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Magnetic coupling is `cheaper than a VSD`

01 May, 2001

Magnetic coupling is `cheaper than a VSD`

A US company has developed a magnetically-based coupling technology that it claims can vary the speed of a driven load at a lower cost than electronic variable speed drives. Seattle-based MagnaDrive claims that its low-maintenance, energy-saving technology has a lifetime cost that is about half that of a typical VSD.

The MagnaDrive coupling consists of two main parts: a copper conductor assembly mounted on the motor shaft; and a precision rotor assembly containing rare-earth magnets, mounted on the drive shaft. There is no mechanical connection between the two parts; the relative motion of the two parts induces a magnetic coupling across the air gap between them, and varying the size of this gap controls the speed of the load. An actuator varies the airgap from 3-25mm in response to 4-20mA, 1-5V DC, 0-10V DC, or 3-15psi control signals.

Because there has to be relative movement between the two parts for magnetic induction to occur, the output speed is always less than the motor speed, with a typical slip at full speed of 1-4%. The output torque of the coupling is always equal to the input torque and the motor only needs to deliver the torque required by the load.

The coupling can also act as a soft-start or stop, allowing the load to be accelerated and decelerated slowly. The motor can thus be sized for optimal operating torque rather than a high starting torque.

MagnaDrive claims that for speeds in the range 90-99% of the rated motor speed, its technology is more efficient than a VSD. It adds that in a fan or pump installation running at 80-100% of full speed, there is little difference between the efficiencies of a MagnaDrive or a VSD. And even though the VSD is more efficient at lower speeds, the company asserts that the MagnaDrive will be more reliable and cost less to maintain over its 15-30 year life. It is particularly attractive to use the technology with MV and HV motors, the company adds.

MagnaDrive claims that its technology has several other advantages. For example, its performance is not affected by minor angular or offset alignments (up to around 5mm) between the motor and the load. Vibration (and hence wear) caused by misalignments is almost eliminated. And, unlike a VSD, the mechanical system does not generate any electromagnetic interference or harmonic distortion. It also has a higher power factor than a VSD-driven motor.

Although the MagnaDrive technology may appear similar to an eddy current drive, the company claims there are significant differences. For example, because an eddy current drive needs power to energise its electromagnets continuously, it is 10-15% less efficient than the magnet-based system. Also, eddy current drives can suffer from vibration.

At present, MagnaDrive is offering its coupling in versions for motors rated from 25-550hp (18.7-410kW) to operate at speeds of 720-3,600 rpm. The couplings cost from $8,000-120,000. The company has recently won a $500,000 grant from the US Energy Department to help raise the power range to 1,500hp (1.12MW).

More than 35 MagnaDrive systems have already been installed at sites including paper mills, food plants, mines, and waterworks. They are said to have produced energy savings of up to 66%.

The company has just moved from one-off assembly to series production and is also marketing dynamometers based on the same technology. A version offering over-torque protection is due soon and the company is studying transport applications such as clutches and brakes.

MagnaDrive was founded in 1999 by Ron Woodward, a former head of Boeing Commercial Airplanes. It has already raised $8m in private financing and $2.1m in funding from the Northwest Energy Efficiency Alliance. Its revenues to date have been around $1.5m and Woodward expects sales of $4-5m this year.

So far, Magnadrive as focussed its efforts on the US market, but plans to enter the European arena by the end of this year.

The MagnaDrive idea was invented by Jerry Lamb whose company, MagnaForce, has a 20% stake in MagnaDrive. Lamb has also licensed his technology to the Invensys subsidiary Rexnord which is marketing a similar product under the Rex MagneLink name.

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