Problem

What is the difference between the "Is variable speed drive?" check box in the "motor" tab of a pump definition, and the "Is variable speed pump?" property of a pump node element?

Solution

The "variable speed drive" check box in the pump definition is used to for energy cost analysis only. With a constant speed pump (Non-VSP) the motor speed, and thus the motor efficiency, is constant. However with a VSP, the motor speed changes, so the user must account for the different motor efficiency for various motor speeds.  This check box allows you to enter a table of speed (relative speed factor) and motor efficiency. For a pump that is a VSP, that check box should be checked, since the speed (relative speed factor) will very likely not be constant (1.00). This variation in motor efficiency impacts the overall efficiency, which impacts energy cost calculations. The overall (wire to water) efficiency of the pump is the pump efficiency (based on the "Efficiency" tab of the pump definition) multiplied by the motor efficiency.

The "is variable speed pump" option in the pump properties enables you to choose if the pump can actually operate as a variable speed pump, where the motor speed can change over the course of the simulation. This part impacts the hydraulics (pump head and flow) as the pump curve scales based on a user-entered pattern of relative speed factor, or based on a desired flow or hydraulic grade setpoint (see Help topics on variable speed pumps)

See Also

Modeling variable speed pumps in parallel with different pump curves

Benefits of using variable speed pump batteries (VSPB) element and how it works