1. The operation of the separator prior to the technical upgrade.

  The sugar refining and separation process at a sugar factory in Yunnan Province is conducted utilizing the centrifugal dewatering method. The centrifugal dewatering method is employed, with the separator comprising a conical rotor and a driving motor. The motor shaft is directly connected to the rotor. Within the conical rotor, solid sugar crystals that have crystallized from high-concentration sugar water undergo dewatering treatment. The operational procedure is as follows: Starting  in a state of equilibrium, the motor gradually accelerates while simultaneously injecting material into the rotor. It takes approximately one minute to reach 210r/min from steadly, during which 1.5 tons of material are injected. At this point, injection ceases and the speed increases to 700r/min for one minute before further increasing to 1000r/min for four minutes. By this stage, all water has been expelled and the speed gradually decreases until it stops completely. The entire process lasts eight minutes.

  Before the upgrade, a 24-8-6-pole multi-pole motor was utilized. The speed transition of the multi-pole motor is abrupt, and frequent fluctuations in speed result in significant impact on both the power grid and mechanical equipment, leading to elevated maintenance expenses and substantial downtime losses. 

2. Variable-frequency speed control for separators

  The utilization of AC variable frequency drives for driving a conventional three-phase squirrel cage motor can fulfill the requirements of the sugar separation process. Considering the mechanical characteristics of the separator, which falls under constant torque and high inertia load category, it is advisable to employ a variable frequency drive with constant torque features and equip it with a braking brake unit. A suitable braking unit has been chosen, along with an appropriate braking resistor to meet the separator's braking demands.

  The C9-19 series variable frequency drive rated at 110kW has been selected. This particular C9-19 variable frequency drive offers functionalities such as low-speed high torque output, torque deviation compensation, AVR automatic voltage regulation, ensuring stable operation. The operational approach remains consistent with the original method by utilizing multi-stage speed control.

3. The Brake Unit and Resistors

a) Energy-absorbing braking: Due to the relatively heavy load (1.5 tons of material), it is necessary to bring it to a stop within 2 minutes after accelerating to 1000 rpm. Therefore, an appropriately sized braking unit must be installed, matching the capacity of the inverter. The braking unit functions as a voltage hysteresis switch. During motor deceleration, the kinetic energy of the load is substantial and generates a magnetic field through voltage applied to the motor rotor. This transforms the motor into a three-phase AC generator, with electricity generated rectified by diodes on the inverter bridge into direct current and stored in capacitors. If observing current flow, its direction will be opposite to that before deceleration, indicating energy being returned from the load back into the inverter system. Once capacitor charging reaches 710V, the braking unit switch opens and directs current flow towards a braking resistor where energy dissipates as heat. As energy is consumed during this process, capacitor voltage drops and stabilizes at around 690V when turning off the braking unit. Throughout continuous brake application until complete stoppage is achieved, opening and closing cycles of the braking unit allow for smooth and rapid deceleration of loads back down to zero.

b) Calculation of braking resistor: When employing a braking resistor, the active power loss within the motor is effectively converted into braking torque, which typically amounts to approximately 20% of the motor's rated torque. The discharge circuit comprising the braking unit and braking resistor imposes limitations on the maximum current IC, as it must adhere to the constraints imposed by the IGBT of said braking unit. Additionally, it is essential to ensure that the minimum allowable value for the braking resistor (Rmin) satisfies Rmin = VC/IC.

c) Determination of braking resistors: The selection of braking resistors varies depending on whether the motor undergoes repeated deceleration. In this particular example, the braking resistor has a rated power of 24kW and utilizes natural cooling. If forced air cooling is employed, the rated power of the resistor can be reduced accordingly. Generally, double-wound non-inductive resistors are recommended for braking applications; however, box-type resistors can also be utilized. It is essential to connect a diode in parallel with the ends of the resistor to facilitate current flow. A fast-recovery diode with a voltage rating of 1000V or higher can be employed.

4. The performance in practical applications.

  After the successful implementation of the separator's transformation, no faults have been encountered, resulting in zero downtime and significant cost savings on maintenance. The successful utilization of Vekont variable-frequency drive on the sugar separator serves as a compelling demonstration that other related industries such as pesticide production, material dehydration, and mineral processing can enhance overall machine performance by incorporating variable-frequency drives.