Deceleration Inhibit Fault Tripping VFD

[philly]

We were recently setting up a 480V 1hp Powerflex 70 VFD on a screw conveyor. After running the drive up to full speed we stopped the drive and as the drive was decellaring we noticed that the drive would trip on a "Deceleration Inhibit Fault". After looking in the manual for the cause of the fault is had to do with the DC bus voltage being too high. The DC level was around 730V DC when trip occured.

For starters we looked at the line voltage on the input to the drive and noticed that it was a little high around 500V L-L. This is because drive is on a new system that is not yet fully loaded. When sitting idle the DC bus voltage on the drive was around 690V DC.

When we would run the drive it would run up to full speed with no problem. When we gave it a stop command the drive would start ramping down with no problem, but when the drive go to around 15hZ the bus voltage would start to increase above 700V. When the drive finally got down to about 2Hz the drive would freeze at 2Hz for a while and the frequency would toggle back and forth around 2Hz until the drive finally tripped on the decel trip.

We tried increasing ramp down time, and had the same issue at 2Hz. The drive was in sensorless vector control mode when this happened. We changed drive to V/Hz and it made it worse, so we changed back to sensorless.

Finally after looking in the manual we found that one of the remedy's for this fault was to "disable the bus regulation" on the drive. After disabling the bus regulation this problem went away and we were able to ramp down the drive completely with no issues although the bus voltage would still go high around low speeds.

The other strange thing was that even when we were running the drive and ramped the drive down from 60Hz to 10HZ, the DC bus voltage would increase to around 740V at 10Hz and would not decrease even with the drive running the motor. As soon as we ramped back up to 60Hz this bus voltage decreased back down to 680V.

Any ideas what is causing this high DC bus level at low frequencies? Was disabling the DC bus regulation a good solution, outside of adding an external brake resistor?

 

[StephenSDH]

Sensor-less vector cannot control a motor at low speeds like 2hz. Why does the drive need to ramp down the screw and how long is the ramp down duration?

We changed drive to V/Hz and it made it worse

Same fault?

Was disabling the DC bus regulation a good solution

You are no longer limiting the voltage on the DC Bus, this will cause more voltage stress on the motor and the drive.

 

[jmgill2]

elevator mechanic

elevator mechanic

This is definetly not a good idea. When the motor that you are trying to control is decelerating, it is acting like a generator pumping voltage back onto the drive. Disabling this will lead to too much bus voltage and damage scr's on the drive. The drive should have a dynamic braking circuit that turns on when bus voltage reaches around 680 volts for a 480 volt l-l drive. Your problem lies in this circuit, either the braking is disabled, or the dynamic braking resistors supplied with the drive are not sufficient.

 

[mcclary's electrical]

If the drive had dynamic braking, try adjusting the deceleration time. We installed a soft start with dynamic braking on a 400 hp. The very first time we started it, it started up fine. Ran fine. The 400hp was driving a wood chipper that had a huge inirtia and would free spin for 1 hour after turning off (that's why they wanted the dynamic brake) Long story short. The first time we pushed STOP,,,,,they decel time was set about 2 seconds. THE DRIVE WAS VIBRATING LIKE A MACHINE GUN. The 750 pad mount xfmr about 20 feet away sounded like it was going to jump ff the pad, then it blew a fuse. Moral of the story, get your decel time set. Also...if there's a certain frequecy that's causing problems, you can most likey program drive to skip that frequency when ramping or decel. Also I would recommend just dropping power once you deceled to around 15 htz. A quick ramp to zero from there should be all you need. Don't try to ramp controlled down to real low frequencies.

 

[ptonsparky]

Increase your deceleration time for testing purposes. Once you get a load on the conveyor you may not have a problem with a shorter dec ramp time.

 

[Jraef]

Increase your deceleration time for testing purposes. Once you get a load on the conveyor you may not have a problem with a shorter dec ramp time.

Assuming it was unloaded of course.

The issue is, as others pointed out, the motor is likely regenerating back into the drive when decelerating to a stop. Decel mode is for when you WANT TO EXTEND the deceleration time of a load that would normally stop quicker on its own if left to coast. That is different from Braking, which is when you want to STOP it quicker than it would on its own. So to Decel, the VFD continues to put out a frequency after you tell it to stop and slowly decreases that frequency with the intent of not allowing the motor to coast. If however the motor has little or no load on it, it may continue spinning FASTER than the frequency being applied to it. If that happens, the motor becomes an induction generator. The VFD can absorb a little bit of that excess energy, but not much because it has no place to dump it other than its own capacitors and a small discharge resistor.

What Dynamic Braking is in a VFD, is this action, done on purpose, and then the VFD is given a place to dump that kinetic energy off of the motor, usually a resistor bank. To accomplish this, the VFD has to have an additional transistor switch, called a Chopper Circuit, that fires whenever the DC bus voltage exceeds a specific limit, pumping the excess into the resistor (otherwise the resistor would be glowing all the time).

