ALTERING TO THREE PHASE:

A ROTARY PHASE CONVERTER IS A COST-EFFECTIVE WAY TO BOOST A SINGLE PHASE INTO THREE

By: Hal Pike

The often forgotten and sometimes misunderstood rotary phase converter is ideal for customers who don't have access to three-phase electrical power. The market is large and diverse. Rotary converters are applicable for agricultural operations, computer numerically controlled operations (such as machines and woodworking shops), welding operations and sewage plants, to name a few.

Three-phase power is superior to single-phase. Three-phase motors are more efficient, cost less, last longer and are more dependable than their single-phase counterparts.

For companies or services that would like to power equipment with three-phase motors, but don't have access to three-phase power, the situation can be frustrating. The acquisition of three phase through conventional methods (the utilities) can be expensive and complicated. For one thing, utility rates are usually much higher for three-phase. Even when the rates are equal, the cost of installation is always higher— sometimes prohibitively so. The solution, and it's a good one, is the three-phase rotary converter.

Simply put, the converter takes an existing single-phase source and converts it into three-phase power.

Phase conversion isn't new. Converters have been in use since the early 1900s. Originally, however, they were the static type and had distinct shortcomings. A static converter is matched to the horsepower rating of a single motor's load. Every time the load fluctuated, the converter would have to be adjusted. And each motor required a separate static converter. These drawbacks kept the converter market from growing.

In the early 1960s, a revolutionary new concept was developed in which converters could operate multiple motors of different horsepower. In addition, the sum of the motors operated could be substantially larger than the largest motor rating of the phase converter. This is made possible because each motor acts in turn as a converter. Motors that are started later in the system have a higher starting torque. So, the largest motor can, if necessary, be started last.

HOW IT WORKS

To convert single-phase to three-phase power, an energy storing device is needed. The kinetic energy in the rotor of the rotary phase converter is utilized to effect this transformation. The high value of capacitance used in the circuit affects a two-phase flux to start the converter. The rotor conductors cut the flux of each of the symmetrically placed three-phase windings of the converter's stator field. As it rotates, there is induced in each of these windings a counter electromotive force. These induced voltages have a time phase that is the same as the space phase of the windings. The converter rotor functions to set up the three-phase field and also acts as an energy reservoir between the single-phase and three-phase systems.

Over the years, more and more industries have recognized the value of rotary converters. Their use has grown steadily. They are found in industries that, a few years ago, wouldn't have considered them applicable.

Over the last five years, new application requests have caused converter manufacturers to improve and apply new technology to their product. The result has been a wider range of better and more versatile models. Most manufacturers now offer models ranging from 230 to 460 volts and 1 to 220 horsepower.

The rotary phase market has grown and will continue to grow, so don't overlook it. There are many timely applications. The agricultural industry is a fine example. Because of the problems caused by last year's drought, and the anticipation of another one this summer, many farmers will be installing or expanding their irrigation systems. Many of the areas to be irrigated don't have access to three-phase lines and, in many areas, the installation costs are prohibitive. In such situations, a rotary converter is a welcome solution.

Computer numerically controlled (CNC) equipment is another example. CNC machines are becoming the norm in many machine and woodworking shops. A converter exists that meets the critical voltage tolerances demanded of CNC machines. Shops that haven't had the means to run CNC can now meet their CNC competition head-on.

New applications are cropping up almost daily, so electrical contractors shouldn't overlook this growing market.

Hal Pike is sales director of ARCO Electric Products Corp., Shelbyville, Ind.

Reprinted from the April 1989 issue of ELECTRICAL CONTRACTOR Magazine

Copyright 1989 National Electrical Contractors Association. Inc.

POWER CONDITIONING / AUXILIARY POWER / UPS