Switching Frequency - Emerson unidrive m200 User Manual

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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 05.018 (dependent on drive size). The
available switching frequencies are shown below.
Table 8-1 Available switching frequencies
Drive 0.667 1 2
Model
size kHz kHz kHz
9 9 9
1 to 6 All
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 section 11.1.1 Power and current ratings (Derating
for switching frequency and temperature) on page 159.
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
0.667, 3, 6, 12
1 kHz kHz
250 μs 167 μs
Level 1
250 μs
Level 2
Level 3 1 ms
Level 4 4 ms
Background
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
Torque
Rated
voltage
Rated speed
Care must be taken to ensure the torque available above base speed is
sufficient for the application to run satisfactorily.
96
Electrical Getting Basic
installation started parameters
3 4 6 8 12
kHz kHz kHz kHz kHz
9 9 9 9 9
2, 4, 8, 16 Open
RFC-A
kHz loop
2 kHz = 250 μs
4 kHz = 125 μs
Current
Peak limit
8 kHz = 125 μs
controllers
16 kHz = 125 μs
Current Speed
limit and controller
ramps and ramps
Voltage controller
Time critical user
interface
Non-time critical user
interface
Speed
Speed
Running the NV Media
Optimization
motor Card
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
16
Hz.
kHz
9
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. 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.
Advanced
Technical data Diagnostics
parameters
Unidrive M200 / M201 User Guide
Issue Number: 4
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