Industrial Automation

Zero Speed Switch Working Principle

Zero Speed Switch

         The Zero Speed Switch (ZSS) identifies whether rotating, reciprocating, or conveying machinery is in motion or not. The probe body is potted with a circuit card and magnet assembly. The ZSS has a set of dry relay contacts that serve as an indication of motion and is powered by line voltage. The probe cannot detect motion unless it is pointed towards a target on the apparatus or a ferromagnetic material. The probe is extremely magnetic. Magneto-sensitive items such as computer discs and audio or video cassettes should be kept away from it.

What is role of Zero Speed Switch?

  • Zero speed switches used to track the beginning and ending of important spinning shafts.
  • In motors, conveyors, power plants, as well as in the manufacturing of sugar, paper, textiles, and cement, among other things, a zero-speed switch, also known as a speed actuating detecting switch, is used to measure the rotational shaft speed.

Symbolic Representation of ZSS

Symbolic rep Zero speed switch

Zero Speed Switch in Conveyor Belt

  • When this switch is utilized with conveyor belts, the conveyor will halt anytime the speed of the belt decreases below the preset speed.
  • Similar to how they may detect slow speed or jamming in an electric motor shaft to stop the motor when utilized in mechanical motors.
  • These switches primarily employ several technologies, including electrical, magnetic, and proximity.
  • In conveyor belts, the ZSS, or zero speed switch, is used to:
  • Stop the conveyor when the belt’s speed falls below the set speed;
  • Trip the motor circuit in the event that the motor is running and the conveyor belt does not run

Parts of ZSS Unit

         ZSS unit generally consists of

  • Controller
  • Proximity sensor

Working principle of ZSS

  • ZSS unit is always positioned close to the conveyor’s tail end. On the conveyor’s tail pulley, four studs or iron pieces are welded at a 90-degree angle along the central axis.
  • When the tail pulley rotates, a proximity sensor detects these metal components and sends signals to the controller. The controller computes the speed of the tail pulley using these signals and continuously monitors the states.
  • The controller’s contact relay changes its contact, whose feedback is used to trip the motor circuit, if the speed drops below the specified speed.

Operation of ZSS

         The fundamental idea behind a speed monitor is to compare the pulses received from the sensor with reference pulses. The control unit and “sensing probe” are the two components of the device. The sensor must be mounted with its sensing face very near a rotating item. Flags on this item must be fixed. Strong electromagnetic waves are produced by the sensor, which are affected by the flags and result in equivalent pulses. These pulses are delivered to the control unit, which compares them to typical pulses to determine the speed.

Working of ZSS controller

A ZSS controller essentially consists of the following: indicating LED’s; a built relay; terminals for voltage supply 110/240 volts; terminals for NO and NC contacts of the relay; terminals for taking the input of proximity sensors; electronic circuits; knobs for adjusting

Setting of a ZSS controller

There are three kinds of settings in ZSS controller: 

  • Speed setting- It determines the minimum speed of running of the conveyor belt, if the speed falls below these setting it drops the inbuilt relay and trips the motor circuit
  • ITD: ITD’s full form is initial time delay. It speaks of the amount of relaxation timing that must be given to the conveyor belt in order for it to increase in speed when the motor is turned on. During the period described in the ITD setting, the relay continues to be picked up at the beginning of the conveyor belt.
  • NTD SETTING-NTD is acronym of nuisance time delay.As the speed of the conveyor suddenly slows down, it prevents the motor from nuisance tripping. The relay continues to be picked up for the time specified in NTD setting when the conveyor’s speed falls below the speed setting. When the timing of NTD is completed then the motor circuit is tripped on The motor circuit is tripped on completion of NTD timing. It counts the timing from the time speed goes below the speed setting and waits for conveyor belt to normalize its speed until the end of NTD time. If belt speed normalizes during the time NTD timer is reset.


  • There is an output relay built into the control unit.
  • This relay will run at a predetermined speed and provide two 5 amp, 240 volt resistive c/o contacts.

Define Initial Time Delay & Nuisance Delay in ZSS.

Initial time delay and nuisance delay: A capability is provided for an initial time delay to enable the rotating mechanism to operate at the required speed. The usual delay is 0–30 seconds. The system can delay the trip action by using a nuisance delay so that it has a chance to regain speed.

Installation of ZSS

         The Zero Speed Switch must be mounted in a location that is safe, within the range of the ambient temperature, and doesn’t corrode the building materials. Below figure shows Zero Speed Switch Outline and Mounting.

Installation of ZSS

         The supplied mounting flange should be used to secure the probe to a structure that is vibration-free. There should be enough space between the probe and the target such that there is no chance of the target harming the probe. The maximum distance between the target’s face and the probe’s face for targets with dimensions of 25 mm x 25 mm x 50 mm is 38 mm (1.5″). (1″ x 1″ x 2″). The magnetic field of the Zero Speed Switch is susceptible to lateral disturbances. Move the Zero Speed Switch or put a ferrous plate (steel) as a shield between it and the interfering target if the Zero Speed Switch is responding to movements from the interfering target.

