Servo Regen Braking (aka Dynamic Braking) - SureServo2 from AutomationDirect

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 and override or have a fault. Regen braking which is used while the motor is  running to create super-fast deceleration ramps  using a regen resistor. And Electro-Mechanical  braking which is used while the motor is stopped  to keep it from spinning. This  video covers regen braking. Most of the time we want to  control the deceleration ramp.   The problem is when you try to slow a motor down  faster than it would normally spin down by itself,  the motor becomes a generator and pushes current  back into the drive. That charges the capacitors  which 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. But we don’t want our  system to fault out, so how do we avoid that? Well, the SureServo 2 drive monitors the bus  voltage and when it starts to get too high,  it automatically re-routes the excess energy  to a large wattage resistor where the energy  gets burned up as heat. If you are familiar with  VFD’s you would probably look at this and say,  “oh, that’s dynamic braking.” Yeah, I agree,  but the SureServo 2 uses the term dynamic braking   for a different braking method where the drive  dynamically adjusts the decel ramp to prevent  the bus voltage from getting too high. So, we’ll  refer to this braking method as regen braking. In dynamic braking, the drive controls  the decel ramp. In regen braking, you do.  And the cool thing about the SureServo 2 system is  regen braking is built-in and enabled by default.  And drives through 3-kilo watts even  have the braking resistor built in!  So, since I’m using a 400-watt drive, we don’t  have to do anything to set this up. Do you have   to use the built-in resistor? Nope. You can  disable it and use your own external resistor  if you want to. We’ll cover how to do that  and why you might want to later in this video.  I’m using the same hardware as we used in the  quick start video except I added a 4-pound  pulley to give us some inertia. I reset the drive to factory default,  so you know exactly where I am starting from  and power cycled the drive. That clears out any residual non-volatile memory locations. I then  changed it to the speed mode that guarantees the first speed is absolute zero, 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 that 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 and since we  are using speed mode 4, this is guaranteed to be a zero. These are the default speeds for the  other three, but I changed this one to 3000 RPM so I could just toggle this one bit to  quickly switch between zero and 3000 RPM. These are the parameters you need to do all of that. And I set the speed mode decel ramp to   1 ms – which tells the drive to decel as fast  as it can. So, we have a 50:1 mismatch on our  load and the fastest possible decel ramp,  yeah, that’s asking a lot of the drive. I also tuned the drive to this load – check out  the auto tune video to learn how to do that. Ok, let’s bring up the SureServo  2 Pro software scope function. I have the commanded speed in white  using 32 bits so it uses two of these,  the actual motor speed in purple also 32 bits,  the bus voltage in red and the motor current  in yellow. Let’s get the scope running. I’ll enable the servo motor and flip this   switch to tell the drive to go to 3000 RPM. The  motor ramps up to speed. I’ll turn the speed   switch back off and sure enough the drive ramps  quickly down to 0 RPM without a fault. Perfect. I’ll stop the scope and zoom in. Look at that  – it took the drive only around 160 ms to bring that heavy load to a nice clean full stop.  That’s amazing. We see the current going back into the drive was a strong 8.5 amps and the bus  voltage rose from 170 volts to almost 320 volts. Keep in mind I’m using a 120-volt drive here.  If you are using a 220-volt drive then your nominal bus voltage will be around 312 volts  and it will rise proportionally due to regen. Ok, so out of morbid curiosity, what  happens if we don’t use a regen resistor?   The drive has SOME capacity to absorb energy, will  it have enough? Let’s find out. I removed this  connector which has this jumper pre-installed  which connects the built-in regen resistor to  the drive’s braking circuit so the current flows  through the resistor. With the connector removed,  this is gone so the drive has no regen resistor to  dump the extra energy to. Let’ see what happens. I’ll restart the scope, switch to 3000 RPM …  and then back to 0 RPM. That’s interesting – it looks like the drive handled this massive  load just fine without a regen resistor – or did it? Look – we got a fault that tells us  something is wrong with the regen circuit. And if we zoom in on the decel ramp we see  something interesting. We got our expected  linear deceleration ramp until here, but when the  bus voltage rose to around 240 volts, the ramp took on a different trajectory and the current  consumption backed way off. What’s going on? Well, this is the drive handling the regen energy  on its own without a resistor attached. But  when the bus voltage got too big for it  to handle without the regen resistor,  the drive switched over to  the