PowerFlex 700 Stopping issues

[Twistertime 0]

I am working on a powerflex 700 AC drive as they are having issues with the stopping control of the drive. The drive works fine on startup and during running but we are having issues when it stops. If I depress the Stop button, it decels with DC braking and stops within the 2 sec. decel time, no faults and all is as it should be.The next time I depress the Stop Button, it coasts to a stop after about 10 seconds.It is always every other time that it coasts. If I watch the HIM, one time it states DC braking when all is well and then the drive goes to Not Enabled when the drive is operating properly. The next time I depress the stop button after a run condition , the drive says Stopped for about 3 seconds then the display reads Not Enabled but the drive is still coasting to a stop. There are never any drive faults or alarms. This has never been right and has acted this way since the install.If I look at drive status 1, the Active bit goes out immediately when the drive coasts yet the active bit stays on until the drive is Not Enabled after a good stop. There are 2 inputs wired to the digital inputs, one is a start button and it is on Digital Input 2 and is programmed for run forward. The next digital input is wired to Digital input #6 and it is an enable from the PILZ relay after all is picked up. This enable stays high after the stop condition for 3 seconds as set in the PILZ relay. Lastly, I have never checked the Hardware enable jumper on the side of the card, is it possible this has been removed whenit should have been left installed? Removing the board is a bit of a pain but if need be...This is a safety concern and Any help would be appreciated. .

[paulengr 44]

Any time someone calls something a "safety concern", it usually is an excuse for "we must get this fixed immediately" problem, whether it's really a safety concern or not. It always raises my eyebrows and my immediate response is "If that's so, what injury will occur if it doesn't work? Second, was this addressed in the risk assessment or do we need to re-evaluate it? Third, how much money are you willing to spend money to make it right?" Usually I get the "safey concern" crap from production all the time when they don't really mean it, can't provide adequate evidence for what they are claiming, and don't want to spend their own money and political capital to fix it. They just use that as a weapon to try to force maintenance into rushing into fixing it. Not sure what your problem is but right off the top of my head, the PILZ relay should be wired directly to the hardware enable input to control it, and the "ENBL" jumper (which disables the ability to disable the hardware enable through software) should be disabled. This is not the same as a safety rated system because you don't have a true safety rated input to the drive but it is pretty darn close because the hardware enable wiring cuts the logic signals to the IGBT gates. I haven't done the calculations but this is probably somewhere between SIL 1 and SIL 2 but it is not going to rate a SIL 2 (for those doing event trees, between 10^-1 and 10^-2 probability of dangerous failure on demand). A true safe off function (in the 700S) uses two different techniques (cut both control signals and gate power) and two different inputs (redundancy). This gives you SIL 3 (aka control reliable, 10^-3 probability of dangerous failure on demand) disabling of the drive. A safety circuit is not a safety circuit just because you used a safety relay if you used an ordinary (non-safety rated) device at the other end, unless you use an event tree analysis to verify that it meets the risk assessment criteria that you've determined. This would be akin to wiring up a safety relay to your PLC and then using the PLC's "run" output signal to stop the drive. Nice attempt but the safety relay would fail to do anything except lower the probability of failure. The reason for using the safety relay in the first place is to eliminate the PLC and drive software as potential points of failure. Note that a ControlLogix PLC CAN achieve SIL 2 on it's own if you follow AB's document on how to design your system for that rating, which eliminates the need for a safety relay in many cases. It does NOT give you control reliable (SIL 3). For that you need either safety relays or Safety PLC's, but for the money, a safety relay system is usually far less expensive. I suggest you check your drive stop parameters carefully. Check what type of STOP is being executed since one is a deceleration to stop and the other is coast-to-stop. You are attempting to execute an emergency stop under power based on your description. This is really bad if it's a safety concern. A coast-to-stop should be adequate for the configuration if that's adequate for an emergency stop. If not, you MUST use a mechanical brake. If you can't tolerate that, then you need to add both a UPS to hold the power up long enough to execute a stop (with redundancy around the UPS), and a safety rated drive (700S, or go to a motion control system). Third, you are using DC braking. This severely heats up the motor. You didn't say how long you are waiting between stop 1 and stop 2 but if you are doing them in rapid succession then what's probably happening is that on the first stop, you are raising the thermal levels in the motor to such a degree that by the time you execute the second stop, the drive can't use DC braking anymore because of thermal overload. Now you probably have an error second state but you have your safety circuit programmed in a very strange way. Standard procedure is that the safety circuit triggers only on demand. You have it programmed to trigger all the time (on every stop). So restarting the drive is a two-motion operation (hit reset, then hit start). Aside from the fact that this throws you from an "on demand" to "continuous" regime in terms of safety systems, it is probably blanking the error message so that you never see why the drive is failing to stop as expected...so it might be tripping due to thermal overload but then the enable gets cut off and the drive is forced to shut down for safety reasons and the error message never appears long enough to see it. It sounds to me like you've confused the terms "stop" and "emergency stop". They can and should be two totally different functions. An emergency (or safety) stop is used to stop due to human life being in danger. In this situation the goal is to stop as quickly as possible no whether the equipment is destroyed or not. At least according to NFPA 79 (and almost all other safety codes) once you trigger an emergency stopping procedure, the stop should be maintained continuously and some sort of manual reset procedure should be necessary to clear the stop. Most of these reset circuits also have a second series signal going through all the safety relays that detects failed relay contacts (since safety relays always have two sets of contacts for redundancy) in the event that either set of contacts has failed, which prevents the safety circuit from being reset. Sounds like you are auto-resetting and not using the safety relay failure detection circuit which is not good. The normal stop is for everything else. Since you are dropping the enable out, clearly, you're not using the normal stop at all. I suggest you get rid of the DC braking. Instead configure the motor for flux vector control mode. This will give you much better starting and stopping overall because the drive will then brake using regenerative braking (using the motor/drive as a generator to pump power back into the mains). Since it regens, you aren't dumping a lot of heat into the motor. DC braking is only marginally faster than regenerative braking if at all, and adds a lot of heat to the motor. If this still isn't fast enough, add an encoder to the drive and use full vector control. If this still isn't fast enough, add a Stearns brake to the motor shaft. Mechanical braking is the fastest way to stop a motor in all cases but mechanically less reliable (there's something else to break) and less energy efficient. Still, you'd be amazed at just how fast a full vector controlled drive can actually stop. Edited 12 Nov 2009 by paulengr

