Application Guide
63-706227
Routes for Emissions
The main route for the RF energy is out through the VFD output terminals. See Fig. 29.
DriveSupply
Motor
Safety Earth Lead
Cable
RF Current
Winding
Capacitance
Machinery and Building
Radiation
u
w
v
Capacitance
Fig. 29. Emission routes.
Without Input Filter
The VFD can be considered as a source of radio frequency (RF) current which leaves the output terminals. The capacitance of
the output cable (motor cables) and the motor windings represents a relatively low impedance path to this
noise
current which
flows back to the input terminals of the VFD via the main safety earth and then the input conductors. Any equipment coupled to
the distribution network will effectively
see
this RF at its power supply input. If this path is not well defined and low impedance
then the route back to the VFD input may be via unexpected path which may disturb nearby sensitive equipment.
To ensure the minimum possibility of disturbance to sensitive equipment, ensure that a low impedance path is provided for this
current via a conductor at least the same cross section as the power conductors.
With Input Filter
When a correctly designed input filter is used with a VFD, then the path back to the VFD, for the RF current in the safety earth,
is greatly reduced. The capacitors in the filter effectively provide a short circuit for the high frequency current.
Drive
Supply
Motor
Safety Earth Cable
Cable
Capacitance
RF Current
Winding
Capacitance
Machinery and Building
Filter
Capacitors
Inductors
u
w
v
Fig. 30. Effect of an input filter.
Shielding Motor Cables
Where further reductions in RF emissions are required, shielded motor cables can be employed. Standard armored cable has
proven to be adequate for this purpose, resulting in 1/30 of the original emissions. The motor chassis should be connected to
the safety earth, in the normal manner, with the both ends of the armored cable also connected to the safety earth. The safety
earth cable should then be taken directly to the VFD earth terminal, before progressing to the main distribution earth. See Fig.
31.
H
H
2
1
21
2
1
Motor
Motor
E
1
E
2
H
1
H
2
Noise Current
Noise Current
Fig. 31. Shielding motor cable.