Electric part-turn actuators - AUMA - #9

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Switching off in the end positions Protective functions Irrespective of whether the actuator is operated in open-close or modulating duty, the device must automatically switch off when reaching an end position. Two different switch-off mechanisms are available and are applied depending on the type of valve. If an excessive torque is applied during travel, e.g. due to a trapped object within the valve, the controls switch off the actuator to protect the valve. Limit seating Thermal motor protection Switching points for end positions are set in the actuator. As soon as the set switching point is reached, controls automatically switch off the actuator. Thermoswitches or PTC thermistors in the motor winding trip as soon as the motor temperature exceeds 140 °C. Embedded within the controls, they optimally protect the motor winding against overheating. Torque seating Thermoswitches or PTC thermistors offer better protection than thermal overload relays, since the temperature rise is measured within the motor winding. This type of seating is used if the valve has to be moved to end position CLOSED at a defined torque to ensure correct seating. As soon as the preset torque is applied at the valve end position, controls automatically switch off the actuator. Overload protection for the valve The type of seating must be programmed within controls, either at external controls or at integral AUMA actuator controls. For actuators with AM or AC integral controls, the type of seating is selected at the actuator and may even differ for the two end positions. Setpoint control for modulating duty Setpoint control is generally made via the valve position. The controls receive a position setpoint from the host DCS, e.g. as 0/4 – 20 mA signal. The integral positioner compares this value with the current valve position and controls the actuator motor as required until the offset value comes close to zero. The current valve position is transmitted to the DCS. |9