Speed, Rotation and Underspeed Monitoring: Choosing the Right Sensor

Speed monitoring covers three distinct jobs that get treated as one: counting pulses to measure rate, detecting that something has stopped or slowed when it should not have, and establishing which way a shaft is turning. The sensor requirements differ, and specifying for the wrong one is a common cause of intermittent faults.

Underspeed detection is protection, not monitoring

Worth separating clearly. A speed readout on an HMI is information. An underspeed trip on a bucket elevator or a mill roller is a protection function — it exists because a stalled or slipping drive under load damages itself, and in dusty plant can start a fire.

Protection functions deserve the fail-safe treatment. Ask what happens when the sensor cable is cut: if the answer is that the machine reads zero speed and trips, good. If it reads nothing and the logic interprets nothing as fine, the protection is not protection.

What limits the maximum countable speed

Three things, and only one of them is the sensor.

The sensor switching frequency. A rated figure in hertz. The sensor cannot produce more transitions per second than this, and running near the limit gives degraded, uneven pulses rather than a clean failure.

The target geometry. This is usually the real constraint and it is under your control. Pulses per second equals teeth multiplied by revolutions per second. A 60-tooth wheel at 3000 rpm demands 3000 Hz; a 6-tooth wheel at the same speed demands 300. If you are near the sensor limit, reducing the tooth count is easier than changing the sensor — provided the resolution still suits the application.

The controller input. An ordinary PLC digital input scanned every 10 ms cannot count 3000 Hz. It needs a high-speed counter input or a dedicated speed monitoring relay. This is the step most often missed, and it produces the characteristic symptom: counts correct at low speed, drifting low as speed rises.

Target design matters more than people expect

For an inductive or magnetic sensor counting a toothed wheel, the tooth and gap need to be comparable in size to the sensing face. Teeth that are too fine relative to the sensor produce a weak, mushy signal because the sensor never fully sees a gap.

Rules of thumb worth applying: make tooth width and gap width roughly equal, keep both at least as wide as the sensor face, and mount at a consistent air gap well within the sensing distance. A wheel that runs out radially gives a varying air gap and an output that is fine at one point of the revolution and marginal at another.

Direction sensing

A single sensor on a toothed wheel tells you the shaft is turning, not which way. Where direction matters — a reversible conveyor, a hoist, a mill that must not run backwards — you need two channels in quadrature, so the controller can compare which one leads.

The speed-and-direction variants in the D33C range provide this in a single sensor rather than requiring two units to be mechanically aligned. Given how difficult it is to hold quadrature alignment between two separately mounted sensors in an industrial environment, that is usually the better engineering answer.

The part numbers carry an SD suffix — D33C-18MD-NM-3S-SD is the M18 version, D33C-30MD-NM-3S-SD the M30.

Mind the output current

One specific and avoidable mistake. The D33C range is rated 30 mA, not the 200 to 300 mA of the general-purpose switching ranges. It is built to feed a controller input, not to drive a contactor coil or a relay directly.

Wiring one to a relay coil will either fail immediately or, worse, work marginally and fail later. If the speed signal needs to switch a load, take it through a controller or a speed monitoring relay. Our part number guide lists the output rating for each series.

Choosing the sensing technology

Inductive — for a ferrous toothed wheel, gear or keyway at moderate speed. Robust, cheap, tolerant of dust between sensor and target. The D30C range covers it.

Magnetic — senses through non-ferrous material, so the sensor can sit outside a stainless or aluminium cover with nothing exposed. This is why magnetic types dominate in wet, dusty and washdown positions.

Photoelectric — where the target is non-metallic or the standoff is large. Needs a clean environment, which rules it out of most speed monitoring positions in heavy industry.

Typical applications

• Bucket elevator underspeed — protection against belt slip, in dusty and fire-sensitive plant
• Conveyor belt slip and drift
• Mill roller underspeed in sugar and steel plants
• Fan and blower rotation confirmation, often interlocked with a process
• Reversible drive direction confirmation
• Production counting on packaging and assembly lines
• Shaft rotation confirmation on pumps and gearboxes

Specifying

Send us the maximum and minimum speed, the target — material, tooth count, tooth and gap dimensions, or whether it is a single keyway — the available air gap, whether direction is needed, the environment, and what the signal feeds. If the application is an underspeed trip rather than a readout, say so, because it changes the fail-safe requirement.