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Fixed speed applications: starting solutions

18 December 2012

It is easy to assume that variable speed drives are the best choice for controlling any motor in any application. However, this may not always be the case, according to Peter Schaffel of Ralspeed. He explains how the right choice of motor starting technology is key to optimising energy efficiency and to maximising plant life.

Electric motors are everywhere, in applications as diverse as driving fans in air-conditioning systems and powering batch mixers used in the rubber and chemical industries. It should come as no surprise, therefore, that there is no ‘one size fits all’ solution for starting and controlling motors.

A great deal of information has been put forward about the energy savings that result from using variable speed drives (VSDs). While mostly true, there is a little caution that is often overlooked – VSDs only deliver energy savings in certain types of application. When used in an inappropriate application they will actually waste energy!

VSDs control the power supplied to the motor using electronic devices, usually insulated gate bipolar transistors (IGBTs) and no electronic device is ever 100% efficient. This means that VSDs inevitably waste a certain amount of power. With most modern drives, this amount of power wasted is relatively small – typically between 2% and 5% – 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. A pump or fan running at 70% of full speed, for example, will only use around 35% of the energy it would use when running at full speed.

Taking into account the fact that many pumps and fans are oversized and are rarely, if ever, required to run at full speed, it becomes clear that the potential for energy saving is huge, and those savings far outweigh the losses in the VSD. Clearly, they are a good 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.

However, in other applications, for example mobile drainage pumps, which always need to deliver maximum output, or industrial plant that runs at a fixed speed. Here 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. Although losses of 2% may not sound a lot, with a 30 kW motor running for 40 hours a week, 50 weeks a year, they amount 1,200 kWh a year.

The alternatives
The simplest and cheapest option for a fixed-speed drive is a direct-on-line (DOL) starter. Modern starters of this type that incorporate electronic overload protection are efficient. However, they draw a big spike of current from the supply – typically 7x or 8x the motor’s full load current – during starting. With large motors, this can be a problem, especially when the supply is derived from a generator or has little spare capacity.

Motors with DOL starters also start with sudden jerk and the resulting mechanical stresses can lead to increased maintenance requirements as well as shortening the life of the plant.

Because of these shortcomings, alternative forms of starter have been developed over the years – for example, star-delta, autotransformer, primary resistance, stator-rotor and Korndörfer – they have limitations, and the only type still commonly used for new equipment is the star-delta starter.

Unfortunately, this form of starter has limitations. Unless it is very carefully matched to the application it can generate big supply transients and also produces mechanical stresses in the load comparable to those produced by DOL starters. An alternative is the electronic soft starter.

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. However, it has no provision for running the motor at anything other than full speed. Because it provides control over acceleration, it can help to minimise mechanical stresses on the plant during motor starting and also limits the peak load on the supply.

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 can, therefore, offer an energy-efficient option in fixed-speed applications, especially as they offer the additional benefits of smooth-starting and low current peaks.

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