Newtown, CT -When a supplier stopped making the custom dc brush motors for Kendro Laboratory Products' medical centrifuges, Kendro faced a pressing challenge. With thousands of installed units worldwide, Kendro had to find a new motor for both field replacement and new production. But without enough time to redesign the machine, controls, or rewrite the software, the company needed a fast solution. So Kendro turned to the Two-Wire Module from Bayside Motion Group (Port Washington, NY) that simplifies control and wiring of brushless dc (BLDC) servomotors. The module let Kendro replace a brush motor with a brushless motor, yet control it with the centrifuge's existing SCR full-wave bridge controller.
Bayside's Two-Wire Module, an electronic commutation device, takes the motor and hall wires from a BLDC motor and provides two input wires that marry with a brush-type amplifier.
The challenge with BLDC motors, however, is that typically the wiring is more complex than brush-type motors. Brushless motors normally require a wire for each of the three motor phases, along with 5 wires for the Hall-effect sensors. Bayside's Two-Wire Module takes the motor and hall wires from a BLDC motor and provides two input wires that marry with a brush-type amplifier. It consists of a signal responsive stator-coil switch circuit, commutation logic, direction-polarity control circuit, and an auxiliary dc power supply. Full four-quadrant operation provides torque control through zero speed without any deadband, according to Bayside Engineering Manager Fred Moritz. Moreover, the brushless motor can operate directly from a simple dc power supply.
Kendro benefited from the brush to brushless switch in many ways, according to Kendro's Development Associate Kumar Das, P.E.. Brushless motors yield higher speed and more power per frame size than conventional brush-type motors. So the replacement motor and module fit into the existing holes in the machine making redesign unnecessary and field replacement easy. Moreover, by removing brush contact, brushless motors provide longer life with lower operating temperatures. Furthermore, the elimination of brush dust is especially advantageous in medical and biotech applications.
Used for blood, cell, and gene separation in medical and bioprocessing applications, Kendro's centrifuges spin a sample mixture, and separate the individual components using centrifugal force (or centripetal acceleration to the engineering purist). An adjustable-speed drive controls the motor, which is directly coupled to a rotating sample tray called the rotor. The rotor holds the samples for spinning, while a refrigeration system maintains constant temperature inside the centrifuge. Users set parameters based on specific protocols and monitor the process through an operator interface.
During floor-model centrifuge operation, the machine is first pre-cooled to a specific temperature with the rotor spinning at low speed. After pre-cooling, the operator loads the samples, sets temperature, speed, and duration parameters, and hits the start button. The rotor begins spinning in a controlled slow-start mode between 0-250 rpm, runs for the specified time at operating speed (much higher), then starts to slow down. From 500-0 rpm, dynamic braking provides a smooth slowdown to preserve the density gradient between separated components and minimize agitation that could mix them back together.
Achieving these smooth slow downs without rewriting the control programs was a key test. Using existing signals from Kendro's software and converting those signals to generate the proper profiles required a PLD (programmable lookup device) to adjust the deceleration profile in response to an analog voltage level from the existing controller.
The brush to brushless conversion on Kendro's desktop model centrifuge was so successful the company has applied Bayside's Two-Wire Module to other models as well. "In two years, we have converted three different floor models in the product line, and developed easy retrofit-kits for each that only take about an hour to install," says Das. "Without good communication with Bayside, this work would have been a nightmare."
Additional details...Contact Fred Moritz, Bayside Motion Group, 27 Seaview Blvd., Port Washington, NY11050; Tel: (516) 484-5482; E-mail: [email protected]
or Circle 501.