PFLX70 Dynamic Brake parameters

unsaint32

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I read in one of PFLX70 manuals that if using dynamic brake, the P155 Stop/Brk Mode has to be set for "Ramp". I am attaching a screenshot of the list of PFLX 70 parameters (from Logix Studio). As you can see, P155 is set for "DC Brake" even though Dynamic brake is used.

There is a dynamic brake resistor on the top of the motor control cabinet which houses the PFLX70.

This set up is not new. This machine had been running for years with the parameters settings. Is this a wrong parameter configuration? Does this simply mean that the dynamic brake is not being used? If we are not using the DB, then shouldn't the Bus Reg mode be set for "Adjust Freq" or "disabled"?

Thanks.

dynamic brake.jpg
 
It may not be "wrong". If the drive is configured with ramp to stop, then the dynamic (snubber is a better term) brake is in effect. If the drive is configured with coast to stop, then the DC injection brake would be in effect. It may be possible to have the drive capable of two different selectable configurations, on with ramp and the other with coast to stop. Then both would be needed side by side.

I'm not familiar enough with the PF70 but it might be configured with snubber braking to stop on a ramp and then DC injection used to hold the motor in position for a short period. Usually that is called DC Hold but AB might not differentiate between the two.
 
It may not be "wrong". If the drive is configured with ramp to stop, then the dynamic (snubber is a better term) brake is in effect. If the drive is configured with coast to stop, then the DC injection brake would be in effect. It may be possible to have the drive capable of two different selectable configurations, on with ramp and the other with coast to stop. Then both would be needed side by side.

I'm not familiar enough with the PF70 but it might be configured with snubber braking to stop on a ramp and then DC injection used to hold the motor in position for a short period. Usually that is called DC Hold but AB might not differentiate between the two.
What do you mean by "both would be needed side by side."? Are you saying that the Stop/Brk Mode A selection can be programmed to change in the middle of a process?
 
I read in one of PFLX70 manuals that if using dynamic brake, the P155 Stop/Brk Mode has to be set for "Ramp". I am attaching a screenshot of the list of PFLX 70 parameters (from Logix Studio). As you can see, P155 is set for "DC Brake" even though Dynamic brake is used.

There is a dynamic brake resistor on the top of the motor control cabinet which houses the PFLX70.

This set up is not new. This machine had been running for years with the parameters settings. Is this a wrong parameter configuration? Does this simply mean that the dynamic brake is not being used? If we are not using the DB, then shouldn't the Bus Reg mode be set for "Adjust Freq" or "disabled"?

Thanks.

I'm thinking you may not understand what the dynamic braking resistor does.

During the process of ramping down the motor can turn into a generator and pump energy back into the drive. This makes the voltage on the DC bus caps rise. If the voltage rises above a safe level the drive will shutdown. To keep this from happening we use braking resistors to dissipate the excess energy.

The drive can handle small amounts of regeneration from the motor without a braking resistor.

I'm suspecting in your case the original intent may be for two different reasons.

1. The installer always installed braking resistors on all of his installs.

2. Sometimes transients on the incoming power can trip a VFD on overvoltage, and a dynamic brake resistor can bleed off a limited amount of this overvoltage.
 
I'm thinking you may not understand what the dynamic braking resistor does.

During the process of ramping down the motor can turn into a generator and pump energy back into the drive. This makes the voltage on the DC bus caps rise. If the voltage rises above a safe level the drive will shutdown. To keep this from happening we use braking resistors to dissipate the excess energy.

The drive can handle small amounts of regeneration from the motor without a braking resistor.

I'm suspecting in your case the original intent may be for two different reasons.

1. The installer always installed braking resistors on all of his installs.

2. Sometimes transients on the incoming power can trip a VFD on overvoltage, and a dynamic brake resistor can bleed off a limited amount of this overvoltage.

So, is it correct to say "if using dynamic brake, the P155 Stop/Brk Mode must
be set for "Ramp." ?
 
unsaint32

In your original post you indicated

This set up is not new. This machine had been running for years with the parameters settings.

If you needed a different stopping mode then your next comment might bear some weight.

Is this a wrong parameter configuration?
Thanks.

As I mentioned in my previous post, there are times when regeneration is not the cause of a overvoltage trip. Sometimes a brake resistor can keep a drive that is seeing transients on the incoming power from tripping. The manner your drive is configured would prevent small to mid size transients on incoming power from tripping the drive on overvoltage.
 
Dynamic or snubber braking requires a ramp stop. DC injection to hold a load stationary (that's DC Hold, not DC Braking) can be used at the end of the ramp-to-stop. In that case, both snubber braking and DC injection would be programmed on the same drive. Note that they are not ever used simultaneously.

As mentioned above, it is unlikely but possible that the snubber resistors are there to suppress input power transients only. In that case, the drive might be configured with coast to stop or even DC injection braking to stop.

