User guide

Xenus Plus User Guide I
2
T Time Limit Algorithm
Copley Controls 132
I
2
T Example: Plot Diagrams
The plots that follow show the response of a drive (configured w/ I
2
T setpoint = 108 A
2
S) to a given
current command. For this example, DC output currents are shown in order to simplify the
waveforms. The algorithm essentially calculates the RMS value of the output current, and thus
operates the same way regardless of the output current frequency and wave shape.
A)
B)
At time 0, plot diagram A shows that the actual output current follows the commanded current.
Note that the current is higher than the continuous current limit setting of 6 A. Under this condition,
the I
2
T Accumulator Variable begins increasing from its initial value of zero. Initially, the output
current linearly increases from 6 A up to 12 A over the course of 1.2 seconds. During this same
period, the I
2
T Accumulator Variable increases in a non-linear fashion because of its dependence
on the square of the current.
At about 1.6 seconds, the I
2
T Accumulator Variable reaches a value equal to the I
2
T setpoint. At
this time, the drive limits the output current to the continuous current limit even though the
commanded current remains at 12 A. The I
2
T Accumulator Variable value remains constant during
the next 2 seconds since the difference between the actual output current and the continuous
current limit is zero.
At approximately 3.5 seconds, the commanded current falls below the continuous current limit and
once again the output current follows the commanded current. Because the actual current is less
than the continuous current, the I
2
T Accumulator Variable value begins to fall incrementally.
The I
2
T Accumulator Variable value continues to fall until at approximately 5.0 seconds when the
commanded current goes above the continuous current limit again. The actual output current
follows the current command until the I
2
T Accumulator Variable value reaches the I
2
T setpoint and
current limiting is invoked.
I
2
T current limit
0
2
4
6
8
10
12
14
16
0 1 2 3 4 5 6 7
Time (S)
Current (A)
I_commanded
I_actual
I
2
T Accumulator
0
20
40
60
80
100
120
0 1 2 3 4 5 6 7
Time (S)
I
2
T energy (A
2
-S)
I^2T Setpoint
I^2T Accumulator