Thursday, 31 July 2014

John Guthrie, Energy Efficiency Manager, ABB writes: You’d expect your drives supplier to be an expert in drives right?

They are, but some of them might not be telling the whole story when it comes to drive savings – in fact, some may have fallen for three big myths that plague our understanding of how motor applications work and how variable-speed drives (VSDs) can help save energy and money.

It’s true that energy savings using drives can be spectacular but you need to know what they are based on to get the real picture.

Myth 1:  Pumps are always operated at full load
Let’s look at the numbers. A 55 kW motor running at full load for 12 hours, 5 days a week for 52 weeks a year will cost £22,308 a year to run at 0.13p/kWh. Right?

Well, in a perfect world, maybe. Factor in design oversizing, throttling and head and the figures begin to look very different.  And if running the motor at full load is essential to your process, how can you slow it down and save any energy?

A more realistic figure would be around 80 percent load, which gives a running cost of £17,846. Any savings we can make with drives must be calculated against this figure. A VSD will still make a real difference in your application as you operate it, but each one should be assessed in its own right, with data not just taken from the motor nameplate.

Myth 2: You can apply the cube law to the motor’s rated power
The cube law is a well-known aspect of motor operation. Most people who work with fans and pumps think they know it. But do they?

Put simply, the cube law states that for variable-torque loads, essentially centrifugal fans & pumps, power varies in proportion to the cube of the speed.

So, by reducing the speed by a certain percentage, the power reduces by the cube of the speed change.

As an example, take a motor operating at 80 percent speed. Plugging in the numbers gives (0.8)3 = 51 percent power. So, following this theory, if you fit a VSD, then slowing down the centrifugal pump or fan by just 20 percent will save you almost 50 percent power.

Sounds fine? Well, at first glance it does, until you realize that there is something not quite right here. It’s too perfect, too simple for the real world. What has been forgotten is design oversizing, throttling and head. And what about the losses in the VSD itself?

All these will alter the results, making the saving not quite as attractive as you’d think at first sight.

This example not only overstates the existing energy usage, it also exaggerates the potential savings that can be made.

Myth 3: You can work with energy saving graphs showing power constant at 100 percent
In some presentations of energy saving potential, the power is not shown as a curve but as a constant at 100 percent across the top. In reality, the graph representing energy usage in a throttled system should look like a rugby ball.

The proof comes when VSD’s are fitted in real applications, where, say, swimming pool pumps are turned down by 20 percent but only managed a 38 percent power saving and NOT the predicted 50 percent using the above approach.

Industries across the world are using VSDs right now to make dramatic energy savings – there is no doubt about that. But be careful. The message is, check the credentials of your supplier to ensure that their engineers are true drive engineers. Only then can they understand your applications and give you a solution that will save what it says it will save.

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