Introduction Parameter x.00
Parameter
description format
Keypad and
display
CT Modbus
RTU
User
programming
CT Soft Menu 0
Advanced parameter
descriptions
Menu 4
Commander SK Advanced User Guide 51
Issue Number: 2 www.controltechniques.com
inherent in the loop. The integral term must be increased enough to counter the effect of the ramp which is still active even in current limit. For
example, if the drive is operating at constant frequency and is overloaded the current limit system will try to reduce the output frequency to reduce the
load. At the same time the ramp will try to increase the frequency back up to the demand level. If the integral gain is increased too far the first signs of
instability will occur when operating around the point where field weakening begins. These oscillations can be reduced by increasing the proportional
gain. A system has been included to prevent regulation because of the opposite actions of the ramps and the current limit. This can reduce the actual
level that the current limit becomes active by 12.5%. This still allows the current to increase up to the current limit set by the user. However the current
limit flag (Pr 10.09) could become active up to 12.5% below the current limit depending on the ramp rate used.
Torque control
Again the controller will normally operate with an integral term only, particularly below the point where field weakening begins. The first signs of
instability will appear around rated speed, and can be reduced by increasing the proportional gain. The controller can be less stable in torque control
mode rather than when it is used for current limiting. This is because load helps to stabilise the controller, and under torque control the drive may
operate with light load. Under current limit the drive is often under heavy load unless the current limits are set at a low level.
Mains loss and controlled standard ramp
The DC bus voltage controller becomes active if mains loss detection is enabled and the drive supply is lost or controlled standard ramp is being used
and the machine is regenerating. The DC bus controller attempts to hold the DC bus voltage at a fixed level by controlling the flow of current from the
drive inverter into its DC bus capacitors. The output of the DC bus controller is a current demand which is fed into the current PI controller as shown
below:
The DC bus controller gain is a function of DC bus capacitance and therefore is fixed internally. It may often be necessary to adjust the current
controller gains to obtain the required performance. If the gains are not suitable it is best to set up the drive in torque control first. Set the gains to a
value that does not cause instability around the point at which field weakening occurs. Then revert back to open loop speed control in standard ramp
mode. To test the controller the supply should be removed whilst the motor is running. It is likely that the gains can be increased further if required
because the DC bus voltage controller has a stabilising effect, provided that the drive is not required to operate in torque control mode.
See Pr 4.16 for details.
0: Trip when threshold reached
1: Reduce current limit when threshold reached
The motor is modelled thermally in a way that is equivalent to the electrical circuit shown below:
4.15 Thermal time constant
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
111
Range 0 to 250 s
Default 89
Second motor
parameter
Pr 21.16
Update rate Background
4.16 Thermal protection mode
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
111
Range 0 to 1
Default 0
Update rate Background
DC Bus
voltage
controller
P Pr
4.13
I Pr
4.14
Frequency
reference
Active current
DC Bus
capacitor
Current
demand
(I /(K x Motor Rated Current)
22
=RCt
Temp
C
R
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