Unidrive M702 User Guide
125
Issue Number: 3
Pr 00.038 {04.013} / Pr 00.039 {04.014}
Current Loop Gains
The current loop gains proportional (Kp) and integral (Ki) gains control the response of the current loop to a change in current (torque) demand. The
default values give satisfactory operation with most motors. However, for optimal performance in dynamic applications it may be necessary to
change the gains to improve the performance. The proportional gain (Pr 04.013) is the most critical value in controlling the performance. The values
for the current loop gains can be calculated by performing a stationary or rotating autotune (see
Autotune
Pr 00.040, earlier in this table) the drive
measures the
Stator Resistance
(05.017) and
Transient Inductance
(05.024) of the motor and calculates the current loop gains.
This will give a step response with minimum overshoot after a step change of current reference. The proportional gain can be increased by a factor
of 1.5 giving a similar increase in bandwidth; however, this gives a step response with approximately 12.5 % overshoot. The equation for the integral
gain gives a conservative value. In some applications where it is necessary for the reference frame used by the drive to dynamically follow the flux
very closely (i.e. high speed Sensorless RFC-A induction motor applications) the integral gain may need to have a significantly higher value.
Speed loop gains
(Pr 00.007 {03.010}, Pr 00.008 {03.011}, Pr 00.009 {03.012})
The speed loop gains control the response of the speed controller to a change in speed demand. The speed 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 speed controller with Pr
03.016
. If Pr
03.016
= 0, gains Kp1, Ki1 and Kd1 (Pr
00.007
to Pr
00.009
) 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. If the load is predominantly
a constant inertia and constant torque, the drive can calculate the required Kp and Ki gains to give a required compliance angle or bandwidth
dependant on the setting of Pr
03.017
.
Speed 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 speed
error to produce a torque reference. Therefore as the motor load increases there will be a difference between the reference and actual speeds. This
effect, called regulation, depends on the level of the proportional gain, the higher the gain the smaller the speed error for a given load. If the
proportional gain is too high either the acoustic noise produced by speed feedback quantization becomes unacceptable, or the stability limit is
reached.
Speed Controller Integral Gain
(Ki), Pr
00.008
{
03.011
} and Pr
03.014
The integral gain is provided to prevent speed regulation. The error is accumulated over a period of time and used to produce the necessary torque
demand without any speed error. Increasing the integral gain reduces the time taken for the speed 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
00.009
{
03.012
} and Pr
03.015
The differential gain is provided in the feedback of the speed 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.
Summary of Contents for Unidrive M702
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