Because the electrical control system in addition to to meeting the production of mechanical processing process, but also to ensure long-term safety, reliability, trouble-free operation of equipment, so the protection link is an integral part of all electrical control systems, the purpose is to protect the motor, power grid, electrical control equipment and personal safety.
Can it be called a complete electrical control system, electrical control system must have at least short-circuit protection, overload protection of these two or more protection links. In general, the commonprotection links in electrical control systems are short-circuit protection, overcurrent protection, overload protection, zero-voltage protection, low voltage protection and weak magnetic protection.
AC contactors and thermal relays are commonly used electrical components in electrical control systems. AC contactors are mainly used in power transmission systems to frequently turn on and cut off main circuits with loads and high-capacity control circuits, while thermal relays are mainly used to protect motors and other loads from overloading and tangent protection of three-phase electric motors. Therefore, as described above, the AC contactor mainly works in the main circuit in the electrical control system, while the thermal relay mainly works in the protection circuit.
Reasons for overload protection
The motor long-term overload operation, the motor winding temperature will exceed its allowable value, resulting in the motor insulation material brittle, shorten the life, serious will cause the motor damage. The greater the overload current, the shorter the time allowed for a temperature rise.
According to the cause of overload, thermal relays are suitable commonly used overload protection elements. Why do you say that? It is known that thermal relays can meet those requirements. When the motor is rated current, the motor is rated heating, the thermal relay is not action, in the overload current is small, the thermal relay will also take a long time to move, overload current is larger, the thermal relay will be after a short period of action.
The thermal relay takes into account some of the requirements it can meet, but also its thermal inertia. The thermal relay does not act instantaneously by the motor's short-term overload shock current or short-circuit current, so short-circuit protection must be applied when using the process thermal relay action overload protection.
Note that the rated current of a fuse melt protected by a short circuit should not exceed the rated current of a heat relay heating element of 4 times. It should be noted here that short circuit, overcurrent, overload protection although all belong to current protection, because the operating value of the fault current, protection characteristics and protection requirements and use of different components, so they are not interchangeable.