This video picks up where the Overvoltage video left off.
In that video we saw a live demo that showed us when the drive tries to rapidly decelerate
a motor with a large inertia load, the motor becomes a generator, and it pushes current
back into the drive and that raises the DC bus and causes an overvoltage fault.
Dynamic Braking simply monitors the DC voltage and when it gets to a braking threshold that
you set, it re-routes the excess current to an off-board braking resistor where the excess
energy is converted to heat.
The Chopper uses IGBT’s which are just big transistors that can quickly switch the high
voltage and current that goes to the motor.
Dynamic Braking uses a fourth IGBT circuit to regulate the current flow to the resistor
using the same chopping method.
Wide pulses send lots of current to the resistor, narrow pulses send a little.
Dynamic Braking is a great way to get fast deceleration times and the WEG drive makes
it easy to do.
I’m using the exact same setup in this video as we did in the overload video, so I’m
not going to repeat that here.
But as a refresher, here are the parameters we used with in that video.
For this video I swapped that A-Frame drive out with a larger B-Frame unit because B-Frame
drives have Dynamic Braking built in.
That’s important: A-Frame drives don’t have Dynamic Braking built in – the package
is too small to fit the extra IGBT Chopper circuitry.
I set up this B-Frame drive with all the same parameters as the A-Frame drive and made sure
that the deceleration ramp was at 1.2 seconds like we had in the previous video when the
drive gave us the overvoltage fault.
Remember, on the A-Frame 120 volt drive the programming manual told us the overvoltage
threshold was 460 volts and we actually saw that on the trend. When the DC bus hit
460 volts, the drive shut down the output.
But look at this!
On this 230 volt B-Frame drive, the overvoltage threshold is only 410 volts!
That would be down here on the trend.
In the previous video we saw that a deceleration ramp of 5 seconds took the DC bus to 420 volts.
So, to avoid an overvoltage fault on this B-Frame drive we would have to slow down the deceleration
ramp to something even longer than 5 seconds to keep the DC bus below 410 volts, right?
OR … we can use the dynamic braking feature built into this B-Frame drive.
And guess what? Dynamic braking is enabled by default in this drive.
You just add a resistor and you are done.
If I search WPS for Braking, we see there are a bunch of DC braking parameters – that’s
a different video – but there is only one Dynamic Braking parameter.
And look, it’s preset at 365 volts, and you can set it anywhere between 325 and 460.
Let’s go back to our trend.
We know the overvoltage fault threshold for THIS drive is at 410 volts.
Dynamic Braking is preset for 365.
If you set the braking threshold below the normal DC bus level then braking would be
on all the time, which we don’t want.
If you set it above the overvoltage threshold then the drive will shut down before getting
to the braking level.
So we want the braking threshold between those two limits, which is what the default value
does, so we will just use that.
During deceleration, we expect to see the DC bus rise until it hits 365 volts.
At that point, the drive should switch the braking resistor in and the DC bus will stop
rising.
All that’s left now is to connect the resistor.
A word of caution.
The DC bus is at several hundred volts.
Do you want the drive powered up while you connect the resistor?
I know I don’t.
So PLEASE be sure to disconnect the power to the drive and let the drive fully dissipate
it’s energy before installing the resistor.
While we are waiting for the power to dissipate, how do you know what size resistor to choose?
The CFW300 user manual has this chart in Appendix B that tells us everything we need to know
about braking.
First we see the A-Frame drive doesn't support braking.
The two CFW300 B-Frame drives do, and they need around a 39 ohm resistor that can handle
around 10 amps.
If you do I squared R that tells us we need almost a four thousand watt resistor, right?
No, not really.
Remember, you aren’t braking 100% of the time.
And you really want to limit braking to less than 10% of the time to give the resistor
time to cool down.
So what you care about is the AVERAGE power.
If we are braking 10 percent of the time, then we could get away with something in the
400 watt range.
So I went and searched the AutomationDirect website for a 400 watt 40 ohm resistor and
found this guy.
Exactly what we need.
Again, I’m doing this in my office so I’m just going to let it dangle here, but in a
real application you would want to mount the braking resistor to a really good heat sink.
Usually the largest piece of metal you can find.
I also added a switch in series with the resistor, so I can quickly switch it in and out for
our demo.
Ok, here we go.
The Dynamic Braking resistor is currently swiched out of the circuit, so nothing has
changed from the previous video.
If we start the trend … then hit run .. wait for it to ramp up to speed over the 5 second
default acceleration time .. let it level out .. then hit stop .. sure enough the drive
gives us an overvoltage fault just like in the previous video.
I’ll reset the fault, stop this massive pulley .. Switch the braking resistor back
in .. hit run .. wait for it to ramp up .. hit stop .. ahh look!
The DC level never got above the braking threshold and the drive stopped without a fault!
All simply because we added a braking resistor which gave the Dynamic Braking circuitry, which
was already enabled, a place to send the excess current.
Perfect!
One side note: It looks like the DC Bus never actually hit the Braking threshold on this
trend.
The issue is the trend is only sampling the DC bus 5 times a second, and the crossing
happened between samples.
So while we don’t actually see the crossing here, it’s pretty obvious it did and that
the braking resistor is definitely doing it’s job.
For the A-Frame drives, it’s the exact same thing, but now you have to provide an EXTERNAL
Dynamic Braking Unit.
There is a separate video that explains how to do that.
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