Motor 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 the fuel usage and reduce carbon emissions. In fact the second approach even found its way into Formula 1 as a way to get a performance boost. Jeff Whiting of Mitsubishi Electric looks at how inverter drive technology, in the form of the regenerative drive, answers real issues in industrial environments, and indeed demonstrates a number of other operational benefits
Until a few years ago, when drivers stopped at traffic lights or a level crossing, they simply left their engines running. But now there are many campaigns to encourage switching off - in California 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.
Historically in industry, 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 power inrush could be up to 12 times the working current of the motor and therefore motors are usually rated with a number of direct starts allowed per hour. Leaving the motor running seemed quite a realistic approach. 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 only run the motor during operational requirements, and to save significant energy by switching the motor on and off.
A inverter drive offers even more energy ‘bang for its 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 sometime 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 in 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, affecting a business' bottom line and ecological in the reduction of CO2 used. In fact it has been calculated that the CO2 savings made by the inverters sold in the UK each year relate to the CO2 used by 100,000 Business cars doing normal mileage.
An inverter doesn't just save energy or allow a process to be optimised for changing loads and requirements. There are many types of industrial processes driven by motors. Some of these applications bring a number of other challenges which are easily addressed by today's high performance inverter drives. Typical of these is where energy in the process overhauls the power of the motor. To keep the process under control, this energy must be dealt with, and if possible used to power other parts of the production cycle. This was the principle of the Kinetic Energy Recovery System used for a short period of time in Formula 1 racing, but finding a far more appreciative audience in today's high efficiency and hybrid cars. Normally, under braking conditions, the weight of the car generates heat in the brake disks. With the latest technology, KERS uses this condition to capture the energy and release it during the driven part of the journey, thereby reducing fuel consumption.
Consider an escalator at a deep London Underground station at rush hour. The ‘up' escalator will be working hard to lift maybe a hundred people over a considerable height. The ‘down' escalator will be carrying just as many people - and it will be creating energy as they descend. 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 for 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 huge repercussion with people thrown every which way - mainly downwards into a big heap of limbs and bodies. People could be hurt and the legal repercussions last for years.
To achieve this continuous control under all load situations, an inverter has to shed this extra energy somewhere. There are many mechanical ways to collect some of this energy - counterweights, winding sprints, etc - but most of them are fairly crude and only partially effective. As this generated energy is in the form of electricity, it is general to dissipate it in that form. In the past, vast banks of braking resistors were used to dissipate the electricity into heat. This could become a considerable fire risk anywhere, but doubly so in a dusty, hot underground machine room. However, a specially designed regenerative drive, such as Mitsubishi's Regenerative A701 drive, controls the load under all conditions and sheds the excess power by converting the kinetic energy into electricity and pumping it safely down the mains or even sharing it with other drives by connecting their power reservoirs together. The energy generated during the lowering stage can be dissipated and lost, or captured and reused. By contrast, a regenerative drive captures all of the energy and feeds it back into supply mains giving welcome savings in electricity bills.
The basic requirements of a soft start-up and stop can be programmed into a regenerative drive quite easily. Throughout a normal day's operation of the escalator, the drive will still be minimising the energy used. As you can imagine, during rush hour the escalators are fully loaded with people rushing to get to and from work, yet for most of the day there will only be a trickle of people using them.
A typical energy strategy 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 where the speed requirements are not so prevalent. The use of a reduction in transfer speed will bring an immediate energy gain, which will be further enhanced by the inverter's innate capability to shed excess power when there are fewer people on the escalator. The next stage in the developing strategy takes its lead from the stop-start strategies beginning to appear in today's high efficiency vehicles. As previously stated, using an inverter means the motor can start and stop the escalator quickly and safely when required. Maximum savings will occur when there are no passenger requirements 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.