Sags, swells, and voltage deviation effects for troubleshooting motors

NEMA states speed and torque performance—as well as motor life—may be negatively affected by voltage variations.
By Nicole Dyess, Motors@work April 3, 2018
Troubleshooting series: Sags, swells, & voltage deviations Courtesy: Motors@WorkNational Electrical Manufacturers’ Association (NEMA) standard MG1 states that “motors shall operate successfully” so long as the supplied voltage is within ±10% of rated [§ 12.44]; i.e., from 414 V up to 506 Volts for a nominally 460 V motor. But “successful” doesn’t mean efficient or reliable operation: in MG1, NEMA states that speed and torque performance — as well as motor life—may be negatively affected by voltage variations.
Voltages outside these ranges are fairly common—if only momentarily. Half of all utilities report their customers experience potentially disruptive voltage deviation issues—excluding major events and total outages—nine to 20 times per year.
So, what happens to motor when voltage deviates from this range? What about momentary—i.e., less than one second—sags, swells, transients, and interruptions?
Here’s a quick primer on how each of these power quality problems affect motors.

Over-voltage events

Transients, swells, and over-voltage deviations occur when RMS voltages exceed 110% of nominal. Per IEEE and IEC standards, we use different terms to describe the event depending on the duration of the deviation:
Transients, unofficially called surges and spikes, are sudden voltage spikes of extremely short duration — less than 50 nanoseconds, or billionths of one second. Transients are classified as impulsive or oscillatory depending on the shape of the waveform [see Figure 1, graphs (E) and (F)], and then by “speed” — a ratio of the time it takes to hit peak voltage versus the time it takes to return to normal voltage. Transients that exceed the voltage rating for the motor epoxy may lead to arcing between windings, reducing the life of your motor’s windings.
Voltage increases that rise and return to normal voltage in less than one second are called swells.
Persistently high voltages that last more than one second are called overvoltage deviations.
Some motor users state they run high voltages (e.g., 490 V) in order to reduce motor current draw, thereby decreasing motor temperature and extending motor life. This is a myth.
FIGURE 1 Depictions of over- and under-voltage events Courtesy: Motors@WorkOperating your motor at higher-than-nominal voltages pushes the motor’s iron core towards magnetic saturation—the maximum magnetization that the material can hold, or maximum magnetic flux it can produce. To maintain equilibrium between voltage, frequency, and magnetization described in Faraday’s Law, a motor operated at higher-than-nominal voltages will continue trying to increase magnetic flux production.

However, as the motor approaches saturation, it becomes harder and harder to produce each incremental unit of magnetic flux. So, as it tries to increase magnetic field strength, the motor draws more current, causing higher core (I2R) losses  (i.e., lower efficiency) and hotter motor temperatures. 

Under-voltage events

On the other hand, interruptions, sags, and under-voltage deviations all occur when RMS voltages fall below 90% of nominal. Like over-voltage events, we use different terms to describe the event depending on its duration:

Interruptions are complete losses of voltage lasting less than two minutes; a loss of power lasting more than two minutes is considered an outage. Interruptions are further classified as instantaneous (less than 30 cycles), momentary (30 cycles to 2 seconds), and temporary (2 seconds to 2 minutes).

When voltage reduces and then return to normal voltage in less than one minute are called sags or dips.

Persistently low voltages that last more than one minute are called undervoltage deviations.

Whereas higher-than-nominal voltages push the motor towards saturation, lower-than-nominal voltages reduce the strength of the motor’s magnetic field and thus its ability to produce torque at rated speeds. Since torque and slip are proportionate to the square of voltage, a 10% reduction in voltage (e.g., operating a 460-V motor at 414 V) reduces torque by 19% (i.e., 90%2 = 81%); a 20% reduction in voltage restricts torque production to 64% of its full-potential potential [see Figure 2].

FIGURE 2 Speed and torque performance of induction motors operated above and below nominal voltage Courtesy: Motors@WorkOperating below rated voltage may have minimal effect on motors running at less than 50% of its rated load and those starting up in low inertia applications (many soft starters work by ramping up voltage as the motor starts) — if anything, these motors may see improved efficiency through lower core losses. However, motors operating near rated load or starting high-inertial loads below rated voltage will experience higher current draws, lower torque, and longer start times — resulting in reduced starting ability, lower load capacity, more overheating, and a shorter motor life.

A 19.1% reduction in voltage means these blowers can only produce about 65% of nominal torque at rated speed. Since air flow, and the horsepower required to produce it, is proportionate to the cube of speed, that means there may not be enough air being produced. After checking air flow levels and the latest voltage measurements, the event appears to have been a sag—voltages and air flow have returned to normal levels. It’s good to make a note to keep an eye on the voltage because a longer or more severe power quality event may cause problems on a high-production day.

Nicole (Kaufman) Dyess has nearly 20 years’ experience optimizing the performance of motor-driven systems. She began her career at Advanced Energy testing thousands of motors, consulting with motor & appliance manufacturers on their designs, and documenting motor management best practices for the US Department of Energy. Subsequently, she managed statewide energy efficiency programs at the NC Department of Commerce and facilitated sustainability and process improvement projects for the City of Raleigh. Now, as Motors@Work’s director of client solutions, Nicole focuses on client implementations and user experience while providing technical support to the sales and development teams. Nicole holds master’s degrees in mechanical engineering and public administration. This article originally appeared on Motors@Work, a CFE Media content partner.

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