The voltage on a fully charged DC link is equal to the peak
voltage of the input AC line. Theoretically, this voltage can
be calculated by multiplying the AC line value by 1.414
(V AC x 1.414). However, since AC ripple voltage is present
on the DC link, the actual DC value is closer to
(V AC x 1.38) under unloaded conditions. The DC value can
drop to (V AC x 1.32) while running under load.
Example
For a frequency converter sitting idle while connected to a
nominal 460 V line, the DC-link voltage is approximately
635 V DC (460 x 1.38). As long as power is applied to the
frequency converter, this voltage is present in the DC link
and the inverter circuit. Voltage is also fed to the switch
mode power supply (SMPS) on the power card which is
used for generating all other low voltage supplies. The
SMPS is activated when the DC-link voltage reaches
approximately 250 V DC.
3.3.3 Inverter Section
The inverter section is made up of six IGBTs, commonly
referred to as switches. One switch is necessary for each
half phase of the 3-phase power, for a total of six. The six
IGBTs are contained in one power module shared with the
rectier. The inverter section receives gate signals from the
MOC.
Once a run command and speed reference are present, the
IGBTs begin switching to create the output waveform, as
shown in Illustration 3.3. Looking at the phase to phase
voltage waveform with an oscilloscope, a train of pulses of
dierent widths is shown. The amplitude of the pulses
measures the DC-link voltage. To view the fundamental
sinusoidal curve, set the oscilloscope to lter out high
harmonic content.
When measuring current, the normal view will be a
sinusoidal curve. The amplitude of the measured current
depends on the loading form.
This waveform, as generated by the frequency converter,
provides optimal performance and minimal losses in the
motor.
Hall eect current sensors monitor the output current and
feed it back to the control. The current signal is used for
two purposes:
•
to compensate for dynamic motor operation.
•
to monitor overcurrent conditions, including
ground faults and phase-to-phase shorts.
During normal operation, the power card and control
monitor various functions within the frequency converter.
The current sensors provide current feedback information.
The DC bus voltage and mains voltage are monitored and
the voltage delivered to the motor. A thermal sensor
mounted inside the IGBT module provides heat sink temp
feedback for the inverter.
Illustration 3.3 Output Voltage and Current Waveforms
3.3.4 Fan Speed Control
IP20
Enclosure H6 H7 H8
Voltage T2T2T2T2T2T2
Power rating
[kW]
15 18.5 22 30 37 45
FAN start
temperature
°
C
45 45 45 45 45 45
FAN maximum
speed
temperature
°
C
60 60 60 60 60 60
FAN stop
temperature
°
C
36 36 36 36 36 36
Table 3.1 Fan Speed Control, IP20, H6–H8, T2
IP20
Enclosure H6 H7 H8
Voltage T4 T4 T4 T4 T4 T4
Power rating
[kW]
30 37 45 55 75 90
FAN start
temperature
°
C
45 45 45 40 40 40
FAN maximum
speed
temperature
°
C
60 60 60 55 55 55
FAN stop
temperature
°
C
41 41 41 30 30 30
Table 3.2 Fan Speed Control, IP20, H6–H8, T4
IP54
Enclosure I6 I7 I8
Voltage T4T4T4T4T4T4T4
Power rating
[kW]
22 30 37 45 55 75 90
FAN start
temperature
°
C
45 45 45 40 40 40 40
Internal Frequency Converte... TR150 and TR170 Service Manual
BAS-SVM04B-EN 02/2017 All rights reserved. 25
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