STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Prime Movers 417
Natural Gas Pipelines. Compressor drivers are usually reciprocating gas engines or
gas turbines, to make use of the energy available in the pipeline. Electric motor drives
use slow-speed synchronous motors for reciprocating compressors and four or sixpole
induction motors with gear increases for high-speed centrifugal compressors.
Motor voltages, types, and enclosures are selected as for oil pipeline pumps. Motors
used with centrifugal compressors must develop sufficient torque at the voltage
available under in-rush conditions to accelerate the high inertia load. They mu51
also have adequate thermal capacity for the long starting time required, which
may be 20 or 30 s.
Motor Control
Efficient use of electric motors requires appropriate control systems. Typical control
systems for AC and DC electric motor operations are now discussed [IO].
Inverter-AC Motor Drives. An adjustable-frequency control of AC motors provide
efficient operation with the use of brushless, high-performance induction, and
synchronous motors. A typical system is shown in Figure 3-14. Such a system consists
of a rectifier (which provides DC power from the AC line) and an inverter (which
converts the DC power to adjustable-frequency AC power for the motor). Inverter
cost per kilowatt is about twice that of controller rectifiers; thus the power convertor
for an AC drive can approach three times the cost of a DC drive.
These AC drive systems require the inverters to operate with either low-slip induction
motors or reluctance-type synchronous-induction motors. Such systems are used where
DC commutator motors are not acceptable. Examples of such applications are motor
operations in hazardous atmospheres and high motor velocities.
The power convertor must provide the AC motor with low-harmonic voltage
waveform and simultaneously allow the amplitude to be adjusted. This avoids magnetic
saturation of the motor as the frequency is adjusted. For constant torque, from
maximum speed to base speed, the voltage is adjusted proportional to frequency.
Above base speed, the motor is usually operated at constant horsepower. In this region
the voltage is held constant and the flux density declines. Also, the convertor must
limit the starting current, ensure operation at favorable slip, and provide a path for
reverse power flow during motor slowdown.
Inverters are designed with various power semiconductor arrangements. Power
semiconductor elements of the inverter operate like switches by synthesizing the motor
voltage waveform from segments of the DC bus voltage. For power ranges up to
about 5 hp, convertors can use power transistors to synthesize six-step (per cycle),
three-phase voltage for frequency ranges from 10 to 120 Hz for standard motors and
from 240 to 1,200 Hz for high-frequency motors. For the conventional drive range
from 5 to 500 hp, thyristor inverters are used to develop either six-step per cycle,
twelve-step per cycle, or pulse-width modulated (pwm) voltages over typical frequency
ranges from 10 to 120 Hz.
i=rt-F@
phase 3 -
line
Adjustable
Rectifier frequency Motor
inverter
Figure 3-14. Typical invester AC motor drive consisting of rectifier-DC link,
adjustable-frequency inverter, and induction of synchronous motor [lo].