Menu 5
Parameter
structure
Keypad and
display
Parameter
x.00
Parameter
description format
Advanced parameter
descriptions
Macros
Serial comms
protocol
Electronic
nameplate
Performance
Feature look-
up table
108 Unidrive SP Advanced User Guide
www.controltechniques.com Issue Number: 7
accurately.
Fixed boost mode: Fd
A linear characteristic is used from 0Hz to rated frequency, and then constant voltage above rated frequency. Low frequency voltage boost as defined
by Pr 5.15 is applied as shown below.
Square law mode: SrE
A square law characteristic is used from 0Hz to rated frequency, and then constant voltage above rated frequency. Low frequency voltage boost
raises the start point of the square law characteristic as shown below.
Closed loop vector
The rated voltage is used by the field controller to limit the voltage applied to the motor. Normally this is set to the nameplate value. So that current
control can be maintained it is necessary for the drive to leave some 'headroom' between the machine terminal voltage and the maximum available
drive output voltage. The drive allows over-modulation of the PWM inverter which can produce a fundamental voltage that is higher than the drive
input voltage, but would cause substantial odd harmonic distortion if used in steady state operation. Therefore the drive uses a headroom limit which
allows the inverter to give a steady state output voltage equivalent to the input voltage minus voltage drops inside the drive. This gives enough
headroom for the current controllers to operate satisfactorily. However, for good transient performance at high speed the rated voltage should be set
below 95% of the minimum supply voltage to the drive.
The rated voltage is also used in conjunction with the motor rated frequency (Pr 5.06) during the rotating auto-tune test (see Pr 5.12 on page 109) and
in the calculations required for automatic optimisation of the rated motor slip. It is important, therefore that the correct rated voltage for the motor is
used. In some applications it may be necessary to restrict the voltage applied to the motor to a level lower than the nameplate rated voltage of the
motor. In this case the rated frequency (Pr 5.06) must be adjusted to maintain the ratio of rated voltage and frequency given on the motor nameplate.
The rated frequency will then be different to the nameplate value, and so the rated speed must be changed from the nameplate value to give the
correct rated slip.
Servo
The rated voltage is used by the field controller to limit the voltage applied to the motor if field weakening operation is required. As in closed-loop
vector mode some headroom must be left for the current controllers to operate, and so the drive will use the voltage level set by this parameter or the
headroom limit whichever is the lower.
Voltage
boost Pr
5.15
Output
voltage
Pr
5.09
Pr / 2
5.09
Pr / 2
5.06
Pr
5.06
Output
frequenc
utput voltage characteristic
Pr
5.09
Pr
5.15
Pr
.
Pr + [(freq/Pr ) x (Pr - Pr )]
5.15 5.06 5.09 5.15
2
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