CABLING AND CONNECTIONS
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6
6.2.1.2
Installation of ferrite rings (+OCM) on the motor cable
Modern PWM Frequency converters based on IGBT transistors generate output pulses having short
rise times (0,1...0,2 µs). Those pulse edges increase the stress for motor insulation and also gener-
ate high frequency common mode voltages on the motor output. Common mode voltage increases
emitted radio interferences and might cause danger of bearing currents when larger motor frames
(> 100kW) are used.
Motor cable selection and installation has to be done according to the requirements of the installa-
tion environment. The screen of the motor cable has to be connected to earth at every end (fre-
quency converter, safety switch, motor etc.) so that high frequency impedance is as low as possible.
The best solution is a 360-degree grounding. If this is difficult to do then connect the PE conductor to
ground or panel wall (metallic) right after the stripping point of jacket of cable (see Figure 6-8, alter-
natives a and b). Influence of common mode voltages can easily be additionally attenuated by install-
ing a common mode choke on the motor output. The simplest way is to use ferrite rings with a di-
ameter large enough to allow the motor phase conductors to be slipped through.
Installation of the ferrite rings:
Slip only the phase conductors through the rings; leave the cable screen below and outside the
rings, see Figure 6-8. Separate the PE conductor. In case of parallel motor cables, reserve an equal
amount of ferrite ring sets for each cable and feed all the phase conductors of one cable through
one set of rings.
Vacon's delivery consists of fixed sets of ferrite rings (option). When ferrite rings are used to
attenuate the risk of bearing damages, use always two ferrite ring sets per motor cable.
Note! The ferrite rings are only additional protection. The basic protection against bearing currents
is an insulated bearing.
Figure 6-8. Installation of ferrite rings on single (left) and parallel (right) motor cables
ud1011k8.fh8
Ferrite rings
Metallic wall
of cabinet
Screen connected
to earth
Motor cable
PE
Mains cable
High frequency
grounding
Motor cable
Ferrite rings
Metallic wall
of cabinet
Screen conneted
to earth
