Heat is a motor's worst nightmare. Submersible motors are rugged and will last many years with proper care and treatment. Their primary enemy is excessive heat that can melt the motor winding insulation, causing the motor to fail.
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 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.
Condition 1 - Setting the pump below the well screen. In this case, the flow of water to the pump inlet is from above the pump. Since the motor is located below the inlet to the pump, adequate cooling will not be provided.
Solution - Reposition the pump above the screen or install a flow-inducing sleeve - a pump shroud, as it is commonly called. A pump shroud for a 4-inch pump can be made from a piece of thin-wall 41⁄2-inch or 5-inch PVC pipe about a foot longer than the pump and motor. 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 of cooling water around the motor (see Figure 1).
Condition 2 - Low-yield, large-diameter wells may result in inadequate flow past the motor for proper cooling.
Solution - Install a shroud to provide the necessary velocity for adequate cooling. Franklin Electric recommends a minimum flow velocity past the motor of 0.25 feet per second. The smaller the inside diameter of the shroud, the faster the flow velocity will be for a given GPM flow rate.
Condition 3 - The well runs dry during pumping cycle, which may damage both the pump and the motor.
Solution - Install a dry-well pump protector device. This can be as simple as a pressure switch with a low pressure cut-out as described last month, or 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 will get into dry-well protection in more detail later in this article.
Condition 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.
Solution - 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.
Condition 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.
Solution - If you have well water above 104 degrees F, it is necessary to reduce the load on the motor by throttling the pump discharge or by switching to a higher horsepower motor. Contact the motor manufacturer for specific advice on pumping hot water.
Standard Submersible Motor Protection
Franklin, the manufacturer of most of the submersible motors used in North America, includes basic motor protection in the form of a heat- and current-sensing device, 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 11⁄2 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 protection are guarded against some minor ambient heat threats, but remember that this protection primarily 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.
Dry Well
One of the worst things you can do to a submersible motor is to let it run dry. As described above, submersible motors count on the flow of water to provide the necessary cooling to dissipate their heat. When the water 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 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 sense changes in the electrical load drawn by the pump motor as conditions, such as running out of water, blocked discharge, overload, etc. Your water systems distributor should stock several brands of pump protection devices that will protect the pump and motor from the various conditions mentioned above (see Figures 2 and 3).
A word of caution: Deadhead - where the discharge is blocked by ice or by some other constriction - can sometimes be difficult to detect, especially in submersible pumps with a flat pump curve. Although most pump protection devices protect for deadhead, there will be some applications, such as deep sets, where they may not without some tweaking. Most electronic dry-well pump protection devices on the market today have a sensitivity adjustment, sometimes hidden, that will allow them to respond to difficult dry-well and deadhead detection conditions. Ask your distributor for advice if you have a slow-producing well with a deep-set pump.
Next month, we will turn our attention to three-phase devices, applications and installations. 'Til then ….
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