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The steady-state heating value of the thermal relay is small, which is very unsuitable for motors with large inertia and light load starting. The method of wiring the thermal relay during starting is often adopted, which not only makes the control system structure complicated and increases the cost, but also has a protective overcurrent protection.

In order to prevent incorrect operation during startup, some motor protectors increase the set current, making the protection in name only. However, most motor protectors use fast saturation voltage transformers, so they generally have the characteristics of long allowable starting time in hot state and quick load protection in hot state. This is exactly in line with the specific protection requirements of motors in factories and mines, and can more reliably protect motor loads.

The test element of the thermal relay is a resistor. Due to the overcurrent impact of the starting current and load, it is easy to cause fatigue of the resistor, resulting in the deviation of the scale value and unstable posture. This is difficult to detect at the manufacturing site, resulting in load instability. The motor protector uses a voltage transformer as the test element, so there is no heating problem. Its posture reliability is a qualitative leap compared with the thermal relay.

The better motor protector test components generally use voltage transformers, and have a through-core structure. Not only is it easy to use, it is completely protected from the main control circuit, which is not likely to affect the main control circuit, and improves its own stability. At the same time, it completely eliminates the problem of heating of the terminal block, and the tolerance is good. However, at present, most countries in the world generally only use the through-core type for large-sized motor protectors, while small-sized models can only use the wiring type.