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VFD Braking Resistor

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  • MikeHenry
    replied
    Forrest,

    OK - sounds like I am fine on the lathe. A partially operational horizontal mill with VFD has been giving me periodic faults, though so now I have a clue on what to check on that one. The VFD was a used model, so it probably has the wrong settings for the mill motor.

    Mike

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  • J Tiers
    replied
    Just as a reminder.........

    The switch element in the VFD is designed to carry the braking resistor current, at the relatively low duty cycle required.

    Since they are designed to a price, and the market is extremely competitive, they are no bigger than they have to be, and have no more heatsink than required.

    If you use a resistor lower in resistance than the manufacturer recommends, OR it is temperature variable, and is too low when cold, you may blow the braking switch element (probably a lower current mosfet). Then you have NO braking resistor, and many VFDs are not very repairable.

    The stove element may be perfect, IF you know the resistance, and that resistance is right for your VFD and load.

    But, it won't be right for a small VFD, almost certainly.

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  • cnccnc
    replied
    Sorry about the late reply,
    I was eyeballing the inner workings of a Haas
    toolroom lathe and there was what I believe to be an electric stove element in a metal box with air holes punched in it, I dont think the element will ever get hot enough to significantly alter it's resistance from coast down current, used as a stove then yes.

    opinions are like _ _ _ holz we all have one, unless we have children or ex wives then maybe more than one!

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  • JeffKranz
    replied
    Well, after searching the internet, Ebay and local electronics shops, I ended up with a 50 OHM 220 Watt wirewound resistor. Not 40 OHM like the manufacturer recommends but I hope it will work. I did have a choice of a 33 OHM 500 watt resistor but it was about 1 foot long.

    Since the 2HP VFD required a 60 OHM and the 5HP VFD required a 40 OHM I figured that I'm only using a 3HP Motor a 50 OHM was probably OK - do you agree?

    Thanks,

    Jeff

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  • J Tiers
    replied
    However, do not be surprised if someday when you have a heavy thing on the faceplate, or the like, you do get an OV fault.

    It is the energy received from the load (the motor) that determines the bus voltage, and how fast it arrives. If you have low mains voltage, you may NOT get an OV fault when if you had higher voltage you would.... if you start low, you have "farther to go".

    For the technically inclined, the energy returned from the motor will appear as excess charge (and thus voltage), on the bus capacitors. The bus voltage varies as the square root of the received energy, since energy in a capacitor (Joules, or watt-seconds) is (C* V^2)/2 where C is the capacitor value.

    So it is perfectly possible to never have an OV fault and still have braking happening, if you don't need to remove much energy. The excess charge is soon used up by powering the motor.

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  • Forrest Addy
    replied
    Mike, the braking resistor is helpful when crisp decels to a stop is desired. Most machine tools have enough power robbing clutter (gears in mesh, belts etc) to stop the spindle in an unsassisted coast in a reasonable time.

    As ususal I got off on a purist rant over VFD resistors which led us into another debate that served us poorly as practical points for the home shop machinist to consider.

    If you set your VFD to decel in two seconds or less from full spindle speed chances are you'll get an "OV" trip. This means the kinetic engegy of the rotating parts was regenerated as electrical power by the switching ctansistors in the VFD and stuffed back into the DC buss. The VFD has a protective circuit that senses this excess voltage and shuts down the drive.

    If your machine works fine as is, don't fix it.

    [This message has been edited by Forrest Addy (edited 08-13-2005).]

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  • MikeHenry
    replied
    Sorry for coming into this late but I happen to have a Clausing 5914 lathe with 2 HP motor that is powered by a Freqrol FR-A220E-2.2K VFD. So far as I know the VFD has no braking resistor and I've had no problems running the lathe & VFD over the past two years. The lathe still has the OEM vari-drive system.

    What would the braking resistor be useful for?

    Mike

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  • Forrest Addy
    replied
    Jerry is right about hot resistance (I hate it when Jerry's right). It is a factor if the resistor develops significant heat in service, particularly if the resistor in question has a low time constant as would an array of light bulbs.

    Most machine tools have little more kinetic energy of rotation than the armature, pulleys, and input shaft except when running in their higher gears. My 5 HP 17" engine lathe's worst case is when I have the 10" three jaw mounted. A single three second stop from 2000 RPM causes the pair of 100 watt ceramic wire wound resistors on its VFD to warm up about 80 degrees over ambient. Obviously YMMV.

    All common resistance materials have a positive temperature coefficient; their resistance goes up with temperature. Most VFD makers offer several Watt ranges of dynamic braking resistors depending on the braking service required by the application.

    An extreme case would be a centrifuge in a sugar mill, a water extractor in a laundry, or a long speedy conveyor loaded with weighty bulk material. Here the dynamic brake may have to dissipate over a span of minutes whatever kinetic energy the motor's accelleration has built up in the load. So a VFD dynamic braking resistor in a 5 HP machine tool could be safely rated at 300 watts whereas an extreme application might require a 10,000 watt resistor. Same resistance; mucho more kinetic energy to dissipate.

    Some fancy VFD's offer a regeneration to line option but I'm not smart ehough on that refinement to do more than mention it.

    [This message has been edited by Forrest Addy (edited 08-11-2005).]

