How to Calculate the Starting Amps for a Compressor on a Freezer


A heat-exchange compressor is a key component of a freezer. Its job is to pressurize the circulating coolant, changing it from a gas to a liquid state, releasing latent heat into the atmosphere as it does. The compressor consumes more electrical power than any other component of the freezer. The compressor's power demand is greatest at start-up, when it must overcome the inertia of the pump. Calculating the starting current of the compressor is easy, if you know the compressor's wattage.

Things You'll Need

  • Flashlight
  • Switch off the power, and disconnect the freezer from the power supply. Make sure the power cannot be reconnected to the appliance while you are working on it.

  • Access the rear of the freezer, and look for a cylindrical object connected to thin pipes. This is the compressor; it may look similar to a pressurized gas cylinder. Locate the manufacturer's plaque attached to the compressor, and read the "Maximum wattage" information.

  • Substitute the maximum wattage and the appliance voltage into Ohm's Law to determine the starting current. Ohm's Law, often expressed as I = P/V, where I is the current, P is the power in watts and V is the voltage, states that the current equals the wattage divided by the voltage. For example, a 720-watt compressor running on 120 volts draws a starting current of 6 amps: 720/120 = 6.

Tips & Warnings

  • The technical data section of the appliance manual may list the maximum compressor wattage. It may be listed for the compressor or as the appliance maximum wattage.
  • The compressor "maximum wattage" is the wattage drawn at start-up and is the value used to determine the start-up current. The normal running wattage is considerably lower than the start-up value.
  • Don't damage any of the small pipes connected to the freezer. They can be brittle; once one is broken, the freezer is useless.

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