Thermal modeling protects motors without overtripping

04/01/2009


Today’s modern microprocessor-based protection relays offer much more than the standard motor protection provided by electromechanical relays. Advanced features include motor-heating, time-to-trip and countdown-to-restart calculations %%MDASSML%% plus real-time diagnostics.

Even with the calculating power of microprocessor-based protection, most relays today still attempt to provide motor protection just by measuring current. The various manufacturers’ models calculate motor heating by thermal capacity or thermal register, where 0% is completely cooled and 100% is the trip threshold.

This thermal capacity is accumulated based on the measured current such that during motor starting, protection is essentially a function of the current squared multiplied by time (an I2t element), with maximum starting time dictated by the hot motor safe-stall time.

Limits of accuracy

While this type of protection dates back to electromechanical relays, the I2t model does not result in the most accurate motor protection, and limits true available horsepower. Thermal time constants and changing motor resistance are critical to proper protection.

Without proper compensation for these motor attributes, the overcurrent model overestimates motor heat, which causes it to trip prematurely. This is especially evident in cyclic overload conditions and in slow-starting, high-inertia load applications.

Cyclic overloads cause a motor to run for some time over the rated load and then for another period at less than the rated load. If the total heat buildup during the overload condition is less than the heat dissipated during the underloaded condition, then the motor thermal capacity is not exceeded. A true thermal model will calculate the heating and cooling time constants and properly apply them to the cyclic overloading condition.

An overcurrent-based protection scheme will overtrip the motor because the model overestimates heat buildup. Adjusting the cooling rate does not solve the problem because it is only correct for one known overcurrent and cyclic period. True thermal-based protection accurately models motor heat and provides accurate, proper protection.

Loading problems

Problems with the I2t model also arise when starting motors with high-inertia loads, as the time required to start the motor may approach %%MDASSML%% or even exceed %%MDASSML%% the hot safe-stall time. Highly accurate thermal-based protection includes a model that calculates motor slip (speed) during the start.

The relay calculates slip based on measured current, voltage and known full-load slip and locked-rotor torque (rated torque) from the nameplate. The relay uses the calculated slip to compute the positive- and negative-sequence rotor resistance throughout the motor start.

Motor resistance changes during starting by a factor of three or more. Without compensating for the resistance reduction during starting, an I2t scheme will trip before true thermal limits are reached, thereby restricting available horsepower during the start. Relays using a thermal model with a slip-based algorithm can automatically calculate maximum safe start times for each unique start sequence. Calculation of rotor resistance allows the relay to accurately reflect motor heating during a start and results in longer allowable acceleration times before tripping.

Motor protection can be greatly enhanced today with microprocessor-based relays with true thermal models that account for dynamic motor attributes. Relays that calculate thermal time constants and apply a thermal-based protection scheme can properly protect a motor during cyclic overloads without overtripping.

The slip-dependent thermal model protects the motor and allows for long acceleration times better than traditional I2t elements and electromechanical relays. Accurate calculation and tracking of motor heat by modern relays are valuable tools for improving motor protection and providing wider operating margins.

True thermal modeling provides motor protection even under cyclic loading from equipment such as this blower for an aerated holding tank at the Moscow, ID wastewater treatment plant.

Accurate thermal modeling provides protection that maximizes motor availability while providing excellent protection from damage.


Author Information

Mark Zeller is director of corporate marketing at




No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Your leaks start here: Take a disciplined approach with your hydraulic system; U.S. presence at Hannover Messe a rousing success
Hannover Messe 2016: Taking hold of the future - Partner Country status spotlights U.S. manufacturing; Honoring manufacturing excellence: The 2015 Product of the Year Winners
Inside IIoT: How technology, strategy can improve your operation; Dry media or web scrubber?; Six steps to design a PM program
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
Special report: U.S. natural gas; LNG transport technologies evolve to meet market demand; Understanding new methane regulations; Predictive maintenance for gas pipeline compressors
Warehouse winter comfort: The HTHV solution; Cooling with natural gas; Plastics industry booming
Managing automation upgrades, retrofits; Making technical, business sense; Ensuring network cyber security
Designing generator systems; Using online commissioning tools; Selective coordination best practices

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
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 Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me