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68 Unidrive M100 User Guide
Issue Number: 1
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.
8.5.2 Maximum frequency
In all operating modes the maximum output frequency is limited to 550
Hz.
8.5.3 Over-modulation
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.
Rated
voltage
Tor q u e
Speed
Speed
Rated speed