Motors use most of the electricity in industry. Yet users often pay scant consideration to this drain on costs, probably because each individual motor is just a tiny part in a giant system. But with two-thirds of industry’s electricity costs being due to motors, significant savings can be made with the right approach.
With rapidly rising electricity costs, users need to consider the energy consumption of electric motors. While each individual motor may not consume an enormous amount of energy, collectively, motors are responsible for two-thirds of electricity use in industry.
High efficiency motors can give significant cost savings if used in the right applications. Compared to a standard motor, a high efficiency model can be up to 3% more efficient. A 90kW high efficiency motor could cost £1,200 more to buy than the standard model, but will save £12,000 during a ten-year service life. The failure to fit a high efficiency motor may therefore cost the company £10,800 – a fairly hefty cost for what at the time may seem a small decision. And think of what a plant with hundreds or even thousands of motors could save.
As well as saving energy costs, high efficiency motors also save on operating costs by bringing better reliability, reduced downtime and lower maintenance costs. Lower losses give the motor a better tolerance to thermal stresses, an improved ability to handle overload conditions, better resistance to abnormal operating conditions, and higher tolerance to inconsistent and noisy voltage and current wave shapes.
But what are the right applications for high efficiency motors? Used with an emergency pump that only runs a couple of times a year, a high efficiency motor will obviously not give a radical reduction of the running costs. But for applications in continuous use, a high efficiency motor is the better choice.
Establishing a formal motor management policy helps make the decision as to whether a motor should be replaced or rewound
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Making these kinds of savings means thinking carefully about your motors and how and when they are replaced. However, when a new motor is needed, all these considerations tend to fall by the wayside. Most motors are bought because the old one has failed, which means the top priority is to get production up and running again, not debating the finer points of energy efficiency.
Cheaper motors are more expensive
Saving a few pounds up front by buying a less efficient motor could end up costing a lot more in the longer term. In terms of total life-cycle costs, the continued energy use of a motor is far more important than its initial purchase price. Generally, motors, irrespective of power rating, consume their own capital cost in energy in just 30 days of continuous operation.
The capital cost of buying a replacement high efficiency motor can often be quickly recouped by savings in operating costs. However, this usually won’t apply to large motors and those running only a few hours each year, as they may not be commercially viable to replace. Whether a failed motor should be replaced is not a decision to be taken when the pressure is on to get the motor back into service.
Motors with long running hours should generally be replaced rather than rewound to save energy losses
As well as ensuring that you have the best efficiency, you must also be sure you have the right sized motors for the job. Many motors are oversized. An application might have changed over the years, and now requires lower capacity. Alternatively, people may have added a “service factor” when designing the system. If the application runs at less than full duty for some of the time, consider the installation of a variable speed drive - a pump or a fan need only have its speed reduced by a couple of percent to give significant savings.
A sound policy
The secret to energy efficiency is to take a step back, look at the plant as a whole and establish a formal motor management policy. This will set out when motors should be replaced and when they can be rewound, as well as give clear guidelines on when to purchase energy efficient motors, based in their running hours. A replace or rewind decision can and should be made long before failure occurs – there needs to be clear guidelines for all responsible personnel.
The time spent creating a motor management policy will be paid for many times over by reduced energy costs and less downtime.
As well as bringing some logic to the replace or rewind debate, a motor management policy helps bring together capital, maintenance and revenue budgets, showing the effect they have on each other when different types of motors are selected. One policy decision should definitely be to select high efficiency motors when purchasing new plant, with specified minimum acceptable efficiency values. Companies benefit from such a policy through reduced energy costs, by upgrading to high efficiency motors at the most cost-effective time. The planning inherent in the practice helps reduce downtime - inventory can also be reduced through a fast track delivery agreement.
Plan for savings
A UK water company cut its motor electricity costs by 6% following the introduction of a motor management policy. The utility uses about £25 million of electricity per year, a large proportion of which is consumed by pump motors.
The policy was devised following an energy monitoring study conducted at three of North West Water’s sites. This involved measuring energy consumption of motors over a seven days period. Each motor was then replaced with a high efficiency unit and monitored for another week. The results identified which motors should be replaced or rewound based on a maximum payback of two years. In several cases, the tests revealed that a working motor could be replaced and still achieve a payback of less than two years.
Another example is a motor management policy developed at BP’s petrochemicals complex at Grangemouth. A consignment stock of motors is stored at a local distributor within a quarter of a mile to the refinery, to ensure a speedy replacement service.
BP’s petrochemicals complex at Grangemouth is making significant savings since developing a motor management policy in co-operation with ABB
Making motor replacement a matter governed by a defined company policy is the best way to keep costs under control, both energy costs and maintenance and replacement costs. A further incentive has been added by the government, in the form of Enhanced Capital Allowances (ECAs). High efficiency motors qualify for ECAs, enabling end users to deduct the full costs of the investment against tax in the year of purchase, instead of writing off the asset over a number of years.
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