Getting it right from the start
01 May 2012
These days, it’s easy to get the impression that variable speed drives are the best choice for controlling any motor in any application. But it’s not that easy, says Peter Schaffel, who explains how choosing the right motor starting technology is the key to optimising energy efficiency and to maximising plant life.
In commerce and industry, electric motors are everywhere, and they’re used for applications as diverse as driving fans in air-conditioning systems to powering the big batch mixers used in the rubber and chemical processing industries. Given this huge range of applications, it’s hardly surprising that there’s no ‘one size fits all’ solution to the problem of starting and controlling motors. Yet you could be forgiven for thinking otherwise.
For several years now, there has been a lot of information put forward about the energy savings that result from using variable speed drives (VSDs). To be fair, most of the things that have been said are true, but there’s a little caution that’s often overlooked; VSDs only deliver energy savings in certain types of application. Use them in an inappropriate application and they will actually waste energy! Here’s why.
VSDs control the power supplied to the motor using electronic devices, usually IGBTs (insulated gate bipolar transistors), and no electronic device is ever 100 percent efficient. This means that VSDs inevitably waste a certain amount of power. With the best modern drives, the amount of power wasted is relatively small – typically between 2 and 5 percent, depending on the power rating of the VSD.
Now lets consider the energy saved by variable speed drives. With a fan or positive displacement pump, reducing the running speed dramatically reduces the energy requirements. For example, a pump or fan running at 70 percent of full speed uses only around 35 percent of the energy it would use when running at full speed.
Taking into account that many pumps and fans are oversized and are rarely, if ever, required to run at full speed, it’s clear that the potential for energy saving is huge, and those savings far outweigh the losses in the VSD. Clearly, VSDs are a great choice for this type of application and, even in existing plant, the cost of retrofitting them can often be recouped in a very short time, after which they simply put money in the bank.
However, let’s now consider applications of a different type – for example mobile drainage pumps that always need to deliver maximum output, or industrial plant that runs at a fixed speed. In these cases, the driving motors always run at full speed, so the energy savings that come from reduced speed operation are no longer available.
Fitting a VSD in this sort of application will waste energy, because of the losses already mentioned. Losses of 2 percent may not sound a lot, but with a 30kW motor running for 40 hours a week, 50 weeks a year, they amount to 1,200kWh a year, which is hardly insignificant. And don’t forget that VSDs are usually the most expensive type of motor control equipment to buy, so fitting them in applications where they deliver no energy or operational benefits is a very poor investment indeed!
So, what are the alternatives? The simplest and cheapest option for a fixed-speed drive is the direct-on-line (DOL) starter. Modern starters of this type that incorporate electronic overload protection are very efficient, but it’s not all good news. DOL starters draw a big spike of current from the supply – typically seven or eight times the motor’s full load current – during starting. With large motors, that can be a problem, especially when the supply is derived from a generator or has little spare capacity.
There’s also another problem: motors with DOL starters start with a sudden jerk, which is not kind to the plant they’re driving. The resulting mechanical stresses can lead to increased maintenance requirements as well as shortening the life of the plant.
Because of these shortcomings, many alternative forms of starter have been developed over the years – star-delta, autotransformer, primary resistance, stator-rotor and Korndöorfer, to mention just a few – but these all have their own limitations, and the only type still commonly used for new equipment is the star-delta starter.
Unfortunately, this form of starter is not free of limitations, and unless it is very carefully matched to the application it can generate big supply transients and also produce mechanical stresses in the load comparable to those produced by DOL starters. Fortunately, however, there is a more modern and much better alternative – the electronic soft starter.
In some senses, the soft starter is a little like a single-function variable speed drive, in that it electronically controls the power supplied to the motor during start up to produce smooth acceleration. The soft starter, however, has no provision for running the motor at anything other than full speed. Because it provides excellent control over acceleration, a soft starter minimises mechanical stresses on the plant during motor starting and also limits the peak load on the supply.
But what about efficiency? Surely, since a soft starter is an electronic device just like a VSD, it must have losses that are similar to those of a VSD? Actually it does, but that’s not the whole story. Unlike a VSD, the soft starter only has a function to perform while the motor is starting and, with some types, while the motor is stopping.
This means that, at all other times, the electronics in the soft starter can be bypassed by a contactor. Under these conditions, losses are minimal. Soft starters are, therefore, a very attractive and energy-efficient option for many fixed speed applications, especially as they offer the additional benefits of smooth starting and low current peaks.
We’ve seen that there are many factors to consider when specifying motor starters if energy efficiency, running costs and plant life are to be optimised. To help choose the right starter for a particular application, expert advice is therefore very useful, but it’s important to be careful about the source of that advice. After all, a company that supplies only one type of starting technology is unlikely to advise against using it!
And it’s worth remembering that, when it comes to motor starting, good advice can pay big dividends not only in direct financial terms throughout the life of the plant, but also by safeguarding the environment.
Peter Schaffel is with Ralspeed
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