Sounds about right to me. I would only add that because of the problems of using the affinity law with pumps that must produce a constant head, the difference in power consumption between VFD and a throttling valve always look better for the VFD on paper than in real life. See this quote from another pump engineer.
“7. Finally, beware using the affinity laws for calculating a new pump diameter or speed for
systems that have a high static head, the affinity laws apply only between two points that
are at the same efficiency.â€
You also would not need to bypass any flow for motor cooling with a throttling valve. With a VFD you are creating a smaller motor from a larger motor, so you still need ample cooling for a smaller motor running at full load. This is why you need a minimum of 1.2 GPM to keep the motor cool.
“A variable frequency drive will introduce harmonic currents into the
motor windings which will cause a nominal increase of five per cent in
motor heating and therefore five percent higher energy losses.â€
Using a throttling valve, you are derating a large motor by making it draw a smaller load. A derated motor can pump hot water without problems, so it needs very little cool water for proper cooling. If the motor is still spinning at full speed and running on smooth sinusoidal power, restricting the flow with a valve will decrease the load enough that only 2/10 of a GPM are required for adequate cooling. This eliminates the need for a bypass as long as at least 2/10 of a GPM are being used, and also proves how much cooler a motor runs when throttled by a valve, compared to when being heated by a VFD. And heat is what destroys a motor, be it sooner or later.
“7. Finally, beware using the affinity laws for calculating a new pump diameter or speed for
systems that have a high static head, the affinity laws apply only between two points that
are at the same efficiency.â€
You also would not need to bypass any flow for motor cooling with a throttling valve. With a VFD you are creating a smaller motor from a larger motor, so you still need ample cooling for a smaller motor running at full load. This is why you need a minimum of 1.2 GPM to keep the motor cool.
“A variable frequency drive will introduce harmonic currents into the
motor windings which will cause a nominal increase of five per cent in
motor heating and therefore five percent higher energy losses.â€
Using a throttling valve, you are derating a large motor by making it draw a smaller load. A derated motor can pump hot water without problems, so it needs very little cool water for proper cooling. If the motor is still spinning at full speed and running on smooth sinusoidal power, restricting the flow with a valve will decrease the load enough that only 2/10 of a GPM are required for adequate cooling. This eliminates the need for a bypass as long as at least 2/10 of a GPM are being used, and also proves how much cooler a motor runs when throttled by a valve, compared to when being heated by a VFD. And heat is what destroys a motor, be it sooner or later.