How Thermal Overload Protection Works in Motor Starters (Bimetal, Trip Classes, Adjustment)
Thermal overload protection is one of the most important safety elements in any motor starter. Its job is simple: protect the motor from overheating caused by prolonged overcurrent. But many technicians still misunderstand how the bimetal mechanism works, what trip classes mean, and how to set the device correctly. This guide explains the essentials in a clear and practical way.
1. What Thermal Overload Protection Actually Does
An electric motor can withstand short overloads, but if the current stays above the rated value for too long, the windings heat up and insulation begins to degrade. Thermal overload relays prevent this by disconnecting the motor before the temperature reaches a dangerous level. Unlike short‑circuit protection (MCB, MCCB, fuses), which reacts instantly, thermal overloads are time‑delayed. They simulate the heating behavior of the motor and trip only when the overload is sustained.
2. The Bimetal Principle: How the Relay Detects Overload
Most classic overload relays use bimetal strips—two metals with different thermal expansion coefficients bonded together.
When current flows through the relay, the bimetal heats up. If the current is too high:
the strip bends,
the bending force releases a mechanical latch,
the relay trips and opens the control circuit of the contactor.
This mechanism is simple, reliable, and requires no electronics. The bending speed depends on the current level: higher overload → faster heating → faster trip.
3. Trip Classes (Class 10, 10A, 20, 30): What They Really Mean
Trip classes define how fast the overload relay must trip under a specific overload condition—usually 600% of the motor’s rated current. Here are the common classes:
Trip Class Typical Trip Time at 600% In
Application
Class 10 / 10A 2–10 seconds Standard motors, general industrial loads Class 20 4–20 seconds High‑inertia loads (fans, conveyors) Class 30 7–30 seconds Very high‑inertia loads, heavy starting
Why the difference matters
A Class 10 relay will trip too fast on a motor with a heavy start. A Class 30 relay may trip too slowly and allow overheating on a small pump. Choosing the correct class ensures the motor is protected without nuisance tripping.
4. Setting the Thermal Overload Relay Correctly
Most overload relays have an adjustable dial, typically covering a range such as 4–6 A or 10–16 A. This must be set according to the motor’s nameplate current (In).
Correct procedure
- Read the motor’s rated current from the nameplate.
- Set the overload relay to exactly that value.
- If the motor operates in a hot environment, increase the setting slightly (5–10%).
- If the motor is frequently overloaded, investigate the cause—do not simply increase the setting.
Common mistakes
- Setting the relay too high “to avoid tripping”
- Using Class 10 for high‑inertia loads
- Forgetting to reset the relay after a trip
- Bypassing the overload relay entirely (dangerous)
5. Manual vs. Automatic Reset
Thermal overloads can reset in two ways:- Manual reset – safer; requires a technician to inspect the cause
- Automatic reset – used in unattended systems, but can cause repeated cycling if the fault persists
6. Why Thermal Protection Is Still Relevant Today
Even with modern electronic motor protection relays, bimetal overloads remain popular because they are:- simple
- inexpensive
- reliable
- easy to maintain
- suitable for most small and medium motors
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