Reference Manual

PMAC 2 Software Reference
100 PMAC I-Variable Specifiation
If you are using differential analog outputs (DAC+ and DAC-), the voltage between the
two outputs is twice the voltage between an output and AGND, so the Ix69 value should
be set to half of what it would be for a single-ended analog output.
This parameter provides a torque (current) limit in systems with current-loop amplifiers, or
when using PMAC’s internal commutation; it provides a velocity limit with velocity-mode
amplifiers. Note that if this limit “kicks in” for any amount of time, the following error
will start increasing.
Use when Commutating: When PMAC is commutating Motor x (Ix01 = 1) but not
closing the current loops (Ix82 = 0), Ix69 corresponds to peak values of the sinusoidal
phase currents. Motor and amplifier current limits are usually given as RMS values. Peak
phase values are √2, or 1.414, times greater than RMS values. For instance, if an amplifier
has a 10 amp (RMS) instantaneous current limit, the instantaneous limit for the peak of the
phase currents is 14.14 amps.
Use with Magnetization Current: When commutating, Ix69 is technically the limit of only
the quadrature, or torque-producing, current. Ix77 sets the magnitude of the direct, or
magnetization current, and the total current limit is the vector sum of these two variables.
If the Ix77 magnetization current for the motor is set to a value other than 0, Ix69 should
be set such that:
767,32
max
I
2
77Ix
2
69Ix ≤≤+
Use in Direct-PWM Mode: When commutating (Ix01 = 1) and closing the current loop
(Ix82 > 0) of a 3-phase motor (Ix72 = 85 or 171), it is important to understand the
relationship between the quadrature current limited by Ix69 and the phase currents
measured by the A/D converters. This difference is due to the nature of the conversion
between direct and quadrature current components, which are 90
o
apart, and the phase
currents, which are 120
o
apart. This difference introduces a factor of cos(30
o
) into the
calculations.
For a given level of DC quadrature current with zero direct (magnetization) current, the
peak value of AC sinusoidal current measured in the phases will be 1/cos(30
o
), or 1.15
times, greater. When quadrature current is commanded at its limit of Ix69, the peak phase
currents can be 15% higher that this value. For instance, with Ix69 at 10,000, and Ix77 at
0, the A/D converters can provide readings (normalized to 16-bit resolution) up to 11,547.
Use with Magnetization Current: With non-zero direct current, the peak value of AC
sinusoidal current measured in the phases will be 1.15 times greater than the vector sum of
the direct and quadrature currents. Therefore, in order not to saturate the current in the
phases, Ix69 should be set such that:
()
377,28866.0*767,32
o
30cos
max
I
2
77Ix
2
69Ix ≤≤≤+
Example
1. Motor 1 is driving a velocity-mode amplifier with differential analog inputs that are
limited to +/-10V between the inputs. This means that the PMAC outputs should each
be limited to +/-5V with respect to the AGND reference. I169 should therefore be
limited to 32,768/2 = 16,384.
2. Motor 3 is driving a DC brush motor amplifier in current (torque) mode with an analog
output. The amplifier has a gain of 2 amps/volt and an instantaneous current limit of
20 amps. The motor has an instantaneous current limit of 15 amps.