Matrix Converter Creates New Opportunities for Centrifuge Retrofit

Motor controller handles frequent power regeneration, harsh environment
By Brendon R. Fritz | May 10, 2012

Chief Ethanol Fuels Inc. was Nebraska’s first dry-mill ethanol plant, beginning production in 1985 in Hastings. The plant currently produces approximately 62 million gallons of ethanol and more than 500,000 tons of feed annually. Throughout the years, Chief Ethanol has remained committed to implementing new technologies to keep pace with evolving industry innovations. For a recent project, Chief wanted to update the back drive system on one of its Sharples Horizontal Centrifuges. The centrifuge was originally supplied with a 150-horsepower main drive motor and an eddy current back drive. The eddy current drive unit had failed and Chief wanted to upgrade the unit to an electric motor control.

Several years earlier, Chief Ethanol had implemented a retrofit with three out of four of its centrifuges, consisting of a closed loop vector variable frequency drive (VFD) and separate regeneration converter to handle the frequent regeneration that occurs on back-drive systems. Due to the complexity and frequent issues with the VFD/regen retrofit, Chief approached Automated Drive Systems LLC (formerly Northwest Electric’s advanced controls group) for recommendations on a simpler, more robust solution.

The project goals included:
• Provide a motor controller with ability to handle frequent power regeneration.
• Eliminate frequent encoder feedback issues due to harsh environment.
• Supply a package capable of withstanding the harsh environment including ambient temperatures of up to 120 degrees Fahrenheit during summer months.
• Keep harmonics to within IEEE 519-1992 recommendations without filtering or isolation transformers.
• Keep programming and troubleshooting familiar for plant personnel.

Centrifuge Operation, Power Regeneration
A centrifuge operates by using centrifugal force to separate solids from liquids in a continuous process. Centrifuges consist of a bowl or main drive and a continuous scroll conveyor or back-drive system. The rotation of the material causes the denser solid particles to be pressed outward against plates while the less dense liquid forms an inner layer, sometimes referred to as a pond. The solid particles are continuously removed by a scroll conveyor, which operates at a slower speed than the bowl. The operator adjusts the speed of the back drive in order to adjust variation of the solids. Due to the forces of the main bowl drive operating at a higher speed, the back drive typically needs to be held back, versus being driven, in order to reach the desired speed. In an electric motor, when the output is driven faster than the operating speed the motor is turned into an induction generator and the motor will generate power back through the motor leads. This is typically referred to as regeneration.

With the project goals established, Automated Drive Systems looked at other common methods of controlling the back drive in centrifuge applications and found two common methods. The first method is using a hydraulic or eddy current braking mechanism similar to that found on the centrifuge when supplied by the manufacturer. These types of systems use hydraulics or an electric braking mechanism to control the speed of the back drive scroll. These systems are usually quite simple although they miss an opportunity to increase the overall efficiency since the braking power is dissipated as heat.

The other common method is to use an electric motor and variable-speed drive to control the speed of the back drive. A major drawback with this option is that variable frequency drives do not handle regeneration without additional equipment working in one of two ways. The first method, which can be used when the main drive is also controlled by a variable speed drive, is to connect the two drives through the internal DC circuits. With this method, when the back drive motor regenerates power, the power is transferred to the main drive VFD and consumed. This method is very expensive, as the main drive motors are typically three to five times the size of the back drive. The other common method is to place a regeneration unit in front of the back drive VFD that allows the regeneration power to flow back to the power grid in the plant. These systems are generally large and add another potential failure point.

A New Opportunity
For this project, an AC motor was recommended to capture the regenerated power and increase the overall efficiency of the centrifuge, using a new technology, the Yaskawa AC7 Matrix Converter. The AC7 is the first low-voltage drive control to use matrix technology. A standard VFD consists of three major power sections: a set of diodes that convert AC power to a fixed DC voltage, a DC bus section which has capacitors that smooth the DC power and, finally, a transistor output section that powers the motor. In contrast, the matrix converter uses nine bidirectional transistor switches that create variable voltage and frequency directly from a 3-phase AC power supply. This design allows for power to flow both directions through the unit, eliminating the need for separate regeneration converter units or connection to another VFD.

The Yaskawa AC7 Matrix converter is capable of both sensorless and closed-loop vector control using an encoder. For this application, it was determined that the sensorless vector control scheme would be used to avoid encoder feedback from the harsh environment. The environmental conditions also posed another design challenge, with ambient temperatures in the area exceeding 120 F at times during the summer. In addition to providing a single package for motoring and regeneration, the AC7 Matrix is highly efficient. With a heat loss of approximately 58 percent versus an installation with a standard VFD, regeneration converter and harmonic filtering, the Matrix converter allowed for selection of a smaller cabinet air conditioner, reducing project cost by nearly $850.

With the unique design of the matrix converter, the unit easily satisfies harmonic guidelines that are important for maintaining a reliable power system. Harmonics are electrical noise phenomena created by digital power devices like VFDs or lighting ballasts. These devices draw power in quick bursts versus in a sine wave pattern like an electric motor. If not corrected, this electrical noise can cause issues with transformers, sensors and other power distribution equipment in a variety of ways. With the matrix converter, each output phase is sourced from three input phases and the current wave form is sinusoidal, unlike the typical double crest waveform for diode-rectified VFDs. This design results in an input current distortion of only 5 to 7 percent THD. This low harmonic distortion eliminates the need for input filtering or isolation, further reducing project costs.

The final project goal of programming and troubleshooting familiarity was easy to satisfy. The AC7 features the same operator display unit as other 7-series Yaskawa products installed at the plant. With the familiar menu structure and many common parameters, the Matrix converter provided a solution requiring minimal operator and technician training.

As part of project commissioning, Chief Ethanol chose Automated Drive Systems to perform an authorized service provider commissioning service, which doubled the Yaskawa Factory warranty. “Using the Yaskawa AC7 Matrix Converter, we were able purchase a package that met all of our requirements in a single unit. This resulted in a system with fewer components, higher efficiency and familiar programming, all at a cost several thousand dollars less than competitive line regeneration VFDs or VFD/Regen packages,” says Jerry Dick, maintenance technician at Chief Ethanol.

Using a Yaskawa AC7 Matrix Converter provides a new opportunity for users of centrifuges to control the speed of the back drive units. This technology successfully provides control, simplicity and reliability in a single, robust solution while maintaining the highest efficiency possible.

Author: Brenden R. Fritz
President, Automated Drive Systems LLC
(402) 858-5560


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