Control principles for variable speed pumping:Soft starting and stopping

Soft starting and stopping

When an induction motor is started direct-on-line, it generates a high level of torque, which causes a very fast breakaway, and it then accelerates up to speed in an uncontrolled fashion.

The effect on the pump is to place mechanical stresses on the rotating components, followed by stresses in the hydraulic system, which may include a high initial draw-down causing a vacuum to be drawn on the suction side, or a surge on the discharge.

Equally when stopping, the rate of deceleration is totally uncontrolled, which can lead to further mechanical stresses and surges in the hydraulic circuit. This can lead to requirements for additional inertia to be added to a pump, generally in the form of a flywheel, or for surge control vessels in the hydraulic system.

The use of electronic starting systems provides smooth acceleration and deceleration of a drive system.

Electronic soft starters reduce the voltage at the motor terminals in a controlled manner, but are generally short time rated devices, while a frequency converter is usually continuously rated and so can be used to give very controlled rates of change.

The only drawback with either electronic scheme is that the equipment must be connected to the network, and therefore problems could arise in case of a power failure, when an uncontrolled deceleration will occur.

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