Accelnet & Stepnet Plus Panels User Guide
16-01339 Rev 07
6 A
PPENDIX
A:
I
2
T
T
IME
L
IMIT
A
LGORITHM
The current loop I
2
T limit specifies the maximum amount of time that the peak current can be applied to the
motor before it must be reduced to the continuous limit or generate a fault. This chapter describes the algorithm
used to implement the I
2
T limit.
I
2
T
O
VERVIEW
The I
2
T current limit algorithm continuously monitors the energy being delivered to the motor using the I
2
T
Accumulator Variable. The value stored in the I
2
T Accumulator Variable is compared with the I
2
T setpoint that
is calculated from the user-entered Peak Current Limit, I
2
T Time Limit, and Continuous Current Limit.
Whenever the energy delivered to the motor exceeds the I
2
T setpoint, the algorithm protects the motor by
limiting the output current or generates a fault.
C
ALCULATING THE
I
2
T
S
ETPOINT
V
ALUE
The I
2
T setpoint value has units of Amperes
2
-seconds (A2S) and is calculated from programmed motor data.
The setpoint is calculated from the Peak Current Limit, the I
2
T Time Limit, and the Continuous Current Limit
as follows:
I
2
T setpoint = (Peak Current Limit
2
– Continuous Current Limit
2
) * I
2
T Time Limit
I
2
T
A
LGORITHM
O
PERATION
During drive operation, the I
2
T algorithm periodically updates the I
2
T Accumulator Variable at a rate related to
the output current Sampling Frequency. The value of the I
2
T Accumulator Variable is incrementally increased
for output currents greater than the Continuous Current Limit and is incrementally decreased for output currents
less than the Continuous Current Limit. The I
2
T Accumulator Variable is not allowed to have a value less than
zero and is initialized to zero upon reset or +24 Vdc logic supply power-cycle.
A
CCUMULATOR
I
NCREMENT
F
ORMULA
At each update, a new value for the I
2
T Accumulator Variable is calculated as follows:
I
2
T Accumulator Variable n+1 = I
2
T Accumulator Variable n
+ ( (Actual Output Current n+1)
2
– Continuous Current Limit
2
)
* Update period
After each sample, the updated value of the I
2
T Accumulator Variable is compared with the I
2
T setpoint. If the
I
2
T Accumulator Variable value is greater than the I
2
T Setpoint value, then the drive limits the output current to
the Continuous Current Limit. When current limiting is active, the output current will be equal to the Continuous
Current Limit if the commanded current is greater than the Continuous Current Limit. If instead the commanded
current is less than or equal to the Continuous Current Limit, the output current will be equal to the commanded
current.