[Eddie Willers 2]

You had to include that last line, didn't you ? I agree that it is likely that the hardware enable jumper is removed, which is correct if Digital Input #6 is used as a non-programmable Enable. See page 1-21 of the User Manual for details on that location and function. You mentioned a Stop button, but did not describe a Stop Input defined in the Digital Inputs on the drive but rather a "Run Forward" whose absence is interpreted by the drive as a Stop. So, the Stop button probably goes to some other circuit, maybe the safety relay, maybe a PLC input. Get a wiring diagram and trace out all the signals visually and electrically so that you understand how the system has been implemented. It sounds like your system mixes a functional "Stop" button with a safety circuit that might be intended for an Emergency Stop. The description of the Digital Input 6 function on page 3-55 of the User Manual says: "Opening an “Enable” input will cause the motor to coast-to-stop, ignoring any programmed Stop modes."

[Eddie Willers 2]

As always, Paul, you provide a truckload of knowledge. Me, I just got a new sig line.

[BobLfoot 302]

AMEN Paul - Thats a grand slam home run and one I'll remember to use next time I'm summoned to a "Safety Emergency" by my shift manager.

[Twistertime 0]

Gentlemen, let me start off by thanking you for your fast replies and secondly, apologizing for using the wording "safety concern". I will give you a little background on this sytem and myself. I am an electrical technician in a local factory and mainly work on Allen Bradley 1394 drives( just starting to get some Control Motion stuff now). I maintain these drives from a hardware and software stand point but have never designed a system. Because of my background ( although limited), a local skihill in my town had asked me to help them out with an issue they have with a drive that was installed a while ago. The hill is a break even setup and is very small with not alot of revenue to work with, I will not be getting paid for this help, strictly volunteer work . The reason I stated it as being a safety concern is that the drive controls a continuous tow bar on a cable with about 30 handles, the small kids hold onto these cables as they are being pulled up a small hill on their skiis. The children grab onto this handle at the bottom of the hill and let go at the top and ski away , the only time the tow stops is when a stop button is pressed at the top or bottom of the tow or if the safety gate is opened as the tow rope comes around the top pulley to come back around. The reason I called it a safety concern was that there are children involved and it should stop very quickly if a situation arose. On that note, although it is a concern this tow will not see any kids in the next month, so I have time to work on it As mentioned earlier, there are 3 stop items that will open up the Input circuit of the PILZ PNOZ XV2 , 2 stop buttons and a safety gate switch. There is a 3 second off delay on the output from the PILZ that is the Enable that goes to Digital input 6 of the powerflex. There is also another off delay output on the PILZ that drops out the contactor for the 3 phase input to the Powerflex. Lastly, there is another Non delayed output from the pilz that is in series with the start button, which picks up a self latched relay which goes to digital input #2 of the powerflex and it is programmed as Run Forward. Therefore, the way the drive drops out is when the Pilz input drops out ( one of the 3 stop devices), which in turn drops out the pilz instant output, the Run Forward bit. 3 seconds later the drive enable to the powerflex and the contactor for the 3 phase opens up. To reply to one of the previous questions asked, these stops are in quick succession and it is always the opposite condition each. If we do 2 or 10, it always alternates on the stop, fast or coast.I have called the operator at the hill and asked him to do a test to see if he could wait a half hour between stops to see if it changes. Next steps for me are to look at the jumper setting and investigate the flux setup. I will not be able to work on this equipment in the next few days but I will post after I continue to troubleshoot and act on your suggestions. Thanks for your help and I look forward to any further suggestions.

