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.


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.

Top Plant
The Top Plant program honors outstanding manufacturing facilities in North America.
Product of the Year
The Product of the Year program recognizes products newly released in the manufacturing industries.
System Integrator of the Year
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
October 2018
Tools vs. sensors, functional safety, compressor rental, an operational network of maintenance and safety
September 2018
2018 Engineering Leaders under 40, Women in Engineering, Six ways to reduce waste in manufacturing, and Four robot implementation challenges.
GAMS preview, 2018 Mid-Year Report, EAM and Safety
October 2018
2018 Product of the Year; Subsurface data methodologies; Digital twins; Well lifecycle data
August 2018
SCADA standardization, capital expenditures, data-driven drilling and execution
June 2018
Machine learning, produced water benefits, programming cavity pumps
Spring 2018
Burners for heat-treating furnaces, CHP, dryers, gas humidification, and more
October 2018
Complex upgrades for system integrators; Process control safety and compliance
September 2018
Effective process analytics; Four reasons why LTE networks are not IIoT ready

Annual Salary Survey

After two years of economic concerns, manufacturing leaders once again have homed in on the single biggest issue facing their operations:

It's the workers—or more specifically, the lack of workers.

The 2017 Plant Engineering Salary Survey looks at not just what plant managers make, but what they think. As they look across their plants today, plant managers say they don’t have the operational depth to take on the new technologies and new challenges of global manufacturing.

Read more: 2017 Salary Survey

The Maintenance and Reliability Coach's blog
Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
One Voice for Manufacturing
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Maintenance and Reliability Professionals Blog
The Society for Maintenance and Reliability Professionals an organization devoted...
Machine Safety
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
Research Analyst Blog
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Marshall on Maintenance
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
Lachance on CMMS
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
Material Handling
This digital report explains how everything from conveyors and robots to automatic picking systems and digital orders have evolved to keep pace with the speed of change in the supply chain.
Electrical Safety Update
This digital report explains how plant engineers need to take greater care when it comes to electrical safety incidents on the plant floor.
IIoT: Machines, Equipment, & Asset Management
Articles in this digital report highlight technologies that enable Industrial Internet of Things, IIoT-related products and strategies.
Randy Steele
Maintenance Manager; California Oils Corp.
Matthew J. Woo, PE, RCDD, LEED AP BD+C
Associate, Electrical Engineering; Wood Harbinger
Randy Oliver
Control Systems Engineer; Robert Bosch Corp.
Data Centers: Impacts of Climate and Cooling Technology
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
Safety First: Arc Flash 101
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
Critical Power: Hospital Electrical Systems
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
Design of Safe and Reliable Hydraulic Systems for Subsea Applications
This eGuide explains how the operation of hydraulic systems for subsea applications requires the user to consider additional aspects because of the unique conditions that apply to the setting
click me