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timgunn

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Posts posted by timgunn

  1. The 37 mbar delivery pressure may well be sufficient on its own to explain the poor performance. Things could be looking up.

     

    There may be some deviation from normal Gas-Safe practice needed: I don't think LPG necessarily falls within the same regs as mains gas (for anyone reading this outside the UK, "Gas-Safe" is the registration body with which anyone working with mains gas in the UK must be registered).

     

    Naturally Aspirated burners use the momentum of gas emerging from a jet to entrain air and mix it with the gas. This means we need a high speed through the gas jet. The gas speed through a jet varies as the square root of the pressure loss across it. Downstream of the jet will be at, or pretty close to, atmospheric pressure. Upstream of the jet will be whatever pressure your regulator supplies. 

     

    37 mbar is about 0.53 PSI. At 2 bar, or 30 PSI, the amount of gas being burned (or at least fed to the forge) will be between 7 and 8 times as great as at 37 mbar.

     

    Varying the pressure controls how much gas is being fed to the forge: effectively how big your flame is.

     

    Varying the ratio of air to fuel controls the temperature of the flame: how hot it is.

     

    Many (most?) Naturally Aspirated burners operate at a single Air:Fuel ratio. Some have an adjustable choke, allowing the ratio to be varied. The most Fuel-Lean mixture is obtained with the choke fully open. Closing down the choke then allows the mixture to be made richer and, in the case of gas forges, the flame to be made cooler.

     

    Probably the biggest change from normal gas-safe practice is that we generally run fuel-rich. This is pretty much unheard-of in the world of heating. The reason for it is that it helps to prevent Oxidation of the workpiece by maintaining a reducing forge atmosphere. Your father may need coffee and aspirin.

     

    Tuning a NA burner is mainly a case of finding a gas jet size that gives the richest mixture ratio consistent with achieving the flame temperature required for the highest-temperature job it needs to do. 

     

    If you are buying a regulator, I recommend a plugged welding regulator for Propane. These are around 25 quid if you shop around, against around 15 quid for the cheapest thing that'll do the job. The welding regs are designed for use by hairy-ar5ed guys wearing welding gloves and are pretty rugged. They have large, grippy adjusting knobs and have the setting scale marked on the body. The scale isn't particularly accurate, but it's probably not much less accurate than the gauges that you'll find on cheap regulators and is plenty good enough for "us". We just need adequate repeatability: I'm forging today, so need X PSI. Welding needs Y PSI, etc. You'll find the values for X and Y for your setup by using it. Gauges have a tendency to get damaged. I use them when they are useful, but feel things are usually safer without them.

     

     

     

     

     

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  2. I'd say that looks just enough like a forge to fool the beginner into hitting the buy button: Prepare for a precipitously steep learning curve.

     

    First off, can you get some photos of the whole lot? Something is obviously wrong. We can see that from the photos of the flame, but we have no information with which to diagnose the cause.

     

    The flame looks horribly rich and therefore cool. There's either too much gas for the amount of air, or not enough air for the amount of gas. Same thing, but not everyone sees this.

     

    My first impression is that the mixture is pants and that there's also not enough of it.

     

    There could be a number of reasons. We need to start somewhere, so we'll start with the stuff that's easy to check

     

    The first thing to check is your cylinder. Propane or Butane? Propane is good. Butane is not. Screw-in regulator, or clip-on? Screw-in is good. Clip-on is not.

     

    If it's a screw-in regulator, what is its outlet pressure range? 0- 2 bar (0-30 PSI) is good. 0-4 (0-60 PSI) bar is perhaps better, though not by much. Some regs are 0.5-4 bar (8-60 PSI) and, whilst they do work, I really loathe them for the lack of control at the bottom end. They also make lighting the forge unnecessarily exciting.

     

    The clip-on regs are fixed low pressure over here, 35 mbar for Propane and 28 mbar for Butane. No good for Naturally Aspirated burners. 

     

    If you are on Propane, it is regulated to a sensible pressure, and are still getting that sort of flame, are there chokes on the burners? If so, are they open? If they are closed, open them fully and try again.

     

    No chokes, or chokes fully open already, we'll need to see some detail of the air intake end of a burner. We also need to know what the gas jets are and what size they are. Mig welder tips are often used. otherwise it's usually drilled holes. The flame photos suggest the gas jet holes need to be smaller: easy if you can swap out mig tips for smaller ones (they are sized for the welding wire diameter. We'll need to know what size is marked on them and also the thread diameter and pitch). Considerably less easy if they are drillings.

     

    Check the above and get some photos. We can probably make it workable.

     

    Where in Somerset are you? What do you want to make initially?

     

     

  3. I usually use 2.5 mm 3-core from the 13A plug to the VFD and 1.5 mm 4-core from the VFD to the motor.  I usually use H07RN-F cable if I can (it has a tough rubber sheath and is nice and flexible). I don't know whether it's in accordance with the current wiring regs though.

     

    When I build a VFD into an IP66 enclosure, I do like to fit a socket for the 3-phase out, mainly so that I can use the VFD for running different machines, just by plugging in the one I want to run.

     

    A few years back, it was relatively cheap and easy to use the blue 230V 3-phase and earth IEC60309 plugs and sockets. They've become hard to find and expensive over the last few years. Unless you have a particularly strong reason for using a plug and socket, it's probably best to just wire direct initially and keep an eye out for affordable plug(s) and a socket.

     

     

  4. Something that is quite likely to happen at some point is that you'll get the flame running back up the burner tube. It can be pretty worrying if you are not expecting it.

     

    The air/fuel mixture needs to be moving towards the forge faster than the flame-front can move through the mixture in the opposite direction. 

