Hydraulic Press brake Dropping Volts HELP - Page 2
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  1. #21
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    When the pressure relief goes off, it shouldn't draw much over 100% on the motor.

    And I'm thinking most press brakes "dwell" at the bottom for a split second, thereby
    they are "blowing off on the relief valve".

    I wonder if this machine got a "screwdriver rebuild" where someone cranked up the relief
    valve pressure. It is common.

    What is the gage saying ?

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    The pressure gauge pegs out at 3000 psi when it hits limit of ram stop. Thats where the voltage drops plus the PSI. The press should reach 3850PSI but i've never got it to hit it.

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    Yes, the amp draw when the machine stalls (I think you said it stalls)

    As I understand your problem, you have a problem that the press ram stalls and does not complete the stroke. And this is a bending brake.

    Ok.

    I assume the metal type and thickness is within the capability of the press, meaning it SHOULD bend the material, but is not finishing the bend.

    I assume that you are not trying to "coin" the metal.... That is, you are bending in a situation where the metal is bending on an open V lower die, and the upper die has clearance at the bottom of the stroke, the metal plus upper die is never solidly filling the lower die. If it DID fill the bottom die then a stall would be expected, and the relief valve would probably operate.

    If the motor you have is the correct one for the machine, it should be able to develop the full design pressure, as long as there are no other issues, and the relief valve is set right. That would mean the brake could bend the maximum full-width metal thickness (if not "coining").

    The problem with motors is that they develop more torque as they slow down under load, up to a maximum torque point. The current (amps) draw increases gradually with increasing load.

    If the load needs more torque than the maximum the motor can produce, then the motor slows further, but the torque no longer increases, it drops, and the motor slows more, decreasing torque even more. It will rapidly stall, and as that is happening, the amp draw increases to maybe 5 times the full load amps, or more. It goes up to the "locked rotor current".

    A 15kVA transformer is a relatively weak source, since that is only 65 amps at 230V. With a weak source, the voltage drops under load. That in turn reduces the motor power (and torque) available.

    The drop is not proportional, it is faster, in other words a 10% drop in voltage causes more than a 10% drop in ultimate power, and it also causes an increase of current for the same power.

    With a weak source, this rapidly can "run away", because the drop in voltage requires an increase of current, which drops voltage more, and requires more increase in current. That can fairly rapidly lead to a stalled motor and a very large current draw. I have seen that when someone tries to run an air compressor on a long extension cord. It may just stall, and draw many times the normal current.

    The questions about the voltage, and the current are directed at determining if that is going on.

    Your situation may be made WORSE by the big, oversized, RPC. The RPC draws current also. You mentioned a draw of 50 or 60A with just the press and RPC, which nearly eats up all your available current from the small 15kVA transformer. If you have other machinery and lights on, the draw may be enough to drop your voltage and cause the problems you have.

    If you can get a clamp-on meter that has a peak current capability, that would be best. With that, the maximum amperage your setup is drawing can be determined, while the press is being used.

    QUESTIONS:

    What is the rating of the brake, vs what you are trying to bend?

    Has the brake ever successfully bent that material anywhere else with a different electrical source?

    Are you certain you do not have a "coining" situation where the dies close up completely on the metal?

    What is the maximum current draw that the press is drawing on the pass-through legs?

    What is it on the generated leg?

    If possible, what is the voltage at the time the press fails to bend the metal? If that is not available/possible, do you notice the lights dimming?

  5. #24
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    Quote Originally Posted by JST View Post
    Yes, the amp draw when the machine stalls (I think you said it stalls)

    As I understand your problem, you have a problem that the press ram stalls and does not complete the stroke. And this is a bending brake.

    Ok.

    I assume the metal type and thickness is within the capability of the press, meaning it SHOULD bend the material, but is not finishing the bend.

    I assume that you are not trying to "coin" the metal.... That is, you are bending in a situation where the metal is bending on an open V lower die, and the upper die has clearance at the bottom of the stroke, the metal plus upper die is never solidly filling the lower die. If it DID fill the bottom die then a stall would be expected, and the relief valve would probably operate.

