Energy Saving with Variable Frequency Drives


Energy Saving with Variable Frequency Drives

One of the big advantages of using variable frequency drives is energy saving. Energy savings can also be made by careful motor selection; energy costs over the life of a motor are fifty or a hundred times the cost of the motor, so choosing a high efficiency motor (IE3 or better, IE4) makes sense. EU regulations now mean a new IE2 motor must be operated with a variable frequency drive in any case.

Constant torque applications (such as conveyers, mixers, machines) will save some energy, but savings really notice with variable torque loads such as pumps or fans. A reduction in speed of 10% here gives a saving of about 25% in energy; 20% will save about half. Fitting a sensor to monitor your process (temperature, pressure, flow rate etc.) and using the closed loop control built into the drive will improve the system performance and save money as well. Payback time is often in months, rather than years. Variable frequency drives such as Invertek’s Eco drive offer many special features to work with pumps and fans as well. All Invertek drives also have an energy saving function (P-06 on E3; P1-06 on Eco and P2 drives) which will save a little more.

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Using Closed Loop Control

We’ve already seen how closed loop control can be used in many applications to regulate temperature, pressure or flow rate. Most variable frequency drives have a built in closed loop control that can be used for this any many other applications.

In a closed loop system, the drive monitors the output (for example, pressure or flow rate) and continually adjusts the motor speed to try to maintain the desired value. Applications such as controlling temperature, pressure and flow rate in air handling systems, or pressure in pumping systems are well known and have been described in an earlier article.

But there are plenty of other possibilities. The drive can control a vacuum level in applications such as milking parlours; this has shown significant savings. Other closed loop systems control humidity in greenhouses (adjusting the water misting system) or Carbon Dioxide levels in buildings.

A neat application is shown in Figure 1. The conveyer feeds stones into the crusher; the crusher runs at fixed speed. The variable frequency drive monitors the load on the crusher (by measuring the crusher motor current) and adjusts the conveyer speed to bring supply or less stone. This allows the crusher to run at optimum load all the time, improving throughput. A real improvement in many applications such as sugar crushing, wood powdering etc.

Torque control and systems with encoders (such as cranes and elevators) are special applications of closed loop control. Getting the best out of solar panels (when used with a drive for remote irrigation pumps) is another application where closed loop control makes a big improvement.

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