There are three kinds of braking used in the SureServo 2 system: Dynamic braking which is used to
stop the motor quickly when you disable the motor, issue an override or get a fault, regen braking
which is used while the motor is running to help create super-fast deceleration ramps
using a regen or braking resistor, and Electro-Mechanical braking which is used
while the motor is stopped to keep it from spinning. This video covers dynamic braking.
When you try to slow down a motor faster than it would normally spin down by itself,
the motor becomes a generator and pushes current back into the drive. That raises the internal bus
voltage and if that voltage gets too high, it can damage the internal components. The drive
won’t let that happen, of course. It will fault out and shutdown before anything bad happens.
Dynamic braking watches that bus voltage and dynamically adjusts the decel ramp
so the bus voltage doesn’t get too high. It’s important to understand that dynamic braking
in the SureServo 2 drive doesn’t use a regen resistor to dump excess energy. It only modifies
the decel ramp to slow down the regen current. Regen braking is covered in a separate video.
This is easiest to see with a live example, so let’s do it.
This is the exact same hardware as the quick start video except I added this
4-pound steel pulley to give us some inertia.
I reset the drive to factory default so you know where I am starting from
and power cycled the drive. That clears out any residual non-volatile memory locations.
I then changed it to speed mode, and power cycled the drive again. That power cycle is real
important because it sets up the drive for the new mode. And here’s a tip: If you can’t get the motor
to spin after a mode change, it’s probably because you forgot to do the power cycle.
In speed mode, you can set up two digital inputs to select one of four preset speeds.
I’m using digital inputs 9 and 10 for that. These are the default speeds, but I changed
this one to 3000 RPM and set the two digital inputs to on so when I enable the servo motor
ramps immediately to 3000 PRM, and when I disable the motor or issue an override or get a fault,
the drive drops to zero from this 3000 RPM. These are the parameters you need to do all that.
Let’s bring up the SureServo 2 Pro software scope function. I’ll hit run and while we are waiting
for that to come up we see we are viewing the commanded speed in white using 32 bits,
so it uses two of these, the actual motor speed in purple also using 32 bits, the bus
voltage in red and the motor current in yellow. When I switch the servo on, the white commanded
speed goes right to 3000 RPM, and the purple actual speed follows that as best as it can.
I haven’t tuned this drive yet, so we have a bit of overshoot.
Watch this video to learn how to fix that. When I disable the motor the white speed command
goes straight to zero and we see dynamic braking is working really hard to bring the
purple actual speed to zero with no alarms or errors. And it did a pretty good job!
I’ll stop the scope and if we zoom in on the decel ramp we see that dynamic braking was able
to get that heavy load close to zero RPM in around 400 ms and fully to zero in around 600 ms. Cool.
We also see while the motor is running at the 3000 RPM, the bus voltage hangs at about 170 volts on
this drive and the current needed to keep the motor spinning is only a couple tenths of an amp.
But when we switched the motor off, the red bus voltage rose from around 170 volts
to 190 volts because the motor pushed the yellow current of almost 5 amps back
into the drive! The minus sign tells us the current is going back into the drive and we
can see the negative current here in yellow. I’m gonna hit this button which maximizes each
trace and gives it its own scale so everything is easier to see. Again, the white commanded
speed went to zero, the motor pushed the yellow current back into the drive, which raised the red
bus voltage. And dynamic braking manages the decel rate to keep that voltage from getting too high.
My nominal bus voltage is around 170 volts because I’m using a 120-volt
supply. If you have a 220-volt supply, then this will be around 312 volts.
Let’s swap out the four-pound pulley for a 2-pound pulley. This will have less than half the inertia,
so let’s see how dynamic braking handles that. Let’s start the scope. I’ll switch the servo on,
wait a second, then switch it off. Stop the scope.
Zoom in on the decel ramp, and sure enough we now see it’s only taking dynamic braking around
100 ms to get close to zero and a total of less than 200ms to completely stop.
And if we optimize that display we can see dynamic braking is still allowing almost
5 amps back into the drive and it’s still allowing the bus voltage to go up to 190 volts
but it’s taking full advantage of that to get a faster decel ramp. Nice.
Of course, you can also disable dynamic braking.
I’ll go to parameter 1.032 and change this to a 1 to disable braking.
I also switched back to the 4-pound pulley to give us a more dramatic result.
I’ll start the scope, switch the motor on, then switch it off
and I’ll speed up the video, so we don’t have to wait for it to spin down. Wow. Looks like
that took that around 14 seconds for the motor to spin down without dynamic braking. Compare that
to the half-second we had with dynamic braking. So yeah, dynamic braking makes a big difference.
And look at this, if we zoom in on the white speed command transition from 3000 to zero rpm,
we see there was no impact on the current or the bus voltage because we weren’t trying to force the
motor to stop, so the motor didn’t become a generator trying to shove current back into the drive.
So we can turn dynamic braking on and we can turn it off to let the motor free spin. Well,
guess what, there is a third mode that does both. It uses dynamic braking down to a certain speed
and then releases the motor to free spin the rest of the way. You put whatever cutoff speed
you want in parameter 1.038. I put 200 RPM in for the cutoff speed and ran that test.
This is what I got. Braking down to here, free spin afterwards. It also sets the zero speed
output bit once it hits this level so other processes know it’s safe to do their thing.
Again, that may be handy for applications where you just need to get the dangerous
speeds shut down as fast as possible and don’t really care about getting it to exactly zero.
That should give you a pretty good feel for how the dynamic braking built in to
the SureServo 2 drive works. Understand that dynamic braking is only used when
you disable the motor or when you issue a motor override via a digital input or have a fault.
The normal decel ramps in the various operating modes use the braking – or
regen resistor automatically to accomplish whatever decel ramp you ask for,
assuming the drive and resistor can handle it. We’ll cover that in this video.
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