an Engineering & Service Bulletin

Technical Topics

J.H. WRIGHT & ASSOCIATES

SUBMERSIBLE PUMP MOTORS

"AIR OR OIL FILLED?"

Air Filled Pumpex Submersible Pump

Oil Filled Barnes Submersible Pump

Since submersible pumps have become so common in both industrial and municipal applications, perhaps a discussion of the two types of motors would be of interest. Manufacturers of these pumps tend to concentrate on either one motor type or another and therefore lack some objectivity regarding the pros and cons of each design. As a distributor of both types, perhaps we can do better.

Any discussion of submersible pump motors should start with the general observation that the best of all possible environments for an electrical motor is under water! That of course is from the viewpoint of heat removal. All other conditions except noise are not so good. So saying, one should also note that many submersible applications invariably run non-submerged at one time or another. The units designed solely to operate submerged may well develop severe heat generation problems. Remember that the efficiency loss of an electrical motor is almost all converted to heat.

Let's list the pros and cons of each type.

Oil Filled Pros:

1. Better conductor of heat. Motor runs cooler with no hot spots.
2. More difficult for pumpage to penetrate into motor.
3. Oil generally considered better bearing lubricant and coolant.

Oil Filled Cons:

1. Hydraulic drag of oil on rotor lowers motor efficiency.
2. Oil to minimize the rotor drag not always the best for bearing lubrication.
3. Messy to rewind.
4. In underground mining applications burning oil may produce some dangerous gasses.
5. Motor insulation must tolerate the oil.

Air Filled Pros:

1. More efficient motor.
2. Cleaner to re-wind.
3. More tolerant of unsubmerged operation (sometimes).
4. Bearings can be regreased without motor disassembly.

Air Filled Cons:

1. Runs hotter on windings and may have hot spots.
2. More expensive to build because of heat dissipation problems.
3. Often cannot run unsubmerged.

One should realize that there are considerable differences in each of the two designs depending on the manufacturer. As examples some circulate the oil through the seals and motor; some circulate pumpage around the motor with a jacket. These design differences can make some of the above Pros and Cons questionable, but generally they should be accurate.

BONE TO PICK OF THE MONTH

Is a submersible pump a pump with a motor driver or a motor with a pump attached? Many users take the latter view and expect their motor re-wind shop to rebuild the pump properly when the motor is in for rewind. Unfortunately, this usually results in another motor re-wind very shortly since many motor shops are experts at motors -- not pumps. The mechanical seals in submersible pumps are the critical heartbeat of the system. The sophistication of these is not something to be handled by the average rewind mechanic. Do yourself a favor -- take your submersible pumps to a pump shop and they will send the motor to a motor shop. That way you will get the unit back with a pump guarantee -- and that is what you really want.

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