Overload Relays for Conveyor Motors

08 Aug, 2026
Posted by: Muhammet KUL

What overload relay setting suits a conveyor motor? A conveyor motor overload relay is set to the motor's nameplate full-load current (FLC) and its trip class is chosen against the belt's loaded run-up time, not a default Class 10 selection, because a long overland belt starting under load can pull inrush current for 8-15 seconds and nuisance-trip a relay picked for a pump. Get the class wrong and the line stops on every cold start, or worse, the relay is too slow and a jammed belt cooks the motor windings before it opens. This covers trip-class selection for loaded starts, FLC dial setting including gearmotors, star-delta sizing on larger drives, phase-loss protection on long outdoor cable runs, manual versus automatic reset, and multi-drive conveyor systems.

Why Conveyor Starting Duty Drives the Trip-Class Choice

A conveyor rarely starts empty. Material sits on the belt at the moment the contactor closes, and the motor has to accelerate that mass plus the belt, idlers, and any incline before it reaches full speed. Run-up time on a short packaging conveyor is often under 2 seconds — light belt, small motor, no load lag. On a long overland belt carrying aggregate or ore, run-up can stretch past 10 seconds while the drive works through belt stretch and material inertia before the head pulley catches full speed.

That spread matters because IEC 60947-4-1 trip classes are tested at 7.2 times the current setting from cold: Class 10A trips in 2-10 s, Class 10 in 4-10 s, Class 20 in 6-20 s, Class 30 in 9-30 s. A relay sized on Class 10A for a 12-second loaded start will open before the belt reaches speed, every time. Pick the class so the run-up current sits inside the trip curve with margin, or the line never gets past startup.

What we see in the field: packaging and short indoor conveyors run fine on Class 10 or 10A. Overland, incline, and any conveyor that starts loaded — no soft-start, no clutch, no back-stop delay — usually needs Class 20, and long high-inertia belts push to Class 30.

Setting the Overload Relay to the Motor's Nameplate FLC

The dial is set to the motor's full-load current from the nameplate, not the conveyor's rated throughput and not a rounded-up "safe" number. Oversizing the setting defeats the protection; undersizing trips the relay on normal load swings as material builds up on the belt.

Formula: Overload relay dial setting — Source: motor nameplate, IEC 60947-4-1

I_set = I_FLC × SF

Symbol	Description	Unit
I_set	Overload relay dial setting	A
I_FLC	Motor full-load current, from nameplate	A
SF	Motor service factor (use 1.0 if not stated)	—

For a gearmotor, the FLC on the nameplate is already referenced to the motor, not the gearbox output shaft — set to that figure, not a torque-derived current. If the drive uses a variable frequency drive instead of a contactor, the VFD's own electronic overload function typically replaces the discrete overload relay; the thermal overload relay engineering guide covers where each protection layer belongs in a direct-on-line starter versus a VFD.

Trip Classes for Conveyor Motors: 10A/10 vs 20/30

Trip class is the IEC 60947-4-1 rating (10A, 10, 20, 30) stating the maximum time an overload relay may take to trip while carrying 7.2 times its current setting from cold — it sets how long a motor is allowed to draw high current before the relay opens.

Class 10A and Class 10 suit a conveyor with a light or empty start: short indoor belts, sortation lines, most packaging equipment. Class 20 covers belts that start loaded against static friction — a stopped belt with material sitting on it takes real torque to break free, and that shows up as extended run-up current. Class 30 is for the long overland or inclined belts where full speed is 10-20 seconds out, or where a soft-start device is absent and the motor takes the full inrush directly.

Key takeaway: Match the trip class to the loaded run-up time you actually measure or estimate, not to whatever ships as the relay's factory default — a Class 10A relay on a loaded overland start will nuisance-trip the line on cold mornings when belt stiffness is highest.

Electronic overload relays with selectable class — Class 10/20/30 on one unit — let a commissioning technician tune the class after watching the actual start current on a clamp meter, which beats guessing from a datasheet when the conveyor's true loaded run-up time is unknown at design stage.

Star-Delta and Reduced-Voltage Starting on Larger Conveyor Drives

Above roughly 15-30 kW, some conveyor motors start star-delta to cut inrush current and mechanical shock on the belt and gearbox. The overload relay in this arrangement sits in the delta leg, downstream of the delta contactor, and it does not see the full line current — it sees the winding current, which is the line FLC divided by the square root of 3, about 0.58 times the nameplate value. Set the relay to that reduced figure, not the motor's full nameplate FLC, or it will never trip on a genuine overload. See overload relay sizing for star-delta starters for the full derivation and a worked example.

