Does a Motor Draw More or Less at lower Voltage?

Have you ever noticed that a blower motor rated for 120V draws about twice the amperage of the same horsepower motor rated at 240V?

That is because motors are rated in watts or horsepower, and according to Watt's law, Watts = Volts x Amps.

To keep the wattage output the same at 120V, it draws twice as much current.

That differs from what happens when you drop the voltage of a motor below its rating.

Here is an experiment I did:

I took a regular 1/6 HP 208-230v condenser fan motor and tested it under normal conditions at my office, and here is what I got:


I then connected the common wire to neutral instead of L1 power, which leads to approximately 120v applied, and here is what I got.


By dropping the voltage by around 50%, the amperage dropped slightly, the wattage went to less than half, the power factor also halved, and the motor slowed way down.

The motor slowing down is due to slip in the motor, meaning that it is running significantly slower than the speed it is designed for.

That means that not only is the motor running inefficiently, but it is also going to get hot because as the motor runs slower, it has lower inductive reactance (the magnetic resistance in the windings). As the inductive reactance drops, the windings have lower resistance, thus getting hotter.

Even after all of this, the motor still consumes less than half the watts.

Rubber meets the road when a motor designed for lower voltage performs the same work as one with higher voltage. The former will draw more amperage to do the same work because it is designed to hit a wattage (horsepower) target at a designed voltage.

When you apply lower voltage, you both decrease the work done and the efficiency and life of the motor because more of the energy goes to heat instead of mechanical work as the motor slips more and more. You also see a higher power factor as the motor begins to slip, resulting in even worse power efficiency.

That is one reason why voltage drop is such an important thing to consider when sizing conductors and why 208-230V units are slightly derated in both capacity and efficiency when installed on 208v.

Pay attention to voltage; it can save a lot of money over time in both power efficiency and motor longevity.

—Bryan

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2 responses to “Does a Motor Draw More or Less at lower Voltage?”

  1. Its road not rode…. just saying ort. Opps typo. Let me corrrct it. Orr. K done.

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