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Test machines utilise PowerRod linear motor system

Edited by the Engineeringtalk editorial team Feb 6, 2009

The PowerRod linear motor system from Parker Hannifin is playing a vital role in a series of test machines for a new generation of spacer dampers designed for use on overhead power transmission lines.

The test machines use the PowerRod motors and Compax 3 servo drive to replace conventional motor and cam mechanisms, helping to minimise setup time and provide high levels of reliability during tests of 20 million cycles, equivalent to around 5,000km of travel.

The spacer dampers are fitted to high-voltage overhead transmission line conductors, used for long-distance transmission of electricity at 400kV from power stations to substations.

Conductors are normally arranged in bundles of two, three or four lines, with specific separations, maintained and protected from wind-induced sub-conductor oscillation energy by spacer dampers.

Spacer dampers protect the line cables from wear and premature fatigue failures, maintaining electrical and mechanical performance during onerous climatic extremes and system faults.

The new generation of Quad and Triple spacer dampers undergoing test forms part of a long-term programme to upgrade the UK power transmission network over the next decade.

The spacer dampers will offer both a longer service life than those currently in use and enhanced performance in terms of their ability to withstand Aeolian vibration, wear and climatic extremes.

A key service requirement for the manufacturer, however, was their ability to offer an operating life of at least 25 years without maintenance or loss of performance.

The reliability of the spacer damper, in particular the performance of the 'elastomer damping' elements, is therefore crucial to the continuous safe and efficient operation of transmission lines.

The spacer dampers have to provide sufficient flexibility to allow the conductors to move due to effects of wind, including expansion and contraction during extreme weather, while controlling and reducing the effects of wind energy.

This includes a phenomenon known as Aeolian vibration, which is caused by streams of air flowing over the surface of each conductor creating vortex shedding, (wind flow disturbance) that results in the downwind cable vibrating.

The semi-conductive spacer dampers are constructed from aluminium alloy, and, in the case of the Quad versions, have four connection arms.

Each of these is linked via a specially designed frame with the arm arrangement incorporating elastomer damping elements.

These attach to each sub-conductor maintaining, for example, a 500mm separation.

The test rigs are primarily designed to prove the performance and operating life of the elastomer damping elements, effectively moving each pivot arm though a predetermined series of cycles.

These include a deflection test, to ensure that the arm will not deviate beyond specific angular displacements, for example 12 either side of its neutral position; deflection forces are initially 325N, followed by continuous flexing at 260N peak, with a return to the maximum load at intermittent intervals at frequencies between 1 and 5 Hz.

These tests are repeated for up to 20 million cycles, which is the equivalent of over 25 years in service.

In the past, test rigs for similar applications tended to use an AC motor, camshaft and various bearings to test each damper.

The problem with this arrangement was that it was noisy and consisted of multiple components, so the potential for mechanical wear and failure was high.

It was also difficult to change applied forces without lengthy adjustments to the cam settings.

The solution was to use the Parker PowerRod PRA linear motor and Compax 3 servo drive solution.

This provides a simple, compact and efficient system that is virtually silent and enables easy adjustment to the applied force.

Perhaps just as importantly the PowerRod motor is supplied as an integrated system complete with encoder so that no additional measurement devices are required.

The Compax 3 interfaces with a PC - via a standard National Instruments' card - so there is simple control of the test rigs' settings.

For example, each PowerRod unit can be cycled between 1 and 5Hz, depending on the unit tested, which equates to a test time of 46 days at maximum speed, and interrupt and restart each test if required.

The PowerRod linear motor system is accurate, enabling the company to ensure that applied loads are repeatable within tight parameters, and is capable of continuous operation for extended periods often equating to many thousands of kilometres of travel, without maintenance.