Brush to brushless adapter solves motor problem

Edited by the Engineeringtalk editorial team Aug 24, 2000

A Bayside adapter module let Kendro Laboratory Products replace a brush motor with a brushless motor, yet control it with an existing SCR full-wave bridge controller

When a supplier stopped making the custom dc brush motors for Kendro Laboratory Products' medical centrifuges, the company faced a pressing challenge.

With thousands of installed units world-wide, Kendro had to quickly find a new motor for both field replacement and new production without involving redesign of the machine, controls, or rewriting the software.

The solution turned out to be an adapter that simplifies both control and wiring for brushless dc (BLDC) servomotors.

Supplied by Bayside Motion Group, the adapter module let Kendro replace a brush motor with a brushless motor, yet control it with the centrifuge's existing SCR full-wave bridge controller.

Normally with BLDC motors the wiring is more complex than for brush-type motors.

Brushless motors typically require a wire for each of the three motor phases, along with 5 wires for the Hall-effect sensors.

However, 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 dead-band, 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, PE Brushless motors yield higher speed and more power per frame size than conventional brush-type motors, consequently the replacement motor and module fit into the existing space envelope in the machine, making redesign unnecessary and field replacement easy.

In addition, by removing brush contact, brushless motors provide longer life with lower operating temperatures, whist also eliminating brush dust, especially advantageous in medical and biotech applications.

Used for blood, cell, and gene separation in medical and bio-processing 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 coupled directly 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 centrifuge 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 minimise 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 analogue 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.".