Overload Protection of Three-Phase Submersible Motors

The characteristics of submersible motors are different from standard, above ground motors. Submersible motors require special overload protection. In order to properly protect a three-phase submersible motor, ambient-compensated, quick-trip overload protection must be used. This can be either a fixed heater or adjustable overload relay, as long as it is ambient-compensated and quick-trip. Franklin Electric?s Subtrol-Plus system can also be used to protect Subtrol-equipped motors.

Ambient-Compensated: Ambient-compensated overload protection must be used to maintain protection in both high and low air temperature areas. Three-phase pump panels are typically suitable for indoor and outdoor applications within temperatures of +14°F (-10°C) to +122°F (50°C). Pump panels should never be mounted in direct sunlight or high temperature locations as this will cause unnecessary tripping of overload protectors. A ventilated enclosure, painted white to reflect heat, is recommended for outdoor high temperature locations.

Quick-Trip: If the motor is stalled or the shaft cannot turn, the overload protector must trip quickly to protect the motor?s windings. In some areas, it is customary to specify that the overload must trip within 10 seconds with 500% normal current (IN). Quick-trip heaters and overload relays shown here, will respond within 10 seconds. Franklin?s Subtrol-Plus responds within 3 seconds. Heaters marked ?Standard Trip? are typically Class 20 or 20 second response time. Standard Trip, Class 20 and some Class 10 overloads will allow submersible motor burn-out before tripping. Quick-trip overload protection must be used.

The time-current trip curve, as shown in Figure 1, shows the response time for different classes of overloads under different running scenarios. The bottom axis of the chart shows different multiples of normal currents. The side axis shows ?Time? in seconds. The first bold vertical line represents the amperage if the power source single-phases on the motor side of the transformer (secondary side). This can occur if a fuse blows or a contact fails. This condition causes the normal line amperage to increase to 173% of normal in two phases and drop to zero in the third phase. The second bold vertical line represents the amperage if the power source single-phases on the incoming side of the transformer (primary side). This can occur if a power line is broken in a storm or car accident. This condition causes the normal line amperage to increase to 230% of normal in one phase and 115% of normal in two phases. The third vertical line represents a locked rotor or bound shaft condition. As you can see from the motor burnout curve or shaded area, the motor must be disconnected with a quick-trip device or severe motor damage can occur. The best protection, as the bold horizontal line shows, is Franklin?s Subtrol-Plus which disconnects the power within 3 seconds. Continue reading