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  • IEC 60034-31:2009

  • 16 –


An analysis of the electrical system will indicate whether an improvement in the power factor is needed and whether capacitors, synchronous motors, or other corrective measures should be used.

When power-factor-correction capacitors are used to improve power factor of the electrical system they should be carefully selected and applied to avoid unsafe operating conditions. It is recommended that the system designer be consulted for the proper value of corrective capacitance.


Matching motors and variable speed drives

The addition of an variable frequency controller adds considerable potential for improved energy efficiency in many electric motor systems. The additional costs of the VFD (typically larger than the higher efficiency motor) and some additional losses (which depends upon size and quality typically 2...5% at nominal torque and speed and 10...30% at 25% torque and speed) require a careful analysis of the application.

The first group of applications is pumps, fans and similar with changing loads where torque increases nearly by the square of the rotating speed of the motor. The electric input power of the motor will increase with the third power of speed when the flow volume in closed ducts and pipes is controlled with dampers and throttles only. The VFD can adjust the electric power input smoothly and continuously to the required flow volume and the losses are reduced accordingly. Traditional load control with multi-speed motors or parallel operated multi-motor schemes are to be considered if they can do the job with lower costs and fewer losses. The cost benefit of a VFD is high because a greater energy efficiency improvement is possible.

The second group of applications are conveyors, escalators, hoists and similar where the torque is more or less independent from speed. The VFD can continuously adjust the speed from almost standstill to full speed without steps and can thus minimize the needed power. The cost and efficiency benefits are smaller compared to the first group of applications because the change of input power is linear with the speed.

The third group of applications includes those which have minimal changes in load and speed but can benefit from a VFD in other ways like soft starting and stopping or the requirement of a high starting torque. The main benefit is not in energy efficiency improvements but in less wear of the machinery involved. There are other technical solutions for soft-starting available which require less cost. However, compared to soft-starting with a variable frequency control, these methods do not save energy.

In some applications, motors are oversized and continuously run at part-load (for example 50% or below). Even though a VFD can improve energy efficiency by reducing the input voltage to the motor, a better sizing of the motor for the necessary load would be more cost effective and save even more energy.

Unless additional sinusoidal filters are used, motors operated on a variable frequency drive will be subject to voltage spikes significantly higher compared to grid operation. Today, most new industrial electric motors have an insulation system that can handle these voltages without problems. When retrofitting older motors in existing applications with VFDs, the manufacturer should be contacted. Also it is important to determine the maximum safe operating speed of the motor in case the grid frequency (50 or 60 Hz) shall be exceeded significantly with a VFD. Such information is normally available in catalogues or the product documentation (manuals).

The VFD has to be selected and programmed based on a clear knowledge of the typical operating conditions listed below. It is important to match the performance of the VFD closely with the required load profile and the electric properties of the motor in order to achieve the full benefit of the VFD.

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