Dynamic Braking on the GS4 Variable Frequency Drive (VFD) from AutomationDirect

If you want to decelerate a motor faster than it would normally decelerate by itself, then you can set the deceleration rate in the GS4 drive. That’s easy and the drive absorbs the energy that the motor would normally dissipate over time. The problem is, if the load has a lot of inertia, the drive electronics may not be able to handle dissipating all that stored energy because the motor is now acting like a generator and trying to push additional voltage back into the drive which causes the internal DC bus voltage to rise which causes an over voltage fault. That’s where Dynamic Braking Units come in to play. They sense the increase in voltage on the DC bus and divert it into a large resistor where the energy can be safely dissipated as heat. The concept is simple, but there are a few things you need to be aware of before using Dynamic Braking Units – which I will also call DBU’s. First, the good news: Most of the GS4 Drives have Dynamic Braking built in – if you need Dynamic Braking, you just attach the resistor shown in this chart. Or if you want to supply your own resistor, then this is the minimum value resistor you can use before the current gets too high for the drive. So you just figure out how much braking torque you need, and choose a drive and braking resistor that can handle it. To wire the smaller drives that have Dynamic Braking built in, connect the Braking Resistor to the B1 and B2 terminals and jumper B1 to the plus DC +1 bus. If I lift this label you can see the terminals. For the larger drives, it’s the same thing, but now YOU have to provide an external Dynamic Braking Unit that can handle the large amperage that the system has to deal with. Now the wiring looks like this where the external DBU monitors the drive’s DC bus and then dumps the excess energy into the resistor. One word of caution. Make sure you get these wired correctly. Iif you reverse those you can damage the drive and the braking unit. In a nutshell, that’s it, BUT there are some things you need to be aware of to protect your system, make things work as efficiently as possible and save money. I can’t emphasize how important it is to add an overload protector to your circuit when using Dynamic Braking. That would go here for the small drives that have dynamic braking units built in and here for the large drives that use the external DBU’s. The overload is wired in series with a contactor that disconnects power from the drive when overload conditions are exceeded. Unfortunately, a lot of folks ignore this and it ends up costing them a lot of time and a lot money. Overload protection is a really small price to pay to save your equipment from damage that cascades out of control really quickly. Check out this video to learn more about using Thermal Overloads and there’s even a section in the DBU user manual that walks you through step-by-step exactly how to choose a thermal overload for your application. Of course, overload and component sizing all depend on duty cycle – that is, what percentage of the time braking is enabled. You need to limit braking to no more than 10% of the time. That give the braking resistor time to dissipate the heat. If something happens that enables braking more often than that, then you run the risk of burning up the braking resistor, and the DBU and the Drive. Of course, you were smart and installed the thermal overload so that can’t happen in your system – right? All of AutomationDirect’s DBU’s have an LED that tells you when the DBU is on and when it’s braking or dumping current into the braking resistor. The smaller DBUs have a single fault output LED while the larger DBUs have an over current and an over heat LED. And they all have an alarm contact you can send the alarm back to your PLC or other devices. Suppose you have another machine in the factory that’s messing with the line voltage and forces a larger voltage than expected into the drive. Well, the DC bus inside the drive is directly proportional to the input line voltage so it rises too. Is there anything preventing the DC bus from rising far enough to cause a fault? Nope. But, if you have a DBU attached, it will automatically dump that excess energy and prevent the fault and possible damage to your drive. So dynamic braking units protect your drive from overvoltage in both directions, How about that? Now if your ac line voltages are large enough to trip the DBU, then you need to find out what is causing that and fix it. DBUs aren’t really a solution for high AC line voltage, just know that they will offer some protection. So how does the DBU know when it’s time to enable braking? You have to tell it at what voltage it needs to brake – or dump energy into the braking resistor. On the external DBUs there is a jumper. This DBU is for a 230VAC system. Suppose we normally see 220VAC on the line, but it fluctuates up to 228 on a regular basis. In that case, we would want to set the jumper just above the 228 – at 230VAC - so those normal line fluctuations don’t enable braking. On the drives with braking built in, you set the voltage via these parameters. Again, using the same guidelines – set it a little higher than the line fluctuations you normally expect to see. If you prefer to measure the DC bus fluctuations, then you can use this table in the DBU user manual to set the jumper. For example: If you want braking enabled for anything over 400VDC, then set the jumper or parameter to the 230VAC setting. Easy. The Dynamic Braking units aren’t large enough to handle the largest loads by themselves. In that case you can use multiple DBUs wired like this: Wire the DBUs in parallel, Wire the Master terminals to the Slave terminals like this, Set the first DBU to be the master and the other DBUs to be the slaves using this jumper on the DBU, and wire the normally closed thermal overloads in series back to the line voltage disconnect contactor. Same thing goes for the braking resistors. There will be times when you need to use multiple resistors to handle the required load. How do you know when it’s appropriate to use different wiring? Easy. Use this table in the user manual. For example, for this guy you need this many DBUs of this type and this many resistors of this type, wired according to this diagram, which looks like this. And look, that section of the table is even repeated here for easy reference. The wirings shown in this manual are the only approved wirings for these loads. If you choose to deviate from this, please make sure the total current out of the drive can’t exceed this Max Total Brake Current shown in the drive user manual. Also, make sure the braking resistors you use can handle the Peak Power shown here for a duration of at least one second. Speaking of Braking resistors, they come in different sizes and form factors – all of which are shown in this table and in the drawings in the GS4 User Manual. Some are simple wire heating elements in metal cages, some are potted bricks like this one and some are just big wire-wound resistors like this one. Be sure to connect the braking unit ground to earth ground and that the wire is at least the same gauge as the +P –N or B1/B2 wire gauges. During braking, these wires generate powerful electromagnetic fields so keep those away from any other low voltage control signals. Don’t do any wiring while power is applied. In other words, just because you removed power from the drive, there is still a lot of internal energy that needs to dissipate. Wait for ALL LEDs to be completely off before messing with the wiring. And finally, one last time, PLEASE use a thermal overload – it can save you a lot of time and money. And since they are inexpensive compared to the rest of your system, there is really no reason not to. Click here to learn more about Variable Frequency Drives and DBUs. The manuals for these guys are really well done and easy to read. If you need more help, contact AutomationDirect’s Free award winning support team – they will be happy to help. And be sure to subscribe to our YouTube video channel so you’re kept up to date on all of our latest tutorial videos!