Control Techniques COMMANDER SE, Руководство для продвинутых пользователей

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Commander SE Advanced User Guide
39
Issue Number: 4
Current limit operation
The current limits will normally operate with an integral term only, particularly below the point where field weakening begins.
The proportional term is 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.
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 base speed, and can be reduced by increasing the proportional gain. The
controller tends to be less stable in torque control mode 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.
The motor is modelled thermally in a way that is equivalent to the following electrical circuit:
Where: Vc represents the temperature of the motor
Vin represents the amount of energy losses in the motor
The energy loss in the motor is proportional to the current squared and it is assumed that the motor reaches rated
temperature at rated current in a time determined by the thermal time constant of the motor.
4.15 defines the time constant of
the circuit above and should be set to the thermal time constant of the motor.
To allow for tolerances in current measurement the trip threshold for Vc is set at (105% of motor rated current)
2
i.e. ( 5.07 x 1.05)
2
.
The drive will trip if the following is true:
I
2
(1 - e
-t/
τ
) = ( 5.07 x 1.05)
2
Therefore time to trip with 150% rated motor current ( 5.07 ) = - 4.15 ln(1 - 105
2
/ 150
2
). If 4.15 = 89s the time to trip = 60s.
If a different value for the time constant is required it can be calculated as follows:
•
The motor thermal time constant can be used directly.
• A specific overload time and current level can be used, i.e. 60s at 150%
4.15 = -time to trip / ln(1 - 105
2
/ 150
2
) = 89
• If the reset time from an overload is assumed to be the time it takes for the motor to cool to within 1% of the ambient
temperature, this can be taken as 5 times the thermal time constant.
There are two alternative modes of operation for motor thermal protection defined by 4.16 :
• If 4.16 =0, trip when the trip level is reached.
• If 4.16 =1, reduction of the current limit to keep the rated current below 100% when the trip level is reached. 5% hysteresis
will be included in this case.
See 4.15 also.
4.16 = 0: trip when threshold reached
4.16 = 1: reduce current limit when threshold reached
4.15 Motor thermal time constant
Ú
0 ~ 250
Ö
89 s RW Uni
4.16 Motor protection mode select
Ú
0 or 1
Ö
0 RW Bit
C
R
Vc
Vin