    If the motor you have is the correct one for the machine, it should be able to develop the full design pressure, as long as there are no other issues, and the relief valve is set right. That would mean the brake could bend the maximum full-width metal thickness (if not "coining").

    The problem with motors is that they develop more torque as they slow down under load, up to a maximum torque point. The current (amps) draw increases gradually with increasing load.

    If the load needs more torque than the maximum the motor can produce, then the motor slows further, but the torque no longer increases, it drops, and the motor slows more, decreasing torque even more. It will rapidly stall, and as that is happening, the amp draw increases to maybe 5 times the full load amps, or more. It goes up to the "locked rotor current".

    A 15kVA transformer is a relatively weak source, since that is only 65 amps at 230V. With a weak source, the voltage drops under load. That in turn reduces the motor power (and torque) available.

    The drop is not proportional, it is faster, in other words a 10% drop in voltage causes more than a 10% drop in ultimate power, and it also causes an increase of current for the same power.

    With a weak source, this rapidly can "run away", because the drop in voltage requires an increase of current, which drops voltage more, and requires more increase in current. That can fairly rapidly lead to a stalled motor and a very large current draw. I have seen that when someone tries to run an air compressor on a long extension cord. It may just stall, and draw many times the normal current.

    The questions about the voltage, and the current are directed at determining if that is going on.

    Your situation may be made WORSE by the big, oversized, RPC. The RPC draws current also. You mentioned a draw of 50 or 60A with just the press and RPC, which nearly eats up all your available current from the small 15kVA transformer. If you have other machinery and lights on, the draw may be enough to drop your voltage and cause the problems you have.

    If you can get a clamp-on meter that has a peak current capability, that would be best. With that, the maximum amperage your setup is drawing can be determined, while the press is being used.

    QUESTIONS:

    What is the rating of the brake, vs what you are trying to bend?

    Has the brake ever successfully bent that material anywhere else with a different electrical source?

    Are you certain you do not have a "coining" situation where the dies close up completely on the metal?

    What is the maximum current draw that the press is drawing on the pass-through legs?

    What is it on the generated leg?

    If possible, what is the voltage at the time the press fails to bend the metal? If that is not available/possible, do you notice the lights dimming?

    I am sure my dies are correct. It is a 88 ton press and i'm trying to bend 32" of 10ga. which should only take 25 tons. It won't bend all the way to the stop. I have to hit it twice. Only builds 1600PSI of pressure than the volts drop and the motor almost stops. Then after i let off the motor comes back to power. I think the generated leg is the one dropping 40 volts. The single phase side is dropping 7-10 volt on my meter. It might be more just not a expensive meter. So i think the transformer could be some of the issue. but i don't think all.

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    you need to be within 5% on the RPC phase to phase, and need to check and adjust with a load on it. Idling and no load should have a higher voltage on the generated leg.
    mine is incoming 238V and the generated leg (PH1-3)was at about 245. I had to play around with the capacitors to get it correct as when first hooked up it was at 268V! on the 3rd leg and kept drawing down my input power unevenly.

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    i can test more tonight and get back with you. thank you for the idea's. its frustrating. I went over board on RPC just so this wouldn't be a issue when i update my press from 50 ton to a 88 ton. Now i can't bend any more than i could before.

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    There is no question that you have a weakish electrical system. The combined current of the idler and the load motor is consuming almost all of the rated current of the pole transformer. That is likely why it is not working well.

    Furthermore, the large voltage drop in the generated leg is turning the 10HP motor into more like a 6.6 HP motor. That will not help.

    The generated leg is always less than the incoming voltage, because of the way it is made in the motor (it is the "back EMF"), which means it MUST be less than the incoming voltage by an amount which allows motor current to flow. So it could be around 5% to 10% low (12 to 24V) just on account of the reduced back EMF. Then there is the incoming voltage drop, which could be another 15 to 20 volts. That accounts for quite a bit of the drop in the generated leg.

