Electric motors produce heat. The heat in motors must be displaced or the heat will build up and cause a failure. Most above ground type motors have a fan that blows air through the motor for cooling purposes.
With submersible motors, the water flowing past the motor before entering the pump, is used to cool the motor. Submerging the motor 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.
Create or De-rate?
When the flow is reduced, the amps drawn by a pump controlled by a VFD, and a pump controlled by a CSV are almost the same. The difference is that a VFD creates a smaller motor from a larger one, and a CSV de-rates the motor load.
When a VFD slows down a 10 HP motor, until it is only drawing a 5 HP load, it "creates" a 5 HP motor from the 10 HP. Even though the motor is only drawing half of a 10 HP load, the harmonic content produced by the VFD increases heat in the motor. The motor must be cooled as if it were a fully loaded 5 HP, which still requires ½ a foot per second flow. According to the submersible motor cooling chart, a 6" motor in an 8" casing needs a minimum of 45 GPM to remain cool. So a 10 HP pump and motor that can deliver 100 GPM, cannot be slowed to less than 45 GPM, or the motor will overheat and be destroyed.
The same 10 HP motor controlled by a CPV doesn't need as much flow to remain cool. This is because the 10 HP motor has been "de-rated" to a 5 HP load. When a CPV restricts a 10 HP pump, the amps can also drop to a 5 HP load. However, the motor is still spinning at 3450 RPM, and is still running on pure sinusoidal AC power, which has no harmonic content. De-rating is described in the chart for motors used in hot water applications. De-rating means a 10 HP motor that is only pulling a 5 HP load, can function properly in water up to 140 degrees. If de-rating a motor means it can remain cool in water up to 140 degrees, then it would take very little flow of 86 degree or cooler water to prevent overheating. This has been tested extensively, and we have found that 5 GPM flow is more than adequate to cool most de-rated motors. The example pump controlled with a VFD can only vary the flow from 100 GPM to 45 GPM. The same pump controlled with a CPV can safely vary the flow from 100 GPM all the way down to 5 GPM.
A smaller 30 GPM pump de-rated with a CPV, can safely vary the flow from 30 GPM all the way down to 1 GPM. This same 4" motor in 5" casing controlled with a VFD, can only reduce the flow from 30 GPM to 7 GPM.
With fan cooled motors, as long as the motor is running at full speed, so is the fan. When a CSV restricts the pump and lowers the motor amperage, the heat produced is decreased. With the fan is still spinning at full RPM, the motor runs much cooler and will provide a long service life.
On the other hand, when controlled with a VFD, a fan in a fan-cooled motor reduces speed along with the motor. This produces less cooling for the motor. Even though the motor is drawing less current, the current a motor receives from a VFD is rich with harmonic content, which increases heat. In most cases an auxiliary fan must be used to maintain cooling flow to the motor. An auxiliary fan adds to the energy consumption and increases the complexity of the control system.