Bret McCormick
Stewart’s Electric Motor Works, Inc.
Orlando, Florida
Technical Education Committee Member
Oil and water just don’t mix — especially when dealing with submersible motors. Most larger sump/submersible pumps have oil filled cavities between the impeller and the motor to prevent water from intruding into the housing that contains the windings. In that cavity are sensors installed as part of the control circuit.
Each manufacturer has different ideas on what kind and how many sensors are used. In this article, we’ll review some of the more common ones. Then I’ll share an experience with one I hadn’t seen before.
If you are like many EASA repair facilities, you work on a number of different types of submersible pump motors. Complicating matters, each manufacturer has differing ideas on how to determine if the integrity of the pump has been compromised by the liquid being moved.
Insulated probe
Usually, an insulated probe is placed into the insulating oil. It is connected to sensing devices in the pump control panel. This probe can be made from brass, tungsten, or stainless steel, with a plastic or ceramic insulator isolating it from the frame. When water displaces the insulating oil or when water mixes with the oil to become emulsified, a current is allowed to flow from the control circuit either to frame ground or back to the control panel. See Figure 1. Other manufacturers use a dry cavity with a float switch to indicate level.
We recently discovered yet another leak detection device. This device looks like a float switch and is mounted on a diagonal in the seal cavity.
Another device has a magnet embedded inside the float with two resistors mounted inside the brass tube. See Figure 2. Each resistor represents a state to the controller. When the float is at rest, the resistance is 1200 ohms. When water enters the cavity, the float reads 430 ohms, triggering an alarm. See Figure 3. The entire circuit also has a normally closed thermostat inside the stator winding in series with the resistors. Also, keep in mind that the presence of moisture usually indicates either a damaged seal or damaged cable assembly.
