When a motor is built as a “totally enclosed” (TE) unit, no free interchange of air can take place between interior and exterior. That’s good protection against entry of solid or liquid contaminants.
But no enclosure can be airtight. As the TE motor heats and cools between “on” and “off,” or from day to night, outside air will “breathe”, in and out, and the water vapor in that air will condense on interior surfaces of windings and bearings. The result will eventually be insulation failure and bearing corrosion.
To prevent that, the motor’s interior needs to be kept warm — above the condensation temperature — whenever it’s not running. This is particularly true for motors that run only a few hours a day, are seasonally idle, or are installed in unusually damp locations. It’s not a function of motor size or type. The same conditions exist, and will be equally damaging, whether the motor is 1 horsepower or 1000.
Two methods of internal heating are possible. For the larger machines, separate electric resistance heating elements can be built into the motor frame. These “space heaters,” energized from (typically) a 120 volt auxiliary circuit, are switched on automatically by the motor starter whenever the motor itself is switched off.
For smaller three-phase machines, particularly at 230 or 460 volts, the control circuit can be arranged to supply a low voltage to one phase of the winding whenever the motor is deenergized. At least one supplier offers an electronic “black box” to do this, mounted in the motor control center, taking its power supply directly from the motor branch circuit and using solid-state devices to convert that voltage to the low value needed for heating the idle winding. That’s less effective for a large motor because the space to be warmed is much larger. Although the winding itself may be adequately heated, the bearings and other metal surfaces throughout the motor interior may not be protected.
To suit whichever of these two methods of heating may be appropriate, a more suitable term for the technology would be “space heating” rather than “space heaters.” The voltage available for separate heaters must be specified by the motor purchaser, and for motors in the 1000-5000 hp size range may be 480 volts threephase. But a single-phase 120 volt circuit is most common.
Little heating power is needed, depending upon the method. Since the alternative can be unexpected winding breakdown, with possibly high downtime and repair costs, “efficiency” of the heaters (used only when the motor is idle) should not be an issue.
Whatever is used, the heating system should not be neglected. Although heating circuits are generally reliable, any maintenance program should include some check of heater operation, because long-term damage resulting from heater failure may not be apparent until too late.
Richard L. Nailen, P.E.