The PID function adds two new alarms and faults to the WEG drives comprehensive set of built
in alarms and faults.
PID can fault and alarm on the process variable getting too high or too low.
An alarm simply notifies you there is an issue via the HMI and in the present alarm parameter.
Faults are also indicated in the HMI and in the present fault parameter, but faults are
also indicated in the inverter status word and they disable the drive's output and allow
the motor to coast to a stop.
Let’s do a quick example.
We are using the same hardware as in all of the videos in this series and this is a scope
traced from one of those videos where we closed a valve and we opened a valve with 1 second
per horizontal division and three quarters of a psi per vertical division.
Let’s assume we want to maintain our pressure within plus or minus half a psi.
And if the pressure falls outside of that window for more than 5 seconds, then we want
a fault which will shut down the drive.
That 5 seconds window will prevent these valve openings and closings from generating faults
and shutting down the drive.
I’m keeping the durations short just to expedite this video – what you use for your durations
will depend on your system.
So, if the pressure drops because we ran out of water in the reservoir or a pipe breaks
or if the sensor shorts to ground for longer than 5 seconds, the drive won’t burn up the
motor or the pump trying to get the pressure back up.
And if the sensor faults with the signal indicating a high pressure for more than 5 seconds, it
will also shut down the drive.
These are the key PID parameters I set up, which is basically what we did in the quick
start video except I increased the P gain to make sure the system ramps up within that
5-second window.
Parameter 924 is the low-level alarm – that will be 5.5 psi.
Notice that this is in the engineering units we specified in parameters 510 and 511.
Parameter 925 is the duration of the condition that will generate a fault– we want that
to be 5 seconds.
Parameter 926 is the high side, so we’ll set that to 6.5 psi.
And Parameter 927 is the duration of the high side.
We want 5 seconds for that one too – it doesn’t have to be the same as the low side,
I just did that for this demo.
I’ve already entered all of those so let’s try it!
This is a live view of the process variable. The drive is in auto mode so PID should start
right away when I switch to run.
And we immediately see an alarm telling us there’s a low pressure – that’s ok we
are ramping up.
Again, if that ramp took longer than 5 seconds, the drive would issue a fault and shut down.
This process ramped up in less than 5 seconds, so we are in good shape and the alarm went
away.
Perfect.
I’ll short the sensor to ground to simulate a sudden drop in pressure and we get the low-pressure
alarm. That occurs 150 msec after the condition starts - and after 5 seconds, we get a fault,
the drive shuts down its output and lets the motor coast to a stop.
Exactly what we expect.
I removed the short to ground so I’ll reset the fault, which resets the drive and since
the RUN switch is still enabled, PID starts right back up.
We get the low-pressure alarm until the pressure ramps up and everything is back to normal.
Ok, this time I’ll short the sensor to 10 volts to simulate overpressure.
We see the high-pressure alarm and after 5 seconds … the drive faults, shuts down its
output and lets the motor coast to a stop.
Perfect.
I removed the short to 10 volts, reset that fault and everything ramps back up to normal
operation.
That’s all we need to know about PID generated alarms.
Of course, you can always set up digital outputs to report the different kinds of alarms and
faults.
And you can view the current fault and the 3 previous faults at any time along with the
current, DC link, frequency and temperature present when the last fault occurred.
In the programming manual, there is a quick reference chart that lists all of the possible
faults and alarms.
Notice that the faults and alarms are in numerical order not alphabetical order and the numbers
are unique – the motor overload alarm is 46 while the fault is 72, for example.
That way you can’t accidentally look up the wrong one.
I really like that.
Finally, remember that the drive has an auto reset function for faults.
It defaults to zero which is off, but if you change it to a non-zero time, the fault will
auto reset after the number of seconds you specify.
And if a fault auto-resets more than three times within 30 seconds after the reset, it
will shut the drive down and won’t auto-reset anymore.
That way an unattended drive won’t sit there resetting itself over and over indefinitely.
That ought to be enough to get you up and running with PID specific alarms and faults.
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