    It looks as if your incoming voltage is around 245V. The capacitors on the generated leg boost that to the 254 to 258V you reported, at no load. That boost goes away when the load motor is connected, because the load motor changes the total equivalent amount of inductance and impedance connected. So you start with about a 10v drop.

    Add to that the 25 to 40 volts drop that could be occurring due to incoming voltage drop, and impedance in the idler, and you almost are accounting for what you report.

    If the voltages are actually different, because you cannot observe them all at once with 1 meter, it seems as if almost all the reported drops can be accounted for.

    Ironically, the oversized idler may be hurting you more than it helps, just due to the extra unloaded current it draws vs a 20 HP one. However, looking again at everything, I think that most of your problem comes from the undersized pole transformer. There may be other contributing causes, but that seems to be a biggie.

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    Quote Originally Posted by flagvan2000 View Post
    Thank you for all you said.

    When you mean current. You mean amps? i know on the single phase side of the RPC i have a large spike of amps when i am using the press.

    I know i was getting 100+ amp spikes when using press last night.

    If you could explain how i might go about testing these i would do it.

    Update on Press. The power company updated my transformer to a 50kw. The single phase side now does not drop any voltage. I am still getting a 40-50 volt drop on the manufactured leg of the rotary converter. They built me a panel to boost the voltage when theres a lose but didn't help. Is there anything else i could try that anyone knows of??

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    OK you get a voltage drop.

    Does the original problem of not working still exist?

    You may get a voltage drop and the machine still can work. If it IS working, but you get a drop in voltage in one part of the machine cycle, that may not be a big problem.


    So....

    What maximum current draw (amps) do you get going to the press motor (on each wire) when you cycle the press under load?

    Will the press cycle OK without metal being bent?

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    The press motor is still stalling out like it was. I also get a 50amp draw when the press is loaded. The motor should only draw 26amps. 10HP 230 volt motor on press. I haven't checked every wire on amp draw but i can.

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    Does it go through the full cycle correctly if there is no metal actually loaded between the dies and being bent?

    The issue here is that this would be easily explained if the dies were set up too tight so you were basically "coining" the metal instead of just bending it.

    It is also explainable if the machine is not going to the slow speed lower flow correctly

    OR if the motor is smaller than the machine should take, perhaps a faster RPM than was originally on it.

    Any of those could cause the motor to stall due to a lack of sufficient torque.

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    I would think if its a mechanical overload ("coining") then the pressure would max higher, like 2500-3000psi. But staying at 1800psi max seems like the pump is never developing full mechanical "power" (flow * pressure).

    From the start it sounded like you have a bad connection somewhere that just can't transmit the current efficiently. Sounds like the single phase is now solid in L1-L2 no matter what the load as a result of the utility's fix. With everything off I'd go back thru every power-side connection and verify torque and pull.

    You might even pull out some "sense" leads which come directly from T1, T2, T3 right on the machine motor itself, for the purpose of clamping well-covered test leads to them and check the actual voltage at the motor. No current but just to meter the voltage. I'm not totally sure where you are measuring.

    One other thought...Have you marked the shafts pump to motor with a sharpie/paint marker to make sure there's no slip in the couplers? Keys can get sheared. It seems counterintutiive to the condition of stalling the motor but could explain the fail for pressure to rise

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    Quote Originally Posted by JST View Post
    Does it go through the full cycle correctly if there is no metal actually loaded between the dies and being bent?

    The issue here is that this would be easily explained if the dies were set up too tight so you were basically "coining" the metal instead of just bending it.

    It is also explainable if the machine is not going to the slow speed lower flow correctly

    OR if the motor is smaller than the machine should take, perhaps a faster RPM than was originally on it.

