Internal Current Regulator; Programming Of Torque Limit And Stop - Danfoss VLT AutomationDrive FC 300 Design Manual

FC 300 Design Guide
Type of Control
PI-control
PID tight control
PID some overshoot
Table 1: Ziegler Nichols tuning for regulator, based on a stability boundary.
Experience has shown that the control setting according to the Ziegler Nichols rule provides
a good closed-loop response for many systems. The process operator can perform final
tuning of the control iteratively to yield satisfactory control.
Step-by-step:
Step 1: Select only Proportional Control, meaning that the Integral time is selected to the
maximum value, while the differentiation time is selected to zero.
Step 2: Increase the value of the proportional gain until the point of instability is reached
(sustained oscillations) and the critical value of gain, K
Step 3: Measure the period of oscillation to obtain the critical time constant, P
Step 4: Use the table above to calculate the necessary PID control parameters.

Internal Current Regulator

"
The adjustable frequency drive features an integral current limit regulator which is activated when the
motor current, and thus the torque, is higher than the torque limits set in par. 4-16 and 4-17.
When the adjustable frequency drive is at the current limit during motor operation or regenerative
operation, the adjustable frequency drive will try to get below the preset torque limits as
quickly as possible without losing control of the motor.
While the current regulator is active, the adjustable frequency drive can only be stopped by means of
a digital terminal if set to Coast, inverse [2] or Coast and Reset, inverse [3]. Other signals on terminals
18-33 will not be active until the adjustable frequency drive is no longer near the current limit.

Programming of Torque Limit and Stop

"
In applications with an external electromechanical brake, such as hoisting applications,
it is possible to stop the adjustable frequency drive via a "standard" stop command and
simultaneously activate the external electromechanical brake.
The example given below illustrates the programming of adjustable frequency drive connections.
The external brake can be connected to relay 1 or 2, see paragraph Control of Mechanical
Brake. Program terminal 27 to Coast, inverse [2] or Coast and Reset, inverse [3], and
program terminal 29 to Torque limit & stop [27].
Description:
If a stop command is active via terminal 18 and the adjustable frequency drive is not
at the torque limit, the motor ramps down to 0 Hz.
If the adjustable frequency drive is at the torque limit and a stop command is activated, terminal 42
Output (programmed to Torque limit and stop [27]) is activated. The signal to terminal 27 l changes
from "logic 1" to "logic 0", and the motor starts to coast, thereby ensuring that the hoist stops even
if the drive itself cannot handle the required torque (i.e. due to excessive overload).
42
Introduction to FC 300
Proportional Gain
0.45 * K
u
0.6 * K
u
0.33 * K
u
MG.33.B3.22 - VLT is a registered Danfoss trademark
Integral Time
0.833 * P
u
0.5 * P
u
0.5 * P
u
, is reached.
u
u
Differentiation Time
-
0.125 * P
u
0.33 * P
u
.