Siemens Servo drive SQN90.201A2790

This servo drive from Siemens positions air dampers and mixing valves in burner and boiler combustion-control systems. It converts a three-point, 0-10 V, or on/off control signal into rotary motion to modulate the air damper or to drive a heating-circuit mixing valve. The unit is designed for full compatibility with the manufacturer's line of control equipment and is used to regulate combustion air flow and valve position during modulation.

Actuators of this type are selected by torque, running time, control-signal format (three-point, 0-10 V, or on/off), shaft or coupling pattern, angle of rotation, and the presence of auxiliary switches. Before ordering, measure the existing damper or valve stem and compare the coupling and mounting pattern to the replacement. Verify that the control signal voltage and type match the actuator input. The actuator drives the valve or damper via the shaft coupling; proper alignment and wiring are necessary to avoid damage during commissioning.

Typical failures in this type of actuator are stripped gear trains, failed end switches, and seized valve stems after prolonged inactivity. When replacing such a unit, inspect the gearing and end switches, and make sure the valve stem rotates freely before installation.