User guide

I
2
T Time Limit Algorithm CME 2 User Guide
192 Copley Controls
D.1: I
2
T Algorithm
D.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.
D.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.