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Cooling a Submersible Motor with a VFD Before water enters the pump, it must flow past the submersible motor to keep the motor cool. Depending on the water temperature, horse power, and casing diameter, a certain amount of flow past the motor is required for proper cooling. There are extremely fast rates of voltage rise, multiple voltage pulses, and harmonic frequencies generated by a Variable Frequency Drive or VFD. These factors cause a motor running on a VFD, to generate more heat than the same motor running on standard power. Therefore, cooling flow past the motor is critical. According to one motor manufacturers cooling chart, a 4" motor, in 5" casing, with 86F water, requires a minimum flow of 7 GPM to maintain proper cooling for the motor. A VFD or Variable Speed Pump, will speed up or slow down to match the amount of water being used. The VFD is trying to maintain a "constant pressure". It has no way of knowing that you are using less than 7 GPM. When you are not using enough water to keep the motor cool, the only indication will be when the motor overheats and shuts down. Hopefully an overload in the controls will shut down the motor before it fails completely. However, anytime a motor gets hot enough to shut down on overload, some damage has already been done. Overheating puts tremendous strain on the motor, and can cause the motor to fail. "Constant Pressure" has many benefits for your water system. "Constant Pressure" is really the only job that a VFD is trying to accomplish. The Cycle Stop Valve or CSV, was designed to mimic the "constant pressure" delivery of a VFD, without needing the computerized controls that create many problems. The best technology is not always electronic. The best technology is one that has been reduced to the simplest form that will accomplish the job at hand. The CSV is newer technology and has been used as a reliable replacement for VFD’s every day since 1993. The CSV reduces energy consumption of a pump as much as varying the speed with a VFD. When a VFD slows a motor down to produce less flow, the amperage is reduced. This does not reduce the minimum flow requirements for motor cooling. When a VFD slows down a 5HP motor until it is only pulling a 3 HP load, the motor has been converted into a 3 HP motor and still requires the minimum flow to cool a fully loaded 3 HP motor. When a CSV chokes back a pump to produce less flow, the amperage is also reduced, the same as when running on a VFD. However, with the CSV, the motor is still spinning at full speed. The 5 HP motor has not been converted into a 3 HP motor, it has been derated. This means that it is still a 5 HP motor, yet it is only pulling a 3 HP load. A derated motor, or a 5 HP motor that is only drawing a 3 HP load, can pump hot water without any damage. If derating will allow the motor to pump any flow rate of hot water without overheating, then very little cool well water is required to properly cool the motor when using a CSV. In other words when running on a CSV, a submersible motor is able to produce a smaller flow rate without overheating, than when running on a VFD. At least 7 GPM flow is required to adequately cool the example motor running on a VFD. The same motor will remain cool with only 2 GPM flow, when using a CSV for control. The CSV controls are also devised to make sure the pump can never produce less than 2 GPM flow, no mater how the homeowner is using the water. The VFD does not know you are using less than 7 GPM, and has no provision to shut down on low flow until the motor overheats. |
| Cycle Stop Valves® is a registered trademark. All right reserved unless prior authorization is obtained. Cycle Stop Valves are patented: Patent number 5,988,984 and other patents pending. |
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