February 2010 Electrical Apparatus

February 2010 Electrical Apparatus

This is a summary of the Electrical Apparatus February 2010 featured technical article,  by Richard L. Nailen, P.E.     

In low-voltage industrial and commercial electric power systems, the need often arises to supply a single- or three-phase load from a power source supplying a voltage between 5% and 25% above or below the load rating. The most cost-effective way of doing so is usually the dry-type buck-boost transformer.

Commonly designed with nominal ratings in the 1 to 10 kVA range, such a unit consists of a primary winding connectable for either 120/240 or 240/480 volts, with secondary windings of 12/24 or 24/48 volts respectively. Rather than being of “insulated” construction, in which no electrical connection exists between primary and secondary, these are normally connected as autotransformers.

Because most of the load current then passes only through the secondary portion, such a transformer can therefore sustain a load far exceeding its kVA rating. The greater portion of the winding, acting as the primary, carries so much less current that smaller conductors can be used. The cost per kVA of this type of transformer is therefore much less than for a conventional insulated type. Direct connection between primary and secondary, which would be unacceptable if the two circuits were at substantially different voltage levels, is generally acceptable when both primary and secondary circuits operate at 600 volts or less. Typical transformations for which boost-buck units are useful include 208 to 230 or 240 volts, 230 or 240 to 277, and 230 or 240 to 208.

The buck-boost transformer cannot be relied upon to smooth out voltage fluctuations; that requires a voltage regulating or “constant voltage” transformer. In addition, autotransformer connections are subject to several limitations in three-phase circuits.

As with conventional transformers, an open-delta connection using two transformers instead of three is a money-saver. However, such a connection requires the circuit supplying the primary voltage to be either wye-connected or a three-wire closed delta. If the primary circuit is connected in delta, autotransformers cannot be used to supply a wye-connected secondary circuit. Several other three-phase options are not usable, largely because the autotransformer network cannot create a true secondary neutral.

To order a back issue with the full article, “When voltages don’t quite match: Buck-boost transformers,” call 312-321-9440 or visit our online webstore.

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