         To prevent condensate buildup in the casing, the probe should ideally be positioned so that the conduit entry points downward. Connection of the probe should be made via flexible conduit for simpler removal or adjustment of the probe. Without using a sun cover, never mount the Zero Speed Switch in the sun.

Timing Circuit in ZSS

  • The timing circuit artificially energizes and holds the alarm relay when power is first introduced to the Zero Speed Switch. For a start-up delay of 10 seconds (or 5 seconds if a jumper is attached across terminal block 1 TB – 7/8), this will imitate the typical operation of the ZSS.
  • The magneto resistive sensor detects the flux distortion as a ferromagnetic object moves across the permanent magnet field of the probe.
  • An internal timing circuit is reset by the sensor’s modulation of the current flowing through it.
  • This keeps the contacts operationally safe and the alarm relay activated. The timing circuit won’t reset if no target or flux change is detected for 10 seconds (or 5 seconds if a jumper is put across terminal block 1TB-7/8). The contacts will change state as a result, and the alarm relay will de-energize. As a result, uniform ferromagnetic masses that do not generate pulses during the time delay on zero speed will not be detected by the ZSS.

Industrial Zero Speed Switch

         Considering zero speed switch in industry, the diagram below illustrates the field wiring.  The power and relay connections to the zero speed switch with typical wiring diagrams are described.

Industrial ZSS
  • Attach the RED wire to 120 VAC power, and the WHITE wire to the neutral wire.
  • The GREEN wire on the should be connected to the equipment safety ground.
  • Connect the YELLOW, BROWN, and/or ORANGE wires to the relay.

Typical Motor Starter Wiring with ZSS

Motor Wiring ZSS

         The zero speed switch turns on when 120 VAC electricity is provided to L1 and L2. The zero speed switch’s N.O. (normally open) contact closes when the start pushbutton is depressed, and the N.C. (normally closed) contact opens as soon as one magnet passes in front of the sensor. By doing so, the motor starter is turned on and the circuit around the start pushbutton is complete. When the monitored shaft speed falls below 1 RPM for roughly 3.8 seconds, at which point the relay de-energizes and the motor starter cuts out, the N.O. contact stays closed and the N.C. contact stays open.

Typical PLC Wiring with ZSS

PLC Wiring ZSS

         The zero speed switch turns on when 120 VAC electricity is provided to L1 and L2. As soon as one magnet moves in front of the sensor, the zero speed switch’s N.O. (normally open) contact shuts and the N.C. (normally closed) contact opens. Until the monitored shaft speed falls below 1 RPM for around 3.8 seconds, the N.O. contact stays closed and the N.C. contact stays open.

Testing Zero Speed Switch

  • Verify that the mounting and electrical connections
  • Locate the RED and GREEN LED lights on the circuit board by removing the back cover of the switch box.
Testing ZSS

LED in Zero Speed Switch

  • The Green LED illuminates to show that the relay is activated, and the Red LED flashes each time a magnet passes in front of the sensor.
  • Connect 120 VAC electricity to the switch, turn on the monitored device, and operate it normally.
  • The Red LED to be on or flashing. This shows that the magnetic target and speed switch sensor are correctly aligned.
  • The Green LED should be on to show that the shaft being monitored is spinning.
  • When the Green LED turns on, the switch relay is turned on. 3.8 seconds after the monitored shaft stops turning, the Green LED turns off.
  • If a magnet pauses in front of the sensing region, the Red LED will stay on. The operation of the switch is unaffected.

Advantages of zero-speed switches

  • Setting a speed is simple.
  • It shields machinery from harm.
  • It uses a non-contact method to determine speed.
  • It has a built-in time delay bypass system.
  • It cuts downtime and maintenance.
  • It includes a sensor that only functions in dust, dirt, water, oil, and chemicals.
  • At a single point, it can calculate the direction and speed of various locations. It is possible to sense directions and track speeds at multiple locations from one location.
  • It runs independently of the machinery, therefore there are no moving parts and minimal maintenance requirements.

Disadvantages of zero-speed switches

  • When the machine is first started, a bypass arrangement is required to check the speed conditions.
  • Equipment may temporarily slow down while operating for a variety of causes. Hence, a built-in time delay is included to prevent tripping.
  • Any device that is powered by a motor may experience changes in speed due to overload, transmission component failure, underload, and a variety of other factors.

Applications of Zero Speed Switch

  • This switch is useful for keeping an eye on the conveyor system’s speed.
  • In industrial machinery like rolling mills, crushers, stirrers, mixers, etc., it is utilized for speed monitoring.
  • Zero-speed switches are utilized in heavy industrial and applications that deal with food product.
  • To control certain control actions like under-speed protection, over-speed protection, and zero speed protection, it is utilized to activate the relay contacts at a predefined speed.
  • This switch is mostly used to keep an eye on the speed of screw conveyors, mixers, stirrers, crushers, conveyors, rolling mills, and so forth.
  • These switches can also be used in hazardous or wash-down settings.
  • They are employed in thermal power plants, steel mills, fertilizer factories, coal mines, tile factories, blowers for heating systems, the food, auto, cement, textile, and fertilizer sectors, among others.

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