A belt that slips on its drive pulley or an elevator that jams can keep the motor running while the material stops moving. A simple pulse counting device on a non driven shaft catches that failure within seconds and trips the drive before damage spreads.
Conveyors, screw feeders and bucket elevators need proof that the shaft is really turning, not just that the motor is energised. This guide explains how speed monitoring works, how to set it and how to troubleshoot it in the field.
What Is a Zero Speed Switch?
A zero speed switch is a motion monitoring device that senses the rotation of a shaft and changes a relay contact when the speed falls below a set value or stops completely. On bulk handling plants it is fitted to the tail pulley of a belt conveyor, the boot shaft of a bucket elevator or the tail of a screw conveyor, alongside other conveyor safety switches.
The key idea is that the motor running signal only proves the motor is energised. It does not prove the belt, chain or screw is moving, so a separate speed proof is needed on the driven equipment.
KWS Manufacturing explains that speed sensors and zero speed switches protect equipment and processes by alerting operators so that a process can be halted before damage spreads. Typical locations are the tail or drive end of screw conveyors, belt conveyors and bucket elevators.
The name is slightly misleading, because most modern units trip on underspeed, not only at a standstill. 4B lists a mechanical slip switch with a single setpoint of 20 percent underspeed.
How Shaft Speed Monitoring Works
Most electronic units use a non contact sensor that faces a rotating target, such as a bolt head, a toothed disc or a slotted collar. Each passing target produces one pulse, much like the sensors described in inductive proximity sensor selection.
The controller measures the time between pulses or counts pulses over a window and compares the result with the setpoint. Some models use a Hall effect sensor with a magnet on the shaft, which allows a larger gap than a plain steel target.
Phares Electric states that its ZS09P all in one sensor covers 0.1 to 25,000 RPM with 1 to 6,000 pulses per revolution. Its manual gives a gap of 1/16 to 1/8 inch when sensing a gear tooth, key stock or bolt head.
The same Phares manual warns that ferrous targets must be high iron content, and that stainless steel and some iron alloys will not work with the sensor. This single detail explains many failed commissioning attempts on food and pharma conveyors.
Mount the sensor on the tail or non driven pulley, never on the motor or gearbox shaft. Only the non driven pulley stops when the belt slips on the drive pulley.
Types of Zero Speed Switch
A flexible coupling drives an internal centrifugal or friction mechanism that opens a contact at low speed.
Sensor and relay in one housing with a trim pot setpoint, facing a target on the shaft.
Separate sensor, target and panel mounted monitor with display and settings.
A shaft encoder feeds pulses to a PLC or speed monitor.
Encoder based monitoring gives true speed, not just a trip, and fits well with a PLC high speed counter. 4B offers encoders with programmable resolution from 1 to 1024 pulses per revolution for this purpose.
Two wire DC and NAMUR sensor versions exist for hazardous areas and intrinsically safe loops, similar to the NAMUR output sensor used on valves. In dusty grain and coal handling, always check the zone rating before choosing a model.
Pulse Rate and Underspeed Setpoint Formula
The pulse frequency seen by the sensor depends on shaft speed and the number of targets. The underspeed trip frequency is simply a chosen percentage of the normal frequency.
Trip frequency ft = f × Trip % ÷ 100
Pulse period at trip = 1 ÷ ft seconds
N = number of targets on the shaft
Example:
RPM = 60, N = 4, Trip = 80 %
f = 60 × 4 ÷ 60 = 4.00 Hz
ft = 4 × 80 ÷ 100 = 3.20 Hz
Pulse period at trip = 1 ÷ 3.2 = 0.31 s
With only one target, the same shaft gives one pulse per second, so the switch needs a full second of silence before it can decide the shaft is slow. More targets mean faster detection, which is why 4B offers its Whirligig target with 1, 2, 4 or 8 targets.
Zero Speed Switch Pulse Calculator
Second Worked Example: Belt Speed to Pulley RPM
Site data often gives belt speed, not shaft speed. Shaft RPM equals belt speed in metres per second times 60, divided by pi times the pulley diameter in metres.
For a belt running at 2.5 m/s over a 0.8 m tail pulley, RPM = 2.5 × 60 ÷ (3.1416 × 0.8) = 59.7 RPM. With four targets this is close to the 4 Hz used in the calculator, so an 80 percent trip gives a period of about 0.31 seconds.
Start Up Delay and Fail Safe Wiring
When the drive starts, the shaft is at zero speed and the switch would trip at once. A start up bypass timer blocks the trip for a set time, often a few seconds to a little more than the run up time of a loaded conveyor.
The output relay should be energised while the shaft runs correctly and de energise on underspeed, loss of power or sensor failure. That fail safe approach is the same logic used in a safety relay, where any fault leads to the safe state.
The contact normally goes into the motor control circuit in the motor control centre or into a PLC input. When it goes to a PLC, confirm the sinking or sourcing type matches the sensor output.
Set the start delay from a timed test with the conveyor fully loaded, then add a small margin. A delay that is much too long lets a jammed elevator run for many seconds before tripping.
