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EC tech­nology or VFD?

The hidden cost of vari­able frequency drives

In an effort to reduce our Carbon emis­sions many Govern­ments around the world encourage the use of vari­able frequency drives (VFD) with induc­tion motors. This deci­sion is made on the under­standing that signif­i­cant energy savings are achieved through the fan and pump ‘power law’. The flaw in this deci­sion is the over­sight of alter­na­tive tech­nolo­gies such as EC motors and that there are hidden costs with vari­able frequency drives.

Actual effi­ciency factor

The power law theory states that the power consumed by the impeller is propor­tional to the cube power of the change in speed; reducing the impeller speed by 50 per cent reduces the power input to 12.5 per cent. If the fan speed can be modu­lated to meet a demand then signif­i­cant energy savings can be achieved. This law should be used with care as theory does not occur in prac­tice. It assumes turbu­lent air flow throughout the control range, and this is unlikely, and it does not consider fixed losses within the motor and drive. Signif­i­cant savings can still be achieved, but not quite as much as the law would indicate.There are some hidden costs with VFD’s, for example they increase motor losses. IEC60034-31, selec­tion of energy-effi­cient motors including vari­able speed appli­ca­tions, advises that the addi­tion of a VFD increase motor losses by 2 to 5 per cent; in adverse circum­stances it can increase motor losses up to 15 to 20 per cent. The VFD also has its own losses with small drives being less effi­cient than large drives. The losses are also depen­dant on the drive switching frequency. The loss is less with low frequency switching, but this produces mechan­ical noise from the motor. The drive is there­fore normally set at a high switching frequency but this increases its losses, for example a 1.1 kW drive is typi­cally 95 per cent effi­cient at low and 92 per cent at high switching frequency. Another loss over­looked is that of power factor correc­tion.

No hidden surprises with EC tech­nology

When these losses are added together and compared to alter­na­tive tech­nology the true cost is revealed. Whereas EC motors include vari­able speed tech­nology and power factor correc­tion the asso­ci­ated losses are not hidden, but shown within the overall figure. For example a 1.1 kW 4 pole motor a high effi­cient (IE2) induc­tion motor is 81.4 per cent effi­cient. When the VFD, filter and increased motor losses are added this leads to an overall effi­ciency of 73.3 per cent, a hidden cost of 150 W. A compa­rable EC motor is 87 per cent and saves 236 W, see figure 1. With a theo­ret­ical energy reduc­tion of 78.5 per cent when the fan is reduced to 50 per cent then it is under­stand­able why vari­a­tion of fan speed is encour­aged through legis­la­tion. However this should not be restricted to vari­able frequency drives as there are hidden costs that are not concealed with alter­na­tive tech­nology, such as EC tech­nology. Which saves even more
energy while being more effi­cient and infi­nitely adjustable.

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  • Rahul Bhargava on said:

    Excel­lent reading.
    Great product ,fantastic tech­nology ,should rule the world.