Single Phase vs Three Phase Power

Single-phase distribution systems are common because single-phase transmission lines costs significantly less than three-phase lines. They consist of one high-voltage line and a neutral. Most residential and rural areas are supplied with single-phase service. Three-phase power cannot be supplied from single-phase service unless a phase converter is used.

Single-phase power is a single voltage that alternates between a positive voltage and a negative voltage for a specific number of times per second (in the U.S., 60 times per second or 60 Hz). Three-phase power is three distinct AC voltages, each shifted in time 120 degrees relative to one another.

Customers are supplied with electricity from the distribution system by placing transformers on the high voltage distribution system to reduce voltage to a level compatible with electric devices, for example, 240 volts. Three-phase service requires three transformers compared to one for single-phase service, and requires different metering equipment as well. Because of this, three-phase service costs more to install, so utilities usually prefer to install single-phase service unless
there is a specific demand for three-phase power at the site.

Three phase power is the most popular method of electric power transmission even though single phase power is more prevalent. When using three phase power you are basically tripling your electricity through three wires without having to increase the thickness of the wires.  Three-phase power is used for most electrical loads greater than ten or fifteen horsepower.

Another consideration for power systems is the reliability of the motor.  The primary failure mechanism of single-phase motors is failure of the switch that controls the secondary winding.  When the contacts eventually become corroded, the switch will fail to close and the motor will not start.  This will overheat the primary winding and may cause destruction of the motor.  Since three-phase motors don’t have this internal switch, they do not have this failure mechanism.  Long motor life is an important consideration for grain legs where the labor cost for replacing the motor can be more than
the cost of the motor itself.

Finally and possibly most important of all is that the cost of a three-phase motor for larger horsepower applications is considerably less than a single-phase motor of equal horsepower.

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