User's Manual

PMAC User Manual
Making Your Application Safe 121
where:
a.
I
q
(quadrature current) is the commanded torque-producing output of the PID filter in
units of a 16-bit DAC;
b.
I
d
(direct current) is the magnetization current command as set by Ix77. This is usually
zero except when PMAC is doing vector control of induction motors.
c.
∆ t
is the time since the last sample in servo cycles
If Sum exceeds Ix58, an I
2
T fault will occur. When commanded current levels are below Ix57, Sum will
decrease, but it will never go below zero.
Example: With command output limit Ix69=32767 (maximum), integrated current limit Ix57=16384
(half of maximum), and magnetization current Ix77=0, the motor hits an obstruction, and the command
output saturates at 32767. The I
2
T function will calculate during this time:
[
]
t75.0Sumt5.001SumSum
222
∆∆ +=−++=
Sum will increase at a rate of 0.75 per servo cycle. At the default servo cycle update rate of 2.25 kHz,
Sum will increase at a rate of 2250*0.75=1688 per second. To trip the motor after three seconds of this
condition, set Ix58 to 1688*3 = 5064.
When an I
2
T fault occurs on a motor, PMAC reacts just as for an amplifier fault error. The offending
motor is killed, and possibly other motors as set by Ix25. PMAC sets the amplifier fault motor status bit.
For an I
2
T fault, PMAC also sets a separate I
2
T fault motor status bit. Both bits are cleared when the
motor is re-enabled.
Note:
When PMAC is not commutating a motor with I
2
T protection, make sure
magnetization current parameter Ix77 is still set to 0. In this setup, Ix77 will not
affect operation, but it will affect I
2
T calculations.
Amplifier Enable and Fault Lines
The use of the amplifier enable (AENA) output and the amplifier fault (FAULTn) input lines for each
motor are important for safe operation. Without the use of the enable line, disabling the amplifier relies
on precise zero offsets in the PMAC analog output and the amplifier's analog input.
Without the use of the fault line, PMAC may not know when an amplifier has shut down and may not
take appropriate action.
If either the +5V supply for the PMAC computational section, or the +15V analog supply is lost, the
amplifier will be disabled automatically, because the output transistor will go into its non-conducting
state. To get this failsafe protection without connecting a signal of this polarity directly to the amplifier,
use intermediate circuitry to change the signal format. With the alternate sourcing drivers, the high-true
enable polarity provides better failsafe protection.
For more details, refer to the Dedicated Digital Output Flags (JMACH, JEQU Ports) and Dedicated
Digital Input Flags sections of this manual.
Note:
With the default sinking drivers for the amplifier enable signals, using the low-true
enable polarity (low voltage – conducting – is enable; high voltage – non-
conducting – is disabled) provides better failsafe protection against loss of power-
supply.