SubMonitor: What it CAN’T do

In the last few Franklin AID posts, we’ve discussed how SubMonitor protects a 3-phase submersible installation against a wide variety of potentially harmful conditions. It does this by continuously monitoring voltage, current, and a heat sensor in Subtrol-equipped motors.

On the mechanical side, however, there are a few things that SubMonitor just can’t protect against. One of these is water hammer. Water hammer most often occurs when a check valve is not used, has been improperly installed, or leaks. As a result, when the pump stops, the water drains back down through the pump inlet and creates a vacuum in the discharge piping. When the pump restarts, water rushes to fill that vacuum at a high velocity. When it strikes the hardware or stationary water above, it causes a hydraulic shock that can split pipes, break joints, and damage the pump/motor. The picture below shows a thrust bearing that was destroyed by water hammer.

Water Hammer

Another mechanical condition that SubMonitor won’t protect against is radial side loads. The most common cause of side loading is misalignment of the pump and motor. This creates a side which, depending on the how extreme that load is, can cause a rapid failure in the radial motor bearing near the top of the pump.

Radial bearing

Finally, another condition that SubMonitor won’t protect against is upthrust. As the name implies, upthrust occurs when a large volume of water pulls the impellers upward. This carries across the pump coupling/motor shaft assembly and pulls the shaft up with it. This generally occurs at start-up, and the pump and motor are designed to handle this on a momentary basis. However, if the pump is operating on the far right-hand side of the curve for long periods of time, the resulting upthrust can damage the motor and pump.

When looking at these three mechanical failure modes, it may seem obvious that no electronic protection device could guard against these types of failures. These situations underscore the importance of using good installation procedures. Combined with the protection that SubMonitor offers against events that you can’t control, your submersible installation will reliably deliver water for years to come.

SubMonitor: Overheat and Rapid Cycling Protection

In previous posts, we’ve covered the details of how SubMonitor protects a 3-phase installation from high/low voltage and under/overload conditions. This time around, we’ll cover overheat and rapid cycling.

All motors generate heat. In order to ensure proper operation and maximum life, a motor must be able to effectively dissipate the heat it generates. In the case of a submersible motor, that method is the cooling flow of water that is drawn past the motor by the pump above it. If that cooling flow is interrupted for any reason, the motor can overheat and fail.

Franklin Electric submersible 6- and 8-inch motors labeled “Subtrol Equipped” contain a built-in sensor that can detect and report an overheat condition when connected to a SubMonitor. When an overheat condition occurs, the sensor sends a series of continuous electrical pulses up the motor lead and drop cable. SubMonitor recognizes this signal and takes the motor offline to protect it.

No temperature adjustment is required if an overheat condition occurs; the sensor is pre-calibrated, and SubMonitor’s default setting for overheat detection is ON. If SubMonitor detects an overheat condition, it will take the motor offline for 10 minutes and then attempt a restart. If the overheat condition is still present on the restart, SubMonitor will take the motor offline again, repeating the cycle until it reaches a maximum number of attempts.  Both the time-out and restart settings can be adjusted manually. This time-out setting can be adjusted from 5 to 60 minutes in 5 minute increments (default is 10), and the restart setting can be adjusted from 0 to 10 restarts (default is 3).


SubMonitor also protects against rapid cycling conditions that can occur, for example, with chattering contacts. If SubMonitor detects more than 10 starts in 10 seconds, it will take the motor offline one minute. After that minute has elapsed, it will attempt to restart the motor 3 more times before requiring a manual reset. This setting is also adjustable, but the default parameter generally covers the vast majority of installations.


Even though SubMonitor protects against many of the circumstances that can harm a submersible motor, there are certain damaging conditions to which SubMonitor does not apply. We’ll cover those in our next post.

Note: Even if a motor does not contain a Subtrol heat sensor, SubMonitor will still protect it from high/low voltage, under/overload, and rapid cycling conditions.