What is Zero Speed Switch : Working & Its Applications

Speed monitoring in motors, and elevators are very essential. So, the speed of any electric device-driven through an electric motor can change because of many reasons like transmission parts breakage, underload, overload, etc. So a speed monitoring device within an open-loop control is very useful for either generating an alarm or turning off the electric motor. But, in a closed-loop control like a V/F drive or D.C drive, feedback is necessary to take appropriate action. This article discusses an overview of a zero-speed switch – working with applications.

What is Zero Speed Switch?

A zero-speed switch or speed actuating sensing switch is used to detect the rotary shaft speed within motors, conveyors, power plants & in industries like the production of sugar, paper, textiles, cement, etc. The zero speed switch symbol is shown below.

Switch Symbols
Zero Speed Switch Symbols

Once this switch is used in conveyor belts, they will stop the conveyor whenever the conveyor belt speed drops under the set speed. Similarly, when these motors are used in electric motors, then they can detect the motor shaft’s slow speed or jamming to stop the motor.

These switches mainly use different technologies like electronic magnetic, proximity, or electronic. Electromechanical speed switches are speed-indicating & direct-coupled devices so these switches simply control a set of contacts by opening or closing mechanically like a function of the rotating speed of the switch shaft.

Similarly, electronic zero-speed switches are also direct-coupled and also speed-indicating devices that control a set of contacts by using a relay or an electronic circuit.

Zero Speed Switch Working Principle

A zero-speed switch includes a sensor & a controller. So the working principle of this switch mainly depends on the working of the sensor and controller. When the sensor is arranged with its detecting face in close proximity to the controller, then the sensor detects if there is any trouble & immediately sends a command to the controller. Thus, based on the received signal an appropriate action is taken through the controller.

The specifications of Zero speed switch mainly include the speed of the input shaft, operating range operating temperature range, trip set-point, field adjustability, operating conditions, relay contact rating, operating voltage, etc. For many applications, the switches used are weatherproof, dustproof, or explosion-proof.

Zero Speed Switch Wiring Diagram & Its Working

Plugging with a zero-speed switch is shown below. Generally, plugging is stopping a motor abruptly or quite suddenly and it is done by throwing it into reverse as soon as you press the stop button. Here we are going to discuss a zero-speed switch and how it is used to plug a circuit.

Zero Speed Switch Circuit Diagram
Zero Speed Switch Circuit Diagram

In the above circuit, we are using a basic start-stop up, forward, and reverse, basic interlocks denoted with ‘R’ and ‘F’ and zero speed switch. Here this zero-speed switch works with centrifugal force.

Once the motor in the circuit moves forward, then the switch is pushed up and if the motor is suddenly gone into reverse, then it’s going to throw it down into this position. So that’s why the arrow symbol in the circuit shows that there is centrifugal motion with the zero-speed switches themselves.

A zero-speed switch used in this circuit is activated through centrifugal forces which are connected either directly or with a belt to the motor’s shaft. Here, plugging action can be provided by using the zero-speed switch. This switch can be connected normally open (NO) so that any motor shaft’s motion will cause the contacts to adjust their condition & close.

When the start button is pushed, the motor works in the forward direction within normal operating conditions. This strengthens the forward coil &closes the normally open contact in parallel through the start button as well as opening the normally open (NO) electrical interlocks in series through the reverse coil.

The motor in the circuit will run continuously & the circuit will work like a standard 3-wire circuit by providing low-voltage protection until an overload occurs or either the stop button is pressed..

When the motor shaft is rotating, the zero-speed switch contacts will adjust their condition & the forward contact will close. This contact is connected with the reverse coil in series, but it cannot energize so long when the electrical interlock opens.

Once the stop button is pressed, the forward coil is de-energized, and then it stops its armature. This causes all the associated contacts with the forwarding coil to come back to their normal state with the normally closed (NC) electrical interlock.

As the motor shaft is still rotating within the forward direction because of the inertia, the contacts of the normally open (NO) zero-speed switch are closed. When the stop button is pressed instantly, the reverse coil gets strengthened, temporarily sending the motor in the reverse direction & canceling out all of its forward force, so bringing the motor to a stop very quickly.

The motor’s shaft starts to move within the reverse direction, then forward contacts of the switch will open & the reverse coil can be de-energized before the motor can increase any energy within the reverse direction. Now the motor has been plugged to stop.

Advantages and Disadvantages

The advantages of zero-speed switches include the following.

  • Speed setting is easy.
  • It protects equipment from damage.
  • It detects speed in a non-contact way.
  • It includes an inbuilt bypass time delay arrangement.
  • It reduces maintenance & downtime.
  • No wear & tears of rollers otherwise misalignment of couplings.
  • Speed setting is achievable from 1 – 5000 RPM within optional ranges.
  • It includes a sensor that simply works in dirt water, oil, chemicals & dust.
  • It can determine the direction and speed of different locations from a single point.
  • Sensing directions & speed monitoring of different locations from a single point is achievable.
  • It operates without any contact with the machinery so there is no moving part and thus less maintenance.

The disadvantages of zero-speed switches include the following.

  • It is necessary to have a bypass arrangement to monitor speed conditions during starting of the machine
  • During machine running, equipment may drop its speed for a moment because of different reasons. So a built-in time delay is provided for avoiding tripping.
  • The speed of any device driven through a motor can change because of overload, transmission parts breakage, underload, and many more.


The applications of zero-speed switches include the following.

  • This switch is helpful in monitoring the speed of the conveyor system.
  • It is used for speed monitoring in Industrial machinery like rolling mills, crushers, stirrers, mixers, etc.
  • It is used to activate the relay contacts at a preset speed to control certain control actions like under-speed protection, over-speed protection & zero speed protection.
  • It is used in many industrial manufacturing plants
  • This switch is mainly used to monitor the speed of crushers, conveyors, rolling mills, mixers, stirrers, agitators, screw conveyors, etc.
  • Zero-speed switches are used in food-grade and heavy industrial-grade applications.
  • These switches are also obtainable for wash-down duty or dangerous environments.
  • These are used in Thermal power stations, steel plants, fertilizer plants, coal factories, Tiles factories, blowers for heating systems, automobile industries, food industries, cement factories, textile industries, etc.

Thus, this is all about a function of a zero-speed switch – working with applications. These switches are available in two types based on the power supply given. The first type is interrupted power type, where the power supply is provided to the zero-speed switches simply for the period for which the power supply is going to the motor. The second type is the power type, which is sued to provide supply continuously to the switch. Installation of these switches mainly depends on the rotating part or shaft construction, wherever the speed has to be observed. Here is a question for you; what is another name for the zero-speed switch?

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