The Motor That Burned (No Overload)
We ran a conveyor motor (a 3-phase AC motor) on a machine. The conveyor jammed (a part got stuck). The motor kept trying (it drew high current). Without an overload relay, the motor overheated (and burned). The problem: no overload protection. The motor had a contactor (it switches on/off) but no overload (thermal). We added an overload relay (a thermal) on the motor. When the conveyor jammed (high current), the overload tripped (it cut the power). The motor was saved. The mistake was not adding an overload relay.
Motor overload relay protects the motor from overheating. This article covers the selection.
The Overload Relay
An overload relay (a thermal) sits between the contactor and the motor. It senses the motor’s current. If the current is too high (over the rated, for a while), it trips (cuts the power). It protects the motor (from a jam, or a stalled rotor).
Step 1: The Rated Current (FLA)
The motor’s nameplate has the rated current (FLA, full-load amps). For a 1 kW motor, FLA is ~2.5 A (3-phase, 400V). Set the overload relay to the FLA (2.5 A). If the motor draws over 2.5 A (for a while), the relay trips.
The overload rule: Set the overload relay to the motor’s FLA (nameplate). The motor that burned had no overload. Add an overload (set to FLA). For a jam (high current), it trips (saves the motor).
Step 2: The Trip Class (10, 20, 30)
The trip class is how fast it trips (at 6× the FLA):
- Class 10: Trips in 10 s (at 6× FLA). For a normal motor (a conveyor, a fan).
- Class 20: Trips in 20 s. For a high-inertia load (a big fan, a crusher).
- Class 30: Trips in 30 s. For a very high-inertia load (a big compressor).
For a conveyor (article 121), Class 10 is fine (the motor starts fast, no high-inertia). For a big fan (high inertia), Class 20 (it starts slowly, the relay shouldn’t trip during start).
Step 3: Thermal or Electronic?
- Thermal (bimetal): A bimetal strip (heats, bends, trips). Cheap. For general. Slightly inaccurate (±10%).
- Electronic: Senses the current (microprocessor). Accurate. For a critical motor. More expensive.
Step 4: Reset (Manual/Auto)
After a trip, the relay resets. Manual (a button) requires the operator to press it (after the jam is cleared). Auto (it resets by itself) might restart (and re-jam). For a machine, use manual (the operator clears the jam, then resets).
| Motor | FLA | Trip Class |
|---|---|---|
| Conveyor (article 121) | ~2.5 A (1 kW) | Class 10 |
| Fan (high inertia) | ~5 A (2.2 kW) | Class 20 |
| Compressor (very high) | ~10 A (5.5 kW) | Class 30 |
An Overload Checklist
- Is there a motor? (3-phase?)
- Is there a contactor? (Switches on/off?)
- Is there an overload relay? (Protects?)
- Is it set to FLA? (Nameplate?)
- Is the trip class right? (10/20/30?)
- Is it manual reset? (Operator clears?)
- Does it trip on a jam? (Test?)
- Is the motor saved? (No burn?)
- Is the contactor rated? (For the motor?)
- Is the wiring right? (Article 181?)
The Bottom Line
Motor overload relay protects the motor from overheating. The motor that burned had no overload. Add an overload (set to FLA). Use Class 10 (conveyor). Manual reset. The motor that survived the jam wasn’t the best motor — it had an overload.
