80
Unidrive M400 User Guide
Issue Number: 2
Frequency Loop Gains
(00.065 {03.010}, Pr 00.066 {03.011})
The frequency loop gains control the response of the frequency controller to a change in frequency demand. The frequency controller includes
proportional (Kp) and integral (Ki) feed forward terms, and a differential (Kd) feedback term. The drive holds two sets of these gains and either set
may be selected for use by the frequency controller with Pr
03.016
. If Pr
03.016
= 0, gains Kp1, Ki1 and Kd1 (Pr
03.010
to Pr
03.012
) are used, and
if Pr
03.016
= 1, gains Kp2, Ki2 and Kd2 (Pr
03.013
to Pr
03.015
) are used. Pr
03.016
may be changed when the drive is enabled or disabled.
Frequency Controller Proportional Gain (Kp), Pr
00.007
{03.010} and Pr
03.013
If the proportional gain has a value and the integral gain is set to zero the controller will only have a proportional term, and there must be a
frequency error to produce a torque reference. Therefore as the motor load increases there will be a difference between the reference and actual
frequencies. This effect, called regulation, depends on the level of the proportional gain, the higher the gain the smaller the frequency error for a
given load. If the proportional gain is too high either the acoustic noise produced by numerical quantization becomes unacceptable, or the stability
limit is reached.
Frequency Controller Integral Gain (Ki), Pr
00.008
{03.011} and Pr
03.014
The integral gain is provided to prevent frequency regulation. The error is accumulated over a period of time and used to produce the necessary
torque demand without any frequency error. Increasing the integral gain reduces the time taken for the frequency to reach the correct level and
increases the stiffness of the system, i.e. it reduces the positional displacement produced by applying a load torque to the motor. Unfortunately
increasing the integral gain also reduces the system damping giving overshoot after a transient. For a given integral gain the damping can be
improved by increasing the proportional gain. A compromise must be reached where the system response, stiffness and damping are all adequate
for the application. For RFC-A Sensorless mode, it is unlikely that the integral gain can be increased much above 0.50.
Differential Gain (Kd), Pr
03.012
and Pr
03.015
The differential gain is provided in the feedback of the frequency controller to
give additional damping. The differential term is implemented in a way that
does not introduce excessive noise normally associated with this type of
function. Increasing the differential term reduces the overshoot produced by
under-damping, however, for most applications the proportional and integral
gains alone are sufficient.
Gain Change Threshold, Pr
03.017
If the Frequency Controller Gain Select (03.016) = 2, gains Kp1, Ki1 and Kd1
(Pr
03.010
to Pr
03.012
) are used while the modulus of the frequency
demand is less than the value held by Gain Change Threshold (
03.017
), else
gains Kp2, Ki2 and Kd2 (Pr
03.013
to Pr
03.015
) will be used.
Tuning the frequency loop gains:
This involves the connecting of an oscilloscope to analog output 1 to monitor
the frequency feedback. Give the drive a step change in frequency reference
and monitor the response of the drive on the oscilloscope.The proportional
gain (Kp) should be set up initially. The value should be increased up to the
point where the frequency overshoots and then reduced slightly. The integral
gain (Ki) should then be increased up to the point where the frequency
becomes unstable and then reduced slightly. It may now be possible to
increase the proportional gain to a higher value and the process should be
repeated until the system response approaches the ideal response as shown.
The diagram shows the effect of incorrect P and I gain settings as well as the
ideal response.
Frequency demand
Insufficient proportional
gain [
]
00.065
Excessive proportional
gain [
00.065]
Excessive integral gain
[
00.066
]
Ideal response
Summary of Contents for Unidrive M400
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