Application Guide

63-706229

Where control signal cables need to cross VFD input and output cables this should be done at right angles with no parallel

runs. See Fig. 33.

Filter

Inverter

Coupling will

Occur here

Filter

Inverter

Correct

Filter

Inverter

Filter

Inverter

Filter

Incorrect

Inverter

Filter

Fig. 33. Locating filter and VFD control signal cables.

Filter Location

Fig. 33 adjacent shows various VFD and filter installations. Where possible the filter should be installed as close as possible to

the VFD and most importantly, the earth conductor from the filter to ground, should be at least the same cross section as the

power conductors. The optimum configuration is 1 filter per VFD. VFD output cables should not be run close to the VFD input

power cables as there will be some pickup from the output cable, even though it may armored or screened in some other way.

VFDs are a major source of interference. However, with good engineering practices, large numbers can be installed in

proximity with sensitive equipment, with no adverse effect. As design technologies advance, more sophisticated control of the

VFD output devices will make significant reductions in the area of RFI emissions. VFDs are here to stay for the foreseeable

future, reducing plant running costs and enhancing our environment.

APPLICATIONS

Fig. 34 indicates the typical duty expected from a fan or a pump used in Heating, Ventilating and Air Conditioning (HVAC)

applications (data supplied by the DTI) It can be seen from the graph that for the majority of the running life of this fan or pump,

it has

over capacity

. The reason for this excess capacity is to accommodate fluctuations in loads due to weather conditions,

changes in occupancy. The excess in flow is usually controlled either by throttling, using dampers or valves, or redirecting the

excess capacity via by-pass ductwork or pipelines. The result of this is type of control is increased running costs and in some

cases increased noise and vibration from the plant. This situation also often occurs in industrial applications, as the through-put

of the process is inevitably variable. In many cases a VFD can be installed to control the speed of a motor and thus the

capacity of the plant, providing reduced running costs and enhanced environmental conditions, as a result of reduced noise

and drafts.

Typical System Load Requirements

% Operating Time

% Flow or Volume

100

Fig. 34. Typical system load requirements.