Variable Frequency Starting vs. Soft Starting of Water Pump

As everyone knows, direct starting of water pump would cause the water hammer effect. Namely when the water pump starts and stops, water impacts the pipe and cause a serious water hammer. When the power is off suddenly or the valve turns off quickly, a water impact wave generates due to inertia of pressure flow, which is like the knock of a hammer, so it’s called water hammer. Sometimes the force generated from water impact wave may be great and damage valve or pump, therefore, to avoid the negative effect of water hammer effect, the pump’s rotation speed should increase to the rated speed slowly when it’s started. Two methods are applicable, one is the variable frequency starting, another is the soft starting.

Variable frequency starting

• When pump starting, transfer the AC current with constant voltage and frequency to the AC current with variable voltage and frequency through variable frequency drive (VFD), so as to reduce the impact load caused by motor start-up. Control motor speed and lengthen start-up time, achieve a soft starting. Meanwhile, it can also improve the efficiency of power grid and motor. Actually, VFD is mainly used in energy saving. By adjusting, it changes the output voltage, current and frequency.
• Disadvantages: high cost, its cost is much expensive than microcomputer protection. Most of current VFDs adopt PWM control, such a pulse modulation makes the VFD to generate harmonic current at the power side when it is running. Due to the harmonic interference, operation efficiency of power system may be affected. Overload leads the frequent tripping of VFD. In case of overload in normal operation, adjusting proper motor parameters would avoid overload because the motor has strong overload capacity. But the VFD has a poor overload capacity, it’s easy to occur overload alarm.

Soft starting

• Soft starter connecting between the power supply and controlled motor, control the internal thyristor through microcomputer to trigger the conduction angle and realize AC voltage regulation, thus gradually increasing the input voltage by predetermined function from zero till the starting ends, the motor is given full voltage. This process is the soft starting. During this process, motor starting torque and speed increases gradually till the thyristor is completely conducted. The motor operates on the mechanical characteristics of rated voltage, so as to achieve smooth start and reduce starting current, avoid overcurrent trip at starting. When the motor reaches the rated revolution, the start-up process ends and provides rated voltage for normal operation of motor.
• Disadvantages: soft starter can’t adjust the power frequency, so it can’t start the motor from zero voltage and zero frequency, thus realizing zero impact starting. It can’t regulate the speed. The soft starting withdraws from the system after starting the motor and loses its protection function.

Differences between variable frequency starting and soft starting of water pump

1. The VFD controls the starting and running of water pump by adjusting the frequency. Soft starting starts high-power motors by changing the resistance in series, it makes the current to change from small to large, so as to reduce the fluctuation of high-power motor direct starting to the power grid, or cause some unimportant load unloading problems.
2. Variable frequency drive is used for speed regulation, its output changes both voltage and frequency.
3. Soft starter is actually a voltage regulator, it protects the motor. When used in low voltage starting of the motor, output of soft starter only changes the voltage, but doesn’t change the frequency.
4. The VFD has all the functions of soft starter, meanwhile, it can control the output pressure and flow of the pump to achieve the goal of saving energy.

All in all, if only considering the control at starting, soft starter is available. But if considering the control in running, variable frequency drive shall be adopted. Choose the one that suits you best according to your requirements. ATO supplys you suitable VFDs 1/2hp to 300hp, soft starters 10hp, 15hp, 20hp to 815hp.