[Eddie Willers 2]

You are in a similar situation as Forum members, but volunteerism, or even anonymity, are not shield from liability. Good for you for trying to help, and I strongly recommend that you provide signed and dated copies of diagrams (number every page), parameter settings, and descriptions of what you did to the system. As Paul mentioned, once a safety relay is tripped, it must be reset. Is there a separate Reset button for the safety relay ? Read the information in the PowerFlex 700 User Manual on page 1-13 about Input and Output contactors. I think that cycling the input power for this drive is going to wear it out prematurely. Your description of the PowerFlex 700's HIM indications sound like it is alternating between "DC Braking" and "Coast To Stop" stop modes. When a DC Braking or Ramp to Stop mode is stopping the drive, the HIM module will show "DC Braking" or "Decelerating" and then "Stopped" when the drive has actually stopped. When a Coast to Stop mode is selected, the drive will show "Stopped" immedately, because the drive is no longer participating in the stop of the motor, even if the motor is coasting. The fact that both stop modes are followed by a "not enabled" is consistent with the PILZ relay removing the Digital Input 6 Enable signal after 3 seconds. Check all the Digital Inputs to be sure only 2 and 6 are connected, and check to see if any of them are programmed for "Stop Mode B". The "Stop Owner" parameter might also give you some information about why the stop mode appears to be alternating.

[Twistertime 0]

Thanks for the heads up on the liability. I will tread lightly and document any potential changes. If this is ever figured out then I will provide the suggestions to the skihill staff and then let them proceed through the proper channels to make sure the changes are made by a certified drive expert and certification facility. A volunteer does not want to loose his shirt for what he thinks is a good deed! As far as a few questions you have: There is a reset and this is in the same pushbutton stations as the stop buttons. The reset is wired to the pilz reset. Once the stop condition is cleared, then the reset button is depressed and the pilz picks up and this gates the 24v enable that goes to digital in #6. I originally checked and no other digital inputs are programmed ( not sure about wired but I will check, print only shows the 2), just the start (latched up run bit) from the pilz to digital in 2 and this is programmed as run forward. The only other bit is Digital Input 6 which is the delay off enable from the pilz, no stop b is programmed. This weekend I will check the stop owner bits to see what is controlling the stop condition. I will also check the enable jumper. Thanks again for the suggestions. I will keep you posted.

[Twistertime 0]

There are a few observations that I will provide that I documented while watching the drive while in the run and stop mode. Remember one time the drive is in DC Braking and the next is Stopped and a coast situation 1) Him Display Screen and Parameter 209 When the drive is in DC Braking the Drive Status Bit #1 (active) does not drop out until the enable drops out after 3 seconds(pilz off delay) .The drive stops in about 2 seconds When the drive coasts ( Him displays stopped) the Drive Status Bit #1 (active) drops out as soon as the stop button is depressed and a coasting situation occurs. 2) Parameter 215 Last Stop Source When the drive stops normally( Dc Braking) the Last stop Source code is 9, Not enabled When the drive coasts the Last Stop Source is code 7 Digital In. 3) If the 24 volt control power is cycled and the drive powers off, on restart it will always be a DC braking controlled stop the first time the drive is run and stopped? 4) I changed the stop mode from DC Braking to Ramp mode with a 2.5 sec. decel time. Everytime, the drive went in to a decellerating mode and stopped in 2.5 to 3 seconds. The problem was the DC Bus went up to 990 VDC. The Bus is 880 VDC in normal run mode and stays at about that in DC Braking. When the enable drops out then the DC Bus drops to about 350 VDC. There is a Dynamic Brake installed and the Bus Reg Mode A and B are set to Dynamic brake /Adjust Freq and also Set for external resistor. The Regen power limit does not display because the Motor is set for Sensorless Vector and not FVC Vector? Does this mean that the dynamic brake is not used at all? Thanks to all that read and/or reply to this post. Edited 17 Nov 2009 by Twistertime