Energy conservation is a popular subject. And a lot of commentary has been offered about the importance of energy conservation in the industrial community. I agree. But the desire for energy conservation needs to be tempered with real cost benefit analysis. Which sometimes gets passed up.
Almost all variable frequency drives are sold on the basis of the amount of energy, and money, they will save as part of the justification for spending the money on the equipment and installation. Fans and pumps can benefit from reduced power consumption, especially when there are hundreds of horsepower of load involved. Mechanical life expectancy increases and maintenance costs are reduced as system speeds are reduced to meet the operating setpoint of the driven load.
Large material handling systems have found ways to reduce costs. Systems that are divided into zones can be monitored for the the presence of product on the conveyor lines. If there is no product present, the conveyor motor is idled which saves energy. When there are large systems, like airport baggage handling systems, with hundreds of horsepower of equipment spread over miles of conveyors, the load requirements are similar and a 20% or 30% reduction in system energy consumption can be very significant.
But plant floor machinery can be very different depending on the industry you are in. In the medical manufacturing arena, there are lots of machines and lots of stepping motors in them. Sometimes 24 axes of stepping motors can be in a very small machine. And medical manufacturing plants can have hundreds, even thousands of motors. So you would think that there are similarities in terms of the energy savings opportunity.
But factory automation systems present different energy usage problems. Stepping motors, for example, are low power consumption systems are always on. The motor may not be doing any work, but the power supply needed to provide DC power is always on. So power is being converted, even though the motors may not be doing any work. But the fact is that stepping motor systems are fairly low power, 200 Watts is typical. So even when there are loads made up of hundreds of stepping motors, its hard to come up with enough energy saving to return the cost of a complex effort to control it.
In the servo world because multiple servo axes are not all on at the same time, average load and power supply sharing have led to a number of servo amps that use common dc bus architecture. This is a great way to say money and reduce equipment size. But even large systems are limited to 10 kilowatt average power. And frequently these systems have the ability to manage the input power so that the AC load is fairly efficient.
But once in a great while, there are applications where excess bus energy is generated, or regeneration is taking place. This is the case when a palletizing machine lowers its load. The weight of the material on the pallets drives the motors instead of the amplifiers. This puts generated electricity onto the input voltage bus. Imagine the surprise of the electrical maintenance department getting an emergency call that the palletizing equipment was generating too much power!
Every prospective energy saving project needs to be considered in the context of the industry, application and cost benefit. A simple Pareto analysis of the major segments of energy consumption can be conducted which will categorized and quantify the opportunities for energy cost reduction. Many of the best opportunities will come from unexpected areas.