Accelnet Plus Micro Modules User Guide
16-01687 Rev 03
Copley Controls
Page 132 of 139
A
PPENDIX
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 (A
2
S) 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.
