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Calculating Minimum Cooling Flows

Electric motors produce heat. The heat in motors must be displaced or the heat will build up in the motor and cause a failure. Most above ground type motors have a fan that blows air through the motor for cooling purposes. As long as the motor is running at full speed, so is the fan. Devices such as a Cycle Stop Valve derate the motor without decreasing the RPM. Derating a motor means that the amperage, load, or heat produced is decreased. When the heat produced is decreased, and the fan is still spinning at full RPM, the motor runs much cooler and will provide a much longer service life. With devices such as a VFD, the amperage or heat produced decreases slightly but, the RPM of the motor and attached fan are also decreased. With slower RPM and a decreased air flow to the motor, heat is not displaced properly, and an auxiliary cooling fan may be needed to prevent premature destruction of the motor.

With submersible motors, the water flowing past the motor before entering the pump, is used to cool the motor. Just because the motor is submerged under water does not guarantee that the flow will pass by the motor before entering the pump. When installing the unit in a large body of water, or when installing above perforations or above the water producing zone is not possible, a flow sleeve is mandatory. Even in situations where the pump is installed above the perforations or water producing zone, a flow sleeve will increase the velocity of flow past the motor. The flow sleeve should be as small a diameter as possible, to increase the velocity past the motor, without causing much friction loss at high flow rates.

Minimum flow rates are figured to maintain a certain velocity past the motor. When running at full service factor load, in water less than 86 degrees, this velocity must be maintained to prevent overheating of the motor. When the water is hotter than 86 degrees, derating the motor is necessary to prevent overheating. Derating a 5 HP submersible motor by as little as 25% will prevent overheating, even with water temperature as high as 140 degrees. In hot water applications, the velocity must also be increased. However, when the motor load is derated, and the water temp is less than 86 degrees, the velocity past the motor can be decreased.

Again, devices such as a Cycle Stop Valve derate the motor without decreasing the RPM. When the motor is derated, and the water temp is less than 86 degrees, very little velocity is needed to maintain adequate cooling for the motor. For instance, a 6" motor inside an 8" casing or sleeve, pumping water less than 86 degrees, and running at full service factor load, needs 45 GPM minimum to maintain adequate cooling. This same motor derated with a Cycle Stop Valve, can maintain adequate cooling with as little as 5 GPM flow past the motor. The cooler the water, and the more derated the load, the lower the minimum flow required to properly cool the motor. This is why in over 17 years of service, there has never been a single motor destroyed from a proper Cycle Stop Valve installation.

While Cycle Stop Valves derate the motor load and reduces the minimum flow required, a VFD does not derate but, "creates" a smaller motor from a larger one. When the RPM of a motor/pump is reduced with a VFD, a 20 HP motor may only be pulling a 10 HP load. However, the harmonic current from the VFD increases the heat in the motor windings. This requires enough cooling flow to adequately cool a 10 HP motor running at full service factor load. It takes much more velocity or flow to cool a fully loaded 10 HP motor that was "created" from a 20 HP motor using a VFD, than it does to cool a 20 HP motor that has been derated to a 10 HP load using a Cycle Stop Valve. This is why a VFD controlled motor still requires as much minimum flow as a motor running at full service factor load, and a Cycle Stop Valve controlled motor can operate safely at a much lower flow rate. Many motor and pump companies also sell VFD's, and do not want you to know that motors last longer and minimum flow rates can be much lower with a Cycle Stop Valve system, than with a VFD controlled system. The "not so minimum flow" required by a VFD system can create many problems when the demand is below a set amount. With a Cycle Stop Valve controlled system, there is no demand too small to cause overheating of the motor.

Motor companies do not disallow warranties for something that "might" happen to a motor. When you bring a motor in for a warranty inspection, the motor company has no idea how the motor was being controlled. Their inspection at that time can easily determine if the motor suffered from a lack of cooling or not. Damage from lack of cooling is obvious and occurs when the pump is allowed to run dry, pumping against a frozen well head, or a completely plugged pipe line. Motors can also be destroyed when pumping from a top feeding well without a cooling sleeve in place, or by running at low flow when controlled with a VFD.

Cycle Stop Valves, Inc. has always offered to cover any motor controlled by a properly installed CSV system, if warranty was denied by the motor manufacturer due to lack of cooling. In more than 17 years with hundreds of thousands of installations, there has never been a single motor destroyed from lack of cooling due to the installation of a Cycle Stop Valve. Some pump and motor companies continually try to convince you that Cycle Stop Valves will cause motor overheating. The truth, is that Cycle Stop Valves have repeatedly proven to increase the life of pumps and motors. Which contrary to what they want you to believe, is exactly why pump and motor manufacturers desperately try to discourage the use of Cycle Stop Valves.

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Cycle Stop Valves, Inc.
10221 CR 6900
Lubbock, Texas 79407
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800-652-0207
Fax: 806-885-1994