If there is a fuse in the motor control circuit, why should a thermal relay be installed

Fuse. It is mainly used for short-circuit protection, thermal relay.

It is mainly used for current protection. When the motor can withstand the current for a short time, if there is an overcurrent and the motor is overloaded, it is protected by a thermal relay, and when the current is stable, it can be automatically put in. In the event of a short circuit in the line or in the motor winding, the fuse is permanently disconnected to protect it from introducing the fault to other lines.

The fuse and thermal relay are installed in the main circuit of the motor, which is the protection measure of the line and the motor under abnormal operating conditions, and both are indispensable.

Thermal relay: belongs to overcurrent protection, the motor is in the normal current operating range, the thermal relay is not active, when the motor is abnormal, the current exceeds the set value of the thermal relay, the thermal relay will act, power off to protect the motor.

Fuse: It belongs to the current quick break protection, the circuit or electrical appliances are short-circuited, the fuse can be quickly fused, prevent the expansion of the accident, and isolate the power supply line from the terminal electrical appliances.

Electrical appliances must have perfect protection measures, do not be greedy for trouble, accidents, regrets.

The action of the thermal relay can cause the main contactor coil to lose power and protect the motor, which the fuse cannot do.

The fuse has a long blowing time.

The fuse protects the entire distribution line, and the thermal relay protects the motor.

The fuse is instantaneous protection, like a fuse, as long as the current immediately executes the protection, while the thermal relay is to play the role of overload protection, overload is a period of time before disconnecting, the two protections are different, will be used for different stages of protection as needed.

The fuse mainly plays the role of short-circuit protection in the motor circuit, and its rated current is generally much larger than the rated current of the motor, and it cannot play the role of overload protection, so the thermal relay is installed to carry out overload protection on the motor.

The fuse in the control circuit is for the protection of the control circuit, the fuse of the main circuit is for short-circuit protection, and the thermal relay is suitable for overload protection, and the function is not the same.

Let's start with thatFuse. It is the use of metal conductors in series to play a short-circuit protection role in the circuit, when there isShort-circuit currentFlows through itself when it passesCalorific valueThe melt is blown to cut off the circuit, thus achieving the protective effect of the circuit. It also works on the principle of the thermal effect of electric current, except that its thermal element is a bimetallic sheet.

This bimetal sheet is made up of two coefficients of thermal expansion.

When it is heated, the bimetal sheet will bend from the large expansion metal sheet to the small expansion metal sheet, and then it will be connected with the dynamic breaking contact to cut off the control circuit, so as to protect the load in the circuit. This shows fuses and thermal relays.

It is two types of protection elements with different protection purposes, and they cannot be used interchangeably in the circuit.

Protection features. Some models of thermal relays also have phase loss protection.

The phase-off protection function of the thermal relay is provided by a differential amplification mechanism composed of internal and external push rods. When the motor is working normally, the current through the thermal relay heating element is normal, and the inner and outer push rods are moved forward to the appropriate position.

When the power supply is disconnected and causes a lack of phase, the phase current is zero, the bimetal sheet of the phase is cooled and reset, so that the inner push rod moves to the right, and the bending degree of the other two phases of the bimetal sheet increases due to the increase of current, so that the outer push rod moves more to the left, due to the differential amplification effect.

In a short period of time after the phase failure failure, the normally closed contact is pushed to disconnect, so that the AC contactor.

Release, the motor is protected when the power is off and stopped.

In electrical control circuits, fuses and thermal relays cannot replace each other.

The fuse is blown when the current is too large, disconnecting the main electrical circuit and protecting the electrical.

When the current is too large, the thermal relay acts, and the trip signal is sent out, so that the contactor of the electrical main circuit trips, disconnects the electrical main circuit, and plays a role in protecting the electrical.

Fuse: Short circuit protection.

Thermal relay: overload protection, play a control role.

That's why, they can't be substituted for each other.

Thermal Relay: Overload Protection.

Fuse: overload, short circuit protection.

No, 1, the focus is different; 2. The internal structure and action principle are different.

For example, thermal relays cannot be short-circuit protected, because it takes a few seconds for the metal sheet in the thermal relay to be heated, and the short circuit is instantaneous.

The fuse plays the role of over-current protection.

Thermal relays are used in control loops and protection loops.

They are not interchangeable with each other.

Now the electrical main circuit of using fuse is less and less, generally speaking, the fuse is used to cut off the short-circuit fault of the distribution circuit, and the thermal relay is used to cut off the overload fault, these two functions can not be substituted for each other, because generally speaking, the fixed value of the thermal relay is only the times of the rated current of the motor, and there is also the delay function, and the fuse is generally selected several times (the load is different, the multiple is different) which can not replace each other.

In practical application, the main circuit of the general motor does not use the fuse as short-circuit protection, and the circuit breaker (air open) is generally used, the reason is that the three phases of the fuse are separated, and it is difficult to ensure that the three fuses are exactly the same fuse current when manufacturing. And even if the three fuses are exactly the same fusing current, the chance of single-phase grounding or two-phase short circuit in the actual circuit is still very large, which will cause one or two fuses to fuse, and then cause the motor to burn out due to lack of phase operation.

