Excessive heat in a single-phase motor can be caused by several factors - high or low voltage, rapid cycling (turning the pump on and off too often), excessive mechanical loads on the motor as would occur if the impellers were dragging, running the pump outside its normal range on the curve, or inadequate flow of cooling water past the motor.
Submersible motors generate a great amount of heat during normal operation, and they depend on the flow of cooling water past the motor housing to provide the necessary cooling. Here are some conditions that can interfere with motor cooling:
1. Setting the pump below the well screen. Here, the flow of water to the pump inlet is from above the pump. Since the motor is located below the inlet to the pump, it will not get adequate cooling.
Solutions - Reposition the pump above the screen or install a flow-inducing sleeve (pump shroud, as it is commonly called). A pump shroud for a 4-inch pump can be made from a piece of thin-wall 4- or 5-inch PVC pipe, about a foot longer than the pump. By simply cutting five or six slots longitudinally about 6 inches down one end of the pipe, the diameter of the shroud at the attachment point can be reduced to fit tightly over the pump housing. Attach it to the top of the pump with a stainless steel clamp so the flow of water to the pump inlet is forced to pass by the motor. Tape the attachment point with heavy duty PVC tape or pipe wrap to make a good seal. It also is a good idea to install three screws in the lower portion of the shroud to center the shroud on the motor, which will provide an even flow, as well as cooling around the motor. Franklin's submersible motor application manual has a good shrouding section.
2. Low-yield, large-diameter wells may result in inadequate flow past the motor for proper cooling.
Solutions - Install a shroud to provide the necessary velocity for adequate cooling. Franklin recommends a minimum flow velocity past the motor of .25 ft./sec. The smaller the inside diameter of the shroud, the faster the flow velocity for a given gallon-per-minute (gpm) flow rate.
3. Well runs dry during pumping cycle, which may damage both the pump and the motor.
Solutions - Install a dry well pump protector device. This can be as simple as a pressure switch with a low-pressure cutout or, better yet, an electronic device that detects the change of load on the motor when the pump runs dry and shuts off the pump for a preset period of time. We get into dry-well protection in more detail later in this article.
4. Loss of flow due to frozen discharge or clogged suction. This can cause the motor to overheat because of a lack of cooling water.
Solutions - Install electronic pump protection that has deadhead capability. Also, wrap the pipes that are exposed to freezing with heat tape or insulation, and/or put a 100-watt light bulb next to the pipes in the pump house.
5. Elevated well water temperature. Four-inch submersible motors, through 3 Hp, can be operated at full load in well water, up to 104 degrees F.
Solutions - If you have water above that temperature, it is necessary to reduce the load on the motor by throttling the pump discharge or switching to a higher horsepower motor. Contact the motor manufacture for specific advice on pumping hot water.
Franklin, the largest manufacturer of submersible motors in North America, includes basic motor protection built into either the motor or control box, depending on the size of the motor. All two-wire single-phase motors, and three-wire single-phase motors up to 1 Hp, have it built into the motor. Three-wire single-phase motors from 1.5 Hp to 15 Hp have the protection in the control box.
The problem with the standard protection is that it primarily responds to motor heating problems related to over-current. Motors relying on the built-in, standard protection are guarded against some minor ambient heat threats, but remember, this protection is designed to safeguard against current problems. In my opinion, additional motor protection as described below is cheap insurance to protect your customer's investment.
One of the worst things you can do to a submersible motor is to run it dry. As described above, submersible motors count on the flow of water to provide the necessary cooling to dissipate their heat. When the flow stops but the motor doesn't, it soon will. A submersible motor, without cooling, can heat up enough to melt plastic well casing. Remember, three-wire motors above 1 Hp have the overloads built into the control box, not the motor. If the well runs dry, the motor will overheat, and the overloads in the control box may not know the difference. Did you ever try to pull a pump from a well where the casing had melted and collapsed around the motor? Bye-bye pump - and well.
Electronic pump protectors turn the pump off in response to changes in the pump motor's electrical load as conditions change, such as running dry, having a blocked discharge, being overloaded, etc. Your water systems distributor should stock several brands of electronic pump protection devices that will protect the pump and motor from the various conditions mentioned above.
A word of caution. Dead head, where the discharge is blocked by ice or by some other means, can be difficult to detect because the load does not vary as much as with dry well, for instance. Although most pump protection device manufacturers claim their units protect against dead head, there will be some pumps, primarily low-flow high-horsepower units, where these devices may not trip on dead head.
Low-flow high-horsepower pumps are also difficult to protect for dry well, particularly if there is a large difference between the static level in the well and the pumping level. This is because the load on the motor changes substantially as the water level drops - often overshadowing the load change when the pump runs dry or is deadheaded. In these instances, calibrating the pump protection unit (if it is calibratable) at the static level when the pump is working the hardest will give the unit its best shot at tripping under dry well or dead head conditions.
With the right equipment and good installation practices, it is possible to provide your customer with a solid long lasting water system even with a less than perfect well.