User guide

Regen Resistor Sizing and Configuration Xenus XTL User Guide
146 Copley Controls Corp.
B.1: I
2
T Algorithm
B.1.1: I
2
T Overview
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.
B.1.2: I
2
T Formulas and Algorithm Operation
Calculating the I
2
T Setpoint Value
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 Algorithm Operation
During amplifier 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.
Accumulator Increment Formula
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
amplifier 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.