December 2010 Electrical Apparatus

December 2010 Electrical Apparatus

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

[Note: This article, as printed in the December 2010 EA, contained a mathematical error. 1/1.73 is equal to 0.578 (not 0.0578, as we printed.) EA regrets the error and thanks N.E.Nilsson of Columbiana, Ohio for bringing it to our attention.]

Among those employed in the electrical industry, involved in installing, maintaining, and operating circuits and apparatus, some persistent uncertainties and misunderstandings exist. These can lead to misuse of tools and equipment, to misinterpreting tests, or to unexpected operating problems. This article deals with just four such issues–not necessarily the most important, but which frequently recur in questions and problems brought up by electricians, contractors, and plant personnel.

Voltage at the terminals of a connected load (particularly a polyphase induction motor) is a common subject of discussion. Unbalance between individual phase voltages is known to degrade motor performance. Its relation to current unbalance, and consequently to winding temperature and load-carrying capability, is often unclear. Voltage unbalance can be calculated in three ways, giving similar answers, although only one is recognized as standard. Unbalance can exist at the three-phase source, not necessarily governed by any enforceable limit. Also, unbalanced motor currents can sometimes be present even when voltages appear to be balanced.

Regardless of phase balance, how motors are affected by terminal voltage below the rated value is widely misunderstood, seeming to contradict basic theory. Under load, current increases when voltage decreases, whereas both rise and fall together during acceleration. Operation of a 230 volt motor at 208 volts can be a problem–or not–depending upon the 208 volt source; this, too, tends to be confusing. For either an unbalanced or low-voltage condition, motor operation depends only upon voltage at the motor terminals –not at the starter or supply transformer.

For either an unbalanced or low-voltage condition, the effect on any electrical load depends only upon the voltage at the load terminals. Measurements at the control, or upstream supply transformer, are not relevant.

Three-phase connections of either source or load can be either wye or delta, which raises several questions: Does the power system connection make a difference to a motor? What about the effect on winding insulation when the delta supply is corner-grounded? Is motor performance different when the winding is designed for one connection rather than the other? None of these issues is a legitimate concern. Motors of certain types or sizes are often said to be “always delta-connected,” which is not true. The winding connection is a manufacturer’s option based on many design, manufacturing, and cost considerations.

To order a back issue with the full article, “The top four electrical misunderstandings,” call 312-321-9440 or visit our online webstore.

Share Button