Ⅰ. Overview

This retrofitting is the agitator tank at the first-stage of extraction process of the xanthan gum production. It is used to stir the xanthan gum fermentation broth, and the faster the stirring speed, the higher the yield of the xanthan gum. In the past, an asynchronous frequency inverter was applied to control a 1500 - rpm asynchronous motor. The motor drove the impeller of the agitator tank by reducing the speed of gearbox to 150 rpm. In this project, this asynchronous inverter will be replaced with a synchronous inverter to control a 160 - rpm permanent magnet synchronous motor to drive the impeller of the agitator tank directly.

Ⅱ. Solution

1. The production process flow of xanthan gum technology

Agitator tank

2. Frequency inverter specification

According to the motor parameters, Yolico has selected the model YDS8500 T4 - E002 inverter.

3. Commissoning

Under the condition of unload on the motor shaft, the equipment proceeds a self-learning action following the motor parameters. Dr30: 2 times phase resistance, Dr31, Dr64: 2 times phase inductance.

The current and voltage waveforms when the frequency inverter runs at 39.3HZ (157rpm)

During the commissioning, it was found that the motor could not reach the rated speed under the heavy load. Even after enabling the field-weakening function, it doesn’t chance the situation. Measurements showed that the back electromotive force of the motor did not match the value provided by the manufacturer. The value was as high as 406V (the back electromotive force of a normal motor is approximately 90% of the rated voltage of the motor). When the input voltage of the frequency inverter remains at 395V, the busbar voltage of the frequency inverter drops from the normal 550V to around 510V under heavy load. Which means, when the motor approaches the rated speed and the output voltage is less than the value of back electromotive force, the motor will be out of control.

Therefore, Yolico suggests when encounter this situation, there are two solutions:
① Install a transformer at the front end of the frequency inverter to increase the input voltage;
② Use a four-quadrant frequency inverter to stabilize the busbar voltage.

The waveform of motor back electromotive force

4. On-site operation situation

Due to the reason of the motor back electromotive force, during normal production, the maximum operating speed of the motor is set at 155 rpm. This speed is higher than the previous 150 rpm, resulting in higher production efficiency. Currently, the equipment has been operating stably for half a month.

The working cycle of the agitator tank is 72 hours. Within each working cycle, the operating speed is 80 rpm in the first 24 hours, 100 rpm in the second 24 hours, and 155 rpm in the third 24 hours.

Ⅲ. Features of the Solution

The permanent magnet synchronous motor can operate in a state of high power factor and high efficiency within the range of 25% to 120% of the rated power, whereas a general three-phase asynchronous motor can usually only maintain this state within the range of 60% to 100% of the rated power. The consumption of the frequency inverter at full load will not exceed 3%.

Ⅳ. Conclusion

There are 30 identical agitator tanks on-site. To observe the energy-saving effect, smart meters were installed both on the retrofitted frequency inverters and the existing frequency inverters. A comparison after a production cycle, it shows 2400 kWh of electricity was saved (800 kWh per day). According to this number , if all the agitator tanks in the factory are replaced with direct-drive permanent magnet synchronous motors, and the industrial electricity price is RMB0.5 per kWh, the annual electricity savings will be 800×30×365×0.5 = RMB438,0000. The energy-saving effect is quite considerable.