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Electrical Slight-of-Hand?


Murphy's Law, "Nothing is as easy as it looks," has a corollary that says, "Any sufficiently advanced technology is indistinguishable from magic." Nowhere is this more apparent than in phase conversion, where 2 equals 1, and adding one produces three (Fig. 2).

Figure 2 - Single-phase has 2 lines but only one voltage.

 By adding a 3rd AC line to single-phase, properly displaced in time from the other two, we now have 3 voltages.

Even though a phase converter adds only one more "leg," it is responsible for 2/3 of the power going to the equipment. Phase converters add a leg--or line--to single-phase and the result is 3 distinct voltages that comprise 3-phase power.

Utility-Supplied 3-Phase

Many readers have already learned that 3-phase is not easily obtained from their local utility supplier. An installation reaching only 1 mile often costs 50 to 80 thousand dollars. There is a good reason for this.

 Since the utility provides 3-phase from the primary, or transmission side, 3 hot wires and 3 transformers are required--along with 3 times the maintenance and installation expense. Single-phase, however, requires only one hot wire and one transformer. So, unless you are in an urban area where a service may be shared with several customers, installation costs may be prohibitive.

 Customers have also been surprised to learn that even if the power company drops in 3-phase for free, many extra costs--in the form of monthly minimum charges, "demand" billing based on peak usage and higher per-kilowatt-hour rates--serve to drive up the price of utility-supplied 3-phase far higher than the investment required for most phase converters.

 Since phase converters work from the secondary side of the power grid (after the utility-supplier's transformer), a very simple installation is involved. Three-phase loads only are applied to the converter. All existing lighting and other wiring to single-phase loads remains the same.

 As for converter costs, a static, or capacitor-type converter may cost less than $150.00. It will operate one or more moderately-loaded motors and install in 30 minutes. Its operating cost is virtually zero.

 A dynamic, or rotary phase converter can be connected to your three-phase loads in one or 2 hours. It can operate any combination of motors, heat loads, welders, or 3-phase rectifiers (AC to DC). The purchase price is usually about $50.00 per operated Hp (on multi-motor applications). This you buy once, and may expect it to last 10 to 20 years with virtually no repairs! And, with an operating cost of only 5% of the operated load it is easy to see that phase conversion is one of the best industrial bargains available.

 The next step is to determine the type of converter that is best suited to your needs.