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Unidrive SP Advanced User Guide
89
Issue Number: 10 www.controltechniques.com
SPMAxxxx and SPMDxxxx drive modules can be connected in parallel provided all power modules have the same voltage and current rating to make
a larger drive. The currents are then defined as follows:
Current scaling (Kc)
Kc is the sum of Kc for all the modules..
Maximum heavy duty current rating
Maximum heavy duty current rating = 0.95 x Sum of maximum heavy duty current rating for all the modules
Maximum rated current
Maximum rated current = 0.95 x Sum of maximum normal duty rated current for all the modules
5.6.1 Open-loop
In this mode the drive operates in the stator flux reference frame under steady state conditions. The absolute maximum controlled motor current is
defined by the peak limit system as 1.75 x Kc. However, the drive does not normally operate at this level, but uses the peak limit system as protection
against over-current trips. Under normal operation the motor current is limited to 1.50 x Kc, allowing a safety margin between the maximum normal
operating current and the peak limit level. Therefore a motor with the same current rating as the drive can produce at least 150% torque when the
drive operates in current limit.
DRIVE_CURRENT_MAX is full scale current feedback, i.e. Kc / 0.45.
The relationship between the voltage and current for open-loop operation is shown in the following vector diagram.
Definitions:
v
s
= motor terminal voltage vector
i
s
= motor current vector
i
sy
= y axis component of current
i
sx
= x axis component of current
v* = no load y axis voltage reference
MOTOR1_CURRENT_LIMIT_MAX is used as the maximum for some parameters such as the user current limits. This is defined in the vector
diagram as follows (with a maximum of 1000%):
Where
Motor rated current is given by Pr
5.07
PF is motor rated power factor given by Pr
5.10
(MOTOR2_CURRENT_LIMIT_MAX is calculated from the motor map 2 parameters)
The Maximum current is either (1.5 x Kc) when the rated current set by Pr
5.07
(or Pr
21.07
if motor map 2 is selected) is less than or equal to the
Maximum Heavy Duty current rating, otherwise it is (1.1 x Maximum Normal Duty rated current).
For example, with a motor of the same rating as the drive and a power factor of 0.85, the maximum current limit is 165.2% for Heavy Duty operation.
The above calculation is based on the assumption that the flux producing current (Pr
4.17
) in the stator flux reference frame does not vary with load
and remains at the level for rated load. This is not the case and the flux producing current will vary as the load is increased. Therefore the maximum
current limit may not be reached before the drive reduces the current limit to prevent the peak limit from becoming active.
v*
Stator flux
(in steady state)
Open-loop
mode
ϕ ≈
cos (PF)
-1
R i
s sx
R i
s sy
i
sy
MOTOR1_CURRENT_LIMIT_MAX
Maximum current
Motor rated current
-------------------------------------------------------
2
PF
(
)
2
1
–
+
PF
--------------------------------------------------------------------------------------------------------
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