Regeneration: a wiser way to manage your energy
11 August 2010
Leading off this week’s newsletter, Mitsubishi Electric’s Jeff Whiting looks at how motor control technology, in the form of the regenerative drive, can provide answers to some of the issues faced by industrial energy users and bring other operational benefits to the enterprise into the bargain. If you need to buff up your knowledge about regenerative drives, Jeff has supplied an explanation at the foot of his article.
Automotive manufacturers have been challenged in today's low carbon environment to target one of the holy grails of the motoring community - energy efficiency. Two significant approaches have found their way into mainstream motoring: automated stopping of the engine when idling at traffic lights, and conserving the energy generated in braking to optimise fuel consumption and reduce carbon emissions. In fact, the second approach even found its way into Formula 1 as a way to get a performance boost.
Until a few years ago, when drivers stopped at traffic lights or at a level crossing, they simply left their engines running. But now there are many campaigns to encourage switching off; in California, for example, it’s already a legal requirement for commercial vehicles. But restarting an engine, even a warm one, requires an extra squirt of fuel, leading to extra CO2 and NOX, so regenerative technologies are being used to capture braking energy that was previously dissipated through hot brake discs and provide a carbon neutral kick start when the lights go green. A number of car manufacturers have automated this approach, bringing clear energy reductions. But now let’s turn our attention to that workhorse of manufacturing industry, the electric motor.
Historically, an electric motor was started and left running throughout the shift. There was often a good reason for this as starting motors usually took a huge energy inrush until it got moving and built up its own resistance. This inrush could be up to twelve times the working current of the motor, so motors are usually rated with a number of direct starts allowed per hour. Leaving the motor running seemed a sensible approach to this problem. However, fitting a motor with an inverter offers a much softer starting regime, and is far less restricted in terms of available starts. This really opens up the opportunity to run the motor only when it is operationally required, and to save significant amounts of energy by switching the motor on and off.
An inverter drive offers even more energy ‘bang for your buck’ by optimising energy used in the electric motor whatever the load, and also by running the process at lower speeds which can also save significant energy and therefore costs. The best savings can normally be made when running a fan or pump, as a slight reduction in speed can really impact the power consumption.
Maybe this isn't a realistic goal of Formula 1, and wouldn't attract much of an audience, but it is well known that a smooth driver uses far less petrol than a boy racer. Uncharacteristically, Jeremy Clarkson and his Top Gear colleagues demonstrated this some time ago by driving large cars from Paris to Liverpool on a single tank of petrol. By maintaining a steady, moderate speed, avoiding stop/start driving, rapid acceleration and hard braking, fuel consumption was kept within the optimum range and the total mileage proved to be way beyond what is normally achieved.
The savings gained by using inverters in real terms are both financial (having a positive effect on the business' bottom line) and ecological (CO2 emissions reduction). In fact, it has been calculated that the reduction in CO2 emissions achieved by inverters sold within the UK each year equate to the CO2 emissions from 100,000 business cars on normal annual mileage. But an inverter doesn’t just save energy or allow a process to be optimised according to changing loads and requirements.
There are many types of industrial processes driven by motors. Some of these applications bring their own challenges, which are easily met by today's high-performance inverter drives. A typical example is where the process energy overhauls the power of the motor. To keep the process under control, this energy imbalance must be addressed and, if possible, used to power other parts of the production cycle.
This was the principle of the Kinetic Energy Recovery System (KERS) used for a short period of time in Formula 1 racing, but now finding a far more appreciative audience in today’s high efficiency and hybrid cars. Normally, under braking conditions, the momentum of the car generates heat in the brake disks. With the latest technology, KERS is able to capture this energy, which would otherwise be lost, and release it during acceleration, thereby reducing fuel consumption.
Consider an escalator serving a deep London Underground station at rush hour. The ‘up’ escalator will be working hard to lift maybe a hundred people over a considerable vertical distance. The ‘down’ escalator will be carrying just as many people - and it will be creating energy on descent. In power terms, the motor requires power to be fed into it to drive the loaded escalator upwards, whereas when descending, the motor has a load driving it, making the motor act as a generator.
Under these conditions the power has to be controlled because the passengers need to descend in a safe manner. This is generally done by using an inverter to ensure safe control and a measured stopping function. Without this, an uncontrolled stop could have enormous repercussions in terms of escalator passenger injury!
Achieving this continuous control under all loads requires the inverter to shed this extra energy. Now there are many mechanical methods of collecting this energy – counterweights, winding springs among them – but most are fairly crude and only partially effective. As this generated energy is in the form of electricity, it is convenient to dissipate it electrically – usually via banks of ‘braking’ resistors, which ultimately dissipate the energy as heat. In a hot and dusty underground machine room, this poses a fire risk.
The alternative is to use a regenerative drive (Mitsubishi's A701 drive is a good example). This controls the load under all conditions and sheds the excess power by converting the kinetic energy into electricity, which is either fed safely back to the mains supply, or shared between other drives by connecting their power reservoirs together. The energy generated during the lowering stage of our escalator example can be dissipated and lost, or captured and reused. A regenerative drive will capture all of the energy and feed it back to the supply to earn a welcome reduction in the electricity bill.
Throughout a normal day’s operation the escalator drive can continue to minimise energy use. During peak periods the escalators will be fully loaded, yet for most of the day there will only be a trickle of people using them. A typical energy strategy for this would be to operate at full loading with optimum transfer speed to get the rush hour passengers through as quickly as possible, and then to slow the escalators slightly for the rest of the day when speed is not so important. Reducing the transfer speed brings an immediate energy gain, which is further enhanced by the inverter's ability to shed excess power when there are fewer people on the escalator.
The next stage of this energy optimisation takes its lead from the stop-start strategies that are increasingly being adopted by the automotive industry. While the inverter safely controls escalator starts and stops, maximum savings will, of course, be made when there are no passengers to carry and the escalator can be stopped. Implementing controls which sense approaching passengers means the inverters can start the escalators and bring them up to speed before a passenger arrives to step onto it.
Industrial electrical engineers have long known of the energy saving benefits of inverters, and although they might not be in a position to teach the likes of Button, Hamilton and Schumacher a thing or two about fast driving, regenerative drives show they know a lot about efficient recovery and use of kinetic energy in the real world.
Regenerative drives – how do they work?
Standard inverter drives have an input section, power reservoir and an output section. In general, they operate in such a way that energy can freely flow in both directions through the inverter (output) sections, but the input section is a diode bridge designed to allow the flow of energy in one direction only.
Regenerative drives maintain the three sections, but to operate in regenerative mode the power needs to flow in both directions in the input section as well as the output section. This is achieved by blending two inverters back to back into a single unit. The additional input inverter allows power to flow from the mains to the power reservoir when needed, and allows unimpeded reverse flow into the mains when the reservoir is above normal operating levels.
When the motor and load requires driving, the input inverter circuitry is automatically operated to allow the mains supply to pass through and maintain the power reservoir at the optimum level. If the motor loading overhauls the motor, the electric motor acts as a generator and energy is passed back through the output inverter section and begins to pump up the power reservoir. Under these conditions the regenerative drive switches the excess power using the input inverter action to return the excess energy to the mains. An important function of the input inverter is to put the power back in synchronisation with the phase rotation of the input phases. JW
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