Electric motor loading is key to energy efficiency. That's the warning from Marek Lukaszczyk, who wants plant engineers to assess their electric motors as a matter of urgency. Brian Tinham reports
 Determining that electric motors are properly loaded enables users to make informed decisions about when to replace them and which to prioritise. That is the advice from Marek Lukaszczyk, European marketing manager with Weg Electric Motors, who adds that, with the right equipment, measuring motor loads is relatively quick and easy – and that plant engineers should do so as part of 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 they are intended to serve," he explains. "Too often, motors are oversized or have been rewound several times, leading to gross inefficiencies in their operation," he adds.
For him, the point is that most electric motors are designed to run at 50% to 100% of their rated load, with maximum efficiency around 75%. Hence, a 10kW motor has an acceptable load range of 5kW to 10kW, with peak efficiency at 7.5 kW. "As a general rule, high efficiency motors garner the maximum savings when they are loaded in excess of 75% of full load, and are operated for more than 4,000 hours a year," he asserts.
And that matters, because the problem with motor efficiency is that it tends to decrease dramatically below about 50% load – which is bad news when as few as 20% of electric motors in the UK are running at full rated input, precisely because of the over-sizing problem. "It is also bad news in terms of energy costs ... a single percentage point increase in efficiency will save lifetime energy costs generally equivalent to the purchase price of the motor," warns Lukaszczyk.
Furthermore, while over-sizing is the most common problem, under-sizing can be just as wasteful, because such motors are likely to be overloaded – causing them to overheat, lose efficiency, and probably fail prematurely, with all the usual cost ramifications for production.
One of the reasons, opines Lukaszczyk, is that the term 'motor service factors' is often interpreted too liberally. "A service factor is a multiplier that indicates how much a motor can be overloaded under ambient conditions," he explains. "For example, a 10kW motor with a 1.15 service factor can handle an 11.5kW load for short periods, without incurring significant damage. However, although many motors have service factors of 1.15, running them continuously above rated load reduces efficiency and motor life."
Incidentally, overloading should never be allowed when the voltage is below nominal, or when cooling is a problem, due to high ambient temperature or dusty motor surfaces. And Lukaszczyk goes on to remind plant engineers that 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 operating at less than 95% of their design voltage. Motors typically lose two to four points of efficiency, and also suffer service temperature increases of up to 7C, greatly reducing motor insulation life and impairing reliability," he advises.
That is why Lukaszczyk is calling for plant engineers to survey and test motors operating for more than 1,000 hours per year. Then, using the analysis results, he says, they should divide the motors into three categories:
Motors that are significantly oversized and under loaded: replace these with more efficient, properly sized models at the next opportunity, such as scheduled plant downtime.
Motors that are moderately oversized and under-loaded – with more efficient, properly sized models but wait until they fail.
Motors that are properly sized but standard efficiency – replace with energy-efficient models, but again, wait until they fail.
One problem with this strategy is that it can be difficult to determine the characteristics of motors that have been in service for some time. Why? Because it is not uncommon for the nameplate on the motor to be lost or painted over. Further, if the motor has been rewound, efficiency may have been reduced.
Lukaszczyk's solution: "When the nameplate data is missing or unreadable, efficiency values must be determined at the operating load point. This involves using power, amperage or slip measurements to identify the load imposed; then obtaining a motor part-load efficiency value. 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 for rewound motors, he suggests that two points be subtracted from a standard motor efficiency on smaller motors [less than 30kW], and one point for larger motors.
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