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148
Unidrive SPM User Guide
www.controltechniques.com Issue Number: 3
10.6.4 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 output frequency and the switching
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.
10.6.5 Maximum speed / frequency
In open loop mode the maximum frequency is 3,000 Hz.
In closed loop vector mode the maximum output frequency is 600 Hz.
In servo mode the maximum output frequency is 1250Hz, however the
speed is limited by the voltage constant (Ke) of the motor. Ke is a
specific constant for the servo motor being used. It can normally be
found on the motor data sheet in V/krpm (volts per 1,000rpm).
10.6.6 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.