Learn how Dynamic braking works in a variable frequency drive and how to get the most out of it in a GS20(X) drive.
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In the overvoltage video, we saw that the GS20 drive can automatically lengthen the deceleration time to help prevent overvoltage faults. But what if you really need the short deceleration time and you don’t want to wait for the drive to lengthen it? That’s where dynamic braking comes in. Remember, when you decelerate the motor faster than it would normally spin down by itself, then the motor becomes a generator and pushes current back into the drive. That raises the DC bus voltage and causes the drive to throw an overvoltage fault and shut down before the overvoltage burns up the internal components. The great thing about the GS20 drives, is every drive in the family has braking circuitry built in. So all you have do is add a braking resistor. So now when the motor tries to push too much current into the drive, the GS20 automatically re-routes that current into the braking resistor so the DC bus doesn’t rise and destroy your drive. The resistor converts the extra energy into heat. I’m using the same hardware that we had in the overvoltage video except I attached a braking resistor to these terminals and added a switch so I could quickly switch the resistor in and out for this demo. I reset the drive to factory default, so you know exactly where I am starting from and entered these parameters for the motor and ramps, and I enabled the frequency control potentiometer. Let’s bring up the free GSOFT2 scope function and monitor the same signals that we had in the overvoltage video. I have the braking resistor switched out for this first test, so we can get a baseline. I’ll rotate the frequency control knob to max, switch to the actual frequency out display and hit run. The output frequency ramps up to 60 Hz over the expected 5 seconds. Hit stop and sure enough we get the dreaded overvoltage fault and the motor is left to freely spin down which with this inertia is going to take a couple minutes. We see on the scope trace that the red DC bus voltage rose rapidly to over 400 volts. Which I saw down here as the test was running. The drive saw that, shut off the blue output frequency and quit trying to decelerate the motor to save itself. I’ll reset the fault and switch the braking resistor in, start the GSOFT2 scope, hit run and wait for it to ramp up to speed. Now when we hit stop – look! We don’t get a fault! And look at this trace. We got a nice linear deceleration ramp. If I move the cursors around, we see it is exactly the 4 seconds we asked for. But most importantly, the DC bus was held to a reasonable level. Perfect. We also see that the pulleys inertia kept the motor spinning even after the deceleration ramp was done. Let’s see how far we can push this. Let’s cut the deceleration time in half to two seconds. Start the scope, hit run. Wait for it to ramp up and then hit stop. Again, no fault and a perfect deceleration ramp. And again, the DC bus was held to a safe level to protect the drive. Of course, the pulleys inertia left the motor free-spinning even faster. OK, let’s go all the way to a deceleration time of zero seconds just to see if we can force an overvoltage fault. Start the scope. Hit run. Let it ramp up. Hit stop. Interesting. Still no fault and the drive issued a deceleration ramp as fast as it could. And even with that, the DC bus I still held to a safe level. Cool. Of course zero seconds isn’t nearly enough time to slow down this cast iron pully’s inertia, so it is going to free spin for quite a while because we don’t have a load on this system. If we did, this thing would stop really quickly. Suppose you have a fan or blower that doesn’t really have a load on it and you don’t want it to free spin. Is there anything you can do to stop that free spinning? Yeah, there is. It’s called DC braking. Watch this video to learn more about that. That’s dynamic braking in a nutshell, but, there are a couple things you need to be aware of before you just plug a braking resistor in. How did I know what size braking resistor to use? Easy, there is a chart like this in the user manual. I’m using this drive, so I need this resistor. And look, they are only around 30 bucks. The recommended braking resistors require you to limit braking to a 10% duty cycle with a max braking time of 10 seconds. If you brake more often than that or break for longer than 10 seconds the braking resistor may overheat. And speaking of heat, be sure to take that into account when installing a braking resistor in a cabinet. Braking can generate a lot of heat. Since we have a braking resistor installed, we can turn off the drive's overvoltage stall prevention that we saw in the overvoltage video by setting it to a zero. It didn’t get in our way here, so I didn’t bother. Finally, with the GS20 family of variable frequency drives you can adjust the braking voltage, current, and start and stop frequencies if you need even more control over the braking action. We mentioned two kinds of braking in this video, so just to be clear: Dynamic Braking – the topic of this video - provides a path to remove energy from the motor which helps the motor come to a rapid stop. DC Braking – also called injection braking - applies a voltage into the motor which creates a magnetic field inside the motor which helps slow, stop and hold a motor from rotating. Each technique has its own considerations and requirements so it’s important to understand what each does and if it is appropriate for your application. Well that ought to be enough to get you going with dynamic braking. Click here to see all of the GS20 variable frequency drive video tutorials. 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