Don’t let power sags stop your motors

A momentary voltage sag can cause contactor (motor starter) dropout, resulting in a stopped process or production line.

By Conrad St. Pierre September 1, 1999

A momentary voltage sag can cause contactor (motor starter) dropout, resulting in a stopped process or production line. The loss of even one motor in a series chain of subprocesses typically results in shutdown of the entire line just as surely as if all motors on the line had tripped off. Too often, plant operators accept these nuisance trips as a cost of doing business. It doesn’t have to be that way.

Three approaches are available to solve the problem.

Improve the system outside the plant . Many voltage sags result from the large exposure of an extensive utility distribution network. Problems due to lightning strikes are a classic example. The serving utility and plant can work together to analyze service reliability and how it can be improved. (See "A 10-step program for improved power quality," PE , April 1999, p 84.) While this approach is important to pursue for the long run, it isn’t likely to yield quick results.

Improve the system within the plant . This approach can be highly effective, but it normally involves some redesign of the plant electrical system, as well as substantial capital investment.

Correct the problem on a case-by-case basis . In many cases of trip-outs due to voltage sags, the motor applications and motor control circuits can be analyzed and the problems corrected individually. When applicable, this approach usually offers the quickest, most cost-effective solution. This article discusses the third approach.

Nature of sags

IEEE Std 1159 indicates that a voltage "sag" is a 10-90% decrease in rms voltage for a time period in the range from 0.5 cycle to 1 min. Figures 1 and 2 present typical data for severity and duration of sags. Of interest are those sags that cause contactor dropout and which can be eliminated in a cost-effective manner.

Figures 1 and 2 show a high percentage of voltage sags less than 50% in magnitude and less than 20 cycles in duration. Enabling the contactor and motor to ride through these sags greatly reduces unwanted shutdowns. (Voltage dips between 0 and 10% of normal voltage are not considered a problem for contactors.)

Before any type of solution is started, it is desirable to know if the source of the disturbances that result in outages is internal or external to the plant. Checking requires connecting monitoring equipment with storage capability to the electrical system for a short period of time. At least two quantities must be monitored to determine the source of a disturbance. If the plant has an incoming transformer, the easiest check is to place a voltage probe on each side of the transformer. The side with the greatest percent sag is the source side of the disturbance.

Nature of contactors

Depending on the particular model, a standard ac contactor drops out and disconnects its motor when the bus voltage drops 15-40% below rated voltage. If there is a sudden drop to 0 voltage, the ac contactor can drop out as fast as one cycle. On the other hand, dc contactor coils tend to be more forgiving than ac coils and hold in at lower voltage for longer duration (see table).

Motor dropout is not always caused by the contactor. Voltage sags may affect electronic controls that, in turn, trip the contactor. To correct a nuisance tripping problem with the least expenditure of time, effort, and money, it is important to know what initiated the outage.

A contactor is a single-phase device. Whether it drops out during a disturbance depends on whether it is connected to a phase involved in the disturbance. An event recorder is useful in determining whether there is a pattern to disturbances and phases involved.

On series-type processes that depend on all equipment operating, taking the operating voltage for all contactors from the same phase at least guarantees all-or-nothing uniformity. On the other hand, in a parallel-type situation such as several pumps handling wastewater, it would be desirable not to have all the motors tripped by a single-phase disturbance. Taking the control voltages for different contactors from different phases yields a better chance that some contactors remain closed.

Modifying contactor circuits

Figure 3 portrays the classic latching circuit for an ac contactor. The start button energizes the coil of the contactor M, closing its contacts and starting the motor. An auxiliary contact pair closes around the start button contacts to keep the contactor energized when the start button is released. A normally-closed stop button and overload relay contacts are placed in series with the supply so that they can de-energize the contactor. Sufficiently low supply voltage reduces the coil magnetic force to a point where the contacts are opened by gravity and/or spring action.

The coil inrush current for a large contactor is beyond the capability of the start button, so the contactor is operated by a control relay capable of handling the inrush current. Either the contactor or the control relay can drop out on low voltage.

There are several quick, simple, cost-effective ways to solve dropout problems. Be sure to examine motor application details to ascertain whether re-energizing the motor during the contactor delay time will cause any problem.

