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Unidrive M200 / M201 Control User Guide 55
Issue Number: 2
8.5 Switching frequency
The default switching frequency is 3 kHz, however this can be increased
up to a maximum of 16 kHz by Pr 37.
If switching frequency is increased from 3 kHz 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 the Power Installation Guide.
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.
Lowest switching frequency in RFC-A mode is 2 kHz.
Table 8-1 Sample rates for various control tasks at each
switching frequency
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 8-3 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 05.029, Pr 05. 030, Pr 05.062
and Pr 05.063) found during the autotune in RFC-A 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 Maximum frequency
In all operating modes the maximum output frequency is limited to
550 Hz.
8.5.3 Over-modulation (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 05.020 (Over-modulation
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.
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.
8.5.4 Switching frequency/Output frequency ratio
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.
Level
0.667,
1 kHz
3, 6, 12
kHz
2, 4, 8, 16
kHz
Open
loop
RFC-A
Level 1 250 s 167 s
2 kHz = 250 s
4 kHz = 125 s
8 kHz = 125 s
16 kHz = 125 s
Peak limit
Current
controllers
Level 2 250 s
Current
limit and
ramps
Speed
controller
and ramps
Level 3 1 ms Voltage controller
Level 4 4 ms
Time critical user
interface
Background
Non-time critical user
interface
Rated
voltage
Tor que
Speed
Speed
Rated speed
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