Removal of Y Capacitor
On ungrounded power supplies, it will be necessary to disconnect the 'Y' capacitor from the inside of
the unit and fit an output choke.
For instructions on the removal of the Y Capacitor please see the operating instructions.
The purpose of the choke is to limit high frequency circulating currents that can flow during ground
faults. These would cause high currents in the output devices, leading to excessive dissipation and
1. Operation on ungrounded power supplies is only possible with unfiltered units. Filtered
units (or units which do not have their 'Y' capacitors disconnected) will be damaged
after running for a short time.
2. An output choke is recommended to limit the fault current during a ground fault. All
qualification tests have been carried out using an output choke.
3. It is often the case that ungrounded power supplies have high levels of transients and
interference. Supplies should be checked for damaging transients before fitting MM4. If
in doubt, an input choke should be installed.
An ungrounded supply is one where the ground is not electrically connected to the inverter input (see
section 2.4). This arrangement is sometimes used in industrial installations because, in the event of an
ground fault occurring, the equipment may continue to run.
Ungrounded does not mean there is no ground connection at all. All Siemens drives
must always be grounded. This is important for safety reasons.
Quick Start Guide