Does a Motor Draw More or Less at lower Voltage?
Have you ever noticed a blower motor rated for 120V draws about twice the amperage of the same horsepower motor rated at 240V?
This is because motors are rated in Watts or Horsepower and according to Watts law Watts = Volts x Amps.
In order to keep the Wattage output the same at 120V, it draws twice as much current.
This is different than 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
The motor slowing down is due to slip in the motor, meaning that the motor is running significantly slower than the speed it is designed for.
This 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 and thus get hotter.
Even after all of this, the motor still consumes less than half the watts.
Rubber meets the road is that when a motor is designed for lower voltage it will draw more amperage to do the same work becasue it is designed to hit a wattage (horsepower) target at a designed voltage.
When you apply lower voltage you both decrease the work done as well as 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 higher power factor as the motor begins to slip resulting in even worse power efficiency.
This is one reason why voltage drop is a such an important thing to consider when sizing conductors and why 208-230V units are slightly derated or n 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 Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade