A variable-speed pump (VSP) is typically powered by a variable-frequency drive (VFD) motor controller or sometimes by a variable-torque transmission mechanism. Variable-frequency motor controllers and soft-starters modify the voltage phase angle using silicon controlled rectifiers to achieve speed variations in pumps. Variable-torque transmissions allow a differential between the motor and driven ends of a pump using special mechanical, magnetic, or hydraulic couplings.

In practice, automatic start and stop sequences can be controlled to achieve any ramp time using a programmable logic controller (PLC). However, there may be limits to the minimum speed or torque which can be achieved. The period of time over which soft-starters can control the motor may be limited. Finally, operational reasons may require that startup, shifting and shutdown sequences be shortened as much as possible—but safely. WaterGEMS CONNECT helps you estimate safe ramp times to make the most of your pump’s capabilities.

In WaterGEMS CONNECT, a variable pump is a prescribed boundary condition which is controlled by setting a time-dependent pattern for its rotational speed or torque. You can enter any speed or torque pattern, including delays, multiple ramps, and periods of continuous pumping.

WaterGEMS CONNECT does not currently model loop-back controllers, which can modify the VFD’s speed or torque to achieve a specific head or flow at some location in the system. This is because the pump may stabilize to a new steady state within a few seconds, including during a power failure or a normal stop or start, for a typical transient event and the loop-back controller is likely not engaged during such operations.