Unbalanced Voltages Increase Cost

Tech Note

The electric 3-phase induction motor is the work horse of industry and represents a significant portion of industry’s electrical utility cost. Plants with a large population of 3-phase induction motors can easily have electric utility bills in excess of several million dollars per year.

As an example of some typical numbers, assume a 24/7 operation with a total motor load of 3000 horsepower, average efficiency of 86%, average lagging power factor of 0.88 (angle of 28.4 degrees), and 10 cents per KWH would have a motor utility energy cost in excess of 2.5 million dollars per year. This cost does not even consider the utility company’s "demand" charge for KW (the rate of KWH consumption) levied on motor loads of this size. A 2% savings in just the energy cost alone would generate at least $50,000 dollars a year, a nice addition to the "bottom line".

Utility companies deliver to our operational facilities 3-phase voltage systems, which we generally assume to be balanced i.e all three individual phase voltages are equal in magnitude and separated by 120 electrical degrees. Now, the fact is that 3-phase voltages are generally not balanced in magnitude upon delivery (utility companies consider 3%-5% balance in 3-phase voltage systems acceptable) and they are typically not balanced throughout an operational facility.

An often overlooked phenomenon is that unbalanced three phase voltages are responsible for adding significant additional operational motor costs. The percent voltage unbalanced is defined as 100 * (maximum deviation from average) / average. For example, in a typical 460 volt 3-phase system with actual phase voltages of 446, 472, and 450 the average is 456, the maximum deviation from average is 10 and the percent unbalance is 2.2%. A few percent unbalance does not, at first, seem like too much; however, the increase in motor operational cost is significant.

As an example, studies from the Electrical Apparatus Service Association
(EASA) show that a 2% increase in voltage unbalance for a 3-phase system can have detrimental effects on 3-phase induction motor operation. Some examples:

  1. increase in motor temperature by 20°C
  2. increase I2R copper losses by as much as 30%
  3. decrease overall efficiency by as much as 2%
  4. winding insulation life expectancy is significantly reduced; rule of thumb is for every 10°C increase in winding temperature the insulation life in halved.

Motor manufacturers can supply, upon request, detail operational specifications and typical test data on products. These examples suggest that monitoring and analyzing 3-phase motor operational conditions could lead to substantial cost savings.

Dataforth has a complete line of signal conditioning modules and data acquisition sub-systems for monitoring temperature, voltages, currents and speed (RPM via frequency) which are available for establishing 3-phase motor monitoring systems. In particular DSCA33, SCM5B33, and SCMVAS product families convert AC potentials into True RMS direct current as well as protect and isolate hazardous voltages from the data acquisition system!

Dataforth True RMS signal conditioner products:

Dataforth Voltage Attenuation System:

Remember, our Application Engineers can assist you with signal conditioner selection over the phone or via fax and email. Call us at our manufacturing facility in Tucson at 520-741-1404 (fax 520-741-0762) or Email us at support@dataforth.com.

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