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Measuring Voltage Drop w/ Jim Bergmann

In this episode, Bryan and Jim discuss measuring voltage drop and what it means. They also cover some other pointers to keep from using parts to bandage a deeper issue.

A common mistake that technicians make is not understanding what an ideal measurement is before making a measurement. For example, they may not know what the refrigerant pressures should be before they attach the gauges. Electrical measurements are the same, and voltage drop falls under that umbrella.

Voltage refers to electrical pressure, and current refers to electrical flow; they are two different values. The voltage will typically be at its full value (e.g., 240v) until you test the system under load. The voltage will drop when the motor begins turning. A motor will generate either motion or heat. When a motor doesn't have enough electrical pressure (voltage) to start, it will generate heat until it trips an internal overload. Upon startup, a standard voltage drop will be around 20% on a properly sized circuit. When the unit drops voltage in excess, the compressor turns more slowly and reduces its output.

Many technicians measure voltage to see if it merely exists. They do not attempt to see if the voltage is at an appropriate level, and that's a major pitfall in our industry. Technicians ought to know the appropriate voltage values so that they can assess if the voltage drops are normal or excessive upon startup and while a compressor is running.

Common issues that cause excessive voltage drop include:

  • Excessive heat loads in the building (from other electrical appliances)
  • Undersized feed wires from the transformer to the pole
  • Loose lugs
  • Corroded connections
  • Conductors that are too small or too long

Before bringing out the hard start kit, make sure you do all of the preliminary checks to make sure you REALLY need it. Make sure the feed wire is the correct size and that your connections are solid. A hard start kit will mask the issues of voltage drop for a short time, but they don't address the core issue.

So, just check voltage drop under load. Voltage drop should not exceed 20% on startup and 3% while running.

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