Jim Bryan
EASA Technical Support Specialist (retired)

From time to time, the subject of space heating comes up to address issues caused by condensation in idle motors (or generators). Many feel that a motor can receive more harm from being idle than if it is in service. This is especially true in humid environ­ments where condensation can have a negative impact on winding insulation material and machined surfaces. 

Condensation forms when the temperature falls below the dew point. This meteorological term refers to the amount of water vapor the air can hold. The higher the humidity, the closer the ambient temperature will be to the dew point. This becomes a condensation problem when warm, moist air cools, especially at nightfall.

Space heating is intended to keep the surfaces of the motor at about 10°C (18°F) above the ambient tempera­ture so the dew point is never reached inside the machine. This is an auxiliary heating source that should only be utilized when the motor is idle. Any­thing that increases the temperature of the motor while it is in operation will decrease the winding life. We will discuss various methods to apply this space heating and their merits and shortcomings.  A September 2001 Cur­rents article, “How to Properly Size Space Heaters,” outlines how to de­termine the correct wattage depending on the motor size. Using the formula from this article, you can calculate the watts required to maintain the required temperature for the methods of space heating described in this article. The formula is:

W = 2DL (Inch)

Or

W = 0.0031DL (Millimeter)

Where

• W = Wattage
• D = Stator Outside Diameter
• L = Stator Length

Cartridge and steel strip space heaters
Cartridge type heaters can be wire wound, resistive or ceramic (see Figure 1).  Steel strip heaters (see Figure 2) are used in a similar man­ner. They offer localized heating and are usually in­stalled  in the end brackets of the machine. They are fairly efficient for enclosed machines, but lose effec­tiveness in open machines since they are not in contact with the affected parts. Often the temperature and wattage are increased in these units to make up for the lack of proximity. This requires adequate space between the winding and the hot surfaces of the heater to avoid damage to the winding insulation. 

One drawback to cartridge heaters is that, since they are often mounted to the end bracket, they heat the bear­ing and lubricant. One manufacturer even installed a shield to minimize this problem. They must be supplied with a separate power supply that is controlled to energize only when the motor is de-energized. Retrofitting with these units requires disassembly of the machine, mounting the heaters with an appropriate bracket and sup­plying a properly controlled voltage supply.

Cartridge type heaters are also available to use in oil sumps to pre­vent condensation from contaminat­ing the oil. To avoid burning the oil, these should be limited to no more than 12 watts per square inch (1.9 watts per cm2) and typically 50-75 watts max.

Silicone rubber strip heaters
Silicone rubber strip heaters (see Figure 3) are more common. The advantage they have is being able to have better proximity to the motor winding which is the more vulnerable part to be protected. They are typically wrapped around the end turns of the winding and laced in place with cord. The watt density of the strip heater should be no more than two watts per square inch (0.3 watts per cm2). If the watt density is greater, insulation such as phase insulation paper should be placed between the end turns and the strip heater to prevent damage to the winding insulation. This insulation will inhibit the effectiveness of the heat transfer to the winding. Many vendors specify the watt density of their heaters. In the event that the required density is not available, heaters can be pow­ered at a lower voltage to limit the watt density. One-half voltage is commonly used in these cases. This will produce one-fourth the watts per square inch or square centimeter and will require adequate surface area to yield sufficient total wattage. Often, users will feel that this will provide a more robust appli­cation and the heater will last longer. Experience has shown that silicone rubber space heaters are very reliable. The inconvenience of dealing with a much larger area in a confined space of the reduced voltage scheme offsets any advantage in extended life.

Like cartridge heaters, strip heaters must also be supplied with a sepa­rate power supply that is controlled to energize only when the motor is de-energized. Retrofitting with these units also requires disassembly of the machine, lacing the strip heaters to the end turns and supplying a properly controlled power supply.

Both cartridge heaters and strip heaters are localized sources of heat. They can be distributed in both ends of the machine to improve performance, but this requires additional parts and effort. But it is not likely that the heat from a single heater will completely permeate the entire core evenly. Often we have seen vertical motors or hori­zontal motors mounted vertically with the strip heater laced to the upper-end turn of the winding. Since heat rises, this will have little impact on the lower part of the motor.

All space heaters should be tested for function and insulation resistance whenever the motor is tested, whether during repairs or routine scheduled maintenance. An appropriate meter scale should be used when measuring the heater’s current. Not doing so has led to erroneous results in the past.

Trickle voltage heating
Another method of space heating is known as trickle voltage heating. This method uses a low voltage; typi­cally 10-20 percent of rated voltage and 25-35 percent of rated current, applied when the machine is not energized.  

A little trial and error with the ap­plied voltage will determine the proper current level. Start at the low end of the voltage range and work up measuring the current at each step. Use the for­mula below with your experimental voltages until the watts are equal to the watts from the formula below. 

W = I x E x P.F.

Where

• I    =    Measured Current
• E   =    Applied Reduced Voltage
• P.F. =   Power factor (Use 0.25)

The circuit in Figure 4 shows how to connect a step-down transformer through auxiliary contacts to provide the voltage determined by the formula only when the unit is off. The transformer should have ±5% and ±10% taps so the voltage can be tuned to the application. The time delay relay should be 10-180 seconds to al­low the magnetic field of the motor to collapse before energizing the single-phase connection. This will prevent re-closure damage to the motor or transformer.

Trickle voltage units are also avail­able commercially which takes a lot of the experimentation out of the process. The vendors listed in Table 1 offer units for motors up to 900 horsepower (670 kW). This short list was found with a quick Internet search using the keywords “motor winding heater.” Ad­ditional sources are available as well; conduct your own search for others. These units offer the ability to “plug in” with the engineering and control already done. This is particularly useful for retrofit situations since the motor will not need to be removed or disas­sembled. All the work can be accom­plished in the motor control cabinet.

Conclusion
Moisture is one of the most detri­mental conditions a motor winding can face. Any time a machine is not energized, it is subject to condensa­tion, whether it be installed and idle or in storage waiting for an opportu­nity to serve. Some sort of auxiliary heating is prudent to achieve the max­imum service life and performance of the machine. 

AVAILABLE IN SPANISH

Categories: Miscellaneous, Space heaters

Tags: Space heaters Motor accessories

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