First, the concept of three-phase asynchronous motor
Three-phase asynchronous motor is a type of motor powered by 380V three-phase AC power supply (phase difference 120 degrees) at the same time. Because the rotor and stator rotating magnetic field of the three-phase asynchronous motor rotate in the same direction and at different speeds, there is a difference rate, so it is called a three-phase asynchronous motor.
Second, what is three-phase asynchronous motor overload
The overload of a three-phase asynchronous motor means that the current flowing through the motor exceeds the rated current within a certain allowable time. As a simple example, a 380v, 7.5KW three-phase asynchronous motor has a rated current of 15A, but in actual use, the current can flow through 20A, and the maximum allowable time can only be 1min. So the overload capacity of the motor is 20/15=133%/1min. The overload capacity of the actual motor is relatively strong, which can often reach twice the rated current, and the duration is 1min.
(The calculation method of the rated current of the three-phase motor: the power of the three-phase motor is P=1.732UIcosφ, and the cosφ (power factor) is between 0.7-0.95.
Then I=P/(1.732*U*cosφ)=4800/(1.732*440*0.8)=7.4A, the 11.5A you mentioned may be the power of the entire compressor or the current you measured with an ammeter, if it is an ammeter Measured, then the motor is overloaded. There is also a voltage of 440V. You can know that your motor is not domestically produced. The domestic motor is generally rated at 380 (400V). Of course, 400V voltage can be used in my country, because the voltage of the motor is a range, and the rated voltage of 400V The motor is at 360V. Can be used between -440V.
There is also the 440V you mentioned. In our country, the general voltage is 380v, and 440V is the rated voltage. Therefore, the above data cannot be used when calculating the current. Instead, 440 should be changed to 380, so that the calculated current is 9.1A. Here is a rough calculation method. Since 1.732*380*0.8=520, which is close to 500, under the three-phase voltage of 380V, the current I=P (kilowatts)/2.
The type of contactor should be selected as long as it is not lower than the rated voltage, and it should be selected according to the number of contacts required by the control loop. Schneider’s contactor model can choose LC-D12, D12 means that the rated current is 12A)
Three, three-phase asynchronous motor overload capacity
The overload capacity of a three-phase asynchronous motor refers to the ratio of the maximum torque per unit value to the rated torque per unit value, which can be understood as the ratio of the maximum load that the three-phase asynchronous motor can actually load to the rated load load. The better the overload capacity, the better the motor performance.
Four, three-phase asynchronous motor overload symptoms:
1. The heat generated by the motor increases;
2 The motor has a low sound, and the vibration is normal;.
3. The motor speed drops, and may even drop to zero;
4. If the load changes drastically, the motor speed will fluctuate;
Five, What are the reasons for the overload of three-phase asynchronous motors?
1. Mechanical factors
After all, the motor is the prime mover. It only makes sense if it has mechanical equipment. Only one motor idling cannot bring benefits. However, there are many types of mechanical problems, and the faults are more complicated. The following examples are common situations, such as: mechanical parts stuck Dead, poor bearing condition, impeller and multi-stage water pump balance disc clearance and other issues will cause overloading of the motor.
2. Connection error
Motors generally have star connection and delta connection, and must be wired in strict accordance with the requirements during installation, but some people at the scene do their work based on experience. For example, if a star-connected motor is directly connected into a delta connection, and its voltage is directly increased from the original 380V to 660V, the current will naturally increase. However, if the delta-connected motor is incorrectly connected to a star connection, the current will decrease at no-load, but the current will increase greatly after full-load. Both of the above conditions will burn the motor.
3. Low power supply voltage
When the motor is fully loaded or started, if the power supply voltage is small, such as 90% of the normal voltage, the current will increase by 110%. This is because the motor power is constant. When the working voltage drops and the shaft power is constant, the motor can only increase the current to ensure the power output. There are two factors that usually cause this to happen.
