Last month’s article was the first of a 10-part series on the electrical portion of pumped water systems. We began with single-phase components, talking about pressure switches, two- and three-wire motors, enclosures and general hook-up considerations. Now we will turn our attention to pump and motor protection in a single-phase system, the potential problems and the available solutions.
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. Heat is a motor’s worst nightmare.
Excessive heat in a single-phase motor can be caused by several factors – high or low voltage, rapid cycling (the pump turning 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.
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 (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- or 5-inch PVC or ABS 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).
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 ft./sec. The smaller the inside diameter of the shroud, the faster the flow velocity will be for a given GPM flow rate.
3. 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 which 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.
4. Loss of flow due to frozen discharge or clogged suction. This can cause the motor to overheat because of lack 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.
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 switching to a higher horsepower motor. Contact the motor manufacturer for specific advice on pumping hot water.
Standard Submersible Motor Protection
Submersible motor manufacturers build basic motor protection, in the form of a heat- and current-sensing device, 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 is primarily designed to safeguard against current problems. Additional motor protection that responds to non-current-related issues 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 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 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? Time to drill a new 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. occur. 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.
A word of caution: Deadhead, where the discharge is blocked by ice or by some other constriction, sometimes can 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 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.
Rapid Cycling
Rapid cycling can be caused by a chattering pressure switch or by a partially waterlogged pressure tank. Regarding the latter, if a pressure tank loses its air charge, either because of a leak in the air chamber or the water chamber, or because someone tampered with the pre-charge pressure, the pump will cycle on and off more often than is recommended by the motor manufacturer, not allowing enough run time to dissipate the extra heat generated during start-up. This not only causes the winding insulation to break down, eventually leading to shorted windings and motor failure, but also leads to excessive wear on the motor/pump coupling. Most of the newer electronic pump protectors provide rapid-cycling protection.Next month, we will turn our attention to three-phase devices, applications and installations. ’Til then....
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