Overload Relays for High-Inertia and Long-Start Loads
What overload relay works for a high-inertia motor? A high-inertia load is one whose starting current stays above roughly 2.5-3x the motor's full-load current (FLC) for longer than about 10 seconds, well past what a standard bimetallic relay tolerates. Large centrifugal fans, crushers, ball mills, big compressors, and pumps with heavy impellers fall into this category, and a Class 10 or 10A relay sized only to FLC will read the long run-up as a fault and drop the contactor before the motor reaches speed. This article covers what makes a load high-inertia, how to match trip class or electronic settings to the actual run-up time, and how to verify the choice against the manufacturer's coordination data — part of the broader thermal overload relay engineering guide.
What Counts as a High-Inertia Load?
Inertia is measured as WR² (or GD² on metric datasheets) — the moment of inertia of everything the motor has to accelerate, referred to the motor shaft. A pump with a small, light impeller reaches full speed in 2-4 seconds. A large induced-draft fan, a crusher, a centrifuge, or a ball mill can take 15-40 seconds, sometimes longer, because the driven equipment itself stores most of the kinetic energy the motor has to supply during run-up. The motor draws 6-8x FLC while accelerating; on a light load that current falls back to FLC within a couple of seconds, but on a heavy fan or mill it stays elevated for the whole run-up.
Typical high-inertia machines: large centrifugal and axial fans (especially with dampers open at start), crushers and shredders, ball and rod mills, centrifuges, large reciprocating and rotary screw compressors started against load, and conveyors with long, loaded belts. What we see in the field: two motors with identical nameplate FLC and horsepower can need two different relay classes, because the driven machine — not the motor — sets the run-up time.
Why a Class 10 Relay Nuisance-Trips on These Motors
A bimetallic overload relay follows an inverse-time (I²t) curve: small overloads take minutes to trip, large ones take seconds. Class 10A is built to trip in 2-10 s at 7.2x the set current, Class 10 in 4-10 s. Both curves are shaped for a motor that reaches full speed in a few seconds — the standard assumption behind most catalog selections. Feed that same relay a 20-second run-up at 6x FLC and it trips on every start, because the bimetal element heats past its trip point before the motor ever gets to speed.
Bumping the dial higher to stop the tripping is the wrong fix — it also weakens protection against a genuine locked-rotor or stall condition once the motor is running. The relay has to change class, not just setting. Key takeaway: a relay that trips on start is telling you the trip class is wrong for the load, not that the motor is oversized.
Matching Trip Class to Run-Up Time
The rule of thumb: pick the class whose trip time at the motor's actual starting current, from cold, exceeds the run-up time with margin — typically 25-50% headroom so a slightly hot restart still starts clean. Class 10/10A covers standard pumps, fans, and machine tools with run-ups under about 6-8 seconds. Class 20 covers most large fans and pumps with run-ups to roughly 15-18 seconds. Class 30 is for crushers, centrifuges, and mills with run-ups past 20 seconds. Manufacturers publish the exact curve (not just the 7.2x reference point) in their technical catalogs — use the curve, not just the class number, for a marginal case.
Formula: Trip Time by Class at 7.2x Setting, From Cold — Source: IEC 60947-4-1, Table 3
ttrip(Class) at I = 7.2 × Iset
| Symbol | Description | Unit |
|---|---|---|
| Class 10A | Trip time range at 7.2x setting, cold | 2-10 s |
| Class 10 | Trip time range at 7.2x setting, cold | 4-10 s |
| Class 20 | Trip time range at 7.2x setting, cold | 6-20 s |
| Class 30 | Trip time range at 7.2x setting, cold | 9-30 s |
These ranges are defined at 7.2x the setting, not at the motor's real starting current, so treat them as a class-selection reference, not the actual run-up trip point. A motor starting at 6x FLC for 18 seconds needs the full manufacturer curve checked at 6x, not just the 7.2x table value, before a Class 20 relay is confirmed as adequate. See our guide on overload relay trip classes for the full curve-reading method.
Bimetal Class 20/30 vs Electronic Overload for Extended Starts
Bimetallic relays in Class 20 and 30 exist across the major lines, but the selection shrinks at the largest frames, and the class is fixed at the factory — changing it later means swapping the relay. Electronic (solid-state) overloads solve the same problem with a field-selectable class, typically 5/10/20/30 on one unit, plus a separate start-inhibit or locked-rotor timer that ignores the run-up current for a set number of seconds regardless of class curve. That timer is the real advantage: it lets the relay hold through a known-long start without weakening the running protection at all, something a bimetal class change cannot do independently.
