Energy Efficiency Opportunities Associated with Induction Motors

Induction motors are often described as the “workhorses” of industry, and can be found in every factory. In many plants they are the largest consumers of electrical energy and contributors to peak demand, and hence it makes sense to evaluate opportunities with respect to motors if one is seeking to reduce electricity costs and emissions.

Every motor has an efficiency level that is determined by its design. High-efficiency variants can be used to replace standard-efficiency motors, and this should result in less input power being required to deliver the same output power. This is the theory, but care must be taken with every application to ensure that speed differentials are accounted for. For motors with the same number of poles, high-efficiency motors tend to run slightly faster than standard-efficiency motors. This speed increase can translate into an increased power requirement for machines such as centrifugal pumps and fans, where power is proportional to the cube of speed. You could therefore have a situation where motor replacement undertaken in the name of energy efficiency actually increases the amount of energy consumed (albeit while using the energy more efficiently).

Motor loading is a key determinant of a motor’s operating efficiency. Loading is simply the amount of power delivered relative to the rated capacity of the motor. The power rating you see indicated on a motor’s nameplate is the amount of power delivered to the shaft. The efficiency level on a motor’s nameplate is that at full load. You can therefore easily calculate the full-load input power, and then calculate the motor’s loading by measuring the actual input power.

Full-load input power = Rated full-load power (at the shaft) / Full-load efficiency (both from nameplate)

Loading (%) = Measured input power / Full-load input power

Like most machines, motors are designed to operate close to rated capacity. Once loading levels get much below 60%, efficiency levels can decline sharply. This depends on the individual motor – larger motors tend to have more stable efficiency levels over a wider loading range, and are also generally more efficient than smaller motors. Power factor also declines with reducing load. Ensure therefore that the motors installed in your facility are not oversized for their duty. Never scrap working standard-efficiency motors. Provided torque and speed characteristics are suitable, you could use an old appropriately sized standard-efficiency motor as a replacement option for an oversized (and hence under-loaded) motor and achieve far higher efficiency gains than would be the case through replacement with a high-efficiency motor. When a motor is suspected of being under-loaded based on a spot check, my advice would be to carefully evaluate whether the load will vary as a consequence of the process being driven before concluding that the motor is under-loaded. You may need to back the spot check up with logging.

Regardless of these motor-specific considerations, your starting point when evaluating motor-related energy efficiency opportunities should always be the system within which a motor operates. An efficient motor driving an inefficient process is of little value. First-order system effects include the efficiency losses associated with gearboxes, misaligned belts/sheaves, certain belt types (cogged and synchronous belts deliver higher efficiencies than standard v-belts for example), coupling problems and misaligned shafts. Beyond these basic system components lies the wider system itself, and here opportunities can be very significant. Through appropriate gearing and pulley configurations (and where there is significant process variation, the use of variable speed drives), motor speed can be reduced, with the potential for significant reductions in energy consumption. This is particularly true for centrifugal pumps and fans, where flow control devices such as valves and dampers can be removed (or fully opened) and flow can instead be controlled with a VSD. The resulting linear relationship between flow and speed often also delivers improved control.

If all of this seems technical, remember also the simple things. The most efficient motor is one that is switched off, and the cumulative effect of switching multiple machines off when they are not required can be surprisingly significant. For sites where production is stopped during employee breaks it can be a simple matter of switching machines off during these times, though where there are long start-up procedures this can be disruptive. Often, to achieve shorter motor running times the focus needs to be on making machines more productive. This could mean reducing rework and idle time, or eliminating upstream and downstream bottlenecks, matters that require analysis at the system level.

A motor inventory is a useful tool for any production site (for both energy efficiency and plant maintenance purposes), and I recommend that you construct a detailed record of every motor on your site, along with its key characteristics and operating hours. Over time you can measure the power consumption for those motors that operate for significant periods of time, assess their loading and then, considering the systems within which they operate, determine options for reducing their energy consumption. You can also use this inventory to assist with decision-making when motors fail.

Copyright © 2017, VWG Consulting, all rights reserved

Leave a Reply