So, generally speaking, now the possibility of designing electrical circuits to use fuses alone as short-circuit protection for motor control loops is becoming less and less, and now the biggest role of the main circuit fuse is not as protection, but as an obvious disconnection point. In order to ensure the power failure of the equipment maintenance process, you can add a large fixed value (the fixed value of this fuse can be 10 20 times the rated current, and the fuse will be on the line), and when the equipment is overhauled, first disconnect the air open, and then unplug the fuse to ensure safety.

Melts early and late.

The function is short-circuit protection. Because the current of the motor is very large when it starts, it is generally rated according to the motor current.

The rated current of the fuse is selected to the time of the time of the fuse. In this way, the fuse will not play a role in the overload protection of the motor.

Thermal relays. The function is overload protection, and the action current is generally set to the rated current of the motor. In this way, as soon as the motor exceeds the rated current for a little longer, the thermal relay will jump off.

In summary, fuses and thermal relays perform their own duties and cannot be substituted for each other.

In electrical control loops, fuses and thermal relays cannot be substituted for each other.

1. Similarities between fuses and thermal relays: both belong to current protection appliances and have the characteristics of inverse time limit.

Second, the difference between fuses and thermal relays:

1. The fuse is mainly used for short-circuit protection, and the thermal relay is used for overload protection;

2. The fuse uses the principle of thermal fuse, which requires the melt to have a high fusing coefficient;

3. The thermal relay uses the principle of thermal expansion, which requires the bimetal sheet to have a high expansion coefficient of the beam;

4. The thermal relay protection has a large delay, and the action of the fuse must be instantaneous when the short-circuit protection is buried.

Hello, such as the clump fuse has overcurrent Zheng Na and short circuit protection for slag burial, thermal relays are mainly overload protection, the role is different, can not replace each other. There is also a difference between overload and overcurrent.

The fuse and thermal relay play the role of short-circuit and overload protection of the circuit respectively in the circuit to achieve different protection functions for the circuit. The fuse mainly plays the role of short-circuit protection, and the thermal relay mainly plays the function of overload protection. Therefore, in the same circuit, not only a thermal relay should be installed, but also a fuse should be installed when guessing the difference.

In motor control circuits, fuses and thermal relays are important electrical components used to protect circuits and motors. The function of the fuse is to detect whether the current exceeds its rated value in the circuit, and once the current exceeds the rated value, the fuse will automatically disconnect the circuit to protect the circuit and electrical appliances from overcurrent damage. If there is a fault such as overload or short circuit when the motor is running, the fuse will immediately cut off the circuit, and the clump will prevent the fault from further expansion and protect the motor from damage.

The thermal relay is a kind of electromagnetic protection seepage device, which detects the current size of the motor when running, when the current exceeds the rated value, the thermal relay will automatically trip, cut off the circuit, in order to protect the motor from overload or short circuit and other faults. The working principle of a thermal relay is that when an electric current passes through it, the current will produce a magnetic field, which will make the thermal elements inside the thermal relay heat, and when the temperature reaches a certain level, the thermal relay will trip and cut off the circuit. In general, fuses and thermal relays are both important components used to protect circuits and motors, they can immediately cut off the circuit when the motor fails, protect the motor from damage, and thus improve the safety and reliability of the motor.

Hello <>, fuse: fuse is an electrical protection device, usually made of a piece of metal wire or lead wire with small resistance, its role is to protect the equipment from damage caused by current overload or short circuit in the electric and combustion circuit, when the current in the circuit exceeds the rated current of the fuse, the wire in the fuse will be instantly heated and fused, cut off the circuit closed section, prevent excessive current from flowing through the equipment, avoid damage to the equipment or cause fire and other dangers, thermal relay (thermal overload relay): thermal relay is a kind of current protection device, widely used in motor control circuits, its role is to detect the working current of the motor, and after the current exceeds the set value or continues to overload the operation for a period of time, automatically cut off the power supply to protect the motor from overheating damage, the thermal relay adopts a thermal element (usually a bimetal sheet or thermistor), when the current is too high or overloaded, the thermal relay absorbs heat through the radiator and deforms or heats up the thermal element, Trigger the relay to cut off the power supply, so as to achieve overload protection for the motor, to sum up, the fuse is mainly used to protect the circuit from current overload or short circuit damage, and the thermal relay is mainly used for the overload protection of the motor to ensure that the motor works within a safe range, these two devices play an important role in the motor control circuit, to ensure the safety of equipment and personnel.

Summary. In the three-phase asynchronous motor control circuit, the fuse plays the role of the main circuit short-circuit protection motor, and the thermal relay plays the role of motor overload protection, both of which must be both.

In the control circuit of the three-phase asynchronous motor, the main circuit has been equipped with a fuse for protection, why should a thermal relay be installed.

In the three-phase asynchronous motor control circuit, the fuse plays the role of the main circuit short-circuit protection motor, and the thermal relay plays the role of motor overload protection, both of which must be both.

A fuse refers to an electrical appliance that melts the melt and disconnects the circuit with the heat generated by itself when the current exceeds the specified value. The fuse melts the melt according to the heat generated by itself after the current exceeds the specified value for a period of time, so that the circuit is disconnected; A kind of current protector made by using this principle. Fuses are widely used in high and low voltage power distribution systems and control systems and electrical equipment, as a short-circuit and over-current protector, is one of the most widely used protection devices.