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100
Unidrive User Guide
www.controltechniques.com Issue Number: 9
8.5.3 Saturation breakpoints
The saturation breakpoint parameters (Pr
5.29
and Pr
5.30
) found during
the autotune in closed loop vector mode ensure the magnetising current
is reduced in the correct proportion for the specific motor.
(In open loop mode the magnetising current is not actively controlled)
8.5.4 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
however this also ensures a symmetrical output waveform which results
in a better quality output than would otherwise result.
8.5.5 Output frequency doubling
(Open loop only)
If this bit is set the motor output frequency is twice the displayed value.
The maximum open loop output frequency increases from 1,000Hz to
2,000Hz.
The following parameters need to be re-scaled when this mode of
operation is used.
For example:-
The real machine is 4 pole, 2,000Hz, 400V, 60,000 rpm, full load speed
58,000 rpm, and the desired maximum speed is 40,000 rpm with a trip at
50,000 rpm. Acceleration is to be 500Hz / sec.
Menu 1:
maximum frequency (Pr
1.06
) should be set to:
0.5 x 2,000 x 40,000 / 60,000 = 667Hz
Menu 2:
the ramp times (Pr
2.11
to
2.29
) need to be set at:
0.5 x 0.2 sec per 100Hz = 0.1
Menu 3:
the over-speed trip threshold (Pr
3.08
) should be set at
0.5 x 2,000 x 50,000 / 60,000 = 833Hz
Menu 5:
the rated motor voltage (Pr
5.09
) = 400V
the rated frequency (Pr
5.06
) = 0.5 x 2,000 = 1,000Hz
the full load speed is (Pr
5.08
) = 0.5 x 58,000 = 29,000rpm
the motor poles (Pr
5.11
) = 4 POLE
Extreme caution should be exercised when setting this bit as the actual
machine speed will be double that indicated.
8.5.6 Maximum speed / frequency
In open loop mode the maximum frequency is 2,000 Hz when output
frequency doubling is used (500 Hz for Unidrive VTC).
In closed loop vector mode the maximum output frequency should be
limited to 400 Hz.
In servo mode field weakening is not possible so the maximum speed is
limited by the voltage constant (K
e
) of the motor. K
e
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).
Summary of Contents for Unidrive 1 Series
Page 208: ......
