7 April 2019. We occasionally get work associated with industrial electric installations. One such job came our way recently was the repair of a variable speed drive that was used to control the opening and closing of an industrial sized garage door. The door measured 4 metres by 3 metres and was driven by  a 5 kilowatt a.c. motor with inbuilt breaking.

Burned Out 1000 Watt Wire Wound Resistor

The door was failing to operate correctly and would stall during the close cycle, so we were called in to investigate. The industrial drive used to control the control motor was over 20 years old and was in generally poor repair. On opening the large stainless steel control box we found that there was a large 1000 watt wire wound resistor used as a current shunt, that had burned out. This resistor was huge, measuring some 60 cm by 20 cm in diameter. It was designed to dissipate electrical power whilst closing the door. The reason the door would not close properly was not due to any mechanical reason, but because the resistor had burned out, meaning that the motor was received no voltage in the close cycle.

Replacement Resistor From Australian Resistors

Finding a suitable power resistor to replace the failed resistor proved fairly easy. We located a local, Sydney based manufacturer of power resistors being Australian Resistors who we called to discuss the problem. They were able to manufacture a 1200 watt power resistor. This resistor was manufactures on a 20 cm ceramic former, wound with resistance wire to the correct resistance, in this case 120 ohms, and then coated in a humidity resistant coating that would ensure long term electrical and mechanical stability, even in a fairly tough industrial environment.

Mechanically Replacing The Failed Resistor

Installing the new resistor proved to be challenging because even though the resistor was housed in a stainless steel enclosure, the mounting brackets of the old resistor had corroded to such a degree that we had to carefully cut the mounting holes to a larger than original size to remove the old resistor. We then drilled new mounting holes to secure the Australian Resistors power resistor into place.

Testing and Reinstatement Of The Door

Once the new resistor was installed we spent some time checking both the electrical and mechanical operation of the garage door. Some minor mechanical ‘tweaks’ were required to ensure the door closed against the mechanical door stop. We also checked the electrical ‘inrush current’ of the motor to check that it did not exceed the current that the newly installed resistor would tolerate. In the end we left with the door working perfectly – job done!