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Build A Brownout Protector

What is a "brownout"? This rather graphically describes what happens to your lights when the AC mains voltage drops dramatically - they get very dim. But apart from dim lights, brownouts are a fatal hazard to induction motors, as used in air conditioners, pumps, dishwashers and a lot of other appliances.

By John Clarke

Main Features

  • Adjustable threshold voltage
  • Switches up to 2300W
  • Power indication
  • Brownout indication
  • Rugged sealed enclosure
  • Specifications

    Standby power consumption: <5W with relay on
    Maximum Control Power: 2300W
    Brownout threshaold voltage: typically set to 200V
    Switch on delay: 5 seconds

    Click for larger image
    Title photo: the Brownout Protector is housed in a rugged ABS plastic case with a clear lid. It can be used with induction motors rated up to 2300W and you will probably need one for each appliance you wish to protect.
    Above photo: Power is applied to the unit via a switched IEC connector attached to one end of the case. Note that this connector and its internal mounting plate must be secured using Nylon screws to ensure safety.

    Years ago, brownouts were quite rare and generally confined to rural districts where the power lines had very long runs. A falling tree or an electrocuted possum might cause the mains voltage to drop to a low level and lights would go dim. This has always been a hazard for the induction motors used in pumps and refrigerators.

    Nowadays though, because the electricity grid is running much closer to total capacity, brownouts can be experienced much more commonly in the cities and suburbs. Our own offices in the Sydney suburb of Brookvale have had brownouts on a number of occasions in the last year or so. On each occasion, we have made sure that the air conditioner, fridges, compressors and other machinery in the building were turned off until full AC mains supply was restored. Had we not done so, all the motors in that equipment were liable to burnout.

    So how many motors in your home are at risk right now if a brownout was to occur? The list can be quite long: fridge, freezer, washing machine, dishwasher, air conditioner, pool pump, spa pump and perhaps one or two garage door openers; typical of many homes. All this equipment could attempt to turn on during a brownout and the motor(s) would probably burn out.

    Maybe your insurance policy covers motor burnouts but you would need to read the fine print. The insurance company might also look askance at your claim if there was more than one motor burnout or if the appliances were more than a few years old.

    Why do motors burn out?

    When induction motors are starting up they draw very heavy current for a second or two and when they are up to speed, the current drops back to reasonable levels. However, if the AC mains voltage is low, the induction motor may not develop enough torque to come up to full speed. In all of the appliances listed above, the motor starts with a heavy load so it is at particular risk if those starting currents do not reduce quickly. Those motors with a starting winding (switched out by a centrifugal switch) are at particular risk because those windings are only intended for very intermittent use.

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