Non-contact transducer measures torque variation at 1500rpm

The Eco Rating stickers that provide potential buyers with a clear indication of white goods' environmental performance now play a major role in retail sales. Manufactures want to achieve the best classification possible, so are redesigning their machines to reduce power consumption.

Technical Solution of Stoke-on-Trent began looking at the issue and came up with several initiatives. One idea involved technology transfer from other fields of engineering.

Technical Director Andrew Stephen explains: "Washing machines' energy is delivered through their main drive motor and we reckoned that there may be some savings using sophisticated motor control. Front-loaders are, of course, horizontal axis machines, so the load - ie the wet laundry - is being lifted on one side of the axis and falling on the other side. We wondered if this offered possibilities for regenerative energy recovery, a technique we have used in many industrial projects."

Test rig

Stephen knew that considerable research would be required, so he built a test rig with which he could subject washing machine system to extensive tests using an industry-standard inverter to simulate the various washing cycles. A critical element of the programme was the ability to make continuous accurate torque measurements, and for this he used a TorqSense transducer from Sensor Technology.

The initial attraction to TorqSense was that it is easy to install and use because it does not use slip rings to contact the motor shaft, but rather a non-contact radio frequency link.

"We have to glue a couple of tiny piezoelectric combs onto each test shaft, but that takes a matter of seconds. The time saving over a big project is measurable and significant, but far more important is the avoidance of frustration. With slip rings, test engineers can end up tearing their hair out."

One of the advantages of the TorqSense's non-contact operation was that it did not add an extra drag load to the results, nor did it affect the dynamic changes in torque as the load went over top-dead-centre. The former is a constant error, which Stephens could have calculated out, but the latter could have made it impossible to collect the highly accurate, highly time-resolved torque data necessary for success.

Switching point

By measuring the torque change he could define the exact moment when to switch the drive from power to regeneration and make the most of the potential energy released by the falling load. Given that the motor could be rotating at up to 1500rpm, this called for very accurate data collection and equally responsive control programmes.

From here he could select the best flux curve for each motor and each washing cycle tested.

Stephen says: "Our findings were that this technique is definitely worth building into next-generation washing machines, and many of the leading manufacturers will be doing this. As you would expect, savings varied between the different motors and cycles; with a half load of lightweight nylon items, the saving is negligible, but with a full load of heavy cottons plus a drum load of water it is a different matter, with 15 per cent energy reduction. This would actually take a few pounds off the average household electricity bill, and help manufacturers achieve Class 1 Eco rating.

"But far more importantly, multiplying it up through the whole population, the annual carbon reductions are very meaningful."

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