Power Quality and Drives LLC

Sag Correction for Small Equipment Without Using UPS Systems
Copyright © 2001 Francis J. Martino

The most common power quality problem encountered is that of voltage sag. All equipment is susceptible to dropping off line if the incoming voltage is low enough for a long enough period of time.

Publications found in the trade journals give the following generalized typical equipment shut-down characteristics. However, finding equipment on the market with AC and DC coils that have the low drop-out characteristics given may be difficult.

* AC coils will drop out with 40 to 70% of voltage rating in less than two cycles.

* DC coils will have a ride-through capability with sags down to 10% of rated voltage.

* Temperature controllers will drop out below 80% of rated voltage for durations longer than one cycle.

* Data processing equipment is susceptible below 70% of normal volts.

* Water solenoids and inductive devices will also trip on sag.

* Boilers with flame sensors may indicate a loss of flame with voltage sags, thus causing needless shutdowns.

* Furnaces will shut down when gas valve solenoids close on sag.

* Variable Frequency Drives have different voltage sag and ride-through capabilities. Drive capabilities vary with design, internal programming, load inertia, and degree of loading.

Sag Correction

Sag correction may be provided on a single coil or piece of equipment. Correction may also be chosen for large portions of a facility or for the entire facility. The selection of sag mitigation technolology will depend on cost verses advantages gained.

Two technologies have been selected for discussion which may be used to reduce sag susceptibility of small electronic and inductive devices. The technologies do not include a UPS and, therefore, the advantage of those technlogies is that there are no batteries to maintain. The disadvantage is that there is no ride-through on power outage of more than a few milliseconds.

DC Coils

AC motor starters are available that have a DC coil and a rectifier. Thus, the incoming 115 VAC, when rectified to 24 VDC, can theoretically sag to 10%, or, 11.5 VAC and still maintain sealing of the coil with 2.4 volts DC on the coil.

Equipment ratings and capability will vary among manufacturers. You may test the ability of your motor starters on the test bench by installing the DC coil and rectifier and then powering the starter through a variable transformer. In the absence of published literature by manufacturers, conducting your own test is the only way to be certain that expenditures on DC coils will be advantageous.

Other inductive devices and PLCs are available to operate on DC power supplies. The ability of a power supply to maintain sufficient DC voltage will depend on its ability to regulate below its rated input. The power supply may have to be oversized to maintain a functional output on voltage sag. Use a power supply with twice the capacity as needed and try it on the test bench. In the absence of published literature, your e-mail of test results will be greatly appreciated by those who read this webpage.

Ferroresonant Transformers

A ferroresonant transformer, also called a constant voltage transformer, will give a constant AC voltage output when the input voltage varies within the rated input voltage range. Typical response for output regulation is plus or minus 3% for input voltage swings that are within the rating of the transformer. Typical response times are 8 millseconds for a 5% line or load variation, and 16 milliseconds for a 10% variation. Generated harmonic distortion will be less than 3%.

A ferroresonant transformer will have a capacitor winding that resonates with the transformer winding. However, some constant voltage transformers may utilize capacitors rather than a capacitor winding.

A capacitor-winding type unit should have no problem when powering a load which is high in harmonic content, i.e, variable frequency drives, SCR controlled DC drives, office equipment, lighting with electronic ballasts, etc.

The constant voltage transformers which use capacitors may resonant with a harmonic load. Both transformer and driven equipment may both be damaged by the resonance. Before applying any constant voltage transformer to a harmonic generating load, be sure to call the transformer manufacturer and discuss the transformer capabilities.

A winding may be added to the ferroresonant transformer which will remove harmonics so that those harmonics will not be reflected to the transformer primary winding. Thus, with high harmonic generating equipment powered from the transformer there will be no harmonic distortion created on the distribution system. Use of the transformer will aid in meeting IEEE 519 requirements for harmonic distortion at the point of common connection.

Commercially Available Equipment

Single-phase power supplies are commercially available that give regulated DC outputs of 5, 12, 15, 24, and 48 volts DC with an input range from 85 to 264 volts AC. Utilizing a system input of 240 VAC, the power supply will give a constant DC voltage output with an input voltage sag down to 85 VAC, which is 36% of the normal 240 VAC line voltage.

Three-phase power supplies are available that will regulate with a sag down to 75% of normal AC voltage.

A voltage sag may be the result of a line fault condition, either within the facility or on the utility distribution system, which will cause one or more phases to momentarily drop to zero volts. If one phase has completely collapsed, the other two phases may be found to be separated by 180 electrical degrees rather than the normal 120 degrees.

The input frequency range of the above power supplies will be a nominal 47 to 63 HZ. The power supplies will therefore regulate if the input frequency is “recognizable” or recovers within the nominal rated 40 millisec (2.4 cycle) ride-through time.

A typical ferroresonant transformer will regulate output voltage under full load between +10% and - 20% of rated voltage. Thus a regulated output of 120 VAC will accept an input range from 96 to 132 VAC.

A ferroresonant transformer will regulate to 65% of nominal if the load does not exceed 60% of the transformer rating. Therefore, to regulate down to 78 volts AC input (65% of 120), oversize the transformer by 67%. Thus a 167 amp transformer will regulate 120 VAC output with 78 VAC input if the load does not exceed 100 amps (60% of 167).

A typical ferroresonant transformer will deliver momentarily 200% of its full load curent rating. Above 200%, the output drops to zero volts, but the unit will not destruct.


Start Small to Solve Sag Problems, Power Quality Assurance Magazine, March/April 1998,
pages 40-44.

AC Voltage Regulation Technologies, Power Quality Assurance Magazine, July/August 1997,
pages 92-97.

Power Quality and Drives LLC

Power Quality and Drives LLC
P.O. Box 83
Middlebury, CT  06762
Phone: (203) 217-2353
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