Motor protection components: three types of overload protection for thermal relays
In the era of electrical automation, electric motors play an important role both in production and in life. During use, their protective measures are also essential. Among them, common thermal relays and motor protectors are the most commonly used protection tools.
As an example, today, thermal relays are used as overload protection components for electric motors in three forms:
1, bimetal type. The bimetal is heated to push the lever to move the contacts.
2, thermistor type. It is made using the characteristics that the resistance value changes with temperature.
3. Fusible alloy type. The overheating current is used to melt the fusible alloy to activate the relay.
The small size, simple structure and low cost are its advantages. It is suitable for overload and phase failure protection of AC motors for long or intermittent operation. It is not suitable for frequent jog operation, and can not be used for short circuit protection. It has its own temperature compensation, automatic and manual reset function.
With the development of people's needs, the thermal relay has fewer functions, no phase failure protection, poor ventilation of the motor, broom, stall, long-term overload; the disadvantages of frequent startup and other failures are not obvious.
Therefore, the motor protector is widely used. It is based on the principle of detecting line current changes (including taking, positive sequence, negative sequence, zero sequence and overcurrent) to detect phase failure or overload signals. In addition to the phase failure protection function, it also has overload and stall protection functions.
The advantage is that the wiring is simple, the main line only needs to pass through the threading hole of the body, there is no large current
wiring, and the contact failure is reduced. The control circuit has four terminals, two power supplies, and the other two terminals are connected in series to the contactor coil circuit. Some of the two control terminals are simpler.
The disadvantage is that after a device failure occurs, most of the control terminals of the control terminals are automatically reset, and the operator may start the device again. Two-wire loads (up to 220 volts), such as small air compressors for single-phase operation, cannot be protected with a protector, while thermal relays can be easily used. In addition, most motor protectors on the market do not provide a set of electrically independent contacts for use as alarms or other circuits. A set of electrically fully independent normally open contacts are provided on the body relative to the thermal relay for use in designing alarms or other circuits.
Nowadays, the motor protection circuit breaker is also used by many people. The rated current is 630A or below (that is, the protected motor is 315kW or less). It is used to start the motor infrequently and break the motor. When the motor is short-circuited or under-voltage, etc. It can be protected (automatically cut off the circuit).
Its instantaneous trip current is 10 times the rated current, taking into account the start and overload protection of the motor. The current is greater than the rated current when the motor is started. The motor-protected circuit breaker, when it is just started, allows a little overcurrent and will not trip.