Star-delta timing also interacts with trip class: the relay has to tolerate the star-to-delta transition current spike without tripping, which is one more reason long-belt star-delta starters often land on Class 20 rather than Class 10.

Phase-Loss Protection on Long Cable Runs and Outdoor Conveyors

Single-phasing is the loss of one supply phase to a running three-phase motor; the two remaining windings pick up roughly 1.7 times their normal current trying to hold the load, and sustained single-phasing burns the motor even though line current on the healthy phases can still look plausible on a meter.

Overland and yard conveyors run motor cable in long, exposed runs — trench crossings, cable trays near moving equipment, connectors at each drive station. A cracked gland, a corroded terminal, or a rodent-damaged conductor can drop one phase without tripping an upstream fuse. A phase-loss sensitive overload relay carries a differential mechanism that reacts to the imbalance between phases and trips faster than a plain thermal element waiting for total current to climb. Specify phase-loss sensitivity on any conveyor overload relay feeding a motor more than a short run from its starter, and pair it with contactors and motor protection circuit breakers already rated for that starter combination — see phase-loss and single-phasing protection in overload relays for the trip-time comparison against a standard element.

Manual Reset and Multi-Drive Conveyor Systems

Set conveyor overload relays to HAND (manual) reset, not AUTO. A tripped conveyor motor often means material has jammed, a bearing is dragging, or someone has cleared debris near the belt — none of those conditions should self-clear and restart the line unattended. Manual reset forces a person to walk to the motor control center, find the fault, and physically reset before the belt moves again.

Key takeaway: AUTO reset belongs on unattended equipment where a restart is safe with nobody nearby — a conveyor with personnel access along its length is not that case.

A conveyor system with multiple drive stations along one belt — common on long overland runs that spread the pulling load across several motors — needs its own overload relay per motor, each set to that motor's individual FLC, even when all the drives are nominally identical.

The three-device starter (SCPD, contactor, overload relay) repeats at every drive station; a single overload relay cannot protect two motors.

Key takeaway: On multi-drive belts, verify each station's overload relay setting against its own motor nameplate — mixed nameplate currents across "identical" gearmotors from different production batches are common enough to check, not assume.

What we see in the field: conveyor overload trips get blamed on the relay before anyone checks whether the belt itself is dragging on a misaligned idler or an overloaded skirt seal. The relay is doing its job; the mechanical fault is upstream of the electrical fix.

Frequently Asked Questions

What trip class should a conveyor motor overload relay use?

Class 10A or 10 for short, lightly loaded starts such as packaging conveyors; Class 20 for belts that start under load against static friction; Class 30 for long overland or inclined belts with extended run-up. Confirm against a measured start current where possible.

Can I use automatic reset on a conveyor overload relay?

Not where personnel have access to the belt. Manual reset forces a physical fault check before restart, which matters on a machine with a long moving surface and exposed pinch points. Reserve auto reset for unattended equipment with no access risk.

Does a soft starter or VFD replace the overload relay on a conveyor?

A VFD's built-in electronic overload function can replace a discrete relay if it is rated and configured for that duty, but a soft starter with a bypass contactor still commonly uses a separate overload relay downstream. Check the drive's motor-overload rating before removing the standalone unit.

How do I size the overload relay on a star-delta conveyor motor?

Set the relay in the delta leg to the motor's line FLC divided by the square root of 3, about 0.58 times nameplate current, because the relay only sees winding current in that configuration, not the full line current.

Why does my conveyor overload relay trip only during startup?

The trip class is too fast for the belt's loaded run-up time. Confirm the actual start current and duration with a clamp meter and step up from Class 10A/10 to Class 20 or 30 if the motor is still accelerating when the relay opens.

Does every drive station on a multi-motor conveyor need its own overload relay?

Yes. Each motor has its own nameplate FLC and its own fault modes, so each drive station needs its own SCPD, contactor, and overload relay set to that specific motor, even on belts with identical-looking gearmotors.

Conclusion

A conveyor motor overload relay does one job well when it is set to the correct nameplate FLC and matched to how the belt actually starts, loaded or empty, short or long. Get the trip class wrong in either direction and the failure mode shows up fast — nuisance trips on every cold start, or a winding that cooks before the relay reacts. For the wider selection and setting logic behind trip class, phase-loss, and reset choice across motor types, see the thermal overload relay engineering guide, or browse thermal overload relays and matching contactors sized to the conveyor motor's FLC.