Volume 1 Number 4 - August 1999 - Air- vs Oil-Filled Submersible Motors

Long, trouble free life may be the most important consideration in specifying and installing a submersible pump system. No component is more important than the motor in attaining this goal. Many design features determine the quality and ruggedness of a submersible motor. One of the most important is the motor type, air filled or oil filled. An air filled motor is superior to an oil filled motor in many ways.

The higher power cost of the oil filled motor is the result of a lower efficiency compared to the air filled motor. The lower efficiency of an oil filled motor is the result of two major issues:

• The oil filled motor has a significant hydraulic drag resulting from the rotor spinning in an oil bath. The extra power required to spin in oil is like walking in water compared to air.
• In addition, the hydraulic drag requires the gap between the rotor and stator to be larger in an oil filled motor. Because the motor operating principle is based on the magnetic field generated between the rotor and stator, a larger gap automatically results in lower efficiency. If the gap was small as in an air filled motor, the hydraulic drag between these two components would be exorbitant.

2) An Air Filled Motor Runs Cooler Than An Oil Filled Motor.

• Because of the lower efficiency discussed in No.1 above, an oil filled motor design generates more heat than an air filled motor design. Most oil filled designs have a layer of oil between the stator and the outer motor wall that is in contact with the cooler surrounding water. This oil acts as a layer of insulation, retarding the dissipation of heat. So, the combination of the additional heat generated due to lower efficiencies and the oil insulation layer between the stator and the outer shell results in a higher winding temperature in an oil filled motor. The higher winding temperature means a shorter life.
• You may have noticed that most oil filled motors have windings with only an insulation Class B. This is because the flash point (boiling point) of the dielectric oil is lower than the Class F insulation temperature of 155°C. So, the oil filled motor runs hotter than the air filled motor, but yet has a lower maximum allowed operating temperature than the air filled motor.

3) Air Filled Motors Have A Longer Bearing Life Than Oil Filled Motors.

There are several major differences between air filled and oil filled motors that determine the life of the bearing:

• The metal surfaces in the bearing (the balls and the raceways) must not contact each other during operation. The role of the lubricant is to maintain a molecular grease film between the two surfaces preventing contact. The higher viscosity grease used in the sealed bearings in the air filled motor will maintain the lubricant layer between the surfaces better than the thin low viscosity dielectric transformer oil used in the oil filled motors. Generally, the transformer oil does an acceptable job of lubrication at lower temperatures but becomes marginal as the temperature approaches 100°C. Many motors operate at or above 100°C (Class A insulation allows 105°C maximum temperature and Class B insulation allows 130°C). If they did not, why would a manufacturer spend money on anything better than a Class A insulation rating?
• Lubrication and cleanliness is of critical importance in assuring long bearing life. The air filled motors maintains a clean lubricant because the bearings are sealed at the bearing manufacturer. Conversely, the bearings used in an oil filled motor are open allowing the contaminated circulating oil to pass through the bearings and between the bearing surfaces. Drain any oil filled motor and you will see the contamination and debris in the oil.
• Sealed bearings used in the air filled motors have another advantage in that a variety of lubricants can be selected based on the application. You are not limited to transformer oils that must serve other purposes in the motor "system". The ability to select translates into a longer bearing life and a more dependable pump.

4) Maintenance Is Easier And Less Costly On An Air Filled Motor.

Maintenance on an air filled motor is as simple as disassembling, rebuilding as necessary and reassembling. Oil filled motors require additional maintenance. The old oil must be drained and properly disposed. It may not be discharged into any sewer. Upon reassembly, the motor must be thoroughly cleaned, including all interior components to eliminate the contamination discussed above that will lead to premature bearing failure. After final assembly, the oil filled motor must be refilled with new oil.

5) An Air Filled Motor Does Not Expose The Engineer, Contractor Or Owner To An Undefined Future Liability Resulting From Environmental Contamination By Oil Leaks From The Motor.

Any operating piece of equipment filled with oil has a potential to develop a leak. An oil leak from a submersible pump will directly contaminate the water in which it operates. Where a discharge of dielectric oil into any sanitary sewer is illegal, it is an especially critical in non-sanitary applications because of the visible nature of an oil slick.

Note: The oil used in the Weil seal chamber is biodegradable food grade oil and will not cause an environmental problem.

6) An Air Filled Motor Is Less Likely To Fail Prematurely.

• Any oil leak in an oil filled motor will result in premature motor failure. The upper bearing will fail immediately after the oil drops below the bearing. The oil filled motor relies on the oil to dissipate the heat. As the oil level drops, the motor will begin to overheat.
• The amount of time the windings are at the operating temperature is one of the determinants of winding life. Longer time means shorter life. In an oil filled motor, the hot oil does not cool quickly when the motor shuts off. The continuing contact of the hot oil with the windings keep the windings hot and has the reverse effect of its purpose of cooling, actually speeding up the deterioration of the coating on the wire of the winding. The result is a shorter life than expected.

Electrical arcing at a bad connection in an oil filled motor can vaporize the surrounding oil, resulting in increasing internal pressure. Either an oil leak or an explosion is possible. This cannot happen in an air filled motor.

Why specify an air filled motor? Compared to an oil filled motor, a Weil air filled motor has many advantages:

• Lower power costs because of higher efficiency
• Operates at a lower temperature
• Has a longer bearing life
• Maintenance is easier and less costly
• No used oil to be disposed of in accordance with EPA regulations
• No premature failure due to oil leaks
• No undefined future liabilities due to environmental contamination and explosions