Which type of overload relay is commonly used in a motor starter?

• A. Bimetallic
• B. Magnetic
• C. Melting alloy
• D. All of the above

Answer: (D)

In motor starters, overload relays are crucial for protecting the motor from excessive currents that can lead to overheating and damage. The reason that all types of mentioned overload relays can commonly be used in a motor starter lies in their distinct operating principles and applications.

Bimetallic overload relays function based on a bimetallic strip that bends when heated due to the current flowing through it. This bending action opens the relay contacts, disconnecting the motor from the power supply when a preset temperature is reached. They are especially favored for their reliability and simple construction.

Magnetic overload relays utilize an electromagnetic mechanism. When the motor experiences an overload, the magnetic force generated exceeds a certain threshold, causing the relay to trip and disconnect the motor circuit. This type of relay is often used in applications where quick response to short-circuit conditions is necessary.

Melting alloy overload relays operate using a thermal fuse that melts at specific temperatures, effectively disconnecting the circuit when the current exceeds safe levels for a sustained duration. This type is less common but can be very effective in providing protection in specific circumstances.

Since all three types of relays can serve to protect motors in different contexts and applications, they collectively form a comprehensive choice for overload protection in motor starters