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Selecting a Drive

3.2.3Ungrounded 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.

3.2.4Low 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.

The current waveform therefore has a relatively high RMS value as high current 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.

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.

MICROMASTER Applications Handbook

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