Western Cape Business News

Send  Share  RSS  Twitter  08 Apr 2010

POWER SOLUTIONS: How To Shave Electricity Bills


Recent Western Cape Business News

IN the past two years, we have seen a sharp increase in electricity tariffs from the state-owned power utility, Eskom. Most industries experienced an increase in operational costs as a direct result of more expensive electrical inputs, at precisely the time when most companies were already fighting to manage cash flows and save jobs.

It is clear that energy saving is no longer a marketing phrase or a ‘nice to have’, but a necessity if companies are to remain profitable. The best way for companies to begin any energy cost savings programme is to pay close attention to their induction motor control, as motors generally consume up to 60% of the total energy bill. The applications in which energy savings can be achieved include pumps, fans, compressors, hoisting systems and conveyors with high duty cycles.

Most pumps and fans are operated at fixed motor speeds to obtain a fixed flow. However, there are many systems that require a variable flow in order to achieve changing process demand. To control this flow, most variable systems employ the throttle valve system or control valve. This method of controlling flow may be compared to regulating the speed of a motor vehicle using only the hand-brake, while the accelerator remains fully depressed.

The engine continues to work at the same maximum power regardless of speed, and fuel consumption remains high. This is what happens when a pump is operated at a fixed speed while flow is regulated by means of a control valve. This type of system simply adds system resistance to restrict flow, while energy consumption remains high regardless of the flow.

How, then, do you save energy costs? The answer lies in using a series inverter. The Mitsubishi FR-F700 inverter with integrated PID control can be compared to regulating the speed of a motor vehicle by making use of the cruise control function, which regulates the speed by adjusting its set point, changing the engine-out to deliver the required power, and varying the fuel consumption accordingly. The FR-F700 inverter works on the same principle, whereby a change in inverter speed reference will yield a change in the system flow. The change in pump speed to achieve the required system flow will result in a significant saving in electricity consumption and therefore cost.

Now, how great a saving can one expect by varying the speed of the pump? This is answered by the following calculation: The system flow rate (m3/h) is directly proportional to the pump-speed (rpm). And the motor power (kW) is directly proportional to the product of speed (rpm) and torque (Nm). The torque of the variable load is directly proportional to the square of the pump speed. Therefore the power consumed by the motor is directly proportional to the cube of the pump speed. This means that a 30% speed reduction will yield to a 65% savings on electricity costs.

The FR-F700 offers drastically reduced power consumption over conventional solutions, particularly in pump and fan applications. It can achieve particularly impressive power savings in low speed ranges, and during acceleration and braking. These inverters are ideal for air extraction systems, fans and blowers, hydraulics systems, compressors, sewerage systems and drains, ground water pumps and heat pumps. The FR-F700 range is available with output power from 0.75 to 630kW.

Additional power savings can also be achieved with Mitsubishi Electric’s Optimum Excitation Control (OEC) technology, incorporated into the FR-F700 to allow these frequency inverter drives to use around 10% less than other drives. OEC technology ensures that the motor always receives the optimum magnetic flux, which means that it always operates at the maximum possible efficiency. In an energy saving mode, the inverter automatically controls the output voltage to minimise it during constant speed operation. As a result, the motor has a long service life and runs more economically, resulting in rapid installation payback.

All Mitsubishi drives incorporate an energy saving monitoring function as a standard feature, delivering read-outs of estimated energy savings and related costs.

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