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Old 12-06-2018, 05:08 PM   #1
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Default Over voltage Fault

I'm using a Benshaw S4 150 VFD to drive a 125 HP motor for a large hammer mill. The hammer mill wheel itself is pretty massive.

I get the over-voltage fault if I try to decelerate to quickly. Using the arrow keys on the front of the drive, I seem to be ok holding the arrow down for 4 seconds and letting go but 5 seconds causes the fault. I guess the 5 second point is where the VFD Freq control moves from counting in tenths to ones. In order to decelerate to the target freq without fault, I need to hold the button for 4 seconds at a time and no longer.

Also, I have the VFD set to "Catch on the fly" but when trying to restart on the fly, the drive will O.V. fault most of the time and I have to reset the fault and try again until it finally catches the rotor and starts to accelerate again.

My accel ramp is currently 10 and I've gone as high as 90 seconds on the decel ramp but no help.

According to tech support, the mass of the hammer wheel is just too much for the internal DB Resistor and I'll need to install an external unit but I figure that it couldn't hurt to post here and get some input.

Other than this issue, the mill works fine.

Thanks,
Jay
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Old 12-06-2018, 06:45 PM   #2
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do you really need to control the de-acceleration or could you just let it coast. Does it matter if it spins up in 10 seconds or 5 minutes. I just do not see that you require a tight speed control on a hammer mill.

There must be something wrong with catch on the fly as technically if you are slowing the load you are not trying to catch on the fly (unless you are doing this after tripping out).

I do not use Benshaw but i know others will be along soon that can help you program the drive to get it to behave (allow the drive more control based on dc buss voltage and other tricks)
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Old 12-06-2018, 06:56 PM   #3
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A motor is a generator when it's stopping.

@JRaef is our jedi master of these things. Hopefully he will be along.
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Old 12-06-2018, 07:23 PM   #4
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Quote:
Originally Posted by Jay Freeman View Post
I'm using a Benshaw S4 150 VFD to drive a 125 HP motor for a large hammer mill. The hammer mill wheel itself is pretty massive.

I get the over-voltage fault if I try to decelerate to quickly. Using the arrow keys on the front of the drive, I seem to be ok holding the arrow down for 4 seconds and letting go but 5 seconds causes the fault. I guess the 5 second point is where the VFD Freq control moves from counting in tenths to ones. In order to decelerate to the target freq without fault, I need to hold the button for 4 seconds at a time and no longer.

Also, I have the VFD set to "Catch on the fly" but when trying to restart on the fly, the drive will O.V. fault most of the time and I have to reset the fault and try again until it finally catches the rotor and starts to accelerate again.

My accel ramp is currently 10 and I've gone as high as 90 seconds on the decel ramp but no help.

According to tech support, the mass of the hammer wheel is just too much for the internal DB Resistor and I'll need to install an external unit but I figure that it couldn't hurt to post here and get some input.

Other than this issue, the mill works fine.

Thanks,
Jay
Not familiar with brand, but I have seen regenerative voltage from the inertia of high mass loads cause similar problems. External resistors were the typical solution.
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Old 12-06-2018, 07:40 PM   #5
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do you really need to control the de-acceleration or could you just let it coast. Does it matter if it spins up in 10 seconds or 5 minutes.
I agree, the time it takes to decel doesn't matter. The decel was set to 5 seconds before I started working on it. I set it to 90 seconds as an experiment but there was no real change. The drive trips out after holding the decel button for 5 seconds. It seems that 90 seconds should be more than enough time. I don't see how setting the ramp to 5 minutes would help but I could try it.

The "Catch on the fly" that I was trying was after the drive tripped and I was restarting it. For some reason, even though the drive is set to CTS, it seems to be trying to bring the motor down to 0 hz and then spin it back up instead of catching on the fly. The drive will fault out will trying to catch on the fly too. There are times when it will catch on the fly though.

However, if I could solve the High Voltage fault problem and be able to decel without issue, the "Catch on the fly" problem wouldn't really come into play.

All we do is start the mill, adjust the speed, and stop the mill. Even though the VFD is sensorless vector, it's set to Linear Auto.

Last edited by Jay Freeman; 12-06-2018 at 07:46 PM.
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Old 12-06-2018, 08:42 PM   #6
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If stopping time does not matter- set VFD to "coast to stop". Otherwise you will need a HUGE breaking resister to burn off the regenerated power. Set your accel ramp to several minutes. Maybe at least 6 to 8 minutes to get the mass up to speed. The accel ramp time also controls the time to change speed, say 50% to 65%. The deccel time will need to be several minutes also to prevent over voltage trips from regen on speed changes.