Siemens 3RB30/3RB31 electronic overloads offer selectable Class 5/10/20/30 with a wide 1:4 setting ratio and, on 3RB31, ground-fault detection alongside the extended-start capability, fitting the SIRIUS load-feeder lineup with 3RT2 contactors. Schneider's LR9 / TeSys T electronic overload range covers a similarly wide ratio with selectable class for the same reason — one relay, several start profiles, no hardware swap. ABB's E-series (EF19 through EF460) covers the electronic side with a 1:3 to 1:4 ratio and selectable Class 10/20/30, while the bimetal TA line tops out lower on inertia tolerance before an E-series unit becomes the practical choice. This depends on how long the motor actually takes to run up under load, not on the nameplate horsepower alone, so measure it before assuming a bimetal Class 30 is enough.
Setting and Verifying the Relay on a High-Inertia Motor
The current dial still gets set to motor nameplate FLC — trip class selection is a separate decision layered on top, not a substitute for correct FLC setting. Confirm the choice three ways: pull the motor's time-current start curve from the vendor or measure it with a clamp meter and stopwatch on a real start, compare that curve against the relay's published trip curve at the actual starting multiple (not just 7.2x), and check the manufacturer's coordination table to confirm the SCPD and contactor still deliver Type 2 protection with the selected relay. Skipping the third step is common and it is where undersized fuses show up during a fault, not during normal starting.
| Criteria | Class 10A | Class 20 | Class 30 |
|---|---|---|---|
| Trip time at 7.2x setting, cold | 2-10 s | 6-20 s | 9-30 s |
| Typical run-up tolerated | Up to ~6-8 s | Up to ~15-18 s | 20 s and beyond |
| Typical loads | Pumps, small fans, machine tools | Large fans, large pumps | Crushers, mills, centrifuges |
| Availability | Bimetal, standard stock item | Bimetal (select frames) and electronic | Mostly electronic; bimetal in limited frames |
Coordination and Nuisance-Trip Troubleshooting
A relay re-classed for a long start still has to satisfy Type 1 or Type 2 coordination with its contactor and short-circuit protective device (SCPD). Moving from Class 10 to Class 30 does not change the coordination table entry on its own — the manufacturer publishes separate tables per class, and a fuse or MPCB sized for Class 10 coordination is not automatically valid for Class 30. Re-check the table whenever the class changes, not just the dial setting. For the coordination background, see IEC 60947-4-1 standards for contactors and motor starters.
If a relay is already re-classed correctly and still trips on start, the usual causes are: mechanical binding making the actual run-up longer than the design curve assumed, a fan running against closed dampers instead of open ones at start, or supply voltage sag stretching the acceleration time. None of those are fixed by moving up another class — they need addressing at the machine or the supply, or the relay ends up oversized for the motor's real overload condition. Some panel builders default every large fan to Class 30 to avoid callbacks, but that trades away fast trip response on a genuine light overload; matching the class to the measured run-up, not the worst case, keeps both protections intact.
High-inertia selection sits alongside two related setting problems worth checking at the same time: motor protection circuit breaker sizing on the same starter (see our motor protection circuit breaker engineering guide), and contactor selection for the same duty (see how to select the right contactor). For the general dial-setting method that applies regardless of class, see how to select and set an overload relay. Stoklink stocks bimetal and electronic thermal overload relays across the Class 10A-30 range from Schneider, ABB, and Siemens.
Frequently Asked Questions
How do I know if my motor is a high-inertia load?
Check the run-up time to full speed under normal starting conditions. If the motor draws elevated current (roughly 2.5-3x FLC or more) for longer than about 10 seconds before settling to running current, treat it as high-inertia and select trip class accordingly.
Can I just set the overload relay dial higher to stop nuisance tripping on start?
No. Raising the current setting weakens protection against a genuine overload or stall once the motor is running. The correct fix is a higher trip class or an electronic relay with a start-inhibit timer, with the dial still set to nameplate FLC.
What trip class do large fans typically need?
Most large centrifugal fans with run-ups in the 10-18 second range use Class 20. Very large fans, especially with dampers not fully open at start, sometimes need Class 30 — confirm with a measured start curve rather than assuming by horsepower.
Is an electronic overload relay always better than bimetal for high-inertia motors?
Not always, but it removes the need to stock or swap a specific class of bimetal relay, and a start-inhibit timer holds through a long run-up without weakening the running trip curve the way a straight class upgrade can. For simple, fixed-duty machines a correctly classed bimetal relay is still adequate and lower cost.
Does changing trip class affect Type 1/Type 2 coordination with the contactor and SCPD?
Yes. Coordination tables are published per trip class, so a fuse or MPCB that coordinates at Class 10 is not automatically valid at Class 20 or 30. Re-check the manufacturer's coordination table whenever the class changes.
Conclusion
High-inertia selection is a run-up time problem before it is a current problem. Measure or obtain the motor's real starting curve, pick a trip class (or an electronic relay with start-inhibit) whose curve clears that run-up with margin, keep the current dial at nameplate FLC, and re-verify Type 1/Type 2 coordination for the new class. Skip the measurement step and the relay ends up either nuisance-tripping on every start or, worse, too slow to protect the motor on a real stall.
