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Seeking higher efficiency? Know your motor load

04 January 2012

Obtaining reliable figures for your motor loading is essential if your goal is to achieve high energy efficiency. Marek Lukaszczyk reminds us that motor efficiency drops off the cliff below 50 per cent load, so avoid the temptation to over-size

Determining that electric motors are properly loaded enables users to make informed decisions about when to replace them; and decisions about which replacements to choose. Measuring motor loads is relatively quick and easy with the right equipment, and every company with a significant motor stock should be looking to perform a motor load and efficiency analysis as part of their preventative maintenance and energy conservation programmes.

Analysis is necessary because there is not much point replacing existing standard ac electric motors with energy efficient types if the motors are mismatched or oversized for the loads that they are intended to serve. Too often, motors are oversized or have been rewound several times, leading to gross inefficiencies in their operation. As a result, it is imperative that before any replacement is considered, the actual loads served by existing motors are determined, together with their annual hours of operation.

Most electric motors are designed to run at 50 per cent to 100 per cent of their rated load, with maximum efficiency around 75 per cent of rated load. Thus, a 10kW motor has an acceptable load range of 5kW to 10kW, with peak efficiency at 7.5kW. As a general rule, high efficiency motors garner the maximum savings when they are loaded in excess of 75 per cent of full load, and are operated more than 4,000 hours a year.

The problem with motor efficiency is that it tends to decrease dramatically below about 50 per cent load, which is bad news when one considers that as little as 20 per cent of the electric motors installed in the UK are running at their full rated input, due to over-sizing. It is also bad news in terms of energy costs, as it has been calculated that a single percentage point increase in efficiency will save lifetime energy costs generally equivalent to the purchase price of the motor.

Whilst over-sizing of motors is the most common problem, under-sizing can be just as damaging, because an under-sized motor is likely to be overloaded causing it to overheat, lose efficiency, and (probably) fail prematurely with costly ramifications for production. This problem can arise where system costs are tight, and users interpret motor service factors too liberally.

A service factor is a multiplier that indicates how much a motor can be overloaded under ideal ambient conditions. For example, a 10kW motor with a 1.15 service factor can handle an 11.5kW load for short periods of time without incurring significant damage. Although many motors have service factors of 1.15, running the motor continuously above rated load reduces efficiency and motor life. In addition, the motor must never be operated in an overloaded state when the voltage is below nominal, or when cooling is impaired by altitude, high ambient temperature, or dirty motor surfaces.

Efficiency is also lost when motors are run either above or below their design voltages. The result of over-voltage is a lower power factor, which reduces overall motor effectiveness. The same is true of motors that are operated at less than 95 per cent of their design voltage. They typically lose two to four points of efficiency, and also suffer service temperature increases of up to 7oC, greatly reducing motor insulation life and impairing reliability.

Armed with all the above knowledge, the recommendation is that motor users should survey and test all of their motors that operate over 1,000 hours per year. Then, using the analysis results, divide the motors into the following categories:

Motors that are significantly over-sized and under-loaded: replace these with more efficient, properly sized models at the next opportunity, such as during scheduled plant downtime.
Motors that are moderately over-sized and under-loaded: replace with more efficient, properly sized models (waiting until they fail).
Motors that are properly sized but standard efficiency: replace most of these with energy-efficient models; again, wait until they fail.

One problem with this strategy is that it is often difficult to determine the characteristics of motors that have been in service for some time. It is not uncommon for the nameplate on the motor to be lost or painted over. In addition, if the motor has been rewound, there is a probability that the motor efficiency has been reduced.

When the nameplate data is missing or unreadable, efficiency values must be determined at the operating load point for the motor. This involves using power, amperage or slip measurements to identify the load imposed on the operating motor, then obtaining a motor part-load efficiency value. Finally, if direct-read power measurements are available, derive a revised load estimate using both the power measurement at the motor terminals and the part-load efficiency value as shown in the following equation:

Load = Pi multiplied by η divided by kW

Where load is the output power as a percentage of rated power; Pi is the three-phase power in kW; η is efficiency as operated in per cent, and ‘kW’ is the nameplate power rating.

For rewound motors, an adjustment to efficiency values is required to reflect that rewound motor efficiency is less than that of the original motor. To reflect typical rewind losses, two points should be subtracted from a standard motor efficiency on smaller motors (less than 30kW), and one point for larger motors. However, it must be stated that rewind companies with the best quality-control practices can often rewind with no significant efficiency degradation at all.

Marek Lukaszczyk is with WEG Electric Motors (UK)

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