# Over voltage Fault



## Jay Freeman (Aug 2, 2017)

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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## gpop (May 14, 2018)

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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## MikeFL (Apr 16, 2016)

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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## Chops146 (Aug 26, 2018)

Jay Freeman said:


> 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.
> 
> ...


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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## Jay Freeman (Aug 2, 2017)

gpop said:


> 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.


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## varmit (Apr 19, 2009)

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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## JRaef (Mar 23, 2009)

varmit said:


> 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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## macmikeman (Jan 23, 2007)

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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## Jay Freeman (Aug 2, 2017)

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/Literature/890035-01-03 RSi S4 Series VFD User Manual.pdf


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## paulengr (Oct 8, 2017)

Jay Freeman said:


> 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.
> 
> ...


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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## varmit (Apr 19, 2009)

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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## gpop (May 14, 2018)

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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## Jay Freeman (Aug 2, 2017)

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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## just the cowboy (Sep 4, 2013)

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. 


Cowboy


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## tmessner (Apr 1, 2013)

Are you trying to control the speed or just using the vfd as a soft start?


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## Cow (Jan 16, 2008)

tmessner said:


> 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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## frenchelectrican (Mar 15, 2007)

tmessner said:


> 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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## tmessner (Apr 1, 2013)

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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## paulengr (Oct 8, 2017)

tmessner said:


> 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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## tmessner (Apr 1, 2013)

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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## gpop (May 14, 2018)

A place i worked had a simple reversing switch (manual) and a disk brake on the mill that you put you foot on a pedal to engage. (old days of y-delta starting)

We used the brake to allow for faster service times. 

Even if we used a more modern approach like a drive im not sure there would be any significant gain in that scenario as we use to pull the screens to check for holes so the mill would have still had to come to a complete stop for servicing before being reversed.

The Op seems to want to adjust the speed. If it just 2 or 3 speeds and they are using the front panel for control i would simple program 3 digital inputs with set speeds then use a switch (use all of the inputs and notch the speed down if need be). I will be honest ive never adjusted a front panel to control speed so i do not know if it over rides decel.


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## paulengr (Oct 8, 2017)

gpop said:


> A place i worked had a simple reversing switch (manual) and a disk brake on the mill that you put you foot on a pedal to engage. (old days of y-delta starting)
> 
> We used the brake to allow for faster service times.
> 
> ...


A drive gets a set point either internally via the keypad or via speed settings, or it reads an analog input or a digital one. Some might also have a PID loop for say level or pressure or flow control. This is the speed reference in a speed control loop. You can also do exactly the same things except have a torque control loop. It may also have limiting on the other variable so you can have speed control with torque limiting or vice versa. Acceleration limiting is a form of torque control that is used often in drive speed controllers but torque control or alternatively current limiting is a little more direct.

And about soft starts not handling high inertia loads if I set the soft start up to current limit at 100% of FLA and it's a centrifugal fan or pump where the load torque is proportional to the square of RPM (hammer mills too) it might take a really long time to get up to speed but it has enough torque to get to full speed and never overloads the soft start or the motor. Since a VFD matches the frequency if set up right you get full torque from 0 to 100% of full speed where a soft start never changes the frequency so as a consequence torque is much weaker. On a relatively constant torque load like conveyor to overcome this the heavy duty soft starts allow 500% of FLA for 30 seconds. It is usually just 1 or 2 sizes oversized from the standard duty one. Generally I find you need to set them around 350-450% of FLA to get up to speed in 30 seconds on these types of loads or it overloads and trips. Strangely smaller 100-250 HP loads seem to need 450% while large motors do just fine at 350%. I do a lot of soft start installs and these settings are what works consistently for me. The worst one I've seen in a while was an 800 HP wood chipper this week, it had to current limit (transformer limitation) at 3000 A with a 1085 A FLA so not even 300%. It's a large hammer mill type load where normally I wouldn't ever go below 350%. It took almost 25 seconds to reach full speed at those settings. Personally I wouldn't have attempted it below 4000 A but they've been starting that way for years.


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## Yellow World (Oct 11, 2017)

Had a similar problem before. Machine is equipped with a flywheel, once you switch off power to the motor, the motor is driven by the flywheel and acts as a generator, that means, it sends voltage to the output side of the VFD. To protect itself, the VFD shuts down and gives out an overvoltage error.

After playing around with the settings for a bunch of days, the final solution was to install an external brake resistor. The resistor catches this voltage and converts it into heat. According to the operator, now it brakes better than before.

Oh right, you said it's a 125HP motor, so you might want to think about some cooling if you install a resistor.


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## Jay Freeman (Aug 2, 2017)

Ok, I had some more time to work with the drive today and so far it's working great.

I ended up setting the Decel ramp to 330 seconds and to my surprise I found that setting too long of an Accel ramp causes Voltage Faults too. The Accel ramp was set to 5 sec to start with. I tried setting it to 20 seconds and found that just wouldn't work. I have it set to 10 seconds now.

After setting the extra long 330 second Decel time, I could see the benefit right away. I could hold the "Arrow Down" button as long as I wanted and the drive just decelerated at it's own pace.... nice and slow with no faults. Acceleration is nice and quick with no faults as well. 

The Mill Operator will be using the 10K pot to adjust speed so I tested that too and had no problems adjusting the pot from30hz to 60hz and back to 30hz... nice quick accel and nice slow decel with no issues.

I had the drive set to CTS but as an experiment, I set it to do a controlled 330 second stop and had no issues with that either... worked perfectly. When I pushed the Stop button, the drive Start button stayed lit while the drive decelerated and after finally reaching a full stop, the Stop button lit up. Controlled stop is nice but the company doesn't care about long stop times so I just set it back to CTS since it's just easier on the drive.

Thanks to everyone for all the input.

Jay


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## tmessner (Apr 1, 2013)

Just curious, What is the purpose of slowing the mill down?


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## Jay Freeman (Aug 2, 2017)

tmessner said:


> Just curious, What is the purpose of slowing the mill down?


I don't have much experience with milling but I'm told that when they need to produce a more coarse product, slowing the mill down helps get the final product in spec.


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## micromind (Aug 11, 2007)

Jay Freeman said:


> I don't have much experience with milling but I'm told that when they need to produce a more coarse product, slowing the mill down helps get the final product in spec.


That's exactly correct. 

The speed of the mill will determine how hard the rocks get hit and therefore, the size they are reduced to.


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