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CSV Cool Submersible Motors Better than Variable Speed Drives Submersible motors stay cool at lower flow rates when throttled back with a valve than when slowed down with a variable speed drive. Reducing the speed of a motor with a VFD creates a smaller motor from a larger one. If a 50 HP motor can be slowed down until drawing a 30 HP load, adequate cooling flow for a fully loaded 30 HP motor must be maintained. A 6" diameter motor, installed in 8" casing, requires a minimum of ½ foot per second or 45 GPM to cool the motor. Increased turn to turn voltage and the reduced current at harmonic frequencies generated by the variable speed drive actually increases heat in the motor as the RPM decreases. All things combined, flow must be adequate to cool a FULLY LOADED 30 HP motor which is 45 GPM minimum. Locking in 45 GPM as the minimum RPM for the variable speed pump will allow flow to vary from 45 GPM to as much as the pump will produce. When flows of less than 45 GPM are used the pressure increases, the drive must ramp the motor down to prevent overheating. With the motor shut off, the pressure will drop as flow is still being used. This signals the drive to ramp the motor back up. Repeatedly ramping up and down is how the drive tries to handle low flow. Some band aids can be applied to the symptoms. A pressure bandwidth can be used. A jockey pump can be added. Large pressure tanks can give some cushion. None of these add ons solve the real problem. The real problem is that ½ foot per second flow must be maintained to cool a submersible motor when using variable speed drives. Unlike a VFD, Cycle Stop Valves derates the motor. Choking back flow from the full speed 50 HP pump the amperage drops to a 30 HP load. When a full speed 50 HP motor is only pulling a 30 HP load, charts from the sub motor manufacturers state that water up to 131 degrees will adequately cool the motor. If any flow rate of 131 degree water will keep the motor cool, how much cold water does it take to keep the motor from overheating? Flow rates as low as 5 GPM of cool water is all that is needed to prevent derated motors from overheating. We currently have several 250 HP submersible motors running that will stay cool at flows down to 5 GPM. The amp draw on these 250 HP motors drops from 320 amps to 190 amps at low flow. At 190 amps the 250 HP motor can safely pump hot water, so 5 GPM of cool water is all that is needed to prevent the motor from overheating. Cooling charts for derated motors have not been made available by motor manufacturers. Motor and pump manufacturers are busy building the next generation of variable speed pumps. These new variable speed pumps, motors, and drives will be smaller, lighter, non-repairable, and can spin pumps at super high RPM. Designed to last a short period of time, be thrown away, and replaced with a more expensive newer model. None of these companies want you to know that Cycle Stop Valves will make standard pumps and motors last longer, run at lower flow rates, and saves as much energy as variable speed pumps. |
| 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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