
Unidrive M / HS Frame 7 to 10 Power Installation Guide
101
Issue Number: 5
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In RFC-S mode the speed is also limited by the voltage constant (Ke) of the motor unless field
weakening operation is enabled. Ke is a specific constant for the servo motor being used. It can
normally be found on the motor data sheet in V/k rpm (volts per 1,000 rpm).
It is recommended that a minimum ratio of 12:1 is maintained between the switching frequency and
the maximum output frequency to maintain the quality of the output waveform. If this minimum ratio is
exceeded, extra motor losses will result due to the increased harmonic content of the output
waveform.
5.1.16
Accuracy and resolution
Unidrive
M600 to M702:
Speed:
The absolute frequency and speed accuracy depends on the accuracy of the crystal used with the
drive microprocessor. The accuracy of the crystal is 100 ppm, and so the absolute frequency/speed
accuracy is 100 ppm (0.01 %) of the reference, when a preset speed is used. If an analog input is
used the absolute accuracy is further limited by the absolute accuracy of the analog input.
The following data applies to the drive only; it does not include the performance of the source of the
control signals.
Open loop resolution:
Preset frequency reference: 0.1 Hz
Precision frequency reference: 0.001 Hz
Closed loop resolution
Preset speed reference: 0.1 rpm
Precision speed reference: 0.001 rpm
Analog input 1: 11 bit plus sign (not applicable to
Unidrive M702
)
Analog input 2: 11 bit plus sign (not applicable to
Unidrive M702
)
Current:
The resolution of the current feedback is 10 bit plus sign.
Accuracy: typical 2 %
worst case 5 %
Unidrive
M200 to M400:
Frequency:
The absolute frequency accuracy depends on the accuracy of the oscillator used with the drive
microprocessor. The accuracy of the oscillator is ± 0.02 %, and so the absolute frequency accuracy
is ± 0.02 % of the reference, when a preset frequency is used. If an analog input is used, the absolute
accuracy is further limited by the absolute accuracy of the analog input.
The following data applies to the drive only; it does not include the performance of the source of the
control signals.
Open & closed loop resolution:
Preset frequency reference: 0.01 Hz
Analog input 1: 11 bit plus sign
Analog input 2: 11 bit
Current:
The resolution of the current feedback is 10 bit plus sign.
Accuracy: typical 2 %
worst case 5 %
Unidrive M frame7 to 10 Power Installation Guide issue5.book Page 101 Tuesday, May 24, 2016 11:52 AM