Striving for high motor efficiency

Optimising the "˜total systems' efficiency of a drive system allows end-users to save energy and maintenance costs, while improving site safety, writes Ian Tribe, product manager of Industrial Gears for drive solutions group SEW-Eurodrive.

Optimal power transmission between motor and driven machine is now a major consideration in modern process engineering applications. Whether for a conveyor, elevator, mixer or other application, the efficient design of the complete drive system, once given scant regard, now attracts close attention due to the heavy impact on energy and maintenance costs, and safety. Indeed, the extra expenses required to realise improved drive efficiency can no longer be justifiably ignored — the potential savings are huge, and the return on investment is realised very quickly.

With a growing awareness that drive efficiency can directly improve the bottom line, many are naturally looking to equipment designed to minimise losses. Thanks to a swathe of drive technology improvements in the last few years — especially in gear units, motors and drive electronics — such high performance products are more accessible and affordable than ever.

But individual component efficiency is only part of the story. The full savings potential can only be achieved once all elements of the drive train are considered together — from the motor through to the drive shaft of the machine that handles the materials.

Multiplying costs
Those who invest in plant infrastructure usually look at the capital outlay when purchasing a drive, but they often don't factor in the energy costs of running it. This is an oversight. Over the working life of a drive system, the costs of power consumption will usually far exceed the original equipment costs. In fact, a motor operating continuously will consume its purchase price in energy quite quickly, sometimes in just a couple of years.

It is important to note that drive train efficiencies are subject to a "˜multiplying effect'; that is, the proportion of energy transmitted is calculated by multiplying together the individual efficiencies of each transmission element, be it motor, gear unit, belt or chain. Because of this, particularly inefficient parts of the drive train — say, a poorly-designed gearbox or a series of couplings — will require the motor to run much harder in order to meet its end-load requirements. Excessive numbers of elements in the drive train will have the same detrimental effect; each inefficient element compounds the overall losses, and thus the costs of wasted energy.

This is an issue that is very pertinent to industries that still utilise V-belts or chains and sprockets, such as quarries and small- to medium-sized manufacturing plants. In the past, the choice of these drive components may have been warranted — gear unit suppliers often had limited numbers of gear ratios available, and the use of belts and chains offered a flexible and cost-effective means to achieve speed adjustment. However, such configurations are not ideal in terms of efficiency and, these days, there are other, more elegant solutions available to provide the necessary flexibility.

In addition to energy costs, ill-suited drive systems have ramifications for both maintenance and safety. Here again, belt and chain drives are the main offenders, with the need for guarding to provide personnel safety, as well as time-consuming maintenance aspects (such as tensioning and guard removal) to ensure smooth and safe operation. Furthermore, when such maintenance is carried out, the materials transport or process machines being driven must themselves be shut down, incurring further costs due to lost operational availability.

It is clear, therefore, that even relatively small steps taken to improve the total systems efficiency of drives promise to pay for themselves in a short amount of time. The question then is: what steps to take?

Motor enhancements
Perhaps the most intuitive starting point is the motor design. The three-phase squirrel-cage induction motor is an indispensable part of most industrial drive systems, and accounts for 30 per cent of total electricity demand in Australia. The efficiencies of these industrial "˜workhorses' typically vary between 80 and 95 per cent, leaving still much room for improvement in many circumstances.

It is not only industrial plants that have a vested interest in motor efficiency. The promise of reduced greenhouse gas emissions, resulting from lower electricity demand, has prompted the Australian government to consider proposed legislation to raise its minimum energy performance standards (MEPS) for motors. If put into effect, the proposal means that all motors from 2006 onwards would need to be built to the same level of efficiency as today's excellent quality, "˜high efficiency' motors.

Motor manufacturers are thus beginning to look at ways to cost-effectively improve motor performance, particularly with a view to minimising all mechanical losses (such as bearing friction and air resistance) and electrical losses (winding resistance and magnetic core losses, among other things).

It should be emphasised, however, that high efficiency motors are best suited to applications in which they are subjected to long periods of continuous running. Where there is a high degree of starting and stopping of the drive train, the new motors may not always provide the best dynamic response. This is because constant acceleration of the higher inertia rotor from a standing start may contribute energy losses that negate the other efficiency benefits.

