Customised for performance: How tailored drives solved a unique engineering challenge
Discover how Power Electronics New Zealand engineered a customised industrial drive solution to overcome a complex technical requirement, delivering improved performance, reliability, and long-term value.
At International Panel & Lumber (IPL), an ageing autotransformer starter for a hogger (a specialised wood waste process machine) was due for replacement. IPL had a SD700 variable speed drive (VSD) on site and contacted Power Electronics New Zealand to see if it could be used to replace the starter. A VSD is not well suited to this type of application, so it was far from a standard upgrade. The retrofit required extensive problem solving and testing to overcome considerable technical challenges for this unique industrial application.
A hogger is a heavy-duty wood chipper used to process waste wood from plywood manufacturing. Plywood is made by peeling logs into thin sheets using a lathe.
When a log is first put into the lathe, naturally, the log isn't perfectly round, resulting in waste wood as the outer, non-uniform parts of the log are peeled off. At IPL, this waste wood is fed into the hogger, which chips the waste wood into finer chips that are used as biofuel to help run the plant.
It’s a heavy, high-inertia piece of kit with significant and variable energy demands.
At IPL, the hogger was originally started using an autotransformer – a traditional but now outdated starter method. IPL had a spare SD700 variable speed drive on hand and wanted to know if it could be used to replace the autotransformer.
The challenge this posed is that hoggers experience high peak currents. Hoggers are a big spinning mass that for periods of time don’t draw a lot of current off the motor. Then, when the waste wood enters the machine, the current surges to a very high level to deal with the load.
The current load can increase to more than two times the rating of the motor as the hogger chews through the wood. This peak far exceeds the overload capacity of most VSDs (typically ~1.5 times rated current).
Using a VSD to continuously run the hogger wasn’t a viable option. However, it could be used to gently start the hogger’s high-inertia motor.
To solve this challenge required addressing the fundamental electrical incompatibilities between the two power sources.
A VSD generates its output by first rectifying mains AC to DC, then inverting this DC back to a synthetic AC waveform. This output has no phase relationship or synchronisation with the mains supply. Directly switching between these unsynchronised sources could cause mayhem and potential equipment damage.
Rather than implementing complex synchronisation systems for this industrial drive solution, the Power Electronics team designed a hybrid system to leverage the strengths of both the VSD and the mains power.
The alternative approach involved:
- VSD for start-up: Using the VSD, the 260 kW hogger motor was gently accelerated through to full speed, managing the high-inertia load without exceeding motor rated current.
- Switch to mains: Once full speed was reached, the VSD output was disconnected, and the motor was transferred to run directly on mains power.
Since the VSD’s synthetic output AC waveform is not synchronised with the mains, directly switching between the two could be destructive.
This solution design required careful timing analysis and testing.
Testing, Testing……123
The Power Electronics team first measured how long it took for residual voltage in the motor windings to decay after disconnecting the VSD. This was recorded at approximately five seconds.
However, this presented another challenge. During this delay, the high inertia hogger would begin to slow down. Switching to mains at this reduced speed would generate a current surge owing to the partial DOL start and negate the soft start benefits of the VSD.
To compensate for this slowdown, the motor was over sped to 111% of its rated speed before disconnecting the VSD.
After five seconds of coasting, the motor naturally slowed to exactly 100% rated speed. A tachometer was used to verify that the rotor was rotating at the correct rpm, at which point it was safe to switch over to the mains with no electrical or mechanical shock.
Sounds good in theory…but let’s protype test…just in case…
Before implementing the approach with the full 200 kW motor, the team prototyped the changeover process using a small test motor. This allowed them to:
- Prove the control system and timing logic.
- Validate the safety of the sequence.
After successful testing on the prototype, it was time to apply the logic to the hogger.
Whoops… didn’t notice that wee mistake!
The first test on the large motor did not go to plan. Unknown to the team, during testing with the small prototype motor, two motor wires had been transposed when changing over from VSD output to the mains. As the smaller test motor lacked inertia and stopped quickly, this reversal went unnoticed.
When this reversal was replicated on the 200 kW hogger motor, it was near-catastrophic. The hogger was spinning at full forward speed when the changeover to mains slammed it into reverse due to the reversed wiring. The result? A large rise in current the motor almost ripped off its mounts, and the switchboard made sounds that had never been heard before!
Rapid diagnosing and problem solving
Fortunately, the team acted quickly to diagnose and fix the issue.
Re-testing again on the small prototype motor confirmed the wiring mistake. Once this simple fix was in place, the full process ran again on the 200 kW hogger – this time seamlessly.
The VSD-assisted start with timed changeover to mains power has now been running seamlessly for IPL since 2007. It’s a great example of deep electrical knowledge, creative problem-solving, and perseverance that delivered reduced maintenance costs with energy-efficient drive systems and real-world trial and error.
While each electrical engineering situation is unique, here are key lessons that are applicable across the board:
- Always double-check wiring. Small test mistakes can scale into major issues on large motors.
- High inertia loads like hoggers need carefully considered drive systems.
- Variable speed drives can be repurposed creatively, even in high-load continuous operation applications.
- Testing with smaller, prototype motors can help validate control logic before applying changes to larger systems.
Need expert guidance for your industrial VSD requirements? Get in touch today to tap into Power Electronics’ experience and optimise your operations.