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Energy saving above and beyond the electric motor

Looking at the entire drive chain, this article from Lenze looks at the obvious and less obvious places where energy can be saved.

We all know that energy costs will continue to rise. We all know that electric motors are an important factor here. After all, electric drives are responsible for about one-third of electricity used in Europe. But the electric motor itself is only part of the picture - a starting point for energy saving and reducing the running costs of machinery. According to the German trade association ZVEI, the energy saving potential in electric motors is 10%. However, ZVEI adds that the saving potential with inverters is three times higher, and that for optimising the complete drive train is six times higher.

There is no rocket science here. The technology needed for these energy savings is proven and reliable. Methods are easy and accessible. It is true that often the initial cost is higher. But payback times are remarkably short, often less than one year. So let's start with the electric motor. The majority used in industrial machinery are standard AC asynchronous designs. Current European regulations require them to meet the IE2 efficiency level which will increase to IE3 in 2015. A higher level of IE4 is on the horizon. What have we achieved with these new regulations? Efficiency gains of between 3% and 8% which directly relate to energy usage. These might look like small numbers, but the savings are significant - for a 7.5kW motor running 5000 hours a year they represent £190 a year with a payback on the additional motor costs in about 6 months. So these motor changes are well worthwhile, but let's see how we can save much more.

Manufacturers of motors are all looking at alternatives to standard asynchronous designs. Permanent magnet motors are one option, good at low speeds and can eliminate gearboxes, but are more expensive. Switched reluctance motors are likely to become more common, but their control is more complex. These and other designs will come along in future years in order to meet the challenge of IE4 and perhaps even higher efficiency levels. However that is the future; we need to focus on energy savings that can be easily made now.

Next up the drive train is often a gearbox. If you select the wrong type you will lose much more than you gained with an energy efficient motor. Worm gears may be quiet and low cost, but they are also inefficient. They should only be used for drives that run infrequently. Changing to bevel, planetary or helical gears will achieve 94 to 98% efficiency and the energy savings can easily be 25%. Yes, they cost more but payback periods will be less than one year. Sometimes the next thing up the drive train are other inefficient components such as fluid couplings and V-belt drives. Look also for pneumatic and hydraulic mechanisms which constantly consume energy to maintain the pressure. Electric drive alternatives can achieve the motion and then be switched off with zero residual energy requirements.

Adding frequency inverter control to the motor can bring significant energy savings. However do not use inverters where the speed and load are constant as they will actually reduce efficiency. Large savings are well known in fluid handling, ie on pumps and fans. These are always designed for highest demand whilst actual demand is usually a bit lower. An inverter that turns down the speed by only 20% in response to actual demand can reduce electricity bills by 50%.

There are many more applications where turning down the speed in response to load is appropriate, for example conveyors, revolving doors, and escalators. These can be slowed or even stopped when there is low demand. A frequency inverter can handle the control side and give massive energy savings. Then there are cases where the speed needs to remain constant but the load varies. Consider an airport luggage system which can be full of suitcases or at another time empty. The nature of asynchronous motors is that their efficiency falls at low loads. This is particularly true below 40% load, and as much as one third of the efficiency can be lost at 20% load. The reason is that the magnetisation current that produces the magnetic flux in the motor remains high as the load reduces. Yet you can buy frequency inverters that include software algorithms that adjust the magnetisation current in proportion to the load. In applications such as these energy savings up to 30% are possible, and at no extra cost.

In some cases the motion in the application puts energy back into the drive. Examples are crank mechanisms, stopping of high inertia loads, vertical motion in cranes, hoists and lifts, and also robotics and winding. Historically the common solution here was to dump the energy as heat into a brake resistor. This may heat the surrounding air but really is a waste of energy. Today there are three better options:
  • If there are two or more axes where one is regenerating whilst the other uses power, the DC bus in the drives can be connected. This means the braking energy is used by a motoring drive. Typical applications are in storage and retrieval units and the savings can be 40% or more.
  • The braking energy can be stored in a capacitor, a similar concept to flywheel storage, and re-used in the next operation. This particularly suits fast cycling drives such as cross-cutters and the energy saving potential is 50%.
  • The surplus energy can be returned to the mains supply, just like a domestic feed-in tariff. A regenerative unit is needed to turn the DC energy in the drive into 230V AC. This is particularly effective with winding applications and tends to be economic for drives above 5kW.

We can see, then, that there are lots of options to save energy in the complete drive train. But how do you make an informed decision? Here there is a further question to consider - is the drive train correctly sized? For best running costs combined with small size and low maintenance, the drive should match the requirements of the application. Not too big, not too small, just right. Very few cases are simple to assess. A solution to this problem lies in selection software that can provide numeric evidence.

A software tool called DSD Drive Solution Designer brings many benefits here. It looks at the entire drive train, single and multi-axis, and brings together product data with real-life application experience. Standard cases are prepared with motion formulae and additional features such as inertia calculators. Variations in ambient conditions are allowed for. Each drive is optimised and graphical displays show how close they are for their performance limits. It is easy to compare alternatives on performance and cost. Finally there is the output of an Energy Certificate that details lifetime costs. DSD is available for six months free trial and also it is free to Universities. Alternatively it can be implemented on site by the manufacturer. Of course every case is different and the powerful optimisation available from DSD is a significant factor in reaching the 60% levels of energy saving proposed by ZVEI.

Really we have no choice. We must save energy from an ecological point of view but also it makes business sense. If you do not have the in-house knowledge, call in the experts. Examine the whole drive train to maximise the savings. The electric motor may give you 10%, but the whole drive could be six times more. Spend a little extra when you upgrade - the payback time is short and from then on it is all savings. Machine manufacturers need to get on board. Making your machine more energy efficient may not cost significantly more, and the changes may get you higher performance and smaller sizes.
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