The Drive for Greater Efficiency

Forward thinking businesses interested in reducing motor energy use can now go further than before, by implementing new tools that enable a.c. inverters (variable speed drives, VSDs) to save even more energy.

Wayne Turtill, Mitsubishi Electric product manager, looks at reducing energy usage with the right a.c. inverter technology.

A.c. inverters have long been used in applications where varying motor output speeds are required, such as fans, pumps and production machines. By controlling voltage and frequency, the key elements of a motor’s electricity supply – a.c. inverters precisely adjust speed and torque to deliver the process requirements of the machine or device.

Using a.c. inverters to reduce energy use makes a huge difference compared to using dampers and valves or gears to adjust the output speed.

Moving Beyond the Usual a.c. Inverter Benefits

Installing an a.c. inverter to operate motors is a good starting point, as it reduces energy consumption and helps to?improve the environmental impact of your industrial activities.

Companies often use fan & pump loads to oversize their motors, maybe because the ideal motor size falls between standard frame sizes or specified one frame size up ‘just in case’.

Avoiding Motor Oversizing

As a general recommendation, oversizing the fan or pump system and particularly the motor should be avoided. Businesses should instead select products that meet the necessary specifications without exceeding them, although it’s not always that simple and easy to do.

While there are situations where it is not possible to avoid oversizing, suitably rated a.c. inverters can still provide the correct level of control and save energy.

In particular, businesses that select a.c. inverters with specific energy reduction capabilities have more to gain. For example, Mitsubishi Electric’s a.c. inverters feature Automatic Energy Optimisation.

This analyses the output to the motor in terms of voltage and current, to reduce the voltage while maintaining the same operating parameters, i.e. speed. The voltage is reduced until the a.c. inverter detects an increase in the current, then increases the voltage to compensate. Using a PID algorithm without setpoints, allows the system to reach the lowest (and most efficient) voltage possible.

These adjustments are conducted a hundred times per second, during steady state operations as well as acceleration and deceleration to support real-time, continuous voltage control.

Next-level Energy Intelligence

Finally, application-specific functions in next-generation Mitsubishi Electric a.c. inverters can help businesses cut energy consumption even further when applied to certain applications, such as pumps and fans, or conveyors. Using Mitsubishi Electric’s E800 range of a.c. inverters, featuring Advanced Magnetic Flux Vector Control to provide optimum dynamic performance.

For example, in a conveyor application the a.c. inverter can automatically switch into energy saving mode when the conveyor is not loaded and maintains this efficient state until a load is applied. This can offer very substantial energy savings compared to more conventional control methods.

Choosing the right equipment for motor-driven applications can deliver significant cost and energy savings, far beyond what you would normally expect for an a.c. inverter application.

By installing state-of-the-art equipment and using all the features available, businesses can reap additional efficiency gains while still benefitting from all the flexibility and productivity advantages of a traditional control philosophy.

How Much Can I Save?

As an example, we will look at a 22kW motor driving a centrifugal pump, constantly, 24 hours per day, 365 days per year.

We are assuming the pump speed can be reduced without affecting the performance of the water system.

According to the Carbon Trust, with an electricity price of 17.2p/kWh, using an a.c. Inverter to reduce the speed of the pump by 20% will save; ? ￡15,333 per year ? 85,661kWh per year ? 36.8 tonnes CO2

OK, but What About the Return on My Investment?

In this example a 22kW Mitsubishi FR-A840 Inverter (￡2,859 list price), gives a payback time of just 68 days!