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New for old: veteran drives get speedy swap-over

02 November 2013

When Portland Dyeing Company of Leek, Staffordshire, needed to replace a couple of legacy drives, what at first appeared to be a potentially complex project was completed in just 24 hours, thanks to some nifty preparation work by the systems integrator.

The inverters – a couple of 25-year old Lenze 8608 units driving a Montforts stenting machine – were experiencing repeated over-current faults, which was having an adverse effect on production.

The situation was made more complex by the fact that the drives were set up with 87Hz technology and some of the machine manuals were no longer available. Fortunately, help was at hand, and Lenze Systems Partner, Cornfield Engineering Solutions stepped in to the breach to engineer up-to-date replacements.

Portland Dyeing’s Montforts stenting machine is used to stretch and set fabric. The stenting process uses heat to give stability to the fabric and prevent subsequent shrinking. It is commonly used for cotton based materials. A web of cloth is connected on each side by hooks to a tractor conveyor. Each side of the conveyor has independent drives so that, while they normally run in synchronisation, they can be advanced and retarded.

The two sides of the conveyor can also be adjusted in width to tension the cloth web. The conveyor takes the cloth through an oven where even and continuous motion is required to prevent burning. The effect of a drive failure is potentially calamitous as the surface of the cloth will burn in the oven resulting in costly waste.

In the original set-up, each 8608 inverter drives a 5.5kW planetary geared motor connected to the conveyor. The motors and drives were selected with a 87Hz setpoint, meaning that the nominal torque is achieved with a 4-pole motor speed of 2,500rpm.

This results in increased power from the motor, up from 5.5 to 9.4kW which, in turn, means the motor is compact and cost saving. The only special requirement for 87Hz operation is that the motor should have connections for 230V Delta. The inverter has to be sized appropriately for the increased power - in this case 11kW models were chosen.

Montforts chose 87Hz technology as it increases the available speed range by 70 percent. The motor nominal torque is generated at 2,500rpmand the inverter can drive it down to 580rpm without cooling, or down to 145rpm with a blower. The motor is effectively standard and runs with a small gain in efficiency at the higher speed. With a 132 frame motor giving the output of a 160 frame, there are further gains in size and purchase cost.

Cornfield Engineering Solutions director, Chris Timbey investigated the application and selected up-to-date replacement inverters – in this case,Lenze  8400 StateLine units rated at 11kW, which respond to an analogue speed reference signal from a higher level control system.

They are easily capable of the 87Hz settings and as they are modern, high quality inverters, Portland Dyeing is assured that they will provide reliable service. Moreover, some 25 years later and the footprint of the new inverter installation has reduced by 78 percent

Cornfield Engineering was able to support their customer from the initial assessment right through to installation and commissioning. In fact, Chris Timbey is proud of how well the project went. "We prepared well with new drawings and programs written with the Lenze L-force Engineer parameterisation software,” he recalls. “The drives were changed and the machine was up and running again in less than one working day".


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