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Drive solution overcomes costly EMC issues

15 November 2016

When a winch controlling a high-powered drill broke down on a ship mining samples off the coast of Japan a rapid solution was needed because downtime was costing the company $30,000 every day.

Due to the difficult location of winch, somewhere off the coast of Japan, sample extraction of rare metals on the ocean bed needs to be planned 20 hours in advance, to ensure that the sea conditions and tides are favourable. The rougher the waves, the more the drill moves and the higher the risk of damage to equipment.

The winch system controls the deployment of the drill to the seabed, which could be up to 3km from the ship. It needs to keep the cable taught during payout. The brake chopper unit, rated at 400KW continuous 600A and 600KW, 900A at peak, dissipates the excess DC BUS voltage fed back from the motor. 

If the winch is unable to dump the energy created during this process into the braking resistors, because of a fault in the brake chopper then the operator is not able to control the cable which could result in the loss, at the bottom of the ocean, of the drill which is worth millions of pounds. So, it was imperative that the crew were confident that the brake chopper was reliable before deploying the drill.  

Once the drill is in the water, the application provides active heave, compensating for the wave movement by automatically driving the winch in the opposite direction at the same speed to stabilise the drill. The drill has to remain at a constant height from the sea floor so that it can clamp in position.  

Problems with the winch began to appear just before the company deployed the drill for the first time. Onboard engineers found the fault to be the result of DC BUS overvoltage issue caused by the brake chopper. The engineers tried, unsuccessfully, to alleviate the problem by replacing the brake chopper like-for-like.  

After calling a representative from the brake chopper manufacturer, it was agreed that an engineer would visit as soon as possible. Unfortunately this did not happen, leaving the project losing tens of thousands of dollars every day and those on board no closer to rectifying the problem. 
When the drill had been out of action for two weeks and the crew were at their wits' end they made the decision to try calling of CP Automation for a solution.

Because time was of the essence, CP Automation flew an engineer to Japan as soon as possible. The application was powered when the boat was docked and the brake chopper exhibited new sets of faults each time it was tested. It was difficult to test under working conditions because it was not possible to deploy the drill, which would work the winch motors. It was, therefore, impossible to confirm the suspicion that the fault was due to electrical noise problems.  

The existing brake chopper was replaced with a CP Automation 100KW unit. The fault did not transfer to this product, but reoccurred when the old unit was reinstated. 

Because the ship's crew identified a window when the sea would be calm enough to live test the drill, we were unable to carry out any more tests in dock. 

The new CP Automation brake choppers were each one-quarter of the rating of the original unit. Consequently, it was necessary had to limit the maximum speed of the winch until it was possible to assess the current required during payout and active heave.

To fit the two brake choppers, the control panel has to modified. This required the DC BUS to be split into two, with drives one and two on DC BUS one and drives three and four on BUS two. This change provided the system with an element of redundancy that it did not have before. There was now the option that, should anything go wrong, the crew could operate the system with only two of the four drives. 

After more tests in mild waves of 1.4m in height, the brake choppers were pulling 30A max per unit during active heave when the drill was at 750m from the ship. The DC bus level was maintained below 750VDC the whole time, which proved that the replacements were a proficient fix. 

The client deployed the drill five times throughout the 12 hours and the system ran smoothly every time with no further issues relating to the brake choppers. The crew was able to extract all the mineral samples that it needed.  

Once back on shore, after further careful testing, CP Automation was able to prove that its brake choppers worked in the drill application because, unlike the original unit, they did not use external power controls. This means that they are not susceptible to external noise. The previous brake chopper had no electromagnetic compatibility (EMC) or harmonic filter installed, so the resultant electrical noise from the drill application and those around it, caused the brake chopper to fault.   

The drilling project that CP Automation contributed to is one of the first of its kind to sample valuable metals from tough volcanic rock on the seabed With the help of more research and investment, it is hoped that method of mining could lead to a flow of essential rare metal resources for many years to come. 

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