In the variable frequency speed regulation system, the speed reduction and shutdown of the motor are realized by gradually reducing the frequency. At the moment when the frequency is reduced, the synchronous speed of the motor decreases. However, due to mechanical inertia, the rotor speed of the motor is not change. When the synchronous speed is less than the rotor speed, the phase of the rotor current almost changes 180 degrees, and the motor changes from the electric state to the power generation state; at the same time, the torque on the motor shaft becomes the braking torque, making the motor speed fast When descending, the motor is in regenerative braking state. The electric energy regenerated by the motor is fed back to the DC circuit after full-wave rectification by the freewheeling diode. Since the electrical energy of the DC circuit cannot be fed back to the grid through the rectifier bridge, it is only absorbed by the capacitor of the inverter itself. Although other parts can consume electrical energy, the capacitor still accumulates for a short period of time, forming a "pumping voltage" and increasing the DC voltage. high. Excessive DC voltage will damage all parts of the device.
Therefore, necessary measures must be taken to deal with this part of the regenerative energy when the load is in the state of dynamic braking. Methods of processing regenerative energy: dynamic braking and feedback braking.
Working mode of dynamic braking
The method of energy consumption braking is to add a discharge resistor unit component on the DC side of the inverter, and consume the regenerative electric energy on the power resistor to achieve braking. This is a direct way to deal with regenerative energy. It consumes the regenerative energy on the resistor through a special energy-consumption braking circuit and converts it into heat. Therefore, it is also called "resistance braking", which includes a braking unit. And brake resistor two parts.
Braking unit
The function of the braking unit is to connect the energy consumption circuit when the voltage Ud of the DC circuit exceeds the specified limit (such as 660V or 710V), so that the DC circuit will release energy in the form of heat after passing through the braking resistor. The braking unit can be divided into two types: built-in type and external type. The former is a general-purpose inverter suitable for low power, and the latter is suitable for high-power inverters or working conditions with special requirements for braking. In principle, there is no difference between the two. Both are used as a "switch" for turning on the braking resistor, which includes a power tube, a voltage sampling and comparison circuit, and a driving circuit.
Braking resistor
The braking resistor is a carrier used to consume the regenerative energy of the motor in the form of thermal energy. It includes two important parameters: resistance value and power capacity. Generally, two types of corrugated resistors and aluminum alloy resistors are often used in engineering: the former uses vertical surface corrugations to help heat dissipation and reduce parasitic inductance, and use high flame retardant inorganic coatings to effectively protect the resistance wire from aging and lengthening Service life; the latter resistors have weather resistance and vibration resistance, which are better than traditional ceramic frame resistors. They are widely used in high-demand and harsh industrial control environments. They are easy to install tightly, easy to attach a radiator, and have a beautiful appearance. The elevator will not be equipped with counterweight safety gear, only in the car club safety gear, and only when the pit is suspended!
The counterweight safety gear is to protect the elevator in case of overspeeding when the counterweight goes up. If the elevator car is on the top floor, the counterweight will generally be in the pit. If there is an overspeed topping, the counterweight will surpass the buffer interval. (The national standard is 150~400mm) and the buffer stroke (different buffer specifications are different). If these two protections fail to break through, then the car will continue to move upward, but our national standard also has a certain stroke The elevator company also reserves distance in addition to the distance, so the car will not rush to the top, and there will be no major safety accidents. The bottom pit is solid and there is no room for people to enter. Even if a large force is applied to the bottom There will be no safety accidents in the pit (except for the suspension of the pit)
The counterweight includes a counterweight frame and a counterweight block. The counterweight block can be placed in the middle of the counterweight frame to adjust the weight of the counterweight, which can be increased or decreased.
The role of the counterweight is to balance the car. There is a traction rope connection between the car and the counterweight frame. The traction rope drives the car up and down by the friction generated by the traction sheave on the roof and the traction rope. The role of the counterweight is to balance the weight of the car, so that the traction wheel only needs to drive the difference between the weight of the car and the counterweight to move the car up and down.
The general material is cast iron, but the weight of each piece is not easy to control (low cost), and there are also cast steel.
For the traction structure elevator, the counterweight should not be too heavy, nor should it be too light. It should be commensurate with the weight of the passenger and load car. That is, the balance coefficient of the elevator should be between 0.4-0.5 according to the regulations, that is, the weight of the counterweight must be balanced with the weight of the car plus 0.4-0.5 times the rated load of the elevator. So what is the physical meaning of the balance coefficient?
The elevator balance coefficient is a parameter that measures the amount of unbalanced state of the elevator in operation. The balance coefficient affects the output torque of the drive motor, thereby affecting the consumption of electric energy. One of the main purposes of using counterweight in traction elevators is to reduce the power of the elevator drive motor. For a traction structure, an 8-floor 8-station elevator with a rated load of one ton and a speed of 1.75m/s, a drive motor with a power of 15kw can be used. After accurate compensation of the traction wire rope, the rated load The 17-floor and 17-station elevators with a speed of 1.75m/s for one ton can also use a drive motor with a power of 15kw. This is because whether it is 8 stations on the 8th floor or 17 stations on the 17th floor, when the two elevators are in operation, the mass imbalance state quantity on the counterweight side and the car side is the same, and the torque difference formed on the traction sheave There is not much difference, so a drive motor with a power of 15kw can also be used.
The electric energy consumed in each operation of the elevator is the integral of the elevator's instantaneous power over the running time and then divided by the efficiency, that is, W=(∫PΔt)/η. It can be seen from the definition of power that the instantaneous power P output by the motor depends on the product of the motor output torque M and the motor speed η. The operating speed curve of each elevator is fixed, so the output torque M of the motor becomes the only variable that affects the output power of the elevator. It can be seen from the elevator structure that the motor output torque is directly affected by the weight of the elevator counterweight and the unbalanced state of the car. If the imbalance on both sides of the traction sheave is large, when the elevator running direction is opposite to this imbalance torque, the motor will have to pay a larger torque, and of course it will consume more electric energy. If the running direction is consistent with it, the motor is in the state of generating electricity. This part of the potential energy is lost by the heating effect of electricity and is consumed in the discharge resistor. When the elevator is running in a mass balance state between the counterweight side and the car side, the motor output torque is small, and its power and consumed electrical energy are also small.
On both sides of the elevator traction sheave, the torque ratio between the counterweight side and the car side, especially the ratio under braking conditions, is an important parameter that determines whether the traction rope and the traction sheave slips, or the elevator runs smoothly. . Then, the balance coefficient describing the unbalanced state of the elevator counterweight side and the car side is also the basis for describing this ratio. The balance coefficient is required to be between 0.4 and 0.5. If it is out of tolerance, the above-mentioned elevator failure phenomenon will be caused. Therefore, the measurement and adjustment of the elevator balance coefficient must be performed again. The method is the same as the adjustment method in the related failure.