The "flywheel effect"of the mill mass is a generator during a speed decrease. Something must consume this excess energy - either a breaking resistor or a full regen VFD. A regen VFD in a 125 HP would be really expensive.
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Old 12-06-2018, 10:46 PM   #7
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Originally Posted by varmit View Post
If stopping time does not matter- set VFD to "coast to stop". Otherwise you will need a HUGE breaking resister to burn off the regenerated power. Set your accel ramp to several minutes. Maybe at least 6 to 8 minutes to get the mass up to speed. The accel ramp time also controls the time to change speed, say 50% to 65%. The deccel time will need to be several minutes also to prevent over voltage trips from regen on speed changes.

The "flywheel effect"of the mill mass is a generator during a speed decrease. Something must consume this excess energy - either a breaking resistor or a full regen VFD. A regen VFD in a 125 HP would be really expensive.
Bingo. On a high inertia load, the load inertia will turn the motor into a generator if you attempt to have a controlled Decel without adequate braking dissipation capacity.


In a VFD, there are 3 kinds of "stopping";

1) Coasting, which means just taking energy off of the motor and letting it spin down on its own



2) Controlled Decel, which means you want the drive to force the deceleration to be LONGER than it would coast


3) Braking, in which case you want the deceleration to be SHORTER than the natural coast time.


By setting a "decel time" you are creating a situation wherein you are choosing option 2 or 3, it will no longer just coast. If there is high friction that WANTS to stop quicker than you would like, then Decel will keep energy on the motor to force it to slow down gradually. But in a high inertia load, the same setting just creates a generator.



An AC induction motor becomes an AC induction generator under the following conditions:
A) The motor windings are energized, meaning the magnetic fields are created and maintained, and

B) The frequency of the power applied to the windings is lower than the relative frequency of the load speed.


So with that high inertia load, when you tell it to decelerate in 10 seconds from 60Hz to 0 Hz, that means the frequency given to the windings is dropping 6 Hz per second. So 3 seconds into it, the frequency applied to the windings is 48Hz. If in that same 3 seconds, the load inertia was so high that the actual speed of the motor has only dropped to the equivalent of 55Hz, you now have a situation where the applied frequency is lower than the actual relative speed frequency, so the induction motor becomes an induction generator and pumps energy into the VFD's DC bus. If that energy has nowhere to go, the drive must trip to keep it from damaging the components.


If it DOES have somewhere to go, such as a Braking Resistor, then the energy will go there, but only to the point at which that resistor can absorb it. Too much and the resistor burns up (or a protection circuits trips).


But as mentioned, if you don't really care about controlling the rate of deceleration, just set the Stop mode to "Coast" and the drive will no longer keep the windings energized (removing condition #A) so it cannot become a generator.


That by the way is the same thing that is interfering with your Flying Start" functionality.
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Old 12-06-2018, 11:05 PM   #8
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Damn, I learned me a lot here at this thread today. And just remember: macmikeman is always learning. One thing I already knew,, Trying to find really big breaking resistors is expensive and difficult to even find them available.
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Old 12-06-2018, 11:13 PM   #9
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The VFD is already set to ART-F/R CTS. The issue is changing speeds, not stopping.

I'm not telling it to decelerate from 60hz to 0hz. I get the fault when trying to go from 60hz to 55hz unless I only hold the decel button down for 4 seconds at a time and repeat until I reach the target frequency.

I tried setting DB Config to "No Dynamic Brake" but that didn't help.

The Decel time is set to 90 seconds but I'll try 5 minutes and see how it goes.

http://www.benshaw.com/uploadedFiles...r%20Manual.pdf

Last edited by Jay Freeman; 12-06-2018 at 11:27 PM.
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Old 12-06-2018, 11:58 PM   #10
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Originally Posted by Jay Freeman View Post
The VFD is already set to ART-F/R CTS. The issue is changing speeds, not stopping.

I'm not telling it to decelerate from 60hz to 0hz. I get the fault when trying to go from 60hz to 55hz unless I only hold the decel button down for 4 seconds at a time and repeat until I reach the target frequency.

I tried setting DB Config to "No Dynamic Brake" but that didn't help.

The Decel time is set to 90 seconds but I'll try 5 minutes and see how it goes.

http://www.benshaw.com/uploadedFiles...r%20Manual.pdf
The Benshaw VFDs are private labelled LSIS. The S4 line are pretty good.

