PRACTICAL ELECTRICAL ENERGY CONSERVATION ON INDUSTRIAL SITES: ARTICLE 2 - INDUCTION MOTORS

Induction motors are the heart and soul of most industrial sites. As modern motors have become more and more efficient, opportunities have arisen to replace older motors with newer ones, thereby harvesting the energy benefits. This replacement, while guaranteed to save energy and possibly reduce maximum demand (for sites that do not have PF correction) is not necessarily always financially viable, and you will need to take some care in investigating opportunities in this area.

Motors driving dosing pumps at a water treatment works

The power factor and efficiency characteristics quoted for induction motors are based on full-load conditions i.e. conditions at which the power delivered by the motor at the shaft is equivalent to the rated capacity of that motor. Motors operated at part-load will have a reduced power factor and efficiency. Don’t therefore make the mistake of simply looking at power factor and efficiency characteristics of two motors for comparison when looking at replacement options. You will need to do field measurements to determine power consumption and ascertain loading, which will then provide additional input to the decision, which may even be that you need to install a smaller motor. An under-loaded energy-efficient motor will not achieve its full load efficiency, and savings may therefore be less than expected. It is also important to assess transient conditions, such as start-up, when power draw can be much higher than during stable running, and variations in power draw which result from the nature of the process being driven by the motor. Whatever motor you install would need to cope with those conditions. 

Aside from the technological issues associated with the motors themselves, there are other system-related issues which are in many cases much more important. Probably the first question I want answered when looking at motor replacement is: “what are the annual running hours of the motor?” Remember that in energy terms, we are looking here at kWh, and while efficiency reduces kW, if the hours (h) of running are small, the total energy savings from retrofitting will also be small. Low-efficiency motors that run continuously in plants that run around the clock are therefore always attractive retrofit candidates, while a motor that runs intermittently in a dayshift-only operation will probably not be interesting, unless it is so under-loaded that its efficiency is only a fraction of full-load efficiency.  In this latter instance, you may even want to install a smaller standard-efficiency motor that is being replaced elsewhere to limit costs while still saving electrical energy.

The next system-related issue to consider in terms of motors are the drive systems associated with each motor. These entail things such as gearboxes, chains, belts, couplings and shafts. You need to consider not only the technological issues here (for example, cogged belts are reported to use 2% less energy than standard v-belts without any changes required to pulley systems) but also the maintenance issues. Key maintenance issues from an energy efficiency perspective could include:

• ·         Lubrication of bearings, chains and gearboxes;
• ·         Alignment of shafts, couplings, sprockets and pulleys;
• ·         Tension levels in belts and chains

It is important to include checks of these types of issues as tasks in your preventive maintenance programme.

The final system-related issue is that of the process being driven by the motor. An efficient motor driving an inefficient process still constitutes an inefficient operation. Hence ask yourself if that agitator is of an efficient design, if the pressure drop in that pumping system as low as it could be or if that fan design is best for the application, as examples. Delve deep into your processes and look at the constraints around your plant, remembering that the overall process only has the capacity of the constraint. It makes no sense to run non-constraining processes at higher throughputs than the constraint, leading to unnecessary circulation, waiting times (with motors running in many cases) or higher flow rates than needed in the case of pumped systems (remember that when pumping fluids, pressure drop varies with the square of flow rate). These types of issues are unique to each individual industrial site, require comprehensive process knowledge, and in my experience are the most neglected when it comes to energy efficiency programmes and projects. They should in fact be addressed first, with motor replacement options only investigated once underlying processes have been optimised.