HOW DOES A THREE-PHASE MOTOR SAVE ENERGY COMPARED TO 1-PHASE MOTOR?

Question by Optimus Prime:
How does a three-phase motor save energy compared to 1-phase motor?
Someone told me that replacing my existing single-phase fan motors to three-phase motors could save me as much as 30% of energy. How is this so?

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Answer by Pete
It’s sort of like gears on a car. In a 3 phase motor the 1st phase (also known as the “starter windings”) is capable of using a higher voltage input, and thus developing more torque to get the rotor accelerated up to the desired speed. This requires overcoming the inertia of the mass of the rotor. The 2nd phase does the same thing, but at a lower torque and thus more efficiently (sort of like 2nd gear). The 3rd phase is the least “powerful” and therefore the most efficient, but is sufficient because it only has to deliver enough power to overcome mechanical friction, not inertia. Hope this helps. Have a nice day. (P.S. You will probably save more than 30%.)

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What do you think? Answer below!

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Comments (2)

#1 written by JD_in_FL
about 1 year ago

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Only if you have three phase power. If not, its all moot.

Three-phase uses three supply wires that carry different voltage, similar to the 220-240 volts supplied to most US homes and one leg is split off to 110-120 volts. The difference is, in three phase, different voltage is delivered on 3 legs, not 2. One example is two legs of 110-120 volts and a third leg of 208 volts. While there are different world standards of “three phase”, It is basically, 3 times the line to neutral voltage ratio.

This saves energy, because the motor does not work as hard, just like 220 volt air conditioners and clothes dryers don’t work as hard as 110 volt models do.

But, a special meter and wiring is required and might not save as much if it is not going to save the costs to install and rewire.

Contact your local electrical company and ask them for a free inspection. They should be able to calculate your savings, if any.
#2 written by Chas EE
about 1 year ago

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Three phase motors are more efficient than single phase motors. The three phases provide a method to produce a magnetic field that rotates smoothly around the motor. It also delivers power at a very constant rate. Single phase motors reverse the polarity of the magnetic field and shift the north of the magnet from one side of the motor to the other but they don’t have anything between the two sides to help the transition of the north pole from one side to the other. To get the motor started, they use an auxiliary winding and a capacitor to create an in between pole position, but that is not as effective as having three equal windings. In three phase motors, there are three equal voltages and three identical windings. Utilities provide three phase only to industrial users. Homes and small businesses can not get it.

Edit 1
30% energy savings is probably an exageration unless the fan speed can be reduced. A variable frequency drive (VFD) with a 3 phase motor can efficiently reduce the fan speed by any desired amount and save energy.