Page 408 - Handbook of Electrical Engineering

VARIABLE SPEED ELECTRICAL DRIVERS 397

14.6 CONTROL SYSTEMS FOR AC MOTORS

In the oil industry the use of variable speed AC motors has become a requirement for several reasons:

• Availability of economical high power inverter systems.
• Improved reliability of power electronic control systems.
• Availability of micro-computers for intelligent control and protection of the rectifier-inverter motor
system.
• The modern emphasis on the conservation of energy.
• Control performance that is superior to non-electrical fluid controllers such as throttle value control
and fluid couplings.
• Standard or ‘near standard’ motors can be used.

The application of speed control to a large AC motor is generally for one of two reasons, or
less frequently a combination of both:
• Steady state speed control over a significant range e.g. 10% to 100%, 50% to 100%, 75% to 110%.
• To restrict the starting and reacceleration currents that the motor requires.

The steady state speed control can be easily achieved by modern control systems and the
regulation about a set speed can be as low as 1% or less. In addition rapid and adequately damped
responses to changes in set points or to process disturbances are standard features of most systems.
The high performance of modern electronic control systems enables the sharing of loads and process
duties between parallel pumps or compressors to be accurately achieved without much difficulty.
These systems also allow scheduling and the admission into or the removal from service of motors
to be achieved in a smooth manner. Modern protective systems for the power electronics and the
motor are very comprehensive, and fast to react if required to do so.
Most oil industry power systems permit, and indeed encourage, direct-on-line starting of motors.
This becomes difficult as the motor ratings are large in relation to the capacity of the main power source
e.g. several gas-turbine driven generators. In large installations such as LNG plants and refineries, and
along bulk oil or gas pipe lines, it is common to find motors with ratings up to 10 MW. In order to
start such large motors it has become the practice to use a variable speed rectifier-inverter system as a
starting device, which is sometimes referred to as a ‘soft start’ system. Whilst the starting problem has
been solved by such a system, it is then a simple matter to take advantage of the variable speed controls
to adjust the motor speed during its normal running operations.
The basic elements of variable speed control systems for an AC motor are shown in
Figure 14.5. In practice there are several variations to the basic system, see Reference 7, Chapters 4
and 6, and Reference 8, Chapter 9. Some of the devices and signal lines, e.g. A or B, may not be
used in all practical systems.
The following comments apply to the various blocks ( ) in the diagram.
The system receives its main power from a circuit breaker (6) or contactor in the upstream
switchboard or motor control centre. This switchgear will contain the main power protective relays