A large automotive components plant was experiencing problems due to unbalanced loading on the drives of its multidrive conveyors. Any proposed solution had to be retrofitted to its existing control panels and Mobile Electro Service, a Mitsubishi Drives Systems Centre, was called in to assist.
The conveyor, which was a kilometre long, was driven from three points using three identical gear motors spaced unequally around the length of the conveyor loop. The motors used one variable speed drive with open loop speed control and common speed input.
Although calibration of the three drives was not identical, they were locked together via the chain conveyor and 'forced' to run at the same speed by forces transmitted through the chain. This caused motor speed slip, which resulted in bunching, snatching and excessive wear of the conveyor chain.
To eliminate bunching and stretching of the conveyor, Mobile Electro Service used the analog output of torque and the PID control features of the Mitsubishi A500 Variable Frequency Drives Series to match the speed of the master while sharing the torque of the two slave drives.
One of the three FR-A540E3.7K-EC drives, with analog output of torque, was set up as the master and this value fed to the setpoints on the PID controllers built into the other two drives. An FR A5AY A500 analog output card was used to make the connection as the 0 to 10 V output and the 0 to 20 mA input were not originally designed for this purpose. Additional signals included alarms from the slaves to ensure orderly shutdown in the event of a problem with any of the three drives.
A 1-second accelerate/decelerate ramp was programmed into the drives to accommodate the fundamentally slow conveyor system and its drives. Other settings were essentially factory defaults.
The result
Once the PI control values were established, the system worked faultlessly. The torque was controlled with only a small residual unbalance of torque calibration being transmitted via the conveyor. In the previous system, full torque was needed to cause the slip. Bunching and snatching did not occur at all at normal operating speeds and even under higher than required speed tests, the conveyor experienced no problems.
According to a senior official at the plant, the solution developed by Mitsubishi provided measurable results, both financially and in terms of conveyor performance.
Circuit Breaker Industries, a company wholly owned by Reunert, is an authorised distributor of the Mitsubishi Electric range of automation products. This range includes programmable logic controllers (PLCs), human machine interfaces (HMIs), variable speed drives (VSDs), servo drives and Mitsubishi motor control equipment (ie, magnetic contactors, medium and high voltage vacuum contactors, thermal overload relays and electronic motor protection relays). A qualified team of sales engineers is able to advise customers and supply back-up support and service. All products carry a 12 month warranty.
For further information contact Eddie Milne, Circuit Breaker Industries, 011 928 2000, [email protected], www.cbi.co.za
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