How Drives Save Money in Pumping Applications

Most modern adjustable speed drives (often called variable frequency or variable speed drives) consist of three major parts – the rectifier section, which converts the single or three-phase AC voltage to a DC voltage, a capacitor bank (DC Bus) to filter and store the electrical energy, and the inverter section, which converts the stored DC voltage into a synthesized AC variable voltage and frequency waveform. This arrangement allows for variable speed operation of an AC induction motor across its speed range. The varying of the frequency essentially results in the varying of the motor speed.

Adjustable speed drives offer cost savings through more efficient use of power that comes from reduced pump speed, when speed reduction is appropriate. The use of control or throttling valves on pumps has been popular as a way of controlling pressure and flow in a fluid systems despite their many disadvantages. The control valve controls pressure and flow while the motor and pump run at 100% speed. As the valve ages, losses increase and result in decreased efficiency and frequent valve failures. Adjustable speed drives offer many advantages over control valves, ranging from better pressure control to energy savings. Varying the speed of the pump to more closely match actual energy consumption to changing system demands, can produce significant energy savings.

Energy Savings
If the speed of a pump is changed, the flow, head and power will change according to the so-called “Affinity Laws.” These laws state that the flow is proportional to the pump speed, head (pressure) is proportional to the square of the speed, and power is proportional to the cube of the speed. Therefore, a reduction of 10% in speed will provide about a 27% (100%, minus 90% cubed) energy savings in a pump application.

Pressure Control
Water pressure may be controlled manually by a potentiometer on the drives for applications with minimal pressure fluctuations. But for typical systems with highly variable pressure and flow fluctuations, a better method is needed. To meet this need, drives from some manufacturers (including Hitachi – the company for whom this author works) feature a built-in PID control function. These drives are connected to a pressure transducer, and receive a 0-10 V or 4-20 mA signal that represents the actual pressure level. The internal PID controller of the drive monitors the pressure fluctuations and adjusts the motor speed to maintain the pressure at the desired set point. As discharge pressure drops, the pump speed increases to increase the pressure and decreases when the pressure rises. When properly tuned, this type of control system has the ability to maintain highly accurate pressure in pumping applications.

Benefits
A drive’s acceleration and deceleration capabilities eliminate pressure surges in pumping applications. So called “water hammer” caused by hydraulic surges can be easily controlled using a drive to start and stop pumps on a ramped or timed basis. The slow starting and stopping of a pump, coupled with varying the speed to maintain predetermined discharge pressure, can virtually eliminate hydraulic surges in the system, preventing water hammer. Mechanical stress is reduced during starting, reducing maintenance of rotating

components, too.

When operating across the line (i.e. without a drive) the in-rush currents at the time of the starting of a pump can range from 6 to 10 times motor-nameplate current. Utility companies often penalize customers for such large power surges through “peak demand” charges. Drives eliminate the problem by starting the pump with minimum current and controlling it as the load accelerates with no danger of exceeding full load current. This eliminates additional electrical charges and in some cases qualifies pump users for subsidies from utility companies.

Technical Papers - VFD Basics