in synchronous
motor than in 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 synchro-
nous 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 Motors It is the lowest cost driver. It provides good starting torque,
usually 60% at breakaway which continuously rises to a peak at about 95%
speed. Higher torques are 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 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 stan-
dard motor with 1.5% or greater slip. Due to the negative effect of pulsating
torque on current pulsation and power factor the hoped for improvement in effi-
ciency may not be achieved and much greater inertia is required in the compres-
sor flywheel to achieve acceptable current pulsation [2b].

Motor 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 stron-
gest harmonic is a function of the number of throws and whether the compressor
is a full load (all cylinders 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. Four-throw strongest harmonic is the
fourth. Six-throw strongest is usually the sixth. The GMRC torsional guideline
[3] 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 four throw 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