How to ensure your variable speed drives running at maximum efficiency?

Variable speed inverter drives (VSDs) offer an important energy saving technology that can save enough electrical energy to give a payback period of less than eighteen months, but only when set to run at their maximum efficiency. So, are end-users getting the most out of their variable speed drives and reaping the benefits, or are they just hoping they are?

It might be assumed, for example, that having fitted a variable speed drive, you have achieved your energy savings potential. Certainly, the drive may be optimising the motor by matching it to the requirements of the application. However, to really deliver the maximum energy savings, variable speed drives must be properly set up at the commissioning stage; this can be as simple as activating one parameter to turn the drive intelligent energy saving mode on, and it can be no more difficult than that.

Improving installation efficiency

The first area to always look at is nature of the load i.e. is it constant torque or variable torque.  To improve installation efficiency, the drive can then be adjusted to match its output to the load, thereby maximising the available energy savings. While a drive in constant torque mode saves energy compared to having no drive on the motor at all, optimum savings will only be achieved in variable torque mode if the connected load is a pump or fan, although it must be stated that some types of pump require the constant torque setting. This needs to be set up when the drive is installed; again, this is normally as simple as adjusting one parameter.

Many variable speed drives have specific energy-saving modes, with dedicated algorithms tailoring the profiles of the output to deliver greater optimisation. As an example, Mitsubishi Electric’s FR-F800 variable speed drive is a product designed for pump, fan and compressor operations. It offers specific energy saving modes that can achieve further energy savings up to 15% compared to standard operating modes. It also provides several advanced features specific to the industries relying on fans, pumps and compressors to deliver further improved performance within the application.

Some users may have shied away from enabling such energy saving modes on variable speed drives, as historically they could make the drive sluggish to respond to changes in the load. However, this is no longer the case, with best of breed products incorporate technologies that will eliminate the problem.

For example, the FR-F800 includes Mitsubishi Electric’s Advanced Optimum Excitation Control technology, which automatically switches the drive between its energy saving mode and its high torque mode to best meet the requirements of the load at any given time. With this mode enabled, users can be assured that they are always achieving the optimum energy saving but without compromising performance.

How much can I save?

It is also fair to say that energy optimisation is a constantly moving target. That means ongoing monitoring of energy usage is vital for identifying areas ripe for optimisation. Even more fundamental than that is the need to perform a post installation survey once a variable speed drive has been fitted. Spending a little time to set up the technology correctly will deliver so much more.

The energy consumed by a motor varies according to the load it experiences at any point in time, so it is not possible to specify exact energy savings. Most VSD suppliers therefore are happy to work with end-users to perform a before and after comparisons. Mitsubishi Electric UK offers free site visits to discuss inverter efficiency.

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