The patented PLUSS technology developed by Fairford Electronics in Ivybridge uses a switching technique to allow current sharing in motor control and can be applied to a wide range of applications from soft starters to thyristor contorllers and heating systems.
The control is implemented on a on a 16bit PIC microcontroller with DSP extensions, although the code is purely in C and does not need these extensions at present, says Gary Stephens, lead engineer at Fairford Electronics (above, right).
“The system simply requires soft starters to be joined with a communications cable,” he said. “The PLUSS software on each soft starter then synchronises each starter to operate alternately on each cycle of the input power. One soft starter takes the load for one half of the cycle and the other soft starter takes over on the rest of the cycle, effectively swopping the load between the two starters up to 100 times a second. The more soft starters connected with PLUSS technology, the greater the load sharing.”
Fairford makes soft starters for large motors, and PLUSS allows multiple soft starters to power a large motor. This enables motor manufacturers to use two or three smaller soft starters rather than one large, expensive one. The current is spread evenly across the soft starters so smaller power devices can be used with less heat generated. For example, two 100 amp soft starters can be linked together with the new PLUSS system to deliver motor control up to 180 amps. “Larger power soft starters are more expensive to manufacture compared to smaller versions because of a lack of economies of scale on production, approvals and testing,” said Mark Shepherd, managing director of Fairford (above left).
This also allow redundancy as a failing soft starter can be replaced without having to shut down the motor. “The technology offers significantproductivity benefits,” said Shepherd. “The PLUSS control system means a spare soft starter can be included on critical applications, ready to take over in the event of a fault on another soft starter. Thermal load sharing between several soft starters working in parallel on one application also boosts reliability and operating lifetime of the control system.”
The technology can also be applied to thyristor based power controllers, DC link circuits and similar applications, allowing lower power, cheaper thyristors and MOSFETs to be used. The company is looking to license the technology, the code and the starter boards.