So you need do do one of three things:
1) As ptonsparky mentioned, if you are testing a screw conveyor unloaded, that may be the only real issue. Screw conveyors with a load on them tend to stop on their own pretty fast because of all the friction. So you may not really have an operational problem, just a testing issue.
2) You don't need Decel at all, just turn it off and let the motor coast to a stop. I can't think of why you would need it on a screw anyway.
3) You need to add a Dynamic Braking unit to this VFD.

Oops, Four; there are FOUR things you could do.
4) Increase the Decel time (assuming you need it for some unknown reason) as suggested.

 

[philly]

Thanks for all the responses guys. I always learn soo much from these great responses.

Yes, as assumed, this screw conveyor is running unloaded and this issue is occuring during testing. I would still think that a screw conveyor is not a high inertia load that would cause regen problems.

So it sounds when stopping a motor with a drive there are three options:

1) Braking - Braking is used when you want to stop the motor faster than it would stop on its own. The drive outputs a freq that is less than the motor is spinning to try to stop the motor. With braking this is where we typically set what is called a "Ramp Down" time which is a time that would be less than the motor would stop on its own. Regen issues can occur during these instances.

2) Deceleration - Deceleration is a form of slowing a motor, that would bring the motor to a stop in a longer time that the motor would typically stop on its own from coasting. When Decelerating, we set what is called a "Deceleration Time" which determines how long the motor will take to stop and is typically longer than the motor coast down time. This form of deceleration still reduces the output freq and can lead to regen if the motor speed is faster than output freq. Is the motor deceleration time different from the motor ramp down time in the drive then? When would you need to have the motor stop in a time longer than it would on its own?

3) The third means of stopping a motor is to let the motor coast to a stop. When doing this the drive releases all output control to the motor, and the motor and load is free to spin on its own in whatever time this takes. During a coast down, the motor is only capable of regening for a couple of seconds while the magnetic field is still present in the rotor of the motor. After a few seconds when the rotor field has decayed, the motor is no longer capable of regenerating, and thus can not regen and cuase a bus voltage increase while it coasts down the rest of the way.

Do I have an understading of these stopping methods or am I just confusing myself even more?

 

[philly]

Sensor-less vector cannot control a motor at low speeds like 2hz. Why does the drive need to ramp down the screw and how long is the ramp down duration?.

Ramp down duration was 10s. We raised to 15s and even 20s and saw the same result.

Same fault?

Yes we got the same fault when in V/Hz mode however this fault occured at a higher frequency. It occured around 10Hz rather than down at 2Hz

You are no longer limiting the voltage on the DC Bus, this will cause more voltage stress on the motor and the drive.

Wont the drive still protect itself by tripping itself on overvoltage at a certain DC bus level. I thought all drives tripped on overvoltage at a certain level to protect themselves?

Why would the drive give the option of disabling bus voltage regulation if it could damage the drive?

Why would the bus level only go really high at the low frequency of 2Hz that we witnessed, as opposed to other frequencies?

Why even when running the drive at low frequencies the bus voltage stays high, and its not until we increase speed on the drive that the bus voltage disipates? Is this because at low frequencies the drive is not outputting as much voltage and therfore as the drive is still producing DC bus voltage there is not enough load voltage to disipate the DC bus level? At higher frequencies does the higher voltage ouput help dissipate the built up voltage on the DC bus?

Should I be looking at installing a brake resistor as opposed to try to deal with this issue with drive paramaters?

 

[Jraef]

Pretty good. You get a cookie...

Minor issues:
A couple of seconds is pretty long for a motor field to collapse, a second is more like it. But even then, there would be no build up of bus voltage in a coast down because you have turned off the output transistors so there is no path for the motor energy to get back into the drive.

Is the motor deceleration time different from the motor ramp down time in the drive then?

Depends on the inertia, friction etc. That's why they make the VFD decel time adjustable Keep in mind the decel time setting also affects how the VFD handles changes in speed as well, not just stopping. Remember your issue of the DC bus voltage climbing when you went from 60 to 10Hz? Same issue.

When would you need to have the motor stop in a time longer than it would on its own?

The biggest application is a pump where you want to eliminate water hammer. Water hammer is cause by the sudden change in energy state of the moving water in a pipe, such as when you shut off a pump and a check valve slams closed. The water can't compress, so the energy gets turned into a shock wave that travels up and down the length of the pipe looking for somewhere to go. It always finds the weakest point eventually. By slowing the motor down gently, you neutralize the pressure in the system that is keeping the check valve open, so it can close slowly and gently and the shock wave is avoided.

Another application is a conveyor where you are moving stacked items, such as into a stretch wrapper. Controlled decel prevents them from toppling.