But, to repeat, you cannot apply AC power and DC power to the same motor at the same time. Component failure or drive fault will occur the instant that happens.
 
Dynamic or snubber braking requires a ramp stop. DC injection to hold a load stationary (that's DC Hold, not DC Braking) can be used at the end of the ramp-to-stop. In that case, both snubber braking and DC injection would be programmed on the same drive.

How do I change the stop/brk mode from ramp-stop to DC injection within a process cycle? I don't think there is a parameter for stop/brk mode source selection that can be programmed to change at a certain point of a process cycle. Is there?
 
Dynamic or snubber braking requires a ramp stop. DC injection to hold a load stationary (that's DC Hold, not DC Braking) can be used at the end of the ramp-to-stop. In that case, both snubber braking and DC injection would be programmed on the same drive. Note that they are not ever used simultaneously.

DickDV, I thought a drive stop mode is set and fixed until someone changes that. But what you said above sounds to me like different stop modes (ramp & DC for example) can be programmed for one drive so that the drive stop mode can change at different stages of a process automatically. That really sounds good, but is that indeed what you are saying? Thanks.
 
There are some drives (not every drive) that give you the capability of mapping the stop control to an external digital input. For example, a zero on the input would give you a coast to stop while a one on the input would give you ramp to stop.

This can be very handy but you have to look for this feature in the drive when you choose the brand and model.
 
This set up is not new. This machine had been running for years with the parameters settings. Thanks.

What is the VFD application?

You are looking at it from the inside-out. Try looking at the whole application from the outside in.
What does the VFD drive?

If the load is a continuously running, then braking does not come into play.

If the motor has a large momentum mass that it must stop, like a vertical elevator car coming down, then yes, the excess energy needs to be dissipated in the form of heat to an external resistor

Get back to the "ROOT CAUSE". If all is well, then you would not be looking at drive parameters.

What problems are you experiencing, after years of successful operation?
 
Ok, time to expand on milldrone's response by mixing DickDVs in with a smattering of specifics to the PF70. It's a long one, but I have the time since I'm at home recuperating after surgery.

In milldrone's response, I think he nailed it as to the possible reason why someone would install a DB resistor and yet not use the DB function. In fact that is a highly likely reason. However, we don't know for sure, because we can't read the mind of the original designer, nor can we see the application. So unless you know FOR SURE that something is wrong, I suggest not messing with it. Now the details.

As DickDV said, there are different modes of electronically braking a load once you have a VFD (although these can also be stand-alone devices, they are typically built-in to most VFDs); Dynamic Braking or DB, and DC Injection Braking or DCIB. But first lets establish that when braking any moving load there is kinetic energy in that load, so because energy cannot be created or destroyed, only transferred, braking involves converting (transmuting) that kinetic energy into some more controllable form. if you have a mechanical braking system, such as on a car, that kinetic energy is transmuted into friction and the heat it creates, and of course wears out the brakes. Electronic braking is the alternative. My wife has a Gen 1 Prius with 280,000 miles on it, we have never had a brake job done yet. That's why industry likes to use electronic braking if they can, and when you already have a VFD, you have it.

With DB, the motor is turned into an induction generator, then the kinetic energy in the load is transmuted into electrical energy and dumped off somewhere. That "somewhere" can be resistors, which transmute it again into heat outside of the system where it is safely radiated, or in the case of a "regenerative" drive, used to power other drives or loads. But to start with, turning an induction motor into an induction generator means you have some specific issues to deal with. For a motor to be a generator, it must have power applied to it, because there are no permanent magnets, they are all electromagnets, this is referred to as "excitation energy". The other necessary component is that the rotor of the generator must be turning faster than the relative rotational frequency of the stator, referred to as "over hauling". So using the frequency control capabilities of the VFD, the stator remains energized so that the motor receives excitation, then the frequency is ramped down in such a way as to ALWAYS be lower than the rotational speed of the rotor, which is connected to the load, so the load is always kept in an over hauling state. The VFD doesn't necessarilly need to actually know the speed of the load, it just watches the DC bus voltage and if it begins to drop, it lowers the stator frequency more and more to keep the energy flow going back INTO the drive. So of you notice my highlight, this can ONLY be accomplished by having the VFD in "ramp mode" for stopping. The other choices involve REMOVING power from the output (coasting), or applying DC only from it, our next form of braking. Neither of those will turn the motor into a generator. The advantage of DB is on being able to very quickly move that kinetic energy out of the motor and load, and put it somewhere else safely. The disadvantage is that it suffers from the law of diminishing returns; the slower the load gets, the less energy remains and the less braking power you have. In fact, a DB braking system alone can never "finish the job" of coming to a complete stop. In the end, it either ends up coasting, or you need some other form of braking. If you have mechanical brakes, you engage them, and because most of the kinetic energy is already gone, they get very little wear. That's what the Prius does. But if you DON'T have a mechanical brake, enter DCIB.

