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Unidrive SP Advanced User Guide 81
Issue Number: 7 www.controltechniques.com
Maximum heavy duty current rating
Maximum heavy duty current rating = Ratio * Module Rated drive current / Total Rated drive current
Where Ratio is the smallest ratio between the Maximum heavy duty current rating and Rated drive current for any of the modules connected in
parallel. The modules share current in proportion to their Rated drive current, and so this ensures that the module with the smallest ratio is at its
Maximum heavy duty current rating when the whole drive is at its Maximum heavy duty current rating.
Maximum rated current
Maximum rated current = Ratio * Module Rated drive current / Total Rated drive current
Where Ratio is the smallest ratio between the Maximum rated current and Rated drive current for any of the modules connected in parallel. The
modules share current in proportion to their Rated drive current, and so this ensures that the module with the smallest ratio is at its Maximum rated
current when the whole drive is at its Maximum rated current.
Open-loop
In open-loop mode the drive operates in the stator flux reference frame under steady state conditions. The absolute maximum motor current is
defined by the peak limit system as 1.75 x rated drive current. 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 rated drive current, 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. rated drive current / 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 Rated drive current) 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 motor 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
v*
Stator flux
in stead
state
Open-loop
mode
ϕ ≈
cos (PF)
-1
Ri
ssx
Ri
ssy
i
sy
MOTOR1_CURRENT_LIMIT_MAX
Maximum current
Motor rated current
-------------------------------------------------------
2
PF()
2
1–+
PF
--------------------------------------------------------------------------------------------------------
100%×=
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