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128
Affinity User Guide
www.controltechniques.com Issue Number: 5
8.4 Switching frequency
The default switching frequency is 3kHz, however this can be increased
up to a maximum of 16kHz by Pr
5.18
(dependent on drive size). The
available switching frequencies are shown below.
Table 8-1 Available switching frequencies
If switching frequency is increased from 3kHz the following apply:
1. Increased heat loss in the drive, which means that derating to the
output current must be applied.
See the derating tables for switching frequency and ambient
temperature in section 12.1.1
Power and current ratings (Derating
for switching frequency and temperature)
2. Reduced heating of the motor - due to improved output waveform
quality.
3. Reduced acoustic noise generated by the motor.
4. Increased sample rate on the speed and current controllers. A trade
off must be made between motor heating, drive heating and the
demands of the application with respect to the sample time required.
Table 8-2 Sample rates for various control tasks at each
switching frequency
8.5 High speed operation
8.5.1 Field weakening (constant power) operation
The drive can be used to run an induction machine above synchronous
speed into the constant power region. The speed continues to increase
and the available shaft torque reduces. The characteristics below show
the torque and output voltage characteristics as the speed is increased
above the rated value.
Figure 8-2 Torque and rated voltage against speed
Care must be taken to ensure the torque available above base speed is
sufficient for the application to run satisfactorily.
The saturation breakpoint parameters (Pr
5.29
and Pr
5.30
) found during
the autotune in RFC mode ensure the magnetizing current is reduced in
the correct proportion for the specific motor. (In open loop mode the
magnetizing current is not actively controlled.)
8.5.2 Switching frequency
With a default switching frequency of 3 kHz the maximum output
frequency should be limited to 250 Hz. Ideally a minimum ratio of 12:1
should be maintained between the switching frequency and the output
frequency. This ensures the number of switchings per cycle is sufficient
to ensure the output waveform quality is maintained at a minimum level.
If this is not possible, quasi-square switching should be enabled (Pr
5.20
=1). The output waveform will be quasi square above base speed
ensuring a symmetrical output waveform, which results in a better quality
output than would otherwise result.
8.5.3 Maximum speed / frequency
In open loop mode the maximum frequency is 3,000 Hz.
In RFC mode the maximum output frequency is 400 Hz.
8.5.4 Quasi-Square wave (open-loop only)
The maximum output voltage level of the drive is normally limited to an
equivalent of the drive input voltage minus voltage drops within the drive
(the drive will also retain a few percent of the voltage in order to maintain
current control). If the motor rated voltage is set at the same level as the
supply voltage, some pulse deletion will occur as the drive output voltage
approaches the rated voltage level. If Pr
5.20
(Quasi-square wave
enable) is set to 1 the modulator will allow over modulation, so that as
the output frequency increases beyond the rated frequency the voltage
continues to increase above the rated voltage. The modulation depth will
increase beyond unity; first producing trapezoidal and then quasi-square
waveforms.
This can be used for example:
•
To obtain high output frequencies with a low switching frequency
which would not be possible with space vector modulation limited to
unity modulation depth,
or
•
In order to maintain a higher output voltage with a low supply
voltage.
The disadvantage is that the machine current will be distorted as the
modulation depth increases above unity, and will contain a significant
amount of low order odd harmonics of the fundamental output frequency.
The additional low order harmonics cause increased losses and heating
in the motor.
Drive
size
Model
3kHz 4kHz 6kHz 8kHz 12kHz 16kHz
1
All
9
9
9
9
9
9
2
All
9
9
9
9
9
9
3
BA320X
9
9
9
9
9
BA3401 &
BA3402
9
9
9
9
9
9
BA3403
9
9
9
9
9
BA350X
9
9
9
9
4
All
9
9
9
9
5
All
9
9
9
9
6
All
9
9
9
3, 6, 12
kHz
4, 8, 16
kHz
Open loop
RFC
Level 1
3kHz = 167
μ
s
6kHz = 83
μ
s
12kHz = 83
μ
s
125
μ
s
Peak limit
Current
controllers
Level 2
250
μ
s
Current limit
and ramps
Speed controller
and ramps
Level 3
1ms
Voltage controller
Level 4
4ms
Time critical user interface
Background
Non-time critical user interface
Rated
voltage
Torque
Speed
Speed
Rated speed
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