Rexroth IndraDrive Requirements to the Mains Connection 10-19
DOK-INDRV*-SYSTEM*****-PR02-EN-P
Cause of Leakage Currents
For the purpose of stepless speed variation with a high degree of
positioning accuracy and dynamic response, certain modulation
procedures are indispensable for drive systems. For physical reasons,
these modulation procedures give rise to inevitable leakage current
produced during normal operation. Especially with unbalanced loads of
the mains phases or a large number of drives it can easily reach some
100 mA of rms value.
The leakage current is not sinusoidal but pulse-shaped. For this reason,
measuring instruments normally dimensioned for alternating currents in
the range of 50 Hz are not suited, as measuring errors occur. Use
measuring instruments with rms value measuring ranges up to at least
150 kHz.
The degree of leakage current depends on the following features of the
installation:
• kind of inrush current limitation
• number, kind and size drives used
• length and cross section of connected motor power cables
• grounding conditions of the mains at the mounting site
• imbalance of the three-phase system
• kind of filters and chokes connected in the incoming circuit
• EMC measures that are taken.
If measures are taken to improve the electromagnetic compatibility (EMC)
of the installation (mains filters, shielded lines), the leakage current in the
ground wire is inevitably increased, especially when switching on or in the
case of mains unbalance. Given these operating conditions, residual-
current-operated circuit-breakers can trigger without an error having
occurred.
The EMC measures are mainly based on capacitive short-circuiting of the
interference currents within the drive system. Inductive filter measures
can reduce the leakage currents, but affect the dynamic response of the
drive and bring about higher construction volume, higher weight and
expensive core material.
Possibilities of Use
We distinguish two kind of residual-current-operated circuit-breakers:
1. Residual-current-operated circuit-breakers sensitive to power pulse
current (type A acc. to IEC 60755)
These are normally used. However, it is only pulsating direct fault currents
of a max. of 5 mA and sinusoidal alternating fault currents that switch off
safely. This is why they are not allowed for devices that can generate
smoothed direct fault currents. In the case of smoothed direct fault
currents that can be produced in power supply units, mains rectifiers and
drive controllers with power converters in B6 circuit, the circuit breaker is
not triggered. The triggering of a residual-current-operated circuit-breaker
sensitive to power pulse current is blocked in the case of ground contact,
i.e. in the case of error.
Residual-current-operated circuit-breakers sensitive to power pulse
current do not provide any protection against inadmissible contact
voltage.
2. Residual-current-operated circuit-breakers sensitive to universal
current (type B acc. to IEC 60755)
These circuit breakers are suited for smoothed direct fault currents, too,
and safely switch off devices with B6 input rectifiers.
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