To show you how current limiting works on this drive, I put a large 12-pound steel sprocket
on this motor to give us some inertia to overcome.
I reset the drive to factory default and setup the motor parameters to match this motor and
set the reference frequency to 60 Hz to tell the drive to ramp up the motor to full
speed over the acceleration time we specified.
The acceleration ramp is currently at the default 5 seconds which is a good place for
us to start, and I’ll reduce the deceleration ramp time to 4 seconds so we’re not spending
a lot of time waiting on the motor to decelerate.
I’m using the free WPS software and this USB com module to talk to my drive from my
PC.
I’ve already created a configuration and a resource and am online with the drive.
We’re just going to use WPS to visualize things, I’ll enter the parameters from the
Drive’s built-in HMI as a reminder that we don’t need the free WPS software and
an optional com module to do this.
Speaking of visualizing things, look at this.
Down here under protections, Inverter – these graphs show us exactly how limiting works.
This side is voltage limiting. We cover that in a different video – and this is
the current limiting we’re doing in this video.
The current limiting threshold is in parameter 135.
That threshold can be used for three things.
First it can be used to disable current limiting.
If we set the threshold really high, the manual says to set it to 1.9 times the drives max
rated current - then there won’t be any current limiting.
Let’s try it.
This drive is rated for 4.2A, so I’ll go to parameter 135 and enter 8 amps.
I’m also going to go to parameter 100 to set the acceleration ramp at a ridiculously
small number to really stress the drive.
Hit run, and of course we get an overcurrent fault.
The second current limiting option is to set the threshold at a reasonable value and employ
one of two current limiting techniques.
The default current limiting threshold for this drive is 6.3 amps – about 50% above the
drive's max rated current of 4.2 amps.
Remember – the drive’s max rated current is how much current the drive can handle continuously.
This threshold is looking for spikes in current.
If we go to the data sheet we see this drive can handle a 150% overload for 60 seconds
and 200% overload for 3 seconds.
So, a 6.3 amp threshold makes sense.
Ok, given that threshold we have two choices.
One is if the current hits the threshold, have the acceleration ramp hold to give the
motor a chance to catch up.
Remember, it’s the acceleration of the motor that creates the demand for the current. The
greater the acceleration, the more current it needs.
So, if the drive holds the ramp constant until the motor catches up then the current demand
falls.
The other current limit option is when the motor current hits the threshold. Instead
of holding the acceleration ramp rate, it actually allows it to decelerate until the
motor catches up.
In both cases, once the motor catches up, the original ramp continues on.
So, what these are really doing is automatically extending the ramp time to prevent an overcurrent
fault.
You could accomplish the same thing yourself by just using a longer ramp time, but then
you would have to figure out how long that needs to be.
By using the current limiting options, you’re letting the drive give you the best possible
ramp time automatically.
Let’s do an example so we can see how this works.
I would love to show you this in the WPS Trend View, but trend view isn’t really fast enough
to show us what we need to see. It’s fastest sample rate is only 50 ms, so instead
I’m using this optional I/O module that has two analog outputs.
And we’ll send the motor current and the output frequency to those two outputs, so
we can view them on a scope.
If I look in the programming manual’s analog output functions table, we see output current
is a 5, and the output speed, which they call the “real speed”, is a 2.
So, I’ll go to parameter 251 for analog output 1’s function and enter a 2 for real
speed and then go to parameter 254 for analog output 2’s function and enter a 5 for the
output current.
I’m monitoring those on an inexpensive USB oscilloscope I bought on the internet.
Ok, before we change anything let’s get a baseline.
Let’s get the acceleration ramp back to a reasonable 5 seconds and put the over current
threshold back at the default value for this drive which is 6.3 amps.
I’ll get the scope running … and hit run on the drive.
Wait the 5 seconds for it to ramp up, give it a couple more seconds to settle out, and
hit stop.
Cool.
We can see the red frequency ramp is pretty much what we expected – and the drive was
able to get the motor up to speed in the 5 seconds we wanted.
And most important, the current looks good.
The analog current output is 0-10 volts where 10 volts is twice the drives rated current
of 4.2 amps.
How did I know that?
It’s in this analog output table in the programming manual where parameter 295 is
the drives rated current.
Which means the current got up around to around 3.2 amps.
And we know from parameter 135 that the current overload threshold is set at 6.3 amps.
So, since the current never got anywhere near that level, it says the 5 second ramp was
easy for the drive to do.
So, let’s challenge the drive by going to parameter 100 and reducing the acceleration ramp to
a ridiculously small time to really stress the drive.
Start the scope, hit run ... stop.
Let’s zoom in so we can see this better.
Now the motor current IS crossing the threshold and the ramp is getting extended but it’s
kind of hard to see, so I’ll speed up the scope AND let’s go to parameter 135 and
lower the current limit threshold to exaggerate the effect.
On our base line we saw the current never got above 3.2 amps, so I’ll lower the threshold
to something just above that – how about 3.6 amps.
Again, you wouldn’t normally do this, I’m just doing it to get current limiting
to kick in sooner, so we can see the effect a little better.
Start the scope, hit run ... Stop.
Our new current limit threshold is here at 3.6 amps.
The bad news is, it looks like the drive is only updating the analog output every 25 ms.
So, we get this stair step effect instead of a smooth curve.
Which means we have to use our imaginations a little bit and just know there are some
fast transitions here that we can’t see on this plot.
But, we can see the end result. The frequency output is definitely going into that ramp
deceleration mode in several places to extend the ramp time and prevent an overcurrent fault.
Looks like the 0.1 second ramp time we asked for was extended to a little over 1.5 seconds.
Let’s go to parameter 150 and switch from ramp decel to ramp hold mode by putting a
2 here.
Start the scope, hit run, stop.
And we got pretty much the same ramp time … but this time instead of decelerating
the ramp to limit the current draw, it was simply held level.
Exactly what we wanted.
And again, while the analog output is too coarse to see the actual threshold crossings,
we can definitely see the ramp hold periods and that the current was held at a reasonable
level.
Ok, well, while we are at it, let’s see what an over current fault looks like.
I’ll go back to parameter 135 and put it at 8 amps again to turn off current limiting.
Start the scope, hit run, stop.
Well, that was pretty quick.
We put the over current threshold way up here, and sure enough, the Motor current was allowed
to spike and in a little over 100 milli-seconds the drive shut the output down.
Cool.
The bottom line is, if you are getting an overcurrent fault like this, you probably
have the current limiting threshold set too high and the fault is kicking in before the
limiting can take effect.
Setting that overcurrent limiting threshold to a value that works best for your application
is usually the key to fixing overcurrent fault issues.
And a good starting point is about 150% of the drive’s max rated current, which is
the default value for these drives.
Click here to learn more about the WEG CFW300 Variable Frequency Drive.
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