First, the power supply voltage itself is low. At this time, it is necessary to adjust the tap changer of the transformer. The tap changer is divided into two types: on-load voltage regulation and no-load voltage regulation. In short, on-load voltage regulation means that the power cannot be cut off, and the working voltage can be adjusted directly. For no-load voltage regulation, the voltage regulation can only be carried out after the transformer is powered off.
Second, the cable between the motor and the power supply is too long. We know that the cables are usually copper cores with small resistance. But small does not mean no, when the cable distance is long, the smaller resistance will also increase. In addition, the cable and the motor are connected in series, and the voltage must be divided in series. When the resistance of the cable is large, the divided voltage will be more, so the voltage applied to the motor will naturally be smaller. To solve the problem at this time, it is usually necessary to increase the cross-sectional area of the cable. Or switch to the nearest power source.
4. Improper selection of motor
For equipment with a long start-up time, such as: ball mills, etc. It is necessary to use a motor with a deep slot double squirrel cage rotor with a small starting current and a large starting torque, or a direct winding motor. Ordinary motor use in the above occasions is easy to burn the motor.
Six, three-phase asynchronous motor overload protection knowledge:
Overload protection of three-phase asynchronous motor: In the control circuit of the motor, a thermal relay composed of bimetallic sheets is often installed. It uses two pieces of metal with different expansion coefficients to bend due to thermal expansion during overload operation, and promote a set of actions. The mechanism makes a pair of constant contacts of the thermal relay disconnected to play an overload protection role. Generally, when the heating element is selected, its operating current is selected according to 1.1~1.25 times of the rated current of the motor.
Seven, Coordination between overload protection device and motor
(1) The action time of the overload protection device should be slightly longer than the starting time of the motor. The characteristics of the motor overload protection device can only ensure its normal operation by avoiding the characteristics of the starting current of the motor; but its action time cannot be too long, and its characteristics can only play an overload protection role under the thermal characteristics of the motor.
(2) The instantaneous operating current of the overload protection device should be slightly larger than the starting impulse current of the motor. If there is a protection device with overload instantaneous action function, its action current should be larger than the peak value of the starting current, so that the motor can start normally.
(3) The action time of the overload protection device should be a little smaller than the thermal characteristics of the wire, so that it can play the function of backup protection of the power supply line.
2. Coordination of overload protection device and short circuit protection device
General overload protection devices do not have the ability to break short-circuit current. Once a short circuit occurs during operation, it is necessary to cut off the circuit by a short circuit protection device (such as a circuit breaker or a fuse, etc.) connected in series in the main circuit. If the fault current is small and belongs to the overload range, the circuit should still be cut off by the overload protection device. Therefore, there should be a choice between the two actions.
The characteristics of the short-circuit protection device are represented by fuses. The intersection current with the overload protection characteristic curve is Ij. If the dispersion of the fuse characteristics is considered, the intersection currents are Is and IB. At this time, Is and the following are required. The overcurrent should be cut off by the overload protection device, and the short-circuit protection device should cut off the circuit by the short-circuit protection device for Ib and above up to the allowable limit short-circuit current to meet the selectivity requirements.
Obviously, it is difficult to ensure selectivity in the Is-IB range. Therefore, the range should be as small as possible. Judging from the current IEC standard, the limit value is Is=0.75Ij, Ib=1.25IJ. At present, the rated making and breaking capacity of the overload protection device is assessed by 0.75IJ, which is obviously lower. The trend of revision from the IEC standard may be assessed by IJ in the future to improve its reliability. Therefore, the above coordination should consider both its selectivity and its rated making and breaking capacity.
The purpose of our three-phase asynchronous motor protection is to “prevent trouble before it happens”, or to prevent the accident from expanding and affecting the power grid and surrounding equipment when an accident occurs. Overload and short-circuit protection are essential for low-voltage three-phase asynchronous motors. So this is a problem that we must pay attention to when using three-phase asynchronous motors.
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