6.3.8 Input Waveform Test
Testing the current waveform on the input of the adjustable frequency drive can assist in trou-
bleshooting line phase loss conditions or suspected problems with the SCR/diode modules. Phase
loss caused by the line power supply can be easily detected. In addition, the rectifier section is
controlled by SCR/diode modules. Should one of the SCR/diode modules become defective or the
gate signal to the SCR lost, the adjustable frequency drive will respond the same as loss of one
of the phases.
The following measurements require an oscilloscope with voltage and current probes.
Under normal operating conditions, the waveform of a single phase of input AC voltage to the
adjustable frequency drive appears as in Figure 6-13.
Figure 6.13: Normal AC Input Voltage Waveform
The waveform shown in Figure 6-14 represents the input current waveform for the same phase
as figure 6-13 while the adjustable frequency drive is running at 40% load. The two positive and
two negative jumps are typical of any 6 diode bridge. It is the same for adjustable frequency drives
with SCR/diode modules.
Figure 6.14: AC Input Current Waveform with Diode Bridge
6 Test Procedures
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
6-26
MG.90.L1.22 - VLT
®
is a registered Danfoss trademark
6
Summary of Contents for VLT HVAC FC 100
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