Modulating control actuators or drives are required to make small corrections in valve trim position or damper vane position to keep the controlled process variable within the desired limits. Large position changes are usually required only during process startup or shutdown, which normally is infrequent.
An average control actuator is required to change position 5 percent of the time or less. However, there are control systems that keep an actuator in almost constant motion close to the desired control position. Although this situation is undesirable it does occur and can continue unnoticed for extended periods of time. So the time spent changing position —which is the modulating duty cycle— can range from practically zero to nearly 100 percent. Since position changes are usually small, it follows that starting and stopping could be very frequent.
How do various control systems handle 100 percent modulating duty cycle? Pneumatic systems under this condition tend to wear out positioners. Electromechanical systems tend to wear drive gears and overheat induction motors (usually good for only 20-25 percent modulating duty cycle). Central electrohydraulic systems are susceptible to relief valve wear, fluid contamination and air entrapment. Self-contained electrohydraulic systems using induction motors are limited to 20-25 percent modulating duty cycle. However, self-contained electrohydraulic systems using stepping motors or servomotors can experience unlimited starts and stops (100 percent modulating duty cycle) without causing undue wear to themselves or other parts of the system.
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