Cool facts about cooling electric motors
It’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.
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.
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.
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.
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.
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, firstname.lastname@example.org