Properly Operating Three-Phase Motor with Single-Phase Power

Properly Operating Three-Phase Motor with Single-Phase Power

Screenshot (106)Electric motors are manufactured and available in two major designs, single- or three-phase. While both types of motors have their merits & demerits, three-phase are more commonly used for industrial applications rather than residential ones, due to the availability of three-phase power.

What happens when you want to use a three-phase motor but only have a single-phase power connection? Will it work?

 

Phantom Leg

Three-phase power is characterized by three sine waves that have a phase shift of 120 degrees with respect to each other. The problem of running three-phase load on single-phase connections isn’t new. The oldest workaround that has been followed for decades is addition of a phantom leg by connecting two of the phases with the incoming 220 Volts connection and the third with a capacitor so that the necessary phase shift is created. The offset is 90 degrees instead of 120, but the arrangement still works.

The important consideration for this method is the size of the capacitor, which must be done according to the load. Otherwise, the current will be balanced leading to malfunctions. Incorrect pairing of the capacitor with the load can also lead to deviations that can lower the torque of the motor.

Rotary Phase Converter

Another method is using a rotary phase converter. The device is commonly used, such as by a wood shop, to run several 3-phase machines on 1-phase power connection. The drawback of this method is the costs associated with it, which can be quite high during the time the rotary phase conversion is occurring, regardless of the operating state of the motors. Furthermore, a balancing problem can also occur, with the current being balanced only when a certain number of motors are operational.

NEMA standards state that motors must be operated from voltage that is balanced within 1% while the percentage current imbalance can be 10 times the voltage imbalance. If the latter rule is applied to a motor operating at 1% voltage imbalance, then the resultant current anomaly could be 10%. This benefits the arrangement as mostly three-phase motors operate between 15 – 50 percent current imbalance.

Variable Frequency Drive

VFDs have become very popular in recent years due to the superior flexibility they offer, but at the same time carry a hefty price tag. The principle of operation involves rectifying each pair of phases to dc and then converting them to three-phase AC output. This allows a VFD to operate three-phase motors on a single-phase connection. With that being said, it is highly recommended that the machine operator work with the manufacturer or machine builder for optimum VFD selection.

Applications where this method is commonly employed are woodworking equipment, compressors and decorative fountains. Instead of buying expensive single-phase motors and dealing with associated speed control issues, it is better to opt for a three-phase motor controlled by a VFD. Additionally, for applications with requirements up to 4kW, suitable VFD can be bought at an affordable price, less than what you would spend on rewinding a three-phase motor.

Three-phase motors are recommended as they are affordable, easier to control and have readily available spare parts. They have become an industrial norm, and therefore, manufacturers usually offer a lot of support. The methods described above can be used to bridge the gap of compatibility, and take advantage of the capabilities of a three-phase motor on a single phase of power.

 

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