What is the main purpose of overload heaters in a motor circuit?

The main purpose of overload heaters in a motor circuit is to prevent overheating. When a motor operates, especially under heavy loads or prolonged use, it can draw more current than it is designed to handle, leading to excessive heat generation. Overload heaters are specifically designed to detect this increased electrical load and heat, and they respond by opening the circuit to stop current flow, effectively protecting the motor from damage due to overheating.

By tripping the circuit in response to high temperature caused by overcurrent, overload heaters help ensure the longevity and reliability of the motor. This protective function is crucial because overheating can lead to insulation breakdown, bearing failure, or complete motor burnout, which would require costly repairs or replacements.

The other options might appear relevant but do not directly align with the primary function of overload heaters. They do not serve the purpose of protecting from overvoltage, which relates to different protective devices, nor do they directly affect the functionality of contactors or manage the motor's speed, both of which are controlled by different mechanisms in the motor circuit.