APPLICATION BULLETIN - FEBRUARY 1994 | BACK TO INDEX  

PDL tackle a tough job with Pan Pacific Forest Industries

Tony Clifford, Pan Pacific's Electrical Projects Engineer inspecting the Log Debarker.

The forestry industry undeniably supplies some of the toughest application tests for technology suppliers and one of the toughest tests for PDL Electronics' ASDi 2000 series motor drives came up recently when Pan Pacific Forest Industries decided to add variable speed to their ring debarker.

Ring debarkers play an important part in modern timber processing, stripping the raw logs at the start of the milling operation so that saw damage can be avoided and bark kept out of the final pulping process where all outer wood waste ends up.

Tony Clifford, Pan Pacific's electrical projects engineer, decided that variable speed on the debarking ring would offer a number of benefits over fixed speed operation.

"Basically we wanted more control over what was happening in the debarking process so we could not only improve debarked log quality, but also knife wear time, throughput and possible extend the working like of the plant by reducing mechanical stress."

Tony carries a weight of heavy industry experience from several years spent in Australia working in the demanding crane and hoist environment whose high torque requirements, he explains, were very similar to those of the ring debarker. "What you've got here is a very heavy (9662kg) spinning ring something like the iris shutter on a camera, carrying five spinning knives which have to adjust to each different log diameter. The sort of torques involved are the highest in any industry with an inertia of 4500 kgm2. That's extremely high by any standard."

"Before we asked PDL in, what we had was a new debarker as supplied from the United States. Traditionally these are either fixed speed or two speed because of the problems in speed control with the torque and inertia. I've only heard of one similar application in New Zealand where someone tried to install another brand AC drive and they were unable in the end to get round the high starting torque and the inertial stress from the ring when ramping up and down."

"I knew from my experience with cranes and hoists in Australia, where we'd also used PDL-supplied motor drives, that the new generation of electronic drives can easily cope with the sort of load shifting you get with a ring debarker. All we had to ensure was that we got the specifications worked out properly so that PDL could come up with a suitable solution for our particular requirements."

"What we wanted basically was more control over the whole debarking process. For example we can have quite a variation in log diameter - from 150mm to 1000mm. We wanted to program four discrete sizes into the PLC so that whatever the log diameter the ring knives would be able to rotate at the correct speed for optimum debarking and wear reduction. The optimum speed is in the range 230 m/min and effectively we had to be able to reduce ring speed as the log diameters increased above 600mm, which was the first size step."

"Light sensors pick up the log size as it is delivered to the ring and the PLC sets the appropriate cutting speed just before the knives contact the butt of the log. We set up a worst case scenario for PDL where the largest log could be followed immediately by the smallest and vice versa. With only four seconds to ramp up or down to the correct speed that required 200% peak torque capability from the motor and inverter combination."

Pan Pacific Forest Industries Sawmill, Whirinaki, Napier.

"Because of the high currents involved at starting and during low speed running and bearing in mind the special demands that would be placed on the drive, we suggested our ASDi 2160 which is designed for a nominal 90kW motor," explains Dave. "This is actually oversize for the job, but it has to be able to cope with other extreme variables besides the size factor which crop up during log processing."

"For example, sometimes a branch stub has been left a bit too long and this places a sudden shock load on the ring as the knives ride over, which the drive has to be able to cope with. You can look at the debarker really as a sort of giant lathe where instead of the log rotating, the tool rotates. Traditionally, lathes have always been a difficult load application for drive manufacturers, but you can deal with any job however many problems it throws up providing you get the specs and design right at the start." 

PDL's application engineer, Richard Murton, dealt closely with the technical aspects of the Pan Pac application. "There's a special problem involved with an application of this nature and that lies particularly with what happens in the drive and the motor when you're ramping down and you've got this tremendous inertia in the spinning ring to deal with. In effect, the mechanical spin energy in the ring turns the motor into a generator producing electrical energy. A dynamic brake - virtually a large resistor - is used in the circuit to absorb the approximately 65 kW of regeneration we could end up with when the ring is ramping down from minimum to maximum log size. This was to deal with the worst case scenario. Because we worked through all the numbers with Tony well beforehand, Pan Pac's installation and commissioning were a breeze." says Richard.

Tony Clifford confirms that the upgraded debarker has already proved the worth of its new drive configuration - virtually a first for any major New Zealand timber plant.

"The ASDi has given us everything we had hoped for at the start - improved quality debarked logs, better throughput and less wear on the knives plus an energy saving."

He's also pleased with the success of what could have been a difficult conversion, but which in the end has proved to be quite the opposite.

"You can get any supplier to tackle the easy jobs, but they always drop out when it comes to the tough jobs. PDL do the easy jobs with ease, but they tackle the hard ones too so that the client gets a successful outcome. That's what really counts in an industry like ours."