Reference Manual

Turbo PMAC/PMAC2 Software Reference
Turbo PMAC Global I-Variables 128
Use when Commutating: When Turbo PMAC is commutating Motor xx, Ixx69 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 (Ixx01 bit 0 = 1), Ixx69 is technically the limit
of only the quadrature, or torque-producing, current. Ixx77 sets the magnitude of the direct, or
magnetization current, and the total current limit is the vector sum of these two variables. If the Ixx77
magnetization current for the motor is set to a value other than 0, Ixx69 should be set such that:
767,32
max
I
2
77Ixx
2
69Ixx 
Use in Direct-PWM Mode: When commutating (Ixx01 bit 0 = 1) and closing the current loop (Ixx82 >
0) of a 3-phase motor (Ixx72 = 683 or 1365), it is important to understand the relationship between the
quadrature current limited by Ixx69 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 Ixx69, the peak phase currents can be 15% higher that this value. For
instance, with Ixx69 at 10,000, and Ixx77 at 0, the A/D converters can provide readings (normalized to
16-bit resolution) up to 11,547.
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, Ixx69 should be set such that:
 
377,28866.0*767,32
o
30cos
max
I
2
77Ixx
2
69Ixx 
Examples:
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.
 PMAC’s maximum output of 32,768, or 10 volts, corresponds to 20 amps.
 The motor has the lower instantaneous current rating, so we use its limit of 15 amps.
 I369 is set to 32,768 * 15/20 = 24,576.
3. Motor 5 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 instantaneous current
limit of 50 amps (RMS). The motor has an instantaneous current limit of 60 amps (RMS).
 PMAC’s maximum output of 32,768, or 10 volts, corresponds to 50 amps (RMS).
 The amplifier has the lower instantaneous current rating, so we use its limit of 50 amps (RMS).
 I569 is set to 32,768 * 50/50 = 32,767 (note that the maximum permitted value is 32,767).
4. Motor 7 is driving a sine-wave amplifier for a brushless servo motor with two analog outputs. The
Ixx77 magnetization current limit is set to 0. The amplifier has a gain on each phase of 4 amps/volt.
The amplifier has an instantaneous current limit of 25 amps (RMS). The motor has an instantaneous
current limit of 30 amps (RMS).
 PMAC’s maximum output of 32,768, or 10 volts, corresponds to 40 amps peak in the phase.