In motor control circuits, should a thermal relay or a current relay be used?

Thermal relays are generally used in low-voltage circuits, as they are simple and reliable； Current relays are used in the high-voltage circuit, and the type of protection mode to be employed is determined based on requirements and the importance of the equipment

For a 380V 30KW motor, which type should be used if it is to be started with a 10-second delay in order to prevent overcurrent? Thermal relays don’t have this function of setting a delay, right?

Thermal relays cannot have such a function; for low-voltage microcomputer-based integrated protectors, the lower-end domestic models cost only a few hundred yuan each

I don’t understand what the purpose of this function is?

The thermal relay merely provides inverse-time protection; if the startup time is long (10 seconds is way too long), this value can be increased slightly. If reliability is a concern, a motor protector can be used instead – such devices are now quite mature

If there are no special requirements, a thermal relay will suffice. The process by which a motor is damaged due to overcurrent is related to factors such as current, time, and ambient temperature. The operating conditions of a thermal relay are very similar to the process of motor damage caused by overcurrent. In other words, the operating principle of the thermal relay is such that it meets the requirement of protecting the motor while avoiding excessive protection as much as possible. It works effectively in cases of startup, overload, and high ambient temperatures. The disadvantage of thermal relays is their lack of precision, whereas the use of time-delay current relays allows for accurate control of the allowable overcurrent duration. However, for most small and medium-sized motors, accurately setting the overcurrent time is another hassle. Especially when different sizes of overcurrents require different delay times, it is often difficult to choose a single delay setting.

All of them can be used; the main consideration is cost. If the cost is acceptable, motor micro-protection can be employed for protection

Thermal relays are generally used for overload protection and are not sensitive to instantaneous currents. Current relays are generally used for short-circuit protection and insulation failure protection; they are sensitive to instantaneous currents but not to long-term overloads. Generally, the two are used together.

Additional note: Current relays are used for timing purposes, mainly to prevent tripping caused by starting current.