First, the operation of the machine before the technical transformation
The sugar separation process of a sugar factory in Yunnan uses centrifugal dehydration method. The separation machine is mainly composed of a cone drum and a drive motor, and the motor shaft is directly connected to the bowl. The solid sugar, crystallized from a high concentration of sugar water, is dehydrated in a cone bowl. The process of processing is: the motor starts to increase the speed from zero, then the material also begins to inject into the drum, it takes about 1 minute to rise to 210r/min, the injected material has reached 1.5 tons, then the injection is stopped, and the speed is raised to 700r/min after running for 1 minute, and then the speed is increased to 1000r/min, after 4 minutes of operation, the water has been all thrown dry, and the speed is gradually reduced to stop, and the speed reduction process is about 2 minutes. The entire process takes 8 minutes.
Prior to the retrofit, the motor of choice was a 2-4-8-6 pole multi-pole motor. The speed change of the multi-pole motor is abrupt, and the frequent speed rise and decrease have a greater impact on the power grid and machinery, and the result is high maintenance costs and large strike losses.
Second, the separation of the machine frequency conversion speed control
The communication inverter is used to drive the general three-phase squirrel cage motor, which can meet the requirements of the sugar separation process.
Considering that the mechanical characteristics of the separation machine belong to the constant torque and large inertia load, the inverter with constant torque characteristics should be selected and the brake brake unit should be configured.
The inverter is Sunland BT40 110kW. This kind of inverter has unique functions such as quasi-superconducting control, low-speed and high-torque output, slip compensation, and AVR active voltage stabilization operation. Consistent with the original operation method, multi-stage speed control is selected.
3. Brake unit and resistance
1. Energy consumption braking: because the load is heavy (1.5 tons of material), after accelerating to 1000r/min, it is necessary to stop within 2 minutes, it is necessary to install a braking unit with a capacity equivalent to the capacity of the inverter, and the braking unit is actually a voltage hysteresis loop switch. When the motor slows down, the kinetic energy of the load is very large, and the voltage added to the rotor of the motor supplies a magnetic field to the motor, and the motor becomes a three-phase communication generator, and the electricity generated by the generator is rectified by the diode on the inverter bridge and becomes DC to charge the capacitor. If the current can be observed, the current flow direction is opposite to before the speed reduction, indicating that the energy is returned to the inverter by the load, when the charging voltage of the capacitor reaches 710V, the brake unit switch is turned over, the current flows to the braking resistor, and the electric energy is dissipated in the form of electric heat. Because the energy is dissipated, the capacitor voltage drops, and when it drops to 680V, the braking unit shuts down. As long as the braking process is not finished, the braking unit will repeatedly turn on and off, so that the load will drop to zero quickly at a steady speed.
2. Accounting of braking resistance: in the case of braking resistance braking, the part of the electric motor with power dissipation loss is converted into braking torque, which is about 20% of the additional torque of the motor. Therefore, the resistance value of the braking resistor can be calculated using the following formula:
where: UC is the action voltage value of the braking unit, which is now 710V; TB is the braking torque; TM is the additional torque of the motor; N is the speed of the motor at the start deceleration, in this case 100r/min.
In the discharge circuit composed of the braking unit and the braking resistor, the maximum current is constrained by the maximum promised current IC of the braking unit ICBG, and the minimum promised value of the braking resistor is:
Rmin=UC/IC
Thus so-so
Rmin﹤RB(5Ω)﹤RBO
3. Determination of braking resistance: depending on whether the motor is repeatedly decelerating, the selection of additional power of braking resistor is different, in this case, the additional power of the resistor is 24kW, natural cooling; If forced air cooling is chosen, the additional power of the resistor can be reduced. The general braking resistor is a non-inductive resistor with two wires and winding, and of course, a general box-type resistor can also be used, but a freewheeling diode must be connected at both ends of the resistor, and the diode is a fast recovery diode, and the withstand voltage is more than 1000V.
Fourth, the actual operation effect
After the successful transformation of the off-line machine, there has never been any problem, and the loss of fault strike has been reduced to zero, saving a lot of maintenance costs. The successful use of Sunland inverter in sugar separation machine fully shows that other industries such as pesticide production, material dehydration, mineral processing and other industries can use Sunland inverter to transform and improve the performance of the whole machine.
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