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Ask an engineer: Three-phase vs. single-phase motors

Are three-phase motors more efficient than single-phase ones? Three-phase motors are slightly more efficient and cost-effective for two reasons: The power delivery mechanism and motor design.

To get technical, three-phase motors have three power lines coming in 120 degrees out of phase from each other, so the peaks of successive waves are closer together. This means less time between each successive peak, and a motor that runs more smoothly and efficiently.

Also, three-phase motors are designed so the coils in the stator (the stationary component through which energy flows) are connected to form three separate windings, one for each power line. Each winding contains one-third the total number of coils in the motor, reducing overall losses within the device.

In larger applications, three-phase motors can be much more efficient than single-phase units. Three-phase motors also typically cost less to purchase since they don't have capacitors (energy storage devices). Because each phase draws less current than a single-phase winding, the wires can be smaller in diameter; this also decreases cost.

Should you switch to a three-phase unit? Despite their cost and performance advantages, three-phase motors aren't a good fit for every situation. Some facilities lack readily available three-phase power. Also, other heavily used equipment, such as lighting and computers, typically runs exclusively on single-phase power.

Rewiring and other costs to install a three-phase system can be significant. Also, if you’ve addressed harmonics and power quality issues with your single-phase motors, you may need to revisit them with a three-phase design.

For applications in which the motor load is relatively light, the potential payback for three-phase may not justify the initial cost. Carefully consider the application, as well as the costs and benefits before making the decision to install a three-phase unit or replace an existing single-phase system.

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