Chuck Yung
EASA Senior Technical Support Specialist

I’ve often commented on how for­tunate we are to work on such a variety of electric motor designs. One day, you are working on a new design some designer has recently created, and the next day you are repairing a motor that could be in a museum. It’s fascinating to see the different ways engineers have devised to do the same thing, and yet reassuring to see how many things remain unchanged even after a century of electric motors. 

One aspect of electric motors that could be placed in both categories is the way an electric motor is cooled. This article takes a look at how motors are cooled and how we can improve cool­ing for some of the special applications we encounter.

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

There are some aspects of the vent ducts that are often taken for granted. In particular, the vent ducts in the stator and rotor use an I-beam spacer to keep the vent ducts open. Those simple spacers do more than just keep the ducts open. They also increase the surface area, much like the ribs on the exterior of a TEFC stator frame. As Fig­ure 1 illustrates, the periph­ery of the vent cross-section without a spacer is roughly doubled by the addition of the I-beam spacer. That extra surface area greatly increases heat transfer to the air flowing through the vent ducts.

Because the usual stack­ing pressure of a stator core is 75 – 125 psi (515-860 kPa), the flange portions of the I-beam shapes are in firm contact with the lamination packets. The web portion of the I-beam spacer serves to increase the surface area of the heat exchanger. The wider flange increases the contact area with the laminations, as well as keeping the spacer square.

One departure from the I-beam design described above was to use pins in place of the I-beams. It seemed like a good idea at the time, as it actually opened up the area for airflow. But the loss of the I-beam’ s contribution to surface area for cooling resulted in increased temperature. It was a great testament to the genius of the early electric motor designers who realized the need to balance airflow and surface area for optimum cooling. 

Anything that obstructs the vent openings, or overly coats the surface, reduces the effec­tiveness of air flow and heat dissipation. For example, if the vent ducts are par­tially blocked with varnish (too many dips & bakes), with dirt (not properly cleaned), or some combination (dipped and baked without being properly cleaned first), the winding temperature will increase. That is a major consideration when a customer wants a large machine cleaned on site. Can we effectively clean those vent ducts without damag­ing the windings or core?

Rust / corrosion cautions
In the rotor, rust corrosion can swell the core and constrict the duct open­ing, especially on the inside diameter of the core where the air enters the ducts. Such blockage may not be no­ticeable unless you use an inspection mirror and light to inspect the inside diameter of the rotor core between the supporting ribs. An alternative is to use a length of welding rod or heavy wire to probe the vent openings from the rotor OD. As steel rusts, there is less ferrous material to carry magnetic flux, and the ability of the motor to develop torque gradually diminishes. Not only does the motor slip more (as it tries to develop the required torque) – which generates more heat – but the swelling laminations squeeze the vent ducts and further reduce airflow through them.

Customizing construction
There are several methods for im­proving airflow that include tips we can borrow from the designers. One method to improve cooling of a TEFC motor is to add air baffles. Properly sized and positioned air baffles (see Figure 2) can reduce winding tem­peratures by 10-15°C, or more.

Some applications are prone to dirt buildup on the exterior of TEFC motors. Cement mills and paper mills are good examples, but there are lots of other tough applications. To prevent the ribs from being coated by dirt, use a rolled steel or fiberglass shroud over the 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. If the ap­plication is unidirectional, the radial fan can be replaced with a directional fan to increase airflow and keep the dirt blown out of the enclosed ribs.
This method can also be used in applications prone to impact damage (e.g., bark hog or saw plat­forms), where motors rou­tinely have ribs broken off. Use 3/16” (4-5 mm) plate steel 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. Add a heat sink to the drive end (DE) shaft extension to reduce the heat transferred along the shaft. If a customer has a really hot location, wrap the motor with copper or stainless steel tubing, with small holes drilled along the side facing the motor. Compressed air, via a pressure regu­lator, can be used to lower the motor temperature. A solenoid valve can be used to turn on the air when the kiln is running.

Longer coil pitch with same chord factor
One little-known method to im­prove cooling of very low speed form-wound motors is to increase the coil pitch. Chord factor can be viewed as a pyramid, with “full pitch” (chord factor = 1.0) at the peak. Consider the example of a 16 pole, 72 slot stator. 72 slots / 16 poles = 4.5; so a “full pitch” of 1-5.5 is impossible. The conventional wisdom calls for a coil pitch of 1-5, which has a chord factor of 0.985:

72/16 = 4.5    For a 1-5 pitch:    Sin((4x90) / 4.5) = 0.985

An alternative, if the motor operat­ing temperature is high, is to use a coil pitch of 1-6 instead. The effective turns, magnetic flux densities and torque remain exactly the same because the chord factor is still 0.985:

Sin((5x90) / 4.5) = 0.985

The advantage gained is that each coil has slightly more exposed exten­sion surface for cooling. The offset is that it takes slightly more copper to produce the wider coils, and the winding resistance increases due to the longer mean length of turn.

Fans
Open synchronous designs offer very little baffling to direct airflow, and most of them operate in only one direction of rotation. When a customer asks what can be done to make the motor run cooler, one common method is to increase the size of the fans bolted onto the rotor hub. Another option is to “pitch” or angle the fan blades, for the direction of rotation, to increase airflow. For example, if the design draws air axi­ally into the rotor, the blades should be leaned into the direction of rota­tion, as in Figure 3. Note:  For more information about fans and affinity laws governing them, see “Fan Law Knowledge Can Help Performance” in the October 2002 issue of Currents.

One of the great satisfactions of what we do lies in finding ways to improve motor performance, custom­izing each motor for the customer’s unique needs. Most of the time, we are simply using some clever technique that a motor manufacturer used on a different motor, for another applica­tion. But still, it is satisfying to know that you can take a very good, reliable electric motor design, and make it just a bit more suitable to a customer’s difficult application.

Categories: AC motor cooling, DC motor cooling, Ventilation/cooling

Tags: Motor temperature Motor cooling

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