Time delay relay

One means to improve the ability of a motor to stay on line is to place a time delay relay (TDR) in the control circuit (Fig. 4). The TDR is energized together with the contactor coil, closing an additional pair of contacts around the start button. During voltage sags severe enough to open the contactor, the TDR contacts remain closed up to their time setting. If voltage returns during the TDR delay time, the contactor is pulled back in and the motor is reconnected to the line. In the case of a large contactor operated by a control relay, the dropout TDR should be added to only the control relay.

The disadvantage of this method is that the motor is disconnected and reconnected, resulting in worst-case inrush current and torque transient on both shaft and load.

DC-coil contactor

Another solution is to replace the contactor’s ac coil with a dc coil and rectifier circuit (Fig. 5). Because the flux in a dc coil is fixed and not varying as in an ac coil, the magnetic force has a longer decay time and the contactor hangs in longer when voltage is lost. Using a capacitor across the coil as shown in Fig. 5 provides additional energy storage to extend the dropout time.

Sizing of the capacitor depends on the current drain of the contactor coil and the length of time the contactor is required to hold in.

DC control with battery

A battery provides the dc supply voltage for the circuit shown in Fig. 6. In this approach, the contactor acts like a breaker. Since the loss of ac voltage does not release the contactor, an ac undervoltage (UV) relay must be included. The undervoltage relay disconnects the motor if an actual outage occurs as compared to a sag. This approach prevents the motor from automatically restarting when voltage is restored after an outage. Design considerations include the battery and necessary associated charger.

Undervoltage relay with resistor

A suitable resistor placed in an ac control circuit allows use of an instantaneous undervoltage relay to provide some ride-through (Fig. 7). A contactor coil with a voltage rating less than the circuit voltage must be used. For example, a 48-V ac coil might be used in a 120-V control circuit with the resistor dropping 72 V.

During a sag, the UV relay contacts short out the resistor. Thus, a higher voltage is provided to the contactor coil, allowing it to briefly ride through lower voltage. For very low voltages of longer duration, the contactor still drops out.

Constant voltage transformer

Another solution is to supply the control circuit from a constant voltage transformer (Fig. 8). This method is not the lowest-cost option, but it’s quick and easy to implement and allows the supply voltage to drop to 40-50% of nominal while still providing enough voltage to hold the contactor closed.

Pros and cons

Approaches that keep the main contactor closed during a sag (Figs. 5-8) as compared to those that allow the contactor to open and then reclose (Fig. 4), offer the advantage that the motor provides some voltage support to the system for a short time. In addition, maintaining the motor connection helps keep the voltages in phase with the system voltages (except for a nearby voltage disturbance), so the torque transients on the motor shaft and load are reduced.

The control circuits presented here show that various options are available to help a motor/contactor ride through many voltage sags. These are not the only possible approaches. Also note that although this article focuses on motor controls, techniques like those described here can often be used to help other types of controls ride through voltage sags.

— Edited by Rick Dunn, Editor, 630-320-7141,

Key concepts

Power sags may cause unwanted motor shutdowns.

The problem can often be cured by a simple, "Band-Aid" fix.

Contactor characteristics

Dropout time, Pickup

Contactor type kV Coil type cycles, at 0 V Minimum hold, V time, cycles

Air magnetic >1 ac 2 65-70% 4

Air magnetic >1 dc 5.5 65-70% 7

Vacuum(1) >1 dc 16-20 65-70% 7

Vacuum(2) >1 dc 10-15 65-70% 21

Size 1-6 &1 ac 0.5-2 40-85% 1-5

Size 1-6 &1 dc 1-5 10-50% 1.5-5

Size 7-9 &1 dc 6-2 030-50% 4-16

(1) Vacuum interrupter in air magnetic frame

(2) Stationary bolt-in design

More info

Mr. St. Pierre is willing to answer technical questions concerning this article. He can be reached at 518-356-9686; e-mail See the "Electrical power distribution and application" channel on www. for more articles related to this topic.

The following papers were used as references in the preparation of this article: "Point of Utilization Power Quality Results" by D.S. Dorr, IEEE IAS Transactions 1A, July/August 1995, pp 658-666; and Electric Power Research Institute (EPRI) Report No. RP3098-1.