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HVAC School - For Techs, By Techs

Mar 31, 2020

In this short podcast episode, Bryan talks about the steady-state voltage problem that can take out inverter boards and what to do about it. Inverter-driven equipment refers to a variable-frequency drive with ECM compressors and fans.

We're talking about modern split-phase equipment rated for 208v or 230v power. However, the split-phase power will yield 240v. Most motors and components for 240v equipment have ratings for 230v AC power. L1 and L2 power can also be significantly higher than 240v, sometimes going as high as 250v.

Inverter boards have a widespread failing problem in locations with high steady-state voltage. Surge protectors only work for spikes in voltage, such as lightning strikes; they don't protect equipment from steady-state high voltage. Inverter boards are rated for 10% voltage over 230v and 5% lower than 208v. The operating range is 197-253v, but consistent overvoltage that doesn't quite reach 253v can still lead to failure.

We attempted to fix the problem by using the ICM493. These protectors have single-phase monitoring and have a NEMA 3R rating (suitable for outdoor usage). You can set the high and low voltage limits and get the benefits of thermally protected MOV surge protection. Although the inverters stopped failing, they started shutting off when they weren't supposed to. We discovered that the power companies were allowed to run up to 252v, which was right on the limit! Power companies may also run voltages 1-2v higher than 252v, so that explained the failures and shutoffs.

If you decide to use the ICM493, you need to calibrate the voltage based on measured voltage (such as from a voltmeter). Then, you set it for 230v +/-10%. If you experience recurring problems with overvoltage, the power company probably won't be much help. In that case, you can use a buck-boost transformer.


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