Reference Manual

Turbo PMAC/PMAC2 Software Reference
Turbo PMAC Global I-Variables 121
actual or commanded current level from Turbo PMAC is above the magnitude of Ixx57 for a significant
period of time, as set by Ixx58, Turbo PMAC will trip this motor on an integrated-current amplifier fault
condition.
The integrated current limit can either be an I
2
T (“I-squared-T”) limit, or an |I|T (I-T) limit. If Ixx57 is set
to a positive value, Turbo PMAC performs I
2
T limiting, squaring the value of current before integrating
and comparing to Ixx58. If Ixx57 is set to a negative value, Turbo PMAC performs |I|T limiting, just
taking the absolute value of the current before integrating and comparing to Ixx58.
I
2
T limiting is best used if the system device with the shortest thermal time constant is resistive (and so
has I
2
R heating), as in motor windings and MOSFET drivers. |I|T limiting is best used if the system
device with the shortest thermal time constant has a fixed voltage drop (and so has IV heating), as in
IGBT drivers.
Ixx57 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, Ixx57 will be set to between 1/3 and 1/2 of the Ixx69
(instantaneous) output limit. Consult the amplifier and motor documentation for their specifications on
instantaneous and continuous current limits.
Technically, Ixx57 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 Ixx77.
In sine-wave output mode (Ixx01 bit 0 = 1, Ixx82 = 0), typically, amplifier gains are given in amperes of
phase current per volt of PMAC output, but motor and typically amplifier limits are 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 (Ixx01 bit 0 = 1, Ixx82 > 0) of 3-phase motors (Ixx72 = 683 or 1365), 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 the top values have been
established in the A/D converters the phase currents on a continuous basis, this value should be multiplied
by cos(30
o
), or 0.866, to get the value for Ixx57. Remember that if current limits are given as RMS
values, multiply these by 2 (1.414) to get peak phase current values.
Examples:
1. Turbo 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 a continuous current limit of 25 amps (RMS).
 PMAC’s maximum output of 32,768, or 10V, corresponds to 50 amps (RMS).
 The amplifier has the lower continuous current rating, so we use its limit of 20 amps (RMS).
 I357 is set to 32,768 * 20/50 = 13,107.
3. Turbo PMAC Motor 4 is driving a sine-wave mode amplifier that has a gain for each phase input of 5
amps/volt. The amplifier has a continuous rating of 20 amps (RMS); the motor has a continuous
rating of 22 amps (RMS).
 PMAC’s maximum output of 32,768, or 10 volts, corresponds to 50 amps peak in a phase.
 The amplifier has the lower continuous current rating, so we use its limit of 20 amps (RMS).
 20 amps (RMS) corresponds to peak phase currents of 20*1.414 = 28.28 amps.
 I457 is set to 32,768 * 28.28 / 50 = 18,534.