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  • J Tiers
    replied
    <font face="Verdana, Arial" size="2">Originally posted by Forrest Addy:

    If you have the right resistance and enough wattage for your machine's braking duty cycle it really doesn't matter what you use for a resistance; light bulbs, stove elements, ceramic coated wire wound resistors, or something inventive so long as it's safe.
    </font>
    True....mostly.....but......

    One possible issue.... non-resistors, i.e. things that are not intended to be resistors of a certain value, meaning light bulbs, stove elements, etc, may have a variable resistance.

    Their resistance 'cold" may be quite different from their resistance "hot". Usually they are HIGHER resistance when hot than when cold.

    This may be a problem for the switch elements in the VFD. They are designed for a certain current, and are made to a price....meaning they didn't put in much extra capacity.

    A light bulb, for instance, may have a resistance "cold" which is 1/10 (or less) of the resistance when fully lit up. So when the internal switch component turns on the current, there may be a LOT more than expected.

    I don't know about stove elements, I hate electric stoves.... I would imagine they do change resistance somewhat, no reason they shouldn't, its cheaper not to make them have a stable resistance, and they are made to a price......

    If your "resistor" is supposed to draw 5 A, and it actually draws 50A as a starting surge, things in the VFD can be messed up.

    I'd recommend using the right stuff, namely a resistor or an array of resistors with a known resistance value, and suitable wattage........ you have been warned.....

    You can get resistors from DIGI-KEY, or Mouser electronics, google them for the URLs.

    EDIT:
    A "real resistor" is made of materials with a very stable resistance over temperature. They have to be, a 10% tolerance resistor can't vary more than that with temperature or any other factor.


    [This message has been edited by J Tiers (edited 08-11-2005).]

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  • Forrest Addy
    replied
    For them that's unfamiliar with the workings of VFD's, the dynamic brakes of a VFD (as was pointed out earlier) is a gadget for dissipating the charge the switching transistor dumps unto the DC buss as heat through a resistor. The regenerated power charges the filter sapacitors raising the voltage across their terminals.

    The dynamic brake has a transistorized chopper that senses the DC buss voltage and cuts the braking resistor in and out of the circuit. The chopped DC energises the resistor to burn up the excess charge as it builds up in the filter caps keeping the buss voltage to a safe level.

    Sizing this resistor is like selecting a lever and placing a fulcrum to move a heavy weight. It's a matter of some judgement (actually calculation) to get the value right. Fortunately the factory had to figure all this out when they designed the VFD so go by theuir values.

    If you have the right resistance and enough wattage for your machine's braking duty cycle it really doesn't matter what you use for a resistance; light bulbs, stove elements, ceramic coated wire wound resistors, or something inventive so long as it's safe.

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  • J Tiers
    replied
    <font face="Verdana, Arial" size="2">Originally posted by Ryobiguy:
    I like the idea of using multiple standard resistors, maybe I'll go that way.

    Lets see, who's in my surplus electonics bookmarks... $4.50 for a 125 ohm 160W resistor:

    Is power dissipation additive when several are put in parallel? Geez, maybe I should go read my basic electronics books again.
    </font>
    It is an excellent idea, if you have room and can make the resulting thing look decent and be safely covered up (it's at line voltage, never mind the 380V DC bus)

    The power is additive among a connection of identical value parallel resistors, yes.

    Since teh resistor SHOULD only be connected whe braking is operating, it does not have to handle as many watts as if it were always connected to the bus. BUT that is already taken into account in teh wattage.... so don't cut it very far....

    A 60 ohm resistor on a 360V bus draws around 2100W, so using a 300W part is already assuming a derating for "duty cycle" (short time on, long time off).

    [This message has been edited by J Tiers (edited 08-10-2005).]

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  • Ryobiguy
    replied
    I like the idea of using multiple standard resistors, maybe I'll go that way.

    Lets see, who's in my surplus electonics bookmarks... $4.50 for a 125 ohm 160W resistor:
    http://www.goldmine-elec-products.co...em=10&mitem=11

    Is power dissipation additive when several are put in parallel? Geez, maybe I should go read my basic electronics books again.

    Leave a comment:


  • JeffKranz
    replied
    At Forrest's request I talked to the manufacturer. They said I need and 40 OHM 300 Watt for the 5HP and a 60 Ohm 300 Watt for the 3HP VFD. He said that the Watts are not that important if you using occastionally but I've found a 40 OHM that is only 25 watts.

    Any other help.

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  • joahmon
    replied
    I belive J Tiers is speaking of resistance.

    The VFD need to "see" sufficent resistance to keep current low enough.

    Wattage only protects the resistor.

    I tried using a 1500 watt water heater element without thinking it thru. 1500 watts @ 240 volts is under 7 ohms. Element turned red and the VFD's transister is shot. No more external BR for that unit :-(

    [This message has been edited by joahmon (edited 08-10-2005).]

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  • mayfieldtm
    replied
    YES! You want breaking. You will love it.
    The factory units are nothing special and extremely over priced.
    I made my own out of [email protected] of 12 Watt resistors, all wired in parallel and I was able to nail the exact value needed.
    If I remember... I needed about 350 watts.
    Used a couple of pieces of un-etched circuit board material about 3" square, drilled 30 holes in each (5x6 about 0.5" apart) made a resistor sandwich. Added a couple of aluminum angles for mounting. Made a nice compact unit that stays cool.
    This was a lot cheaper than the wire-wound Vitreous Power Resistors.
    Don't forget, if you use the adjustable wire-wounds, derate the wattage by the amount of adjustment.

    Tom M.

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