Cool facts about cooling electric motors

Whether it's an old or new design, the process of lowering motor temperatures is based on same principles.


Chuck Yung, senior technical support specialist, EASA. Courtesy: EASAIt's fascinating to see the different ways engineers have devised to do the same thing, yet reassuring to know how much remains unchanged even after a century of electric motors. One aspect of motors that could fit in both categories is the way they are cooled. This article takes a look at various cooling methods and how they can be improved for some special applications.

Heat dissipation methods depend on the machine's enclosure (ODP, WP or TEFC; see definitions below). When vent ducts are used, as in most ODP or WP enclosures, air is drawn into the ends of the rotor, centrifugally moved through its vent ducts to vent ducts in the stator, and then exhausted from the motor frame.

Some aspects of stator and rotor vent ducts are often taken for granted. For example, the simple I-beam spacers that keep the ducts open also provide more surface area for heat transfer, like the exterior ribs of a TEFC stator frame. As Figure 1 illustrates, the addition of an I-beam spacer roughly doubles the periphery of a vent cross-section without a spacer, greatly increasing heat transfer to the air flowing through the vent ducts.

The I-beam spacer aids in heat dissipation by doubling the surface area within each vent passage. Courtesy: EASA

Because stator core stacking pressure is usually 75-125 psi (515-860 kPa), the wider flange portions of the I-beam spacers are in firm contact with the lamination packets, which keeps the spacers square while maximizing the contact area. The web portions of the I-beam spacers serve to increase the surface area of the heat exchanger.

Obstructed vent openings and heavily coated surfaces in stator air passages can reduce airflow and heat dissipation, causing the winding temperature to increase. This includes stator vent ducts that are partially blocked with varnish (e.g., too many dips and bakes), dirt, debris or some combination thereof. Before having a large machine cleaned on site, however, make sure the vent ducts and air passages can be cleaned effectively this way without damaging the windings or the core.

The position of the air baffle relative to the fan blades is important.

Rust / corrosion cautions

In the rotor, rust corrosion can swell the core and constrict duct openings, especially on the inside diameter (ID) between the supporting ribs where the air enters the ducts. Detecting such blockage usually requires checking the rotor ID with an inspection light and mirror, or probing the vent openings from the outside diameter (OD) with a length of welding rod or heavy wire. Rust-swollen laminations also squeeze the vent ducts, further reducing airflow.

As rust progresses, the rotor will have less ferrous material to carry magnetic flux, gradually diminishing the motor's ability to develop torque. The motor will slip more as it tries to develop the required torque, which will generate more heat.

Customizing construction

There are several ways to maintain adequate airflow or even improve it, including tips borrowed from motor designers. With TEFC motors, for example, adding properly sized and positioned air baffles can reduce winding temperatures by 10-15°C or more.

Lots of tough TEFC applications like cement mills and paper mills are prone to buildup of dirt and debris on the exterior of motors, making it difficult to maintain adequate cooling. One solution is to install a rolled steel or fiberglass shroud over the motor's ribs. The best fit will butt against the edge of the fan cover (resting just atop the ribs) and extend to the end of the frame on the drive end (DE). On unidirectional applications, replacing the radial fan with a directional fan can increase airflow and keep dirt and debris out of the enclosed ribs.

This solution is also useful for applications subject to impact damage (e.g., bark hog or saw platforms), where broken ribs are common. In such cases, 3/16" (4-5 mm) plate steel can be used for the rolled cover to protect against high-impact damage.

Kiln applications

In kiln or oven applications, Class H insulation and high-temperature grease only go so far in extending motor life. Substitute C4 internal clearance bearings if the bearings are the thermal weak link. Adding a heat sink to the DE shaft extension can also reduce the heat transferred along the shaft. If the location is really hot, wrap the motor with copper or stainless steel tubing that has small holes drilled along the side facing the motor and lower the motor's temperature with pressure-regulated compressed air. A solenoid valve can be used to turn on the air when the kiln is running.

Cooling of synchronous motors can be improved by adding fan blades, or by angling existing blades for the direction of rotation. Courtesy: EASA

Cooling of synchronous motors can be improved by adding fan blades, or by angling existing blades for the direction of rotation. Courtesy: EASA


Open synchronous designs offer very little baffling to direct airflow, and most of them operate in only one direction of rotation. One common method for making these motors run cooler is to increase the size of the fans that are bolted to the rotor hub. Another option is to increase airflow by "pitching" or angling the fan blades for the direction of rotation. For example, if the design draws air axially into the rotor, tilt the blades in the direction of rotation.

It's always satisfying to discover ways to improve motor performance for an application. Usually, it involves borrowing a clever technique that a motor manufacturer used on one design and customizing it to the unique needs of another application. But still, it is useful to know it's possible to take a very good, reliable electric motor design, and make it just a bit more suitable for a difficult application.

-Chuck Yung is a senior technical support specialist at the Electrical Apparatus Service Application (EASA). EASA is a CFE Media content partner. Edited by Erin Dunne, production coordinator,

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...
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
Strategic outsourcing delivers efficiency; Sleeve bearing clearance; Causes of water hammer; Improve air quality; Maintenance safety; GAMS preview
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

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 of plant safety and offers advice on best practices.
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