Selection of Thermal Relay Protectors

Selecting a thermal relay protector requires matching the relay's current rating, trip characteristics, heating type, and reset mode to the motor's operational profile and protection requirements. Key Selection Factors1. Current Rating: The thermal relay's rated current should correspond to the motor's rated current.

For motors with short starting times and low impact loads, the heating element current is typically set equal to the motor's rated current. For motors with longer starting times, impact loads, or continuous operation, the heating element is set to 1. 1–1.

15 times the motor's rated current to prevent nuisance tripping while ensuring protection. 2. Trip Characteristics: Thermal relays operate on inverse-time characteristics, meaning higher currents cause faster tripping.

Trip curves are classified by protection class (e.g., Class 10 for IEC applications, Class 20 for NEMA applications). Selecting the appropriate class ensures the relay trips before the motor reaches its thermal limit while accommodating normal overloads. 3.

Heating Element Type: Bimetallic relays use two metals with different expansion coefficients to bend under heat, providing reliable protection for standard industrial loads and inherent thermal memory for repeated start-stop cycles. Eutectic alloy relays use a phase-change alloy that melts at a specific temperature, offering precise trip points for high-accuracy applications. 4.

Reset Mode: Manual reset relays require operator intervention after tripping, ensuring the motor has cooled before restarting. Automatic reset relays reset once the temperature drops, suitable for applications where continuous operation is critical but may risk repeated cycling. 5.

Phase-

Loss Protection: For three-phase motors, relays with phase-loss protection prevent damage due to single-phase operation. Standard three-pole relays are used for star-connected motors, while delta-connected motors may require phase-loss protective relays. 6.

Compliance with Standards: Thermal relays should comply with IEC 60947-4-1 for motor starters and overload protection, ensuring consistent performance, reliability, and safety across manufacturers. 7. Connection and Wiring: Proper wire sizing is essential.

Wires that are too thick or too thin can affect heat dissipation and relay performance. Follow manufacturer guidelines or electrical handbooks for correct wire selection.

Practical Considerations

Motor Starting Conditions: Consider starting current and duration; high inrush currents may require adjusting the heating element setting. Operational Environment: Frequent start-stop cycles, ambient temperature, and load variations influence relay selection. Maintenance and Safety: Manual reset relays enhance safety by preventing immediate restarts, while solid-state relays offer faster response and suitability for explosive environments.

By carefully evaluating these criteria, engineers can select a thermal relay that protects the motor effectively, minimizes downtime, and ensures compliance with international standards.