 

[philly]

Depends on the inertia, friction etc. That's why they make the VFD decel time adjustable Keep in mind the decel time setting also affects how the VFD handles changes in speed as well, not just stopping. Remember your issue of the DC bus voltage climbing when you went from 60 to 10Hz? Same issue.

QUOTE]

So then the ramp down time, and deceleration time are two different settings within the drive? Should only one of these be set depending on which one you are using while the other is disabled?

When going between different frequency setpoints do you use a ramp down time, or a deceleration time?

 

[ptonsparky]

Depends on the inertia, friction etc. That's why they make the VFD decel time adjustable Keep in mind the decel time setting also affects how the VFD handles changes in speed as well, not just stopping. Remember your issue of the DC bus voltage climbing when you went from 60 to 10Hz? Same issue.

QUOTE]

So then the ramp down time, and deceleration time are two different settings within the drive? Should only one of these be set depending on which one you are using while the other is disabled?

When going between different frequency setpoints do you use a ramp down time, or a deceleration time?

No, they mean the same thing. When you set the ramp time it uses that time to acc from say 0 to 15 hz. Then uses that same ramp time to acc from 15 to 17.5 if you change the speed request. It will also use that time to go from 0-60. A ramp down from 60 to 50 may not cause you grief if the time is set for 3 seconds but may when you ask it to go from 60 to 0 in the same 3 seconds. Even the cheaper drives I have used offer a 2nd ramp time but I have never had the need to use it.

 

[philly]

No, they mean the same thing. When you set the ramp time it uses that time to acc from say 0 to 15 hz. Then uses that same ramp time to acc from 15 to 17.5 if you change the speed request. It will also use that time to go from 0-60. A ramp down from 60 to 50 may not cause you grief if the time is set for 3 seconds but may when you ask it to go from 60 to 0 in the same 3 seconds. Even the cheaper drives I have used offer a 2nd ramp time but I have never had the need to use it.

O.k. so the selected time is the same for both, however they are referred to as different things, (braking, and deceleration) depending on how the selected time relates to the time it takes for the motor/load to stop on its own?

 

[ptonsparky]

The accel time and decel time are separate parameters. They may be turned on or off and may or not be linear depending on how you set them up.

 

[philly]

I'm really curious as to why this bus overvolage problem would be attenuated as low frequencies, and when operating at low frequencies the bus voltage stays high even with motor running. Not unitl ouput frequency is increased does the DC bus voltage decrease down to normal levels?

Is this due the the low voltage output at reduced frequencies?

 

[Besoeker]

I'm really curious as to why this bus overvolage problem would be attenuated as low frequencies, and when operating at low frequencies the bus voltage stays high even with motor running. Not unitl ouput frequency is increased does the DC bus voltage decrease down to normal levels?

Is this due the the low voltage output at reduced frequencies?

I would think more likely to be a combination of the reduced loading on the supply resulting in lower voltage drop (so higher voltage) and reduced loading on the DC link allowing the DC link to charge closer to peak than mean DC voltage.
You may also have some noise on the supply and, at light load*, the bucket capacitor absorbs this resulting in a voltage increase.
*At 2 Hz it is very light load on both the supply and the capacitor.

 

[Jraef]

I'm really curious as to why this bus overvolage problem would be attenuated as low frequencies, and when operating at low frequencies the bus voltage stays high even with motor running. Not unitl ouput frequency is increased does the DC bus voltage decrease down to normal levels?

Is this due the the low voltage output at reduced frequencies?

It's not the protection that's turned off. DC Bus Regulation is not an industry standard term I am familiar with, I think it's an A-B feature. Ironically, from the description it appears as though it is supposed to prevent the very think that is happening, so I think it's interesting that their tech support says to disable it!

BUS REGULATION:
DC Bus regulation is available to reduce the possibility of overvoltage trips due to
regenerative conditions. The reaction to a bus voltage increase is programmable as follows.

• Disabled: faults on Overvoltage Fault
• Adjust Frequency : adjusts the output frequency to maintain bus voltage at a
• predetermined regulation level.
• Dynamic Brake: dissipates the excess energy on the bus through the internal DB
• chopper and connected resistor.
• Both ? DB 1st begins by actuating the dynamic brake. If more response is
• needed, it will then adjust the output frequency.
• Both ? FRQ 1st begins by adjusting the output frequency. If more response is
• needed, the it will then actuate the dynamic brake
• Alternating between two of the above modes during operation is accomplished through
• digital input functions

 

[philly]

It's not the protection that's turned off. DC Bus Regulation is not an industry standard term I am familiar with, I think it's an A-B feature. Ironically, from the description it appears as though it is supposed to prevent the very think that is happening, so I think it's interesting that their tech support says to disable it!

So do you recommend not disabling the DC bus regulation?

 

[philly]

I would think more likely to be a combination of the reduced loading on the supply resulting in lower voltage drop (so higher voltage) and reduced loading on the DC link allowing the DC link to charge closer to peak than mean DC voltage.

Very interesting. Can you explain this?