What are the reasons for motor terminal block burn of submersible pumps?

A very common problem affecting submersible motors is over-temperature. Causes for over-temperature include pumping hot water, overloading of the motor by the pump, loss of cooling flow past the motor, ochre or scale buildup and frequent motor starts and stops.

Submersible motors somehow must cool themselves. This is accomplished almost universally by transferring the motor's internally generated heat to the water that is flowing past the motor and into the pump. Most standard water well motors are designed to do this but add little safety margin (safety margins add cost).

In submersible motors, the thrust bearing supports the pump's thrust weight of the water column being lifted by the pump. In standard water well motors this thrust bearing is a water lubricated "Kingsbury" type of bearing. A very small film of water between the main elements of the thrust bearing provides lubrication between the two bearing surfaces. If the motor overheats for any reason, this water film can approach its boiling point. If it boils, the lubricating film is lost. At this point, the bearing surfaces come into contact with each other and rapid heating takes place. Catastrophic failure of the thrust bearing is likely to occur.

Stator failure is another problem that occurs when motors overheat. Typical wet wound, water-filled submersibles use a PVC insulation that insulates the copper windings while immersed in water. This wire usually has a maximum usable temperature of between 70º C for standard motors to about 100º C for higher temperature motors. Once these temperatures are exceeded, the insulation system is damaged and a winding turn to turn, winding phase to phase or winding phase to ground fault becomes likely. Once these faults develop, failure of the motor is unavoidable.

Another problematic area for submersibles in municipal applications is hydraulic shock loading or water hammer. Water hammer occurs when a rapidly moving column of water encounters an obstacle or suddenly changes velocity. The use of multiple pumps on a common supply manifold is a prime cause of water hammer. When a pump turns on or off, water hammer is generated. Check valves in the pump discharge string and at the well head are recommended by all manufactures to reduce water hammer. Unfortunately, check valves may be of the wrong type (swing vs. spring-loaded), may be simply not used or may become corroded over time (spring corrosion is a common problem).

Submersible motors, like any electric motor, require a good voltage supply at the motor terminals. A leading cause of submersible motor failure is under-voltage or voltage spikes.

Under-voltage typically is caused by sizing the drop cables (supply cables) too small or by the utility grid supplying low voltage to the site. If the setting of the motor and pump are deep, very large and very expensive cables must be used in order to provide the rated voltage at the motor's terminals.

Voltage Spikes
A very serious problem for all induction motors is voltage spikes. These spikes typically are very short in duration and high in voltage. These spikes can be generated by lightning, other motors turning off or utility switch-gear opening. There is little that motor manufactures can do to protect against severe voltage spike problems. For this reason, external surge protection mounted near the motor starter is recommended.