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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4 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.

  2. Your experiment showed that current decreased when the voltage \\\\\\\\was halved and the load was constant. Voltage is required to create current. Lower voltage is lower current. Reduced efficiency\ may account for a higher motor temp as the motor slows and the BackEMF goes down. Current would rise toward LRA as the BackEMF went down, but the reduced voltage will prevent current from getting to the levels as when the voltage is normal. Input watts, minus efficiency of device equal output watts. The load will get whatever output is available but can’t draw more than that. A higher proportion will be wasted as heat due to lower efficiency. The inductive reactance is determined by the induction(Henries) of the coil and the frequency of the applied voltage. The inductive reactance is constant. Current varies by the voltage applied.

  3. The voltage is the determining factor in how much power a device uses. If the voltage is below system design the power output of the device will be lower. It can’t draw more current to maintain power. Voltage is the only force to drive current. In your experiment, halving the voltage, reduced current and wattage fell to less than half. Less power is less work done. The power, heat, may be redistributed but the consumption is less. The motor cannot do the same work at low voltage as it did at full voltage. It can only do the amount of work that the available input power provides. Inductive reactance is a product of the winding inductance, in Henries, and the applied frequency, 60Hz. The inductive reactance of the motor is constant. Current in the winding varies with applied voltage, less voltage lower current and Back EMF(volts). Back EMF goes down with rotor speed. As Back EMF goes down current goes up toward LRA but the reduced voltage offsets the current rise so it can’t get to the high current of a stalled motor with full voltage. In your experiment, voltage went down by half but current only fell 20%. That current was less than normal current for the motor and the wattage fell by 60%. Only watts can make heat so there is much less power consumption but, a higher portion is going into heating the motor instead of useful work. The higher power factor is a measure of the resulting inefficiency of the motor. Power factor does not cause more inefficiency, it is only the expression of that which exists.

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