    Any of those could cause the motor to stall due to a lack of sufficient torque.
    I have run through full stroke. Without metal. It runs good through the fast down and the slow 2nd stage. Till it hit the stop for depth then the motor stalls. Or if i put in materiel that is longer still in the thickness to die chart range you have to hit it twice to get full bend. The one set-up in 10ga at 32inch long. should take 30 tons and it won't do it in full stroke. The press is rated 88 ton.

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    Quote Originally Posted by JST View Post
    Does it go through the full cycle correctly if there is no metal actually loaded between the dies and being bent?

    The issue here is that this would be easily explained if the dies were set up too tight so you were basically "coining" the metal instead of just bending it.

    It is also explainable if the machine is not going to the slow speed lower flow correctly

    OR if the motor is smaller than the machine should take, perhaps a faster RPM than was originally on it.

    Any of those could cause the motor to stall due to a lack of sufficient torque.

    Can a 3 phase motor still run but not have power if its still wired 440 volt but ran with 230 volt? I had a rotary company tell me this could happen.

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    Quote Originally Posted by flagvan2000 View Post
    Can a 3 phase motor still run but not have power if its still wired 440 volt but ran with 230 volt? I had a rotary company tell me this could happen.

    Absolutely.

    I thought you had checked that, but may have mixed up this thread with another..... it happens sometimes.

    Yes it will act pretty much like a motor running on half voltage, generally because that is exactly what is happening.

    Is your press set up to go until it hits a hard stop that pops the relief? If so, how does it switch to return stroke?

    I am not particularly familiar with specific press types, I just know about the actual bending aspect.... our products had sheet metal enclosures that we designed.

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    Quote Originally Posted by JST View Post
    Absolutely.

    I thought you had checked that, but may have mixed up this thread with another..... it happens sometimes.

    Yes it will act pretty much like a motor running on half voltage, generally because that is exactly what is happening.

    Is your press set up to go until it hits a hard stop that pops the relief? If so, how does it switch to return stroke?

    I am not particularly familiar with specific press types, I just know about the actual bending aspect.... our products had sheet metal enclosures that we designed.
    So a update. Still having issue with the generated leg dropping 40 volts when press has a load. What i did find out is the press seems to be good. I used a 65kw diesel generator and ran the press. Worked great!! Just don't know what to do about the rotsry converter.

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    Quote Originally Posted by flagvan2000 View Post
    So a update. Still having issue with the generated leg dropping 40 volts when press has a load. What i did find out is the press seems to be good. I used a 65kw diesel generator and ran the press. Worked great!! Just don't know what to do about the rotsry converter.
    Call the manuf ?

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    Yes. I have been going back and fourth with them. They added another panel that was suppose to balance but just hasn't worked. Still loosing about 20 volts and when i run through the step down transformer it looses bending power even quicker.

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    Is the pump direct drive?

    If not, if there is a gear/belt whatever that you can change the ratio on, it might be just as simple in the long run to change the ratio to reduce the motor torque, which would be slower, but finish the cycle in one hit.

    The other suggestion is to add a flywheel to the motor. If it does not finish in one hit, but it will finish on the second hit, it may simply be that the motor is storing enough energy to get you through before it slows down too much and loads the RPC.

    Commonly done on punch presses, where the motor is considerably smaller than what would be needed to power through the hit.

    Yeah, it is a make-do workaround, but you just want to get the press bending metal without huge expense.

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    Quote Originally Posted by JST View Post
    Is the pump direct drive?

    If not, if there is a gear/belt whatever that you can change the ratio on, it might be just as simple in the long run to change the ratio to reduce the motor torque, which would be slower, but finish the cycle in one hit.

    The other suggestion is to add a flywheel to the motor. If it does not finish in one hit, but it will finish on the second hit, it may simply be that the motor is storing enough energy to get you through before it slows down too much and loads the RPC.

    Commonly done on punch presses, where the motor is considerably smaller than what would be needed to power through the hit.

    Yeah, it is a make-do workaround, but you just want to get the press bending metal without huge expense.
    It is direct drive hydraulic pump. nowhere to install flywheel


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