Energy savings inverter technology without extra cost [November 2012]

November 22, 2012 1:03 pm

Lenze 8400 series of frequency inverters can reduce running costs by up to 30 per cent, without any additional costs where there are substantial variations in the load
A factor that is slowing the take-up of energy saving technology is the reluctance for companies to invest, even though the payback times are usually short. However for some types of machinery, designers can incorporate drives that reduce running costs by up to 30 per cent, without additional costs. This is possible for conveyors, pumps, fans and other cases where there are substantial variations in the load. Here the Lenze 8400 series of frequency inverters can give these major savings by adapting the motor magnetisation currents to match the varying loads. The feature is called VFC eco and it is standard for the 8400 range at no extra cost.
It is a fact of life with standard AC asynchronous motors that optimum efficiency is achieved around full load, and the efficiency falls steeply at partial loads, particularly below 40 per cent. The reason relates to the magnetisation currents. For such cases running directly online or even in vector control by frequency inverters, the magnetisation current remains constant when the load reduces. Magnetisation currents result in copper and iron losses which are the main factor that decides the motor efficiency. Where the running load falls to 20 per cent of full load, the loss in efficiency is often greater than 30 per cent.
The VFC eco feature is a switchable software option included in the 8400 series of cabinet and decentralised inverters. Effectively it is a loss-optimised algorithm that maintains a constant power factor Ф either at fixed or variable frequencies. It is most effective in a range between 5 and 60 per cent of full load. Here there are significant savings, for example a typical motor efficiency at 20 per cent load might be 53 per cent without VFC eco, and 70 per cent with.
The diagrams illustrate the changes in motor currents as the load changes. Normally when a motor goes from full to partial load, the stator current Iq (also known as the torque producing current) reduces. However the motor current Id, which is the magnetisation current, remains unchanged. With inverter control the applied voltage Us will also decrease. Note that at partial load the angle Ф increases. As the efficiency of the motor is proportional to the Cos Ф (the Power Factor), the efficiency has therefore decreased.

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