Zero speed switches detect the stoppage of a rotating shaft. They typically have one or more relays and may provide analog outputs.

 

Operation and Types

 

Typical zero speed switches use electromechanical, electronic, or magnetic proximity technologies.

 

zero speed switches selection guide

Electromechanical speed switches are direct-coupled, speed-indicating devices that open or close a set of contacts mechanically as a function of the rotational speed of the switch shaft. The construction of the switch and adjustments to internal components determine the speed at which an electromechanical switch trips.

 

Electronic zero speed switches are direct-coupled, speed-indicating devices that open or close a set of contacts through a relay and electronic circuit. User-defined settings and the rotational input speed of the shaft determine the speed at which an electronic switch trips.

 

Magnetic proximity switches are non-contact devices that use a proximity sensor to detect a rotating piece of ferrous metal. These switches use a microcomputer to count between sensor impulses, and circuitry to analyze discrepancies in pulse time. If the impulses depart from preprogrammed limits, a relay opens or closes a set of contacts. 

 

Connections and Contacts

 

Zero speed switches use alternating current (AC) or direct current (DC). Depending on the power supply, the AC voltage rating or DC voltage rating is an important parameter to consider.

 

Switches may feature one of a variety of contact arrangements, as described below.

 

  • Single-pole, single-throw (SPST) switches make or break the connection of a single conductor in a single branch circuit. They typically have two terminals and are called "single pole" switches.
  • Single-pole, double-throw (SPDT) switches make or break the connection of a single conductor with either of two other single conductors. SPDT switches typically have three terminals which are commonly used in pairs.
  • Double-pole, single-throw (DPST) switches make or break the connection of two conductors to two separate circuits. DPST switches typically have four terminals.
  • Double-pole, double-throw (DPDT) switches make or break the connection of two conductors to two separate circuits each. Typically, DPDT switches have six terminals. Contact choices for zero speed switches also include solid-state electronics, which replace mechanical relays and do not contain moving parts.

zero speed switches selection guide

A visual representation of switch pole/throw combinations.

Image credit: diyeffects

 

Performance Specifications

 zero speed switches selection guide

Zero speed switches differ in terms of performance specifications, mounting styles, and features. Important specifications include operating range or input shaft speed, trip point, operating temperature, and field adjustability. Some zero speed switches use external hardware such as screws, bolts, or rails for base mounting. Others are equipped with integral flanges, or are designed for installation in a standard, plug-in socket with 8-pin termination. Zero speed switches with a threaded barrel body are also available.

 

For specialized applications, suppliers can provide zero speed switches that are explosion proof, weather proof, dust proof, or capable of over-speed sensing. Explosion proof switches have NEMA 7/9 and 4X ratings, standards from the National Electrical Manufacturers Association (NEMA). Weather proof and dust proof switches have NEMA 4/5 and 4X ratings.

 

Resources

 

KWS—Features and benefits of speed sensors and zero speed switches

 

NEMA—NEMA enclosure types

  

Image credits:

Electro-Sensors, Inc. | Siemens Process Instrumentation