Menu 4 − Torque and Current control
Mode: Open‑Loop
In Open-loop mode it is possible to control the motor speed or the motor torque. When the speed of the motor is controlled (Torque Mode Selector
(04.011) = 0) the Post Ramp Reference (02.001) is defined by the ramp system unless the current limits are active. The Post Ramp Reference (02.001)
can directly define the output frequency of the drive or this can be modified to compensate for motor slip. If the current limits are active
(Current Limit Active (10.009) = 1) the ramp system remains active, but the output of the current controller is added to the ramp output so that the
frequency applied to the motor is modified to try and reduce the torque producing current in the motor. For example, if the Post Ramp Reference
(02.001) is positive (i.e. motor is running forwards) and the motor is overloaded with a motoring load a positive Iq, Torque Producing Current (04.002) is
generated that will exceed the Final Current Limit (04.018). This gives a negative error which attempts to reduce the ramp output causing the motor to
slow down.
When motor torque is being controlled (Torque Mode Selector (04.011) = 1) the Final Torque Reference (04.003) is converted to a current reference
and the current limits are applied giving the Final Current Reference (04.004). This is used as the reference input to the PI controller that regulates the
torque producing current in the motor. The output of the PI controller is the Post Ramp Reference (02.001), and so the torque is controlled by increasing
or decreasing the motor frequency. This system gives only moderate dynamic performance. For better torque control in a system without position
feedback RFC-A mode should be used.
Throughout this section Rated Current (05.007) and other parameters related to motor 1 are used. It should be noted that this applies if
Select Motor 2 Parameters (11.045) = 0. If Select Motor 2 Parameters (11.045) = 1 then M2 Rated Current (21.007) and other parameter associated
with motor 2 should be used instead.
The full scale current is the maximum current that the drive can measure and if the current exceeds this level the drive may produce an over current trip.
Kc is the current scaling for the drive and is used in determining the control performance of the drive. This is given in Full Scale Current Kc (11.061) and
Kc is equal the full scale current in r.m.s. Amps. (Note that this is a change from Unidrive SP which used the full scale current multiplied by 0.45 for Kc.)
The current is limited by the peak limit system to a maximum level of 0.8 x Kc. The peak limit controls the total current, however the control system
current limit only applies to the torque producing current and would limit the maximum current to I
MaxRef
if the magnetising current remains constant at
its rated level. I
MaxRef
must give headroom below the peak limit and is set at 0.7 x Kc, provided Rated Current (05.007) is set to the
Maximum Heavy Duty Rating (11.032) or less. If Rated Current (05.007) is set to a higher level then the current limits can be adjusted so that the
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