Common protection links in electrical control systems include short-circuit protection, overload protection, phase loss protection, undervoltage protection, and phase sequence protection. Which electrical appliances are used to complete these protections in the control circuit?

Short circuit protection

Common short-circuit protection devices include fuses and circuit breakers.

Fuses are more suitable for systems that have poor requirements for action accuracy and automation, such as small-capacity cage motors, general ordinary AC power supplies, etc. However, the fuse has a disadvantage. When a short circuit occurs, one phase of the fuse may blow, causing the equipment to operate without a phase.

fuse

The circuit breaker is also called the air switch. When a short-circuit fault occurs in the circuit, its electromagnetic release will automatically trip for short-circuit protection, and directly cut off the three-phase power supply at the same time to protect the safety of the circuit and equipment. Therefore, it is widely used in electrical control systems. .

breaker

under voltage protection

Undervoltage generally uses a contactor as protection. When the main motor control circuit, power supply voltage is too low or the power is cut off, the contactor coil is released. At this time, its main contact and auxiliary contact are opened at the same time, so that the motor power is cut off and self-locking is lost.

contactor

Phase sequence protection

Generally, a phase sequence protector is used as a phase sequence protection. The phase sequence protector can prevent the motor from starting when the phase sequence is incorrect, and the motor will start normally when the phase sequence is correct, thereby avoiding accidents that may be caused by reverse rotation. It is generally used in conjunction with a contactor.

Phase sequence protector

It is mainly used for mobile motors that require correct phase sequence. Such as air compressors, fans, water pumps, oil pumps, central air-conditioning units, electric control boxes, cranes and other equipment.

Overload protection or thermal protection

To put it bluntly, overload protection means that the current of the motor exceeds the rated current. Common protection devices include thermal relays, integrated motor protectors, and motor protective circuit breakers.

The advantages of thermal relays are simple structure and low price, but the disadvantage is that there are few protection functions, and sometimes they cannot protect the motor when there are faults such as motor stall, phase loss, long-term overload, and start-up timeout.

thermal relay

The comprehensive motor protector is actually a comprehensive protector that integrates protections such as phase failure, overload, locked rotor, and three-phase unbalance. Hotspots and points of failure. However, the disadvantages are also obvious. The integrated motor protector needs to work continuously, so the failure rate is also very high due to factors such as grid voltage fluctuations, interference, and self-heating.

Integrated motor protector

The advantages of motor protection circuit breakers are stable protection, convenient use, sensitive action, high precision, good repeatability, and complete protection functions, etc. The disadvantage is that the cost is relatively high.

Motor Protective Circuit Breaker

At present, for small equipment or general control, considering the cost, most thermal relays are still used as overload protection, and motor integrated protectors are used in a small number of demanding equipment. Large-scale equipment or automatic control equipment mainly still use motor protection circuit breakers as overload protection.