One of the most common, and vexing, problems that arises in control systems for small OEM process cooling is a momentary power line brownout due to compressor startup. A short-duration brownout can easily reach dips in power line voltage of up to 50 percent and can cause numerous symptoms in the control, such as changed setpoints, blanked or scrambled displays, locked control buttons, load lockup or complete shutdown.
Most process controls can withstand dips in power line voltage of up to 20 to 30 percent for tens or hundreds of milliseconds. However, there is always a threshold where problems begin. Even if the equipment vendor defines this threshold, measuring the actual fluctuations on a power line can be difficult.
The best solution is to be aware of compressor power requirements and provide ample wiring capacity directly to the compressor. Control power should be wired on a completely separate circuit and monitored with an oscilloscope as the compressor is cycled to ensure that voltage transients are less than 10 percent and of a short duration.
A Common Cooling Control Pitfall
October 1, 2008