Siemens A1 Manual De Usuario
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Selecting a Drive
MICROMASTER Applications Handbook
11
3.2.3 Ungrounded
Supplies
Certain industrial installations operate with supplies that are isolated from the
protective earth (IT supply). This permits equipment to continue to run following an
earth fault. However, MICROMASTERs are designed to operate on grounded
supplies and are fitted with interference suppression capacitors between the supply
and ground. Hence operation on ungrounded supplies may be restricted. Some
inverters are designed to allow the removal of these capacitors and enable limited
operation with ungrounded supplies. Please consult Siemens for clarification.
protective earth (IT supply). This permits equipment to continue to run following an
earth fault. However, MICROMASTERs are designed to operate on grounded
supplies and are fitted with interference suppression capacitors between the supply
and ground. Hence operation on ungrounded supplies may be restricted. Some
inverters are designed to allow the removal of these capacitors and enable limited
operation with ungrounded supplies. Please consult Siemens for clarification.
3.2.4
Low Frequency Harmonics
The inverter converts the AC supply to DC using an uncontrolled diode rectifier
bridge. The DC link voltage is close to the peak AC supply voltage, so the diodes
only conduct for a short time at the peak of the AC waveform.
bridge. The DC link voltage is close to the peak AC supply voltage, so the diodes
only conduct for a short time at the peak of the AC waveform.
The current waveform therefore has a relatively high RMS value as high current
flows from the supply for a short time.
flows from the supply for a short time.
Input Voltage
Input Current
DC Link Voltage
Single Phase
Three Phase
Figure 3-2
Rectifier Input Voltages and Currents
This means that the current waveform is consists of a series of low frequency
harmonics, and this may in turn cause voltage harmonic distortion, depending on
the supply impedance.
harmonics, and this may in turn cause voltage harmonic distortion, depending on
the supply impedance.
Sometimes these harmonics need to be assessed in order to ensure that certain
levels are not exceeded. Excessive harmonic levels can cause high losses in
transformers, and may interfere with other equipment. In any case, the rating and
selection of cabling and protection equipment must take these high RMS levels into
account. Some measured harmonic levels are shown In Figure 3-3.
levels are not exceeded. Excessive harmonic levels can cause high losses in
transformers, and may interfere with other equipment. In any case, the rating and
selection of cabling and protection equipment must take these high RMS levels into
account. Some measured harmonic levels are shown In Figure 3-3.