Regenerative approach
January 1st 2011

Inverter drive technology, in the form of the regenerative drive, demonstrates a number of operational benefits in industrial environments, as Jeff Whiting of Mitsubishi Electric explains

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 opens up the opportunity to only run the motor during operational requirements, and to save energy by switching the motor on and off.

An inverter drive offers more energy 'bang for its buck' by optimising energy used in the electric motor, and by running the process at lower speeds which can save 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.

The savings gained by using inverters in real terms are both financial and ecological in the reduction of CO2 used. 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 needs. 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 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 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 a rush hour escalator. The 'up' escalator will be working hard to lift maybe 100 people over a considerable height. The 'down' escalator will carry just as many people and create 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.

This is generally done by using an inverter to ensure safe control and a measured stopping function. Similarly, a soft start can be provided, reducing spikes in energy demand and wear and tear.

To achieve continuous control under all load situations, an inverter has to shed this extra energy. There are many mechanical ways to collect some of this energy – counter-weights, winding springs, 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, banks of braking resistors were used to dissipate the electricity into heat. This could become a considerable fire risk.

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 minimise energy used. 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.

Using an inverter means the motor can start and stop the escalator quickly and safely.

Maximum savings will occur when there are no passenger requirements and the escalator can be stopped. Implementing controls that sense approaching passengers means inverters can start the escalators and bring them up to speed before a passenger arrives.

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 and Hamilton 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.

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