A centrifugal dryer is installed in the lithium slag recycling of a catalyst workshop in Henan Province. Its working principle is similar to that of the dryer in a household washing machine. It is specially responsible for dehydrating and drying lithium carbonate, a product containing a lot of water. The dryer was originally equipped with a 4-pole asynchronous motor of 5.5kW, and the speed of the drying barrel was about 1200r/min, but the drying effect was not ideal. Later, the speed was increased (1600r/min) and the motor power was increased (7.5kW). Although the dehydration effect was good, the motor often tripped due to overload.
After inspection and measurement, the motor's operating current reached 30A after starting, which lasted for about 2 minutes, and then the current dropped and stabilized at about 10A. This is because at the beginning of the motor starting, the product has a high water content, so the load is heavy and the motor operating current is also large. After dehydration, the load is reduced, and the motor operating current is also reduced. In practice, the motor is overloaded only during a period of time when the product has a high water content. We imagine: if we can use variable-frequency drive (VFD) to make the motor run at a lower speed during a period of time when the product has a high water content, so that it can first remove some water, and then increase the motor speed when its load is reduced; or appropriately extend the motor starting process, during the long motor starting process, first remove some water from the product, and then the motor enters normal operation to ensure the dehydration effect. Fuji VFD was used, and the plan of extending the motor starting time was used, so that there is no need to replay the control line, and the original motor control button can be used.
Improvement of existing problems
Soon after the variable frequency speed control device was put into use, the dryer often had an overvoltage fault signal during the shutdown process. Once an overvoltage fault signal appears, the fault signal must be reset before it can continue to operate. There are two ways to stop the dryer, free shutdown and brake shutdown. The overvoltage fault signal appears 3 to 5 seconds after the motor stops freely, that is, the stop button is pressed.
The analysis shows that the centrifugal dryer has a large moment of inertia. When it stops freely, the motor decelerates according to the deceleration time set in the VFD, but the spin dryer still runs at the original speed. The motor is forced to run at a speed higher than the corresponding output frequency. At this time, the motor is actually running in the generator state, so the motor sends power back to the VFD.
The inverter immediately cuts off the power for its own protection needs and sends an overvoltage fault signal. When the brake stops, the speed of the spin dryer is rapidly reduced, which will not cause the motor to run in the generator state, so there will be no overvoltage fault. If the motor deceleration time is appropriately extended, the overvoltage fault problem is solved, but it is very unfavorable for the brake stop of the dryer. Because after pressing the stop button, the motor is not actually powered off, but is in the process of gradually decreasing voltage. At this time, mechanical braking is used, which is not only difficult to brake, but may also cause the motor to have an overcurrent fault during the shutdown process. We use the method of free parking plus mechanical braking to better solve this problem.