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Preferred Instruments

Preferred Instruments manufactures microprocessor based controllers, instruments, and electric actuators for combustion and process applications. Preferred Instruments also manufactures oil tank gauging and leak detection equipment. A major component of the Instrument group sales is from control systems integration and personal computer based Supervisory Control and Data Acquisition systems. Preferred Utilities' industrial factory is located in Danbury, CT

Model SM, Servo Actuator

The Model SM Servo Actuator provides directly connected and precise modulating control of fuel valves and combustion air & flue gas control dampers.

Servo Actuator, Model SM – Datasheet     SDI-SM Installation Manual


The Preferred SM series rotary actuator works on a triac input. Switching SPDT or floating contacts provide power directly to the CW or CCW motor windings. A 5000 Ohm feedback potentiometer provides precise servo position data back to the control device.

Two adjustable Auxilliary switches provide an additional level of safety proving actuator position.


The Model SM Servo Actuator provides directly connected and precise modulating control of fuel valves and combustion air & flue gas control dampers. The actuator opens, modulates, and closes the valves and/or dampers in accordance with the integrated combustion control and burner management system programs.  An integral interlock switch provides proof of open or closed position for safe burner operation.  In addition, the feedback potentiometer provides indication of the motor’s shaft position.  The SM Actuator is used on boiler control applications that have a maximum of 37 ft-lbs torque required.  For heavier-duty service consult the factory.


  • 90° Rotary Electric Actuator (Adjustable Over 270°)
  • 3, 15 or 37 ft-lb. Torque Output
  • Designed For Modulation Service
  • Self-Locking Drive System To Hold In Last Position
  • Electrically Isolated Feedback Potentiometer Provides Indication Of Shaft Position
  • Two Auxiliary Limit Switch Position Output

Power: 120 VAC – 50 / 60 Hz

Ambient Temperature: 14° F to 104° F, (-10° C to + 40° C)

Input: Switching SPDT or Floating Contacts

Torque: 3, 15 or 37 ft-lbs (depending on model selected)

Duty cycle: 100%

Linearity:  ± 1%

Rotation:  90° in 30 Seconds, Maximum Rotation 270 Degrees

Feedback Potentiometer: 5000 OHM Change For 270° Stroke

Auxiliary Switch: Two (2) Integral SPDT, Adjustable

Integral Brake: Fail-Safe, Self Locking

Installation Position: Any

Output Shaft: Hardened Steel (⅜” Sq-Dbl. Ended)

Motor: Reversible, Split Phase, Capacitor Type, Oil Submerged

Enclosure: SM-3: NEMA 2, SM-17 & SM-37: NEMA 4


Provide a separate direct acting Servo Actuator for the fuel gas and fuel oil flow control valves and the combustion air damper.  The servo actuator shall modulate the fuel valves without the use of linkage.  The actuator shall be totally enclosed in a dust-tight, splash-proof housing, have two (2) integral, snap-action, travel limit proving switches, electrically isolated shaft position feedback potentiometer, integral brake, 90° rotation in 30 seconds and maintenance-free oil immersed motor and gear train.  The actuator shall be capable of being stopped, started, or instantly reversed without loss of power or overloading. The actuator shall be a Preferred Instruments, Danbury, CT, Model SM-x [ ‘x’ = 3, 15 or 37 rated torque in ft-lbs].