The desire for accurate speed control of Banbury mixers is nothing new, but making a choice between AC and DC drives has become more of a challenge. The introduction of GE Toshiba's Dura-Bilt5i MV drive is helping key decision makers in the rubber mixing business to make that choice. The picture below shows a 900hp (671kW) Dura-Bilt5i MV drive including an integral transformer, fused switch and vacuum contactor. Dimensions for the drive shown below are 104" H x 74" W x 44" D (2634mm x 1880mm x 1102mm).
The application of adjustable speed AC motors to Banbury mixers has been shown, through experience, to have many advantages over DC motors. Reduced maintenance and increased efficiency are two advantages of AC adjustable speed motors versus DC adjustable speed drives. Historically, DC motors have been used in constant torque applications, such as Banbury mixers because accurate speed control was required. Recently, however, the development of AC drive vector control technology to constant torque processes requiring accurate speed control has greatly improved. This technology, combined with the excellent performance of AC drive systems in the hostile rubber mixing environment makes the AC adjustable speed drive the clear choice today.
BETTER ENVIRONMENTAL FIT
Experience has shown that AC motors are better suited for hostile environments, like the mixing area of a rubber processing plant, than DC motors. Carbon dust pervades even the cleanest rubber mixing environments and degrades the life of electrical equipment because carbon is an electrical conductor. A DC motor, which has brushes and a commutator, is particularly susceptible to the damaging effects of carbon dust. Another disadvantage of the DC motor in the carbon inundated atmosphere of rubber processing plants is that is necessary to replace brushes more frequently than in most other environments. Therefore, the use of AC motors in the rubber mixing industry eliminates the need for the disrupting replacement and maintenance of brushes. As the electrical maintenance personnel of any rubber mixing plant can attest to, DC motors used on Banbury mixers require more maintenance than other applications in the plant. More inconvenient than the brush maintenance required are failures that can be caused by the carbon dust buildup in a DC motor. AC motors, which don't have brushes or commutators, are proven to have extremely high reliability and require virtually no maintenance beyond routine motor maintenance.
ADJUSTABLE SPEED DRIVES
The Dura-Bilt5i MV drive utilizes MV IGBTs, which are selectively turned on and off in order to produce a voltage that resembles a sine wave to the motor. This technique is called Pulse Width Modulation, more commonly known as "PWM".
The Dura-Bilt5i MV drive can be used to achieve adjustable speed on new and existing Banbury mixers in the range of 400 to 5,000 hp at voltage levels of 2300 to 4160 volts. Higher ratings are also available by utilizing two drives, synchronized to provide the higher power that is required. The Dura-Bilt5i MV drive can be used with either an induction or a synchronous motor. The Dura-Bilt5i MV PWM drive minimizes harmonic currents, which add heat, in the motor. The Dura-Bilt5i MV PWM drive also uses a multi-level step output, which helps to eliminate voltage stresses on the motor insulation compared to conventional PWM drives. The figure below shows the voltage and current waveform of a motor with PWM multi-level output.
This combination of PWM control and multi-level output establishes the Dura-Bilt5i MV as the best medium voltage AC adjustable speed drive in the industry today.
IMPROVED POWER CONSUMPTION
The GE Toshiba Dura-Bilt5i MV drive maintains a power factor of .95 or greater over the entire speed range of the motor. In contrast, a DC drive will have a typical power factor of .50 to .85 over the nominal mixer speed range. In some cases, the utility may place an additional charge on the user because of the poor power factor. Also, in a new installation, the poorer power factor may require larger, more expensive cables to be pulled. Additionally, GE Toshiba's Dura-Bilt5i MV adjustable speed drive is capable of providing the same high starting torque to start the mixer while it is loaded, as DC drive systems of the past were able to do.
The Dura-Bilt5i MV drive uses a 24-pulse input diode rectifier to convert the utility power into a DC buss for the inversion to adjustable frequency by the inverter. This 24-pulse input is significant as it produces less than 3% total harmonic distortion, to the point of common coupling, in most applications.
SUPERIOR MIX CONTROL
Two key challenges in rubber mixing are overcome with superior performance by adjustable speed drives. These two challenges are (1) constant speed ride through of impact loading and (2) speed-change on the fly.
Constant speed ride-through of impact loading helps ensure that all the ingredients are thoroughly mixed. Thorough mixing of the batch is necessary to give the final product uniform pliability with no soft or brittle spots. Batch consistency is very important to the quality of the end product. Because the Dura-Bilt5i MV adjustable speed drive is a vector controlled drive, it will keep mixer speed within less than 0.5% of set speed with impact loading of up to 1.5 p.u. (which is more than adequate for nearly all mixer applications). This is all done without the need for a speed feedback device. Torque greater than 1.5 p.u. is available as required by the mixer manufacturer's design.
The ability to change speed during the mixing process allows the operator to prevent premature vulcanization and it enhances the quality of the rubber used for the end products. Because of the vector control, the Dura-Bilt5i MV can respond to speed reference changes over the full operation range quicker than previous designs of AC adjustable speed drives. The process speed can be set manually by an operator, an analog process signal, a PLC, or other controller utilizing the built in LAN connection in the drive. This LAN connection can be used to obtain data from the drive and send it back to the controller for process controls or for diagnostic purposes.
PROVEN EXPERIENCE
You can count on GE Toshiba to have the unmatched application and drive knowledge for your process, having provided more than 30 medium voltage drives for rubber mixers over the past decade. This knowledge is backed up by one of the largest service organizations available to take care of any needs that arise once the drive is installed. In addition to the service organization, GE's remote diagnostic center is available 24/7 to complement your service engineering needs. With optional on-site service available with the Drua-Bilt5i MV drive, the GE OnSite SupportSM remote monitoring and diagnostics center can connect, via modem, to your drive and aid you in troubleshooting, should the need arise.
Every Dura-Bilt5i MV is backed with GE Toshiba's 5-year parts and labor warranty.*
*GE Toshiba Automation Systems' warranty applies only to installations in North America.
Bill Laurenson is a Senior Application Engineer with GE Toshiba Automation Systems and has a 36-year background in the manufacturing, application and sales of adjustable speed drives for processing industries.
Banbury is a registered trademark of Farrel Corporation
Dura-Bilt 5i MV is a registered trademark of GE Toshiba Automation Systems, LLC
OnSite Support is a servicemark of General Electric Company
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