You are decelerating too fast. Increase deceleration. 90 seconds is too slow. Keep doubling it until no more faults. The rate is from 60 to 0. So if it's say 60 seconds it drops 1 Hz per second. So 5 seconds to do 60 to 55. 120 seconds means it takes 10 seconds. That's where your issue lies.

Flying start for all drives is hit or miss with large loads. It's hard for it to see the bump if it is decreasing the frequency and the l8ad regens so bad it hides the counter EMF null. Common issue.

Also did you remove the coupling and auto tune before attempting vector control? Don't assume you can do zero tuning like you can in V/Hz.

Also can you turn regen off? I don't remember but on some drives you can just force coasting.


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Old 12-07-2018, 05:48 AM   #11
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On high inertia machines, 20 to 30 minute ramp times are not uncommon.

On machines that require a high starting torque, sometimes a "step ramp" is required. In this mode, on a start, the VFD output immediately steps to some value (say 25%) to provide enough torque to start the load moving. The timed ramp then begins. These type machines are almost always coast to stop.
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Last edited by varmit; 12-07-2018 at 05:55 AM.
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Old 12-07-2018, 06:21 AM   #12
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you can always set the stop to coast then run the machine up to speed and turn it off. Measure the time it takes for it to coast to a stop. Set the decel to that time.

Another thing is to engage any (programmed) safetys that allow the drive to react to dc buss voltage. There are settings on most drives that allow the drive to ignore speed control (with in limits).

Incoming voltage can be another problem. If you are rich then the dc buss is already high which gives you very little room to play with and may force you to install a braking resistor.
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Old 12-07-2018, 11:22 AM   #13
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I'm off for long weekend but I'll get back to it on Tuesday and post an update.

Thanks for all the input.

Regarding Auto-Tune... besides calculating the RS, what else does the VFD do exactly?

How much does it matter if the drive is not being used in Vector mode?

Also, the flywheel was coupled to the motor at the time they did the autotune. The RS result was/is .028 ohms.

Should I go ahead and de-couple the flywheel and redo the Auto-tune?
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Old 12-07-2018, 11:31 AM   #14
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This sounds like hit or miss. Rethink time. Can you add a small PLC for speed change?
Reduce speed by x Hz's with a step delay. So if you say drop 10 hz it does the 4 that works waits, does the next four and waits then then last two. A timer circuit will work or you can add feedback to sense speed.


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Old 12-07-2018, 01:09 PM   #15
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Are you trying to control the speed or just using the vfd as a soft start?
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Old 12-07-2018, 01:53 PM   #16
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Are you trying to control the speed or just using the vfd as a soft start?

I'm curious as well.


The mills we service use softstarts, since the customer has no reason to vary the speed.
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Old 12-07-2018, 05:39 PM   #17
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Are you trying to control the speed or just using the vfd as a soft start?
I have work on few larger hammer mills and they will take time to get up to running speed and once they hit the running speed they pretty much leave it alone ( max motor rpm speed )

I am not too well famuair with OP's VSD unit but just enough to know what the heck is going on.

If you really want to slow down a good clip of speed then you will need exteral braking restistors to slow the mill speed down.

The one I worked on it do have separated braking restistor on it so when the VSD sense a braking command signal it will active the braking resistor and the contractors too ( yuh it is tied to either motor output connection or dc buss depending on set up ) the braking restoior I have was yanked out of the locomovite that do the trick and yuh make sure the cooling fan is on when it kick on.

How long it stay on ? genrally if set up right it will take little longer than what it bring up to full speed.
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Old 12-07-2018, 10:05 PM   #18
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I just thought of this. A reason for a vfd on a hammer mill is to reverse rotation regularly so the hammers wear evenly. A mill has to run at full speed to do its job.
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Old 12-08-2018, 04:06 AM   #19
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I just thought of this. A reason for a vfd on a hammer mill is to reverse rotation regularly so the hammers wear evenly. A mill has to run at full speed to do its job.
Use a reversing contactor. Still beats the 300% premium over a soft start. They do make some very simple 2 phase contactors that only have 2 positions and just reverse two leads. Very compact. Look on Becker mining components.

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Old 12-08-2018, 11:15 AM   #20
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It takes a very robust soft start to start a very high inertia load like a mill or a large centrifugal fan. We could not get the fan up to speed fast enough for the saft start. When it went to full speed the overload would drop out. we ended up putting a vfd to control the start.
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