Although the operation of three-phase asynchronous motor is generally stable and the accident rate is small, it does not mean that there is no fault. If the load it drives exceeds the rated power of the motor, the motor will be overloaded.
First，What is overload of three-phase asynchronous motor
Generally, the motor has a fixed operating power, which is called the rated power, and the unit is watt (W). If the actual power of the motor exceeds the rated power of the motor under certain circumstances, this phenomenon is called motor overload.
For a simple example, 380V, 7.5kW three-phase asynchronous motor has a rated current of 15a, but in practical use, the current can flow through 20a, and the maximum allowable time can only be 1min. Therefore, 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 last for 1min.
It can be understood that the better the per unit ratio of the rated torque of the three-phase motor to the maximum rated load of the asynchronous motor, that is, the better the per unit ratio of the rated torque of the three-phase motor to the actual load.
Second, the harm of overload of three-phase asynchronous motor
1. The motor is overheated, resulting in the reduction of winding insulation and finally the motor is burned;
2. The overload operation of control elements and lines for a long time will cause the contact of contactor and circuit breaker to heat up and reduce the service life. In serious cases, the contact will be burned out and the motor will be burned out in phase loss operation;
3. The control of frequency converter will make the internal power components at full load and easy to be damaged;
4. The long-term overload operation causes the motor bearing to heat up and shorten the service life.
Third, overload symptoms of three-phase asynchronous motor
If the motor is overloaded for a long time, it is not only prone to overheating, resulting in the reduction of winding insulation and affecting efficiency, but also may shorten its service life. But for many equipment manufacturers, how to judge whether the motor is overloaded has become a difficult problem.
The phenomena after motor overload are as follows:
1. The motor body is seriously heated.
2. The motor current exceeds its rated current.
3. The motor makes abnormal vibration or sound.
4. The speed of the motor decreases, and the motor is locked in serious cases.
Fourth, what are the causes of overload of three-phase asynchronous motor
So what is the reason for the above phenomenon?
1. Mechanical reasons. The motor is only the prime mover, which is meaningful only with the mechanical part. Idling only one motor can not produce benefits, but there are many kinds of mechanical faults, and the faults are more complex. Therefore, here are only some common examples, such as: the mechanical part is stuck, the bearing state is not good, and the clearance between the impeller and the balance disc of multi-stage water pump will lead to the overload of the motor.
2. Connection error. Motors generally have star connection and triangle connection, and the wiring needs to be in strict accordance with the requirements during installation, but some people always do things on site based on experience. For example, if the motor with star connection method is directly connected into a triangular connection method, and its voltage rises directly from the original 220V to 380V, the current will certainly rise. However, if the motor with triangular connection is wrongly connected into star connection, the current will decrease under no-load, but the current will increase greatly after full load. Both of the above conditions will burn the motor.
3. The power supply voltage is low. 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 certain. When the voltage drops and the shaft power is certain, the motor can only ensure the power output by increasing the current. There are generally two reasons for this.
① 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 on-load voltage regulation and off-load voltage regulation. As the name suggests, on load voltage regulation means that the power can not be cut off, and the voltage can be adjusted directly. For no-load voltage regulation, the transformer must be powered off before voltage regulation.
② The cable between the motor and the power supply is too long. We know that generally, the cable is copper core and the resistance is small. But smaller does not mean No. when the cable distance is long, the smaller resistance will also increase. Moreover, the cable and the motor are connected in series, so the voltage should be divided in series. When the cable resistance is large, the voltage is also more, so the voltage used on the motor is naturally small. At this time, in order to solve the problem, it is generally necessary to increase the cross-sectional area of the cable. Or switch to a nearby power supply.
4. Improper motor selection. For equipment with long start-up time, such as ball mill, etc. It is necessary to use the motor with deep slot double squirrel cage rotor with small starting current and large starting torque, or directly wound motor. Ordinary motors are easy to burn out when used in the above occasions.
Fifth，overload protection of three-phase asynchronous motor
As an important measure of motor protection, overload protection is widely used. Its principle is that when the motor is overloaded, the current increases and the winding is overheated. If it takes too long, the insulation will be damaged.
The function of overload protection is to install protective devices in the control circuit of the motor, cut off the power supply in time and limit the overheating time of the motor to prevent insulation damage.
It is divided into two ways. One is the action of thermal effect element to control the connection and disconnection of contact. Its typical representative is the ordinary thermal relay operated by bimetallic sheet. It uses two pieces of metal with different expansion coefficient to heat, expand and bend during overload operation, and push a set of action mechanism to disconnect a pair of constant contact of thermal relay and play the role of overload protection. The other is to use the overcurrent detection circuit to directly detect the current, and finally drive the electromagnetic relay or solid-state relay to disconnect the power supply. Its typical representatives are overcurrent relay and various types of motor protector. It also has the function of short circuit protection.
Generally, when selecting thermal elements, the action current shall be selected according to 1.1 ~ 1.25 times of the rated current of the motor.
