How to set the overload protection of an electric motor
Let's first look at the overload protection of low-voltage distribution lines, which needs to follow two conditions:
Condition 1: Ic≤In≤Iz
Condition 2: I2≤1.45Iz
Among them, Ic represents the calculated current of the line, Iz is the current-carrying capacity of the cable, and I2 is the agreed operating (agreed melting) current of the circuit breaker or fuse.
For household micro-circuit breakers, the agreed tripping current is 1.45In, as shown in Figure 1
For molded case and frame circuit breakers, the agreed tripping current is 1.3Ir, as shown in Figure 2.
In Formula 1, in is its rated current for micro-break (see Figure 3).
The setting current Ir for molded case circuit breakers or frame circuit breakers as trip units (see Figure 4).
If low-voltage circuit breakers are set up in accordance with the requirements of Condition 1, they can provide appropriate overload protection for low-voltage distribution lines. From the perspective of the characteristics of circuit breakers, meeting condition 1 naturally meets condition 2.
If it is an electric motor circuit, how should its overload protection be set?
Overload protection for electric motors typically employs overload protection thermal relays, as shown in Figure 5. Inside, there is a bimetallic strip (Figure 6), which bends and operates under three-phase overload and phase loss conditions of the motor.
Under three-phase overload conditions, the bimetallic strip bends simultaneously to push the release plate to act (Figure 7). If there is a phase loss, the non-faulty phase bimetallic strip bends to push the release plate to act (Figure 8). When the overload and phase loss protection actions, the NO contact becomes normally closed and the NC contact becomes normally open. The NC contact disconnects the power supply to the contactor coil, and the contactor breaks the overload current.
The protective action characteristics of the thermal relay must comply with GB/T14048.4. It should not act within 2 hours at 1.05 times the set current but must act within 2 hours at 1.2 times the set current to protect the motor.
The overload protection setting of an electric motor does not need to meet two conditions like that of a distribution line. According to the "GB50055-2011 Code for Design of Distribution of General Electrical Equipment", the overload protection setting of an electric motor should meet the following requirements: the setting current of the thermal overload relay or overload trip unit should be close to but not less than the rated current of the motor.
If the rated power of a certain motor is 18.5kW and the rated current is 32A, select the appropriate thermal relay for it and set it.
After checking the Schneider LRD thermal relay samples, two types of thermal relays meet the requirements of GB50055:
The first model is LRD3353C, whose thermal setting range is 23A to 32A. For an 18.5kW motor, the setting value can be adjusted to 32A to meet the requirements.
The second model is LRD3355C, with a thermal protection setting range of 30A to 40A. For 18.5kW motors, the setting value can also be adjusted to 32A to meet the requirements.
However, we give priority to the latter. Because if both are set to 32A, if the thermal relay malfunctions due to other factors on site such as excessively high ambient temperature, there is still a possibility to increase the LRD3355C, but the LRD3353C can only be replaced.
The functions on the thermal relay panel are introduced as follows. See the figure below:
1- Represents the model of the thermal relay. The illustration shows Schneider's LRD thermal relay.
The protection setting range of the thermal relay: 4A to 6A, continuously adjustable.
3- Normally open contact NO97-98 is in an open state under normal circumstances. After the thermal relay operates, it becomes in a conducting state to feed back the status of the thermal relay, or it can be used to connect the buzzer or alarm light when the overload alarm in the fire protection circuit does not trip.
4- Normally closed contact NC95-96, under normal circumstances, is connected to the contactor coil control circuit. When the motor is overloaded, has a phase loss or is locked, the normally closed contact becomes normally open, disconnecting the power supply to the contactor coil, and the contactor breaks the fault current.
5-A stands for Auto automatic reset. Only when there is a special need will the on-site personnel select the automatic reset mode by themselves. After the thermal relay operates, after a certain cooling time, the thermal relay automatically resets, the states of NO and NC contacts return to normal, and the motor restarts.
The red Stop button, when pressed, disconnects NC95-96 while NO97-98 remains unaffected. It is used for on-site simulation of the loss of power in the contactor coil after NC95-96 disconnects.
7-H represents Hand manual reset. The default manual reset position of the thermal relay at the factory is H. After the thermal relay operates, after a certain cooling time, when the on-site personnel press the reset button, the thermal relay resets, and the states of NO and NC contacts return to normal.
For the test button, push it to the left with a "T" letter. A "T" letter will pop up, indicating the Trip trip (a "T" letter will also pop up after the thermal relay is overloaded). This can simulate the overload protection trip. At this time, the states of NC95-96 and NO97-98 will both change.
9- Blue reset button. After the thermal relay operates, after a certain cooling time, when pressed, the NO and NC points return to the normal state.
10-Plug-in terminals: The LRD thermal relay can be directly installed at the output terminal of the Schneider LC1D contactor.
Through the previous analysis, it can be seen that:
From the perspective of design specifications, the overload protection setting requirements for electric motors are different from those for distribution lines. The former is just a paragraph in the design specifications, while the latter needs to satisfy two inequalities.
From the perspective of products, for overload protection of motors, thermal relays or motor protection type circuit breakers are generally selected. However, for overload protection of distribution lines, household micro-circuit breakers, distribution type molded case circuit breakers and frame circuit breakers need to be chosen.
From the perspective of protection characteristics, the overload protection characteristics of thermal relays are different from those of miniature circuit breakers, molded case circuit breakers, and frame circuit breakers. Thermal relays are more suitable for protecting motors.
