Motor Overload Protection: Thermal vs Electronic Relays & PTC Thermistors

Electric motors fail primarily due to thermal overheating. For every 10°C increase in motor winding temperature above its rated thermal class limit (e.g. Class F 155°C), the insulation lifespan is halved. Selecting the right combination of current-sensing overload relays and embedded temperature sensors prevents catastrophic motor burnout.

The Overload Technologies Compared

1. Bimetallic Thermal Overload Relays

The traditional electromechanical workhorse. Line current heats resistive bimetal strips. Unequal expansion bends the strips, tripping an auxiliary contact when current exceeds the set threshold for a specified duration.

• Pros: Low cost, simple, self-powered.
• Cons: Susceptible to ambient temperature drift in hot control panels; limited accuracy on non-sinusoidal waveforms.

2. Electronic Solid-State Overload Relays

Microprocessor-driven relays using current transformers (CTs) to sample phase currents with high accuracy:

• Phase Loss & Single-Phasing Protection: Detects phase unbalance > 30% and trips in under 3 seconds, preventing immediate motor burnout when an upstream supply fuse blows.
• Thermal Memory: Models motor heating and cooling curves accurately, preventing operators from repeatedly resetting tripped starters on hot windings.

Understanding Overload Trip Classes (BS EN 60947-4-1)

The Trip Class defines the maximum trip time (in seconds) when carrying 7.2 times rated full-load current ($I_n$) starting cold:

• Class 10 (Trips in ≤ 10 s): Standard duty for domestic/light commercial pumps, fans, conveyors with low starting inertia.
• Class 20 (Trips in ≤ 20 s): Industrial duty for positive displacement pumps, heavy conveyors, and compressors.
• Class 30 (Trips in ≤ 30 s): Heavy industrial starting with extreme inertia (large centrifuges, industrial crushers, ball mills).

Embedded PTC Thermistor Protection (BS EN 60034-11)

Current-sensing relays cannot detect overheating caused by clogged ventilation cowls, low running speeds on VFDs, or high ambient temperatures.