Therefore, caution should always prevail when selecting a motor, with due consideration given to its expected operational duty and the equipment to which it is connected. At any rate, more substantial efficiency gains are actually to be made by focusing on drive train elements downstream of the motor. And the closer to the load the drive element is, the more effect it will have on the total power demand.

Close coupling
The pressure on companies to minimise operating costs, while satisfying environmental and safety legislation, mean that there is a growing preference for "˜close-coupled' drives, in which speed reduction is achieved by a single, customised gear unit.

Today, there are many more gear configurations available than in the past, and the use of an integrated gearbox obviates the need for couplings. Not only are the overall efficiency losses lower, but also the enclosed gear unit has no exposed moving parts — and therefore requires no guards — making it safer and easier to maintain.

The selective use of different gear types and arrangements is also paramount — inefficient configurations should be avoided at all costs. For example, if a worm gear is required for space-saving reasons, a high gear ratio (perhaps 100 to 1) could lead to very poor efficiencies (of the order of 50 to 60 per cent). The problem can be solved by using a worm gear in a low numerical ratio, in conjunction with appropriate helical primary stage, or alternatively a much higher efficiency helical-bevel gearing, to achieve the desired reduction.

Advantages can also be derived from gear units that are as compact as possible for their power rating.

Engineering confidence
All the benefits from using such efficient drive equipment, however, can be seriously undermined if not suitably matched to the application requirements. A motor's efficiency varies, and is generally optimum when running between three-quarters and full load.

The mistake that some designers make is to select oversized equipment in order to cover extreme demands, but which means that the motor is running most of the time at half-load or less. So, the imperative to make conservative estimates, particularly when there is uncertainty as to the expected loading, may well lead to an application which, normally needing no more than a 7.5 kW motor, ends up with a 15 kW motor fitted instead.

What is often not appreciated is that this overestimating of load requirements comes at a high cost to those who operate the plant. The "˜guess a size and double it' approach means that not only will the customer pay more for the oversized equipment, they will also pay more through its life. An oversized motor on partial load will use more power than the correct sized motor on full load.

Transmission control
Once the drive train has been streamlined to ensure the smoothest possible mechanical transmission, there is one more powerful tool available to promote higher efficiency. Drive electronics, once reserved for more sophisticated process applications, are extending their reach to simpler systems, with a growing prevalence in materials handling. Behind the trend is the increasing affordability of the technology coupled with the benefits it imparts — greater control over speed adjustment and gains in motor efficiency.

The most popular product used for drive control is the variable frequency inverter, through which the end-user is able to vary precisely the motor speed, while automatically adjusting the torque in accordance with the load demand. Even the simplest of inverters can contribute to big energy savings. For machines where changing load conditions are ever present, such as fans and pumps, variable speed capability is an attractive alternative to fixed speed, reigning in excessive energy consumption.

Drive electronics also offer a very cost-effective means to achieve soft-starting and braking, and in a much more versatile and compact package than equivalent mechanical systems — such as a flexible or fluid couplings. For motors subjected to frequent run/stop cycling, the addition of an inverter will not only reduce losses caused by high in-rush currents on startup, but will also be gentler on the equipment.

Unlike direct online (DOL) motor configurations, which have a power factor of the order 0.7 or 0.8, inverters are always able to maintain the motor power factor at close to unity. Significantly, at partial loads, inverters are able to offer greatly improved efficiency compared to DOL motors (even at full load, the inverter-controlled motor will draw less current than its DOL counterpart).

Driving convergence
Overall, the demand for high efficiency is leading to innovations at every point of the drive train, and is inspiring undeniable trends towards close-coupled and controlled drive solutions. SEW-Eurodrive is seeing another development: customers are seeking these in the form of a customised drive package, in which all drive elements — including motor, brake and gear unit — are provided pre-assembled and mounted to a single fabricated steel base. Inverters are also being packaged with these systems, right up to 132 kW applications at the larger end.

A simple realisation is at work here. Excessive plant downtime, in maintenance and replacement of parts, can easily undo the savings from efficient running. Plant owners need to be sure that what has been supplied will work well, and what may one day break down will be easily fixed. By taking responsibility for the entire drive solution, the manufacturer furnishes the customer with the confidence that the drive will not only be efficient, but will also be easy to support. In so doing, reliability of the process engineering operation becomes one more item to be removed from the plant's list of driving concerns.

SEW-Eurodrive Pty Ltd
PO Box 59, Tullamarine 3043