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issue: October 2006 APPLIANCE Magazine - Part 2: Motors & Air-Moving Devices

Motors and Air-Moving Devices
A Motor Ahead of its Time


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by Erin Biesen, Associate Editor

APPLIANCE magazine first reported on the Smart Drive motor in the August 1992 Motors and Air-Moving Devices Part II. Over the years, the motor has proved it could withstand the test of time and even cross the Pacific Ocean to broaden its market.

The Smart Drive motor is a direct drive, three-phase permanent magnet motor that has withstood the test of time.

Knocking Noise

While there have been some modifications over the years, the general concept of the motor has remained the same.
Fifteen years ago, Auckland, New Zealand-based Fisher and Paykel Appliances Ltd. set out to design a range of products to suit various markets, one of them being the U.S. The company brought its products to the U.S. in the 1990s and grew the business until the U.S. became its biggest regional market. It made sense to move manufacturing to the country where it sells most of its products. In February 2006 the company opened its Clyde, Ohio, U.S. plant, which manufacturers washing machines, clothes dryers and Smart Drive motors.
“We designed a range of sizes, models of washers and equipment that would be able to make that complete range. The production machinery side of the company also developed the machinery in a modular form so that it could be shipped practically,” says Nairn Henderson, engineering manager of the Clyde factory. “The strategy has been for a long time to be able to build plants in New Zealand and move them to the markets where demand is strong, and our market growth in the U.S. has justified us doing that. We moved the washer plant from Australia and the motor and dryer plants from New Zealand to support the demand and reduce the shipping costs.”
When the company began to design this washing machine platform, it wanted to completely understand the wash process. It quickly learned that due to various types of load sizes and wash requirements there was no one way to wash clothes. “It lead us in the direction of wanting to have a computer-controlled motor, and we went looking for a production version,” says Henderson. But in the early 1990s, electronically controlled washer drives were not common. “We eventually came to the conclusion that there is actually no one around that was going down that path, so we said ‘Ok, no one is doing it, we will do it ourselves’.”
The Smart Drive motor is a direct drive, three-phase permanent magnet motor, with the stator integrated on to the outer bowl of the washing tub. The rotor is directly coupled to both the agitator and the spin bowl. The spin bowl is de-coupled from the drive system by a method the company developed, which involves floating the bowl in the wash water for the agitation phase of the wash.
“The wash action is controlled electronically using sensors that are mounted in the stator itself, and they are always monitoring the position of the rotor or the magnetic field of the rotor,” explains Keith Ferguson, product design engineer for electric motors in New Zealand. “Through the electronics the sensors control the speed and direction of the rotor for agitation.”
These basic functions are unchanged from the original Smart Drive motor. Two changes that have been made were in the materials.
One of the more unique features of the Smart Drive motor is that it has an all-plastic construction. “When I designed it to be constructed in plastic, everyone said ‘Are you crazy? That will never work’,” Ferguson says. “And that is always an incentive to make sure you achieve your goal, and we did achieve it.”
In 1994 the company made a cost savings change when it switched from a polyethylene terephthalate (PET) material to a polybutylene terephthalate material for the molding that coats the steel stator.
Then the company changed the rotor from the PET material to a glass filled polypropylene. “That was a cost savings, but it was also done in conjunction with a change of design from neodynium magnets to ferrite magnets,” notes Ferguson. “ The ferrite magnets gave us a substantial strength increase in the rotor so we were able to go to a much less expensive material.”
The cross section of magnetic wire has gone down in size twice over the years. First it went from 1 mm to 0.71 mm, and then eventually from 0.71 mm to 0.53 mm. Electronic devices improved and began to switch at higher voltages, which allowed the company to use a lower current because it had more turns per pole with the smaller wire, and reducing the switching current even more resulting in the opportunity to use even less expensive electronic devices.
Finally, the company changed the number of magnetic poles on the rotor in relation to the wound poles on the stator, which changed the ratio from 42:56 to 36:48 poles to reduce the cogging torque. Fisher and Paykel uses a similar motor in its dish drawers and says it could be used in clothes dryer applications as well.
In 15 years the Smart Drive motor has experienced several changes and modifications, and the technology is still ahead of its time. “In our machine you can load the clothes in, add some detergent, push the go button, and the machine will determine the rest for you,” says Henderson. “It will sort out the water level, it will sort out what cycle best suits those garments and wash them and spin them out appropriately.”

Suppliers mentioned in this article:
Fisher and Paykel Appliances, Inc.
 

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