In a bucket elevator, belt slip on the head pulley can heat the belt and pulley lagging quickly. That is why elevator standards and insurers commonly expect speed monitoring on the boot shaft.
7 Proven Setup Steps
Record the setpoint, delay and test result in the loop folder. When the trip is also used for a first out display, follow the pattern in first out alarm PLC logic so operators see the real cause of the stop.
Zero Speed Switch Applications
On large drives, the underspeed signal often trips earlier than the motor protection because a stalled machine takes time to raise current. It complements, not replaces, the motor protection relay and upstream sequence interlocks.
- Proves real movement of the driven equipment.
- Non contact sensing avoids wear.
- Detects slip, breakage and stall quickly.
- Simple relay output fits any control system.
- Low cost compared with the damage it prevents.
- Needs a correct start up bypass timer.
- Targets can loosen or get covered by material.
- Stainless steel targets may not be detected.
- Too few targets slow down detection.
- Mechanical types need coupling maintenance.
Troubleshooting Speed Switch Trips
- Check the sensor LED flashes once per target at slow speed.
- Measure the gap and clean off material build up.
- Confirm targets are tight and made of high iron steel.
- Verify the start delay covers the loaded run up time.
- Check the setpoint against the current normal speed.
- Inspect cable, gland and terminal tightness.
- Test that power loss drops the relay to the trip state.
Nuisance trips at start up usually mean the start delay is too short, especially after a belt is re tensioned or the load increases. Random trips while running often come from a loose target or noise on long cables, similar to issues covered in inductive and capacitive proximity sensors.
If the VFD speed has been reduced for a new product, the trip setpoint must be reviewed too. Otherwise the zero speed switch sees the new normal speed as underspeed, a common surprise after VFD retuning.
Selection Guide
Match the housing to the site: dusty coal and cement plants need a high IP rating and a sturdy mounting bracket. Hazardous dust zones need a certified device, so confirm the area using hazardous area classification.
Also choose the supply voltage, contact type and whether you need a 4 to 20 mA speed output. Where the trip forms part of a safety function, ask the vendor for failure rate data and a safety manual.
Paint a white stripe on the tail pulley face during commissioning. It lets anyone confirm pulley rotation at a glance when a trip is being investigated.
Phares Zero Speed Switch Manual
Zero Speed Switch Demonstration Video
Zero Speed Switch FAQ
It senses shaft rotation on a conveyor, elevator or feeder and opens a contact when the speed drops too low. The drive and upstream equipment can then stop before damage spreads.
It proves that the driven equipment is really moving. A motor running signal alone cannot show belt slip, a broken chain or a sheared pin.
Fit the zero speed switch sensor on the tail pulley, boot shaft or another non driven shaft. These shafts stop turning quickly when the belt slips on the drive pulley.
Avoid the motor shaft and gearbox output shaft for slip detection. Those shafts keep running at full speed even when the belt itself is completely stalled.
At start the shaft begins at zero speed, so the switch would trip immediately without a bypass. The start delay blocks the trip while the machine accelerates.
Set the delay from a timed start with a full load on the conveyor. Add a small margin, but avoid a long delay that hides a genuine jam or belt slip.
Many plants set the zero speed switch to trip at 70 to 90 percent of normal running speed. 4B even offers a mechanical slip switch with a fixed setpoint of 20 percent underspeed.
The right value depends on the process and how much speed variation is normal. Review the setpoint whenever a drive speed is changed or a new product is introduced.
Inductive sensors need ferrous targets such as mild steel bolt heads, keys or toothed discs. Phares states that high iron content is required and stainless steel will not work with its sensor.
Hall effect models can use magnets, which allow much larger gaps. Space targets evenly so the pulse period stays steady at constant speed.
Wire it fail safe, so the relay stays energised during correct running. Loss of power, a broken cable or underspeed then opens the contact and stops the drive.
Connect the contact to the motor control circuit or a PLC input with a clear alarm. Test the trip during commissioning by stopping the shaft and watching the relay.
Common causes are a loose or covered target, an increased sensing gap, or electrical noise on long cables. A reduced VFD speed below the old setpoint also gives the same symptom.
Check the sensor LED at slow speed and clean the target area. Then confirm the setpoint and start delay of the zero speed switch against the present operating speed.
Related Articles
- Conveyor Safety Switches Types
- Inductive Proximity Sensor Selection
- NPN and PNP Proximity Sensors
- PLC High Speed Counter and Encoder
- What Is a Safety Relay
External References
- User Manual, Zero Speed Switch Sensor Model ZS09P, Phares Electric
- Speed Switches, Proximity, Inductive and Shaft Encoders, 4B Braime
- Conveyor Belt, Wikipedia
What We Learn Today
- A zero speed switch proves that a conveyor, elevator or feeder shaft is really turning and trips the drive when speed falls below the setpoint.
- Pulse frequency equals RPM times the number of targets divided by 60, so 60 RPM with four targets gives 4 Hz at normal speed.
- Mount the sensor on a non driven shaft, use high iron targets, wire the relay fail safe and set a start delay from a loaded run up test.