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- Thermal relay overload protection
– Working principle :
– Thermal relays are mainly composed of heating elements, bimetal sheets and contacts. The heating element is connected in series in the main circuit of the motor, and when the motor is working normally, the current generates heat through the heating element. A bimetallic sheet is made up of two metal sheets with different thermal expansion coefficients, for example, one is an iron-nickel-chromium alloy and the other is a manganic-nickel-copper alloy.
– When the motor is overloaded, the current in the main circuit increases, the heat generated by the heating element increases, and the bimetal sheet will bend and deform after being heated due to the different coefficient of thermal expansion of the two metals. When the deformation reaches a certain degree, the bimetal sheet pushes the contact action, so that the contactor coil in the control circuit loses power, thus cutting off the main circuit of the motor and realizing the overload protection of the motor.
– Features :
– The action of the thermal relay has an inverse time limit characteristic, that is, the greater the overload current, the shorter the operation time. This is because the greater the overload current, the faster the heating element generates heat, and the faster the deformation rate of the bimetal sheet. This characteristic is in line with the actual needs of motor overload protection, and can effectively prevent the motor from being damaged due to long-term overload.
– However, the thermal inertia of the thermal relay is large, and it will not act immediately due to short-term overload of the motor (such as a large current shock when the motor is started). This is because in the case of short-term overload such as motor start-up, although the current is large, the duration is short, and the heat generated by the heating element is not enough to cause enough deformation of the bimetal sheet to trigger the action.
- Fuse overload protection
– Working principle :
– The fuse is mainly composed of the melt and the insulated tube (or seat) on which the melt is installed. The melt is the core component of the fuse, which is usually made of metal materials with low melting point (such as lead-tin alloy, zinc, etc.). When the motor is working normally, the current passes through the melt, and the heat generated by it can be dissipated through the insulated tube.
– When the motor is overloaded, the current in the circuit increases, and the heat generated by the melt will increase rapidly. Once the current exceeds a certain multiple of the rated current of the melt (which varies according to the specifications of the fuse), the melt will fuse due to overheating, thus cutting off the motor circuit and playing the role of overload protection.
– Features :
Fuses operate relatively fast, which can quickly cut off the circuit in the event of a serious overload or short circuit failure of the motor, protecting the motor and other electrical equipment. However, the fuse is a one-time protection device, and once the melt is blown, a new fuse is needed to restore the circuit connection.
– And, because the fuse’s fuse characteristics are relatively “hard”, that is, it is not sensitive to the size of the overload current, as long as the current exceeds a certain multiple of the rated value will fuse, so in some occasions that require accurate overload protection (such as frequent motor startup may lead to short-term overload), the use of fuses alone may not be appropriate.
- Electronic overload protector working principle
– Working principle :
– The electronic overload protector detects the working current of the motor through the current transformer. The current transformer converts the large current in the main circuit into a small current signal in proportion and then feeds it into the electronic circuit. The electronic circuit amplifies, filters and compares the input current signal.
– When the detected current signal exceeds the set overload current threshold, the electronic circuit will output a control signal that causes the contactor or relay to operate and cut off the main circuit of the motor. At the same time, some electronic overload protectors also have a memory function, which can record overload times, overload time and other information, which is convenient for users to analyze the operation of the motor.
– Features :
– Electronic overload protector with high precision and high sensitivity. It can accurately detect the current change of the motor, and can flexibly set the overload current threshold, operation time and other parameters according to the actual characteristics and working requirements of the motor.
– Compared with thermal relays and fuses, electronic overload protectors can also achieve more functions, such as overload alarm, remote monitoring, etc. However, the cost of electronic overload protectors is relatively high, and their electronic components are susceptible to environmental factors (such as temperature, humidity, electromagnetic interference, etc.).
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