The Sugar Mill Process the crystals are separated from the syrup by means of a centrifuge. The separation is accomplished by the centrifugal forces set up. The centrifuge is started to a speed of around 200 rpm at which the charging of syrup takes place. During charging the motor is disconnected from the supply. The centrifuge is spun at speeds of 500 and 1000 rpm. The speed is then reduced in steps to about 50 rpm, at which ploughing takes place. To reduce the energy lost during starting and braking of the motor the centrifugal action is performed at different speeds.
The motor used to drive the centrifuge must be a variable speed motor. The centrifugal action is performed to reduce the energy loss due to acceleration and braking. The regenerative braking may be advantageously employed. Normally two stage acceleration and braking are employed. While reducing the speed the regenerative braking is employed till the lowest speed is obtained, where ploughing takes place.
Pole changing induction motors are suitable for the purpose. A large pole winding is switched on to get a speed of 200 rpm, at which charging takes place. After the charging the next pole winding is switched on to get the next higher speed. In two stages the highest spinning speed is achieved. The motor can then be switched on to lower speeds and the regenerative brakings may be obtained in stages. Thus pole change motors give definite speeds of operation. Also, when switched from higher speed to lower speed regenerative braking may be accomplished.
A high resistance rotor induction motor with stator voltage control is suitable to drive the centrifuge of the Sugar Mill Process. Due to high rotor resistance, stable operation of motor is possible in the complete speed range from zero to rated (synchronous) speed. The voltage control is used for speed control in the first quadrant. Solid rotor induction motors have high inherent rotor resistance and are economical when employed as drives for centrifuges. The stator voltage control can be accomplished by means of an ac voltage controller using antiparallel thyristors. The motor can be controlled in speed and run at desired speed; it can be accelerated and braked.
The high rotor resistance of a squirrel cage motor, even though it provides a stable characteristic, has poor efficiency. Hence slip ring induction motors with variable resistance in the rotor circuit and variable voltage on the stator are a satisfactory drive system giving a reasonable efficiency under running conditions. The braking when these drives are employed can be dc dynamic braking. No regenerative braking is possible since the energy is lost in the external resistance and cannot be regenerated to the mains.
With all the types discussed above an automatic speed control system to suit the duty cycle can be employed.
Alternately, converter fed induction or synchronous motors may also be used. A static frequency converter (dc link or cycloconverter) may be employed to control the drive motor. By this, soft starting and operation at definite speeds are possible. The speed control range is wide and regenerative braking is possible.
Motor design When pole change motors are used, they require special design to withstand the fluctuations of current when switching over takes place. The line also is subjected to disturbances. Special starting equipment is required. When pole change motors or high resistance cage motors are used the current fluctuations may cause heating of the motor. Protective devices may be required against overheating. Special starting current peaks may be there when an induction motor controlled from a voltage controller is employed. Because of low flux levels at lower speeds, considerable derating of the motor is required. The derating is also due to non-sinusoidal currents. A considerably large size motor is required. Special starting may not be required as the voltage controller employed can be used for starting also. A slip ring induction motor with rotor resistance does not have high peak currents at starting. No special design may be required.
When fed from a variable frequency supply using static frequency controller, the motor is versatile and free from all the difficulties stated above. The motor does not require any special design No protective devices are required if there is no heating.
The mounting of the motor is vertical. As oscillations are present on the rotor, the air gap must be large. The oscillations may be present in pole change motors due to switching. In motors supplied from thyristor converters these are presented by the non-sinusoidal current waveforms. The motors must have humid proof insulation.