Here's how to select and install resistance heaters for reliable operation of motors in damp locations.
You can avoid condensation problems in motors by maintaining a winding temperature 5 [degrees] C to 10 [degrees] C above the surrounding air temperature. For 3-phase AC motors operating in damp conditions, the best way to prevent condensation is to install resistance heaters inside the motor. The installation of flexible resistance heating strips or coils is by far the simplest and least expensive method. For example, a heater and its installation in a typical 60-hp, 364T-frame motor will cost approximately $210. These strip space heaters typically are installed inside the stator housing near the bottom of the motor or in the air chamber above the motor's feet. They should be at least 1/2 in. from the windings.
All motor manufacturers publish a recommended heater wattage for each frame size that varies with manufacturer for any given frame. The differences are small enough to allow reference to a standardized table, such as shown here. Also, you can calculate the approximate wattage required by using the equation W=2DL, where "W" is the heat in watts, "D" is the outside diameter of the stator laminations in inches, and "L" is the length of the stator core in inches.
If you don't have sufficient clearance for rigid strip heaters, flexible heaters can be laced to the outside diameter of the end turns of varnished windings. They should not overlap or be double layered. Larger motors often require a heater in each end.
Heater leads are brought out through the motor terminal box and connected to the available single-phase voltage.
A heater typically is activated by means of a motor control circuit relay contact, which energizes the heater when power to the motor is disconnected. Timers sometimes are used.
Trickle heating can be tricky
Another method is the application of a controlled low voltage to the windings. Although this application of AC or DC power to "trickle heat" the windings may be a viable alternative in a pinch, it's not practical as a permanent solution. Trickle heating with single-phase 115V or 230V AC power will draw excessively high currents, thus requiring a large wire size, greater energy consumption, and potential degradation of the motor's insulation. For example, a 60-hp, 364T-frame, 1770-rpm motor tested on single-phase AC power draws 64A at 120V, 60A at 115V, 50A at 100V, and 15A at 40V. The amount of heat that would be generated by a sustained draw of even 50A quickly begins to degrade the winding's insulation.
Furthermore, this author is not aware of any generally accepted method to calculate the amperage that will produce the desired stabilized temperature, nor any technique to calculate the voltage that will produce that amperage. Therefore, a variable power source is required, and a time-intensive trial-and-error process must be carried out to find each and every motor's individually correct voltage, amperage, and resulting rise in temperature. As a permanent installation, the use of a contactor between the low-voltage power source and motor is required, in addition to the relay at the motor control source.
You'll encounter the same difficulties when trickle heating with a DC power source (which would need to be individually variable at each motor or group of identical motors). Also with DC, the amperage drawn goes up more quickly per volt than with AC.
Where DC trickle heating can come in handy is as a last-resort, quick-fix to dry out occasional winding dampness. For example (again using a 60-hp motor), if insulation resistance readings suggest winding moisture, the motor can be connected to a readily available temporary DC supply, such as a welder, whose rheostat can be adjusted until the ammeter shows an initial pull of 5A to 10A. What amperage to start at is a judgment call or an educated guess. (The above is a hypothetical example, not a recommendation.)
Adjusting the amperage up or down from your starting point and determining how long to apply the current are also risky hit-or-miss guesses, even with temperature probes. Nonetheless, after a certain period of time at a certain amperage, test insulation resistance again, and then repeat the process until either acceptable readings are obtained.