Reference Manual

PMAC 2 Software Reference
92 PMAC I-Variable Specifiation
Motor Servo Loop Modifiers
These I-variables modify the action of the basic PID servo algorithm. They are not available with the
Option 6 Extended Servo Algorithm firmware.
Ix57 Motor x Continuous Current Limit
Range
0 .. 32,767
Units
16-bit DAC/ADC bit equivalent
Default
0
Remarks
Ix57 sets the maximum continuous current limit for PMAC’s I
2
T integrated current
limiting function, when that function is active (Ix58 must be greater than 0 for I
2
T to be
active). If PMAC is closing a digital current loop for the motor, it uses actual current
measurements for this function; otherwise it uses commanded current values. If the actual
or commanded current level from PMAC is above Ix57 for a significant period of time, as
set by Ix58, PMAC will trip this motor on an I
2
T amplifier fault condition.
Ix57 is in units of a 16-bit DAC or ADC (maximum possible value of 32,767), even if the
actual output or input device has a different resolution. Typically Ix57 will be set to
between 1/3 and 1/2 of the Ix69 (instantaneous) output limit. Consult your amplifier and
motor documentation for their specifications on instantaneous and continuous current
limits.
Technically, Ix57 is the continuous limit of the vector sum of the quadrature and direct
currents. The quadrature (torque-producing) current is the output of the position/velocity-
loop servo. The direct (magnetization) current is set by Ix77.
In sine-wave output mode (Ix01 = 1, Ix82 = 0), amplifier gains are typically given in
amperes of phase current per volt of PMAC output, but motor and amplifier limits are
typically given in RMS amperage values. In this case, it is important to realize that peak
phase current values are √2 (1.414) times greater than the RMS values.
In direct-PWM mode (Ix01 = 1, Ix82 > 0) of 3-phase motors (Ix72 = 85 or 171), the
corresponding top values of the sinusoidal phase-current ADC readings will be 1/cos(30
o
),
or 1.15, times greater than the vector sum of quadrature and direct current. Therefore,
once you have established the top values you want to see in the A/D converters your phase
currents on a continuous basis, this value should be multiplied by cos(30
o
), or 0.866, to get
your value for Ix57. Remember that if current limits are given as RMS values, you should
multiply these by √2 (1.414) to get peak phase current values.
Example
1. PMAC Motor 1 is driving a torque-mode DC brush-motor amplifier that has a gain of
3 amps/volt with a single analog output voltage. The amplifier has a continuous
current rating of 10 amps; the motor has a continuous current rating of 12 amps.
• PMAC’s maximum output of 32,768, or 10 volts, corresponds to 30 amps.
• The amplifier has the lower continuous current rating, so we use its limit of 10
amps.
• I157 is set to 32,768 * 10 / 30 = 10,589.
2. Motor 3 is driving a self-commutating brushless-motor amplifier in current (torque)
mode with a single analog output. The amplifier has a gain of 5 amps(RMS)/volt and
an continuous current limit of 20 amps (RMS). The motor has an continuous current
limit of 25 amps (RMS).
• PMAC’s maximum output of 32,768, or 10 volts, corresponds to 50 amps (RMS).
• The amplifier has the lower continuous current rating, so we use its limit of 20
amps (RMS).