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Advantages of using variable speed drives

Manufacturing engineers have been using variable speed drives in their factories for many years, as have some HVAC specialists, the water industry, and designers of pump, fan and conveying systems.

The food manufacturing industries have used drives since their inception 20 years ago, but there are still many machines and processes that would benefit from an upgrade to variable speed capabilities.

This is particularly true with the most modern drives that have many clever features just waiting to be exploited.

In essence, drives control the speed of motors, and thus the machines, pumps, fans, mixers, conveyors to which they are attached.

Normally a motor will run at one fixed speed, dictated by the way it is built and the frequency of the electricity supply feeding it.

In the UK our mains frequency is fixed at 50Hz.

A drive effectively varies the mains frequency just before it is fed into the motor, thus making the motor turn at different speeds.

This technique is called waveform inversion, hence the other name for a drive-an 'inverter'.

With a drive you can simply change the running speed of any motor-driven machine or equipment from its 'natural' speed to the speed you actually require.

This can be done either manually or automatically to meet changing conditions.

For instance, a sugar centrifuge can be programmed to start slowly then gradually accelerate to a high speed, hold this speed for a set time then decelerate rapidly to a far slower speed for emptying, and finally stop altogether to be reloaded ready for the next cycle.

This programme would be time-based, with the drive changing the speed after set time periods.

In other cases the speed profile can be varied according to parameters other than time.

For instance, to mix batter you need to start slowly then speed up as the ingredients combine and the batter changes state.

The best way to do this is not to run the drive on a time base but to use a sensor to measure the torque on the mixer.

As the ingredients combine the torque drops away and the sensor signals to the drive to steadily increase the mixer's speed.

In this way optimum speed is always maintained.

Many types of equipment benefit from the ability to run at different speeds so that they can match precisely the needs of the moment.

For instance, if the temperature in a walk-in refrigerator is too high the ventilation fan will need to run faster to draw out the warm air, but once the temperature is lowered the fan can be slowed down.

It can even be stopped completely during off shifts when there is no traffic through the refrigerator.

However, the motor may need to quickly run up to maximum speed if for instance a large batch of ingredients/food has to be chilled down quickly.

Fortunately a drive can make these changes happen, either manually or under automatic control.

In this case, the primary control is time-based and follows the shift patterns, but this can be overridden by temperature sensors should the need arise.

Taking fans again, but this time in an office, you could have a basic timer that switches the fan off at night, churns it over at low speed from 08.00-09.00GMT to freshen the air prior to the arrival of staff and goes to normal operating speed at 09.00GMT.

Working in conjunction with this, presence sensors, temperature monitors and fire alarms can take over and readjust the speed to adjust for hot/cold weather, non-occupancy, high occupancy.

The examples looked at so far have been relatively simple, but most drives have on-board computer intelligence which helps them respond effectively and automatically to the local conditions and requirements.

They can be made even more intelligent by using them in conjunction with a control unit such as a personal computer (PC) or a programmable logic controller (PLC), which will use sensors to monitor the situation, make decisions and adjust the drive.

Consider the example of a robot arm set up to pick up a cherry from a hopper and place it dead centre on a Bakewell tart as it passes beneath on a conveyor.

If the tarts pass at irregular intervals or are not always centred on the conveyor belt, a systems of sensors can adjust the drive in 'real time' to match the movement of the robot arm to the passage of the tarts and 'score a bullseye' with the cherry every time.

This is an example of a drive responding immediately to adjust the speed of the motor it is controlling.

This characteristic can be exploited to create precise speed profiles that are very complex in nature.

This is useful when performing automated tasks requiring relatively complex movements, such as often found at packing and wrapping stations.

Drives are also regularly used with pumps.

Large process lines, for example, will often include a booster pump in the basement, the job of which is to keep a header tank of water above the process line filled.

But water demand may be characterised by sudden surges, so the header tank could empty and the pump may not be able to refill it promptly.

One way to overcome this is to use a very large tank, but this may not be physically possible and it can also lead to problems with stagnating water.

Another way is to oversize the pump or cascade of several pumps, with progressively more kicking-in as the surge continues, but this is expensive, maintenance-heavy and provides only crude control.

Far better control can be achieved by fitting a variable speed drive to the pump's motor so that the flow can be continually adjusted to keep the tank nicely topped up automatically.

This not only improves the water system but also brings us to one of the great advantages of a drive - energy saving.

Pumps, fans and indeed all electrically driven equipment, consume power, but turning the speed down even slightly can result in substantial energy savings.

Because they are moving volumes, pumps and fans comply with a 'cube law': if you trim their speed to say 80% maximum, the energy consumption is reduced by 0.8 x 0.8 x 0.8, or 51%.

By the same token by running at half-speed power consumption is cut to 12.5%.

Given that pumps and fans often have to be sized to cope with occasional massive demand (such as may be required in an emergency), the energy savings achievable by trimming their speed constantly to match actual demand can be enormous.

In these days of increasing environmental awareness and accompanying new standards, such as ISO14000, drives can only become more and more attractive.

Additionally, analysts all agree that electricity costs will rise steadily for the next decade or more beyond the post-privatisation price squeezes that the supply companies endured to win market share.

In summary, drives work on two levels.

Firstly they make electrically driven equipment far more responsive, user-friendly, and intelligent.

Secondly they achieve significant energy saving.

Without drives intelligent buildings cannot exist; with them, even a humble mixer, pump or conveyor can have features that were considered to be science fiction as recently as last millennium.

Nov 7, 2005
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