Today we're going to talk about when and how to select a braking resistor.
Now in a past video we talked about how a regen unit can be used to dissipate the
excess energy from a decelerating or overhauling load. This video will be about a braking resistor;
a simpler, but lower cost way, to achieve the same result.
How are braking resistors sized?
Now as a quick refresher, anytime the motor is decelerating or overhauling it's generating
energy back towards the drive, which, thus causes the drive's dc bus voltage to rise. You can see an
example of that in the graph here. On the x-axis we have time, and the y-axis is the drive's DC
bus voltage. The yellow line is an example of a drive without a regen or braking resistor unit.
The risk here is if the yellow line becomes too high it can reach the drive's
over voltage level, causing it to trip out into an error. With the addition of a braking resistor
you have the blue line here, which is its switch on off level. Once the DC bus reaches this level
the braking resistor is engaged, any excess energy is dissipated across the resistor,
and the DC bus value will plateau. This is shown with the pink line. The DC bus rises, it hits the
blue line, it plateaus, when the braking resistor is being used, it drops back down, it's used again
here, and continues on normally. Thus the braking resistor is used to dissipate the excess energy
and prevent the drive from tripping into an over voltage fault. So, now that we've talked about
when a braking resistor should be used, we need to talk about how to select a braking resistor.
There are two things to consider when selecting a braking resistor. The first is the resistance value.
Now the reason we have to first talk about the resistance of the braking resistor is actually due
to the braking transistor. Not the braking resistor, but the braking transistor. So the braking resistor
is external to the drive. The braking transistor is internal to the KEB drive, and is what switches the
current on and off to the resistor. So to calculate what we need for resistance we look at this basic
electrical equation of resistance equals voltage over current. Now within this application the
voltage and current are actually known values. So then we can calculate the resistance.
The voltage is the max DC bus voltage within the drive. In our case it's 880 volts DC.
The current value will depend on the drive, but for a basic example we can use this 44 amps.
But again this will be published within any drive series. And so, this is a known value.
So with these two known values of the voltage and current, we can then calculate the resistance.
And in this case it's just 20 ohms. And again, this is a minimum resistance value.
If you use a resistor with a resistance lower than this
your current will then become too high and you risk damaging your braking transistor. So now the
second thing you need to look at when sizing the braking resistor is the required power dissipation.
So again we've talked about the minimum resistance value but now we're talking about
the power dissipation. The reason this needs to be calculated is because when the braking resistor
is sized properly and working correctly your DC bus voltage will plateau like you see up here.
Assuming it's working properly. But if it's undersized it can rather than plateau here
continue to rise up because it doesn't have the available power dissipation for what the motor is
generating. Thus it's crucial to calculate what is needed beforehand. You can do this two ways.
The first is a little bit more involved and because of that we're not going to write out
all the equations here, but if you would like you can reference them in our breaking resistor manual.
Those equations require three variables. The first is the inertia of the system.
So this includes both the motor and the load. Second is your change in speed so this could be
you know from full speed of 1800 rpm down to zero or if your operation requires smaller changes of
speed. And finally, it's just your time so it's the amount of time required to make that speed change.
So again, these are the only three variables you need but the equations are a little bit
more involved, but if you'd like you can reference them in our breaking resistor manual. So the other
simpler way to determine your required power dissipation is using our COMBIVIS 6 software. So
within the software when you're connected to the drive you can actually scope out this exact graph
of voltage over time throughout the operation. This allows you to track the DC bus voltage of
the drive, and see how much it is increasing when you're decelerating or overhauling. This allows you
to get a better idea of how to select the correct size of braking resistor. Now it's a little bit
more trial and error, but it's a lot simpler and easier to determine how big of a resistor you need.
We've got a lot of manuals and resources available for you on our website, or if you'd like to speak
to someone in person, you can reach out to an application engineer through kebamerica.com