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Did they tell you this about higher efficiency motors?

23 March 2009

We’ve all read articles in the trade press about how much electricity could be spared if the manufacturing industry would only get its act together and install high efficiency motors and variable speed drives. The move would be as good for energy budgets as for the environment. It seems so obvious, let’s get out there and do it.

However, the message apparently isn’t getting through to factory managers. According to a survey conducted by ABB and reported by the Drives & Controls web site (www.drives.co.uk/fullstory.asp?id=2336)
engineering managers in the U.K. placed inverters at the bottom of the list of the ten most effective measures for cutting energy bills. In fact more than half of them believe that changing electricity suppliers is the best way to reduce their companies’ energy costs.

After that, clamping down on visible waste, such as inefficient lighting and compressed air leaks, is high on their list. Investing in equipment that makes industrial processes more efficient came at the bottom. So much for technological advantages.

NOT SO FAST

As desirable as energy-efficient motors and drives may appear to be, the issue cannot be reduced to a mere purchasing decision to get rid of the old and bring in the new.

Our feature story in the February 2009 issue written by Robin Cowley makes it abundantly clear that engineers will have to be involved and they will have to do some thinking about the process.

For example, it is unlikely the higher efficiency motor will be the same size and shape as the old one, so there are some immediate engineering and logistical considerations that will have to be taken into account. These are fairly obvious, but may need to be pointed out to a zealous purchasing agent.

In fact, here’s where ‘old timers’ may be the leaders. The longer the maintenance engineers have been around the plant, the better, because the motor’s history can be important. Mr. Cowley’s advice is to always optimise the motor to the driven load. The process may have been modified since the original installation and it could be the motor is not the best motor for the current operation. Who might know that?

If a motor is oversized there may be a good reason. Motors can be oversized because of starting requirements or possible overload conditions to prevent stalling. Knowledge of the process and history can be important.

Higher efficiency motors operate at higher RPM for the same load. On fans and pumps this increase in impeller speed may mean an increase in load and potentially a higher current draw. Pump impellers may need to be ‘trimmed’ and fans and blower systems rebalanced to prevent overload. The higher speed may benefit the process (it may increase the volume of product delivered, for instance) but the issue needs to be reviewed.

Even electrical considerations need review. Premium efficient motor may draw less full load current but the inrush current can be 6-8 times full load. That could make a big difference in overload protection!


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