With DCIB, the output of the VFD is changed to be DC only, applied to two poles. This creates a stationary magnetic field in the stator. As the rotor passes through it, it still has its fields induced, but now they are going to be counter rotating to the direction of the load. That will pull the rotor into a stationary position in line with the stator DC field and the load stops. The advantage to this is that it can finish the job, bring it to a complete stop and even hold it there temporarily. The disadvantage is, all that kinetic energy is trapped as heat INSIDE OF THE MOTOR. So DCIB has a relatively severely limited duty cycle in that every braking operation looks the same, thermally, as a starting cycle as far as the motor thermal damage curve is concerned. So the rule of thumb is, when using DCIB, cut the motor Starts-Per-Hour rating in half. For small motors rated at 20 Starts-Per-Hour, might not be a problem but for large motors that might be rated for 2 Starts-Per-Hour, you can be forced to wait up to an hour before restarting it depending on when it was started and braked. In addition, full braking torque requires full current, including what would have been STARTING current had the motor been started Across-The-Line, and unless the VFD is over sized by about 4X, it's not likely to be able to deliver that. So instead, the VFD provides a lower level of DC current, what the transistors can safely handle, which translates to longer stopping times.

So the "perfect" combination to stopping a load with a VFD is to remove MOST of the kinetic energy with DB and put it somewhere else, then FINISH the job with DCIB once you lose the effectiveness of the DB action. For THAT reason, DCIB is usually programmed to engage at a programmable trigger speed, i.e. the final 10-30% of rotational speed after DB is through with it. Using this, there is usually no reason to need to select DCIB with an input, because once enabled, you must then also program the trigger speed and current values you want, which makes it automatic. But remember, the VFD is not capable of more continuous current than the power devices are rated for. So if for example you set the trigger speed for 100% of the max speed, the instant you hit the stop button, the VFD is braking the load. But if you have the DCIB current set for 300%, the drive can only deliver that for a second or so, and will disengage the DCIB to protect itself. If you set the current limit to 100% it can do that all day, but your load may take a long time to stop. If you are using DB first, the Stop mode is set to Ramp, which keeps the motor powered, you set the Decel ramp for the stop time you want, and again within the limits of the VFD components, it pumps the energy off into the resistors and when it gets to the trigger speed of the DCIB, the ramp is turned off and the DC is injected, finishing the job.

Now the specifics of the PF70 as it relates to milldrone's theory. The PF70 has a feature called DC Bus Regulation in which it will attempt to deal with excess DC bus voltage in the way you program it to. The excess bus voltage can come from line voltage swings, but also from the load, most famously from eccentric loads like pump jacks and vibrating machines, where an eccentric weight requires motoring as the weight is lifted, then as it falls, the motor regenerates. So this is DIFFERENT from braking in that it is for use when RUNNING, not stopping, but the same issues remain; what to do with the energy. One option on DC Bus Reg is to have the drive modulate the frequency to prevent it from happening; don't allow the motor to become a generator by not allowing negative slip. But that is difficult to implement with constantly changing loads and can't help with line voltage surges anyway. So the other option is to dump into the resistors as needed WHILE RUNNING, not just when braking. THAT is what your programming is indicating was the choice here. Most likely the load did NOT need Dynamic Braking because coasting o a stop might happen quickly anyway, but the still wanted to ensure it completely stops, so the DCIB is triggered at some low speed. But for other reasons, they have the DC Bus Reg programmed to utilize the resistors as a means to dissipate any excess DC bus voltage.

So again, unless you can get into the mind of whomever made these decisions, I suggest not messing with anything that has been working fine for years. Now hopefully you know why.
 
Nice job on that, jraef. Always seems "simple" is just not possible, there's always those other possibilities. Takes years of getting your nose bumped to gain a full spectrum of understanding in any field, certainly VFD's.

Hope whatever surgery you just had fixes what it was intended for and especially that you will heal up promptly. A double dose of patience usually helps the healing process.

Best wishes, dickdv
 
Most likely the load did NOT need Dynamic Braking because coasting o a stop might happen quickly anyway, but the still wanted to ensure it completely stops, so the DCIB is triggered at some low speed. But for other reasons, they have the DC Bus Reg programmed to utilize the resistors as a means to dissipate any excess DC bus voltage.
This application is for uncoiling a giant spool of plastic tubing, and it stars and stops every two minutes as the uncoiling tubing gets cut and packaged. After the original posting, it made sense to me to use the DCIB, since the reel must stop on a dime in order not to give away too much free tubing to the customers (each second of ramp decel would probably dispense 3 feet of tubing). But I still didn't get purpose of the external resistor. After reading your explanation, I checked the parameter 161 Bus Reg Mode A. It was set for Dynamic Brake, which I assume to mean the external reistors, and it is used during the run for the DC bus regulation. Thanks for the education. I wish you fast recovery.
 

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