Overload symptoms and causes of three-phase asynchronous motor
Release time：2020-05-08 16:24Number of views：Originate：
1、 The concept of three phase asynchronous motor
A three-phase asynchronous motor that operates as a motor. The rotor speed of three-phase asynchronous motor is lower than that of rotating magnetic field. Due to the relative motion between the rotor winding and the magnetic field, the rotor winding induces the electromotive force and current, and interacts with the magnetic field to generate electromagnetic torque, thus realizing the energy conversion. With the three-phase asynchronous motor manufacturers to understand the three-phase asynchronous motor overload reasons it has.
2、 What is three-phase asynchronous motor overload
Three phase asynchronous motor overload refers to the current flowing through the motor exceeds the rated current within a certain allowable time. For a simple example, the rated current of 380V, 7.5kW three-phase asynchronous motor is 15a, but in actual use, the current can flow through 20a, and the maximum allowable time is only 1 minute. Therefore, the overload capacity of the motor is 20 / 15 = 133% / 1 minute. The overload capacity of the actual motor is relatively strong, usually reaching twice the rated current within 1 minute. (calculation method of rated current of three-phase motor: power P of three-phase motor is 1.732 UI cos φ, cos φ (power factor) is between 0.7-0.95. The higher the power of the motor, the greater the power factor, usually 0.85. So I = P / (1.732 * u * cos φ) = 4800 / (1.732 * 440 * 0.8) = 7.4a, the 11.5a you mentioned may be the power of the whole compressor or the current measured by the ammeter. If the ammeter is used to measure the current, the motor will be overloaded. There's another 440 Volts. You can know that your motor is not made in China. Generally, the domestic rated voltage is 380 (400V). Of course, 400V can be used in China, because the motor voltage is within a certain range. Motors rated at 400 Volts can be used between 360 and 440 Volts. You mentioned 440 Volts. In China, the general voltage is 380V, and the rated voltage is 440V. Therefore, the above data cannot be used to calculate the current. Instead, 440 should be changed to 380, so the calculation current is 9.1a.. this is a rough calculation method. As 1.732 * 380 * 0.8 = 520, close to 500, the three-phase voltage is 380V,
Current I = P (kw) / 2. As long as the type of contactor is not lower than the rated voltage, it also needs to be selected according to the number of contacts required by the control circuit.
3、 Overload capacity of three-phase asynchronous motor
The overload capacity of three-phase asynchronous motor refers to the ratio of maximum torque standard value to rated torque standard value. It can be understood that the higher the ratio, the better the overload capacity and the better the motor performance.
4、 Overload symptoms of three-phase asynchronous motor
The heating capacity of the motor increases;
The motor has low chirp and general vibration..
The speed of the motor drops, or even drops to zero;
If the load changes rapidly, the motor speed will rise and fall;
5、 What is the cause of three-phase asynchronous motor overload?
The tractor is faulty
When the dragged machine fails, is not flexible or jammed, it will overload the motor and overheat the motor winding. Therefore, when the motor is overhauled to overheating, the load factor cannot be ignored.
The load of the dragging machine is not working properly
Although the equipment is matched, the mechanical load dragged by the equipment is not normal, and the load is large and small during operation. For example, when the feeding quantity of thresher is too large, the motor is overloaded and generates heat.
When the equipment does not match and the load power of the motor is greater than the rated power of the motor, the motor will be overloaded for a long time (i.e. the pony pulls the car), resulting in the motor overheating. When repairing the overheated motor, check whether the load power is consistent with the motor power to prevent blind and purposeless disassembly.
Check the voltage of external power supply. If the voltage is too low, it may cause this phenomenon.
The maximum torque of the motor is proportional to the square, and the starting torque is proportional to the square. Asynchronous motor is very sensitive to the fluctuation of power supply voltage. If the voltage drop is too large, its overload capacity and starting torque will be reduced, and even the phenomenon of unknown load or no starting at all will appear.
In order to ensure the normal operation and effective protection of asynchronous motor, the cooperation between asynchronous motor and protection device must be considered. Especially when using small capacity asynchronous motor in large capacity power grid, the protection coordination is more prominent.
6、 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 bimetal is usually installed. It uses two metals with different expansion coefficient to expand and bend when they are overloaded, pushes a set of action mechanism, disconnects a pair of thermal relays in normal contact mode, and plays the role of overload protection. Generally speaking, when selecting thermal element, its working current should be 1.1 ~ 1.25 times of the rated current of motor.
7、 Coordination between overload protection device and motor
(1) The action time of the overload protection device shall be slightly longer than the starting time of the motor. The characteristics of motor overload protection device can only avoid the characteristics of motor starting current to ensure its normal operation. However, its action time can not be too long, and its characteristics can only play an overload protection role under the thermal characteristics of the motor.
(2) The instantaneous action current of the overload protection device shall be slightly greater than the starting impulse current of the motor. If some protective devices have the function of overload instantaneous action, the action current shall be greater than the peak value of dynamic current to enable the motor to start normally.
(3) The action time of the overload protection device shall be less than the thermal characteristics of the conductor to play the backup protection function of the power supply line.