dynamic braking method   which dynamically manages the decel ramp to  reduce the regen current which allows the  bus voltage to relax. I say “relax” but look,  the bus voltage got up to over 400 volts! We can prove that’s the dynamic braking  taking over by simply turning it off. I’ll  reset the alarm, and go to parameter  1.032 and disable dynamic braking. Start the scope, ramp up to 3000 RPM, then  switch back to zero rpm. Look, the drive is  free spinning down. I’ll fast forward the video  so we don’t have to wait. I’ll stop the scope  and adjust the display. Sure enough, the drive  tries to consume the regen energy, but when the bus voltage hits around 240 volts, it gave up  and passes deceleration off to dynamic braking. We turned dynamic braking off, so the  motor just free spins down by itself. I’m going to put the regen jumper back  in and run one more trial to make sure  everything is back to normal. Yep. Looks great. But, the bus voltage takes a long time to settle  out – does that mean I have to wait for it to  settle before attempting another deceleration?  No, not at all. I ran several trials in  a row. As you can see, as soon as the acceleration ramp kicks in it consumes all  that voltage to get the drive up to speed. Ok, my drive has a regen resistor built-in.  You can see what the drive expects for a regen resistor to be in parameter 1.052 which  is the resistance in ohms – this drive   is a 100 ohms - and parameter 1.053 which is  the power in watts – we have a 10-watt resistor.  How do I know if that’s the right  value for my application? Or,  if I have a drive without an internal resistor,  how do I know what size resistor to use? You can figure that out by going to this table  in section 2.8 of the user manual. It tells us it takes this much energy to take the motors spindle  by itself from 3000 to 0 RPM. We know our load is  a 50:1 load so we need 50 more of these to account  for that – or a total of 51 of these E zero guys. This chart also tells us that the drive-by itself  can absorb this much energy, so if we subtract   that from the total energy we have to dissipate,  then this much energy is left for us shove into  a regen resistor. Suppose your cycle time is 400  milliseconds. Divide the energy by time and you get this many watts. If you have two deceleration  times during that cycle time – one for the trip  out and one for the trip back for example -  then you need to multiply the energy required, by two. And that’s a good estimate of the minimum  wattage your resistor needs to do this over and over continuously. But consider this a starting  point then add some common sense and testing   to see if it works for you. Just know that if  you run the system with a lot of braking, this  resistor can get hot. And I mean hot to touch, so  beware. If you aren’t running a 100% duty cycle, then you could back off on the requirements  proportionally. Again, use your judgment here. The internal regen resistor is connected  like this. To use an external regen resistor,   pull this jumper, and put the external resistor  here. I happen to have a 91 ohm, two-hundred-watt  resistor laying around. Is it ok to use that?  Well, 200 watts is more than enough, and  according to this chart in the manual, I’m allowed  to use down to 60 ohms, so yeah, 91 ohms is fine. Could you use a combination of resistors  to get whatever value you need?  Sure! Just make sure the combination meets the  minimum resistor ohms and wattage requirements. I’ll plug that into the drive  and modify parameter 1.052 with the new resistance value and parameter  1.053 with the new wattage so the drive knows exactly what it has to work with and  can plan out that linear ramp accordingly.  I’ll start the scope, ramp the  motor up, and then back down again. Zoom in on the deceleration ramp. Looks like  we got basically the same results as before. Almost 9 amps of regen current, bus voltage rose  to around 300 volts and the ramp time is roughly 160 ms all of which is pretty much what we  saw with the built-in resistor. Perfect. One closing thought. Regen braking can create a  lot of energy which gets dissipated as heat. So,  it’s important you have really good heat sinking  and a lot of forced air to help cool things down  especially when relying on the internal braking  resistor. In fact, even if you have a drive with   an internal resistor, you may want to disable  that and use an external resistor instead  just so you can mount it away from the drive  to provide better cooling and heat sinking. Again, use your judgment and do  what’s best for your system. That should give you a good idea of how regen  braking works in a SureServo 2 drive when  decelerating the motor during normal operations. And remember – having regen braking built into  the drive saves you time, footprint in your  cabinet, and money. Just one more reason the SureServo 2 system is such a great value. Click here to learn more about the  SureServo 2 drive and to find more videos like  this from AutomationDirect.com. Click here to  subscribe to our YouTube channel so you get  notified when we publish new videos like this  and click here to learn about AutomationDirect’s  free award-winning support options.