iDVM 510 Quick Tips: Meter Ranging and Voltage Drops under a Load
Published at : 11 Feb 2021
When your compressor won't start, the first thing you might want to do install a start assist device, but that might actually be the last thing that is really needed.
Start assist devices are OK for tight compressors, and on systems where the voltage is low, like 208 volts vs 240 volts or even when there is a hard shutoff TXV that is not allowing full system equitization. But they are not always the right solution without a lot of investigation into the root of the problem, and in many cases, they can shorten the life of the system by either masking the true problem or simply by straining the compressor windings at startup. In some cases, not diagnosing the problem correctly can even lead to system failure or even a fire.
Before installing a start assist, start boost or similar device it is imperative that you make sure that electrical pressure (voltage) is not dropping more than 3% when the system is under a load. This requires measuring the static voltage and then repeating the measurement when the system is operating and preferably under full load. What you are looking for at this point is "voltage drop", typically due to an undersized conductor, a loose connection, or high resistance in a contactor or other component in series with the compressor. That voltage drop should not exceed 3% of the static voltage reading.
What can cause excessive voltage drops?
-Loose wires at the breakers
-Undersized feed wires, or cut or nicked wires
-Loose wire connections at the disconnect
-Loose connections at the disconnect blades
-Loose fuses if fuse equipped
-Undersized wires in the whip
-Loose connections at the contactor
-Bad contactor contacts
-Loose or bad stake-on terminals at the contactor or compressor.
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