     

    The flame-front speed is not a constant. It varies with temperature, pressure and air:fuel ratio. 

     

    With your burner (which is a Naturally Aspirated design without an adjustable choke), the air: fuel ratio is effectively fixed. The mixture pressure (as distinct from the gas pressure upstream of the gas jet, which is what your regulator controls) is reasonably constant: it will vary with atmospheric pressure and you may see another inch or so of Water Column in the forge if you are giving it the beans and have the opening restricted.

     

    When you start with a cold forge, it can be difficult to get the flamefront speed high enough (or the mixture speed low enough) to keep the flame stable. As the forge gets hotter, the flamefront speed increases and the flame stabilizes. 

     

    At some point, things can get hot enough that the flamefront moves faster than the mixture and runs back along the burner tube until it runs out of mixture to burn and goes out. Gas will keep flowing and will draw more air and mix with it. The mixture will flow along the burner tube until it reaches the hot forge, ignites and repeats the cycle. As the flamefront moves along the burner tube, it will heat it up a little. This will make the mixture a little warmer the next time, increasing the speed of the flamefront. 

     

    The noise is something like a pulsejet. If you catch it quickly enough, you can often turn up the gas pressure to increase the mixture flow and break the cycle. Don't pussy-foot about if it happens. Double the gas pressure for starters, which will give a 40% increase in mixture flow (Gas flow through a jet varies as the square root of the pressure doubling the pressure will increase flow by a factor of root2; 1.41). 

     

    Once you know what you are looking for, you can establish the minimum pressure your forge will run without burning back without unnecessary anxiety.

     

     

  5. Kast-O-Lite 30: The number needs specifying.

     

    Kast-O-Lite is the trade name of a range of insulating castables. The number that follows is the rated temperature in hundreds of degF. Kast-O-Lite 30 is rated for 3000 degF. 

     

    As a general rule, the higher the temperature rating, the higher the Alumina content (Alumina content is a big factor in flux-resistance) and the higher the thermal conductivity (higher conductivity = poorer insulator). 

     

    There are certainly a Kast-o-lite 23 and a 25, probably others too. I'd hate for someone to search for Kast-O-Lite, buy 23 because they don't know the difference, and end up with a dribbly mess instead of a forge lining.

    • Like 1
  6. The length of the burner tube matters. There is a pressure loss (resistance to flow)  associated with the length of the pipe and the flowrate. The maths gets pretty complex pretty quickly. 

     

    The gas issuing from the jet generates a low-pressure zone around the gas stream that draws air in. The gas and air mix with the gas slowing down and the overall gas/air mixture retaining the momentum of the original gas flow. This effectively results in a (very) small overpressure that drives the turbulent mixture along the burner tube towards the forge. "Some" length of burner tube is necessary to get adequate mixing, but the longer the tube is, the greater the pressure loss along the tube. "About" 8 pipe diameters seems to be the length that most of the homebuilt NA burner designers have found works best. Elbows and other fittings offer a restriction to flow that can be expressed (for "simplicity", though that is a relative term) as an equivalent length of pipe.

     

    The combination of pipe lengths and the elbow looks, to my strictly inexpert eye, to be equivalent to about 5 or 6 times the "works best" value.

     

    The (fuel) gas pressure is so high that the gas will flow regardless. The extra resistance to flow will therefore affect the airflow almost exclusively, reducing the amount of air relative to fuel.

     

    Going to a smaller gas jet should help to get the mixture back towards a "good" mixture.

     

    If the resistance to flow in your long pipe is high, the mixture speed will be relatively slow. The mixture needs to move towards the forge faster than the flame-front can move through the mixture in the opposite direction. If the flamefront moves faster through the mixture, the flame will run back down the burner tube until it runs out of mixture to burn (somewhere near the gas jet, where the gas and air have not had the time/distance to form a flammable mixture). The flame will go out, mixture will start to flow. After a time, it will reach the hot forge chamber and ignite, whereupon the process will repeat. I am guessing this may be the pulse jet effect you mention. 

     

     

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  7. It would be nice to see the position of the gas jet relative to the end of the long threaded nipple with everything assembled.

     

    Looking at the photo, it seems that with the brass sleeve-thing inserted into the threaded nipple with enough sticking out at the back end to get the hose on, the jet will be somewhere close to flush with the end of the threaded nipple? I suspect that'll cause all kinds of turbulence right where you don't want it. 

     

    I think you probably want the full length of the mig tip exposed to allow air to be drawn relatively smoothly into the low-pressure zone created by the gas coming out of the jet.

     

    Once you've got it something like, there are plenty of adjustments available with that design.

     

    If the installed burner is similar to the right-hand one in the first pic, there's a very long length (in total) of pipe on it plus an elbow. A screwed 90-degree elbow is usually considered to be equivalent to "about" 30 pipe diameters in terms of pressure loss IIRC. I think the left-hand one in the top pic is probably pretty close to convention at "about" 8 pipe diameters. The one on the right looks to be equivalent to perhaps 40-50 diameters. 

    • Thanks 1
  8. There was a group buy of 2" x 36" Multitool attachments on the now-defunct British Blades forum a few years ago. I think it was an importers stock clearance, but the price was a fraction of list (I think they were about 60 quid each). I bought 3 of them, and a 4" x 36.

     

    One of the guys on the forum came up with a kit to fit the MT to an IEC metric 80-frame foot-and-face-mount motor. It was a pretty simple kit: basically a few bits of plasma-cut 6mm plate, a few fasteners and a bit of 1mm shim stock to wrap round the 19mm motor shaft and adapt it to the 3/4" bore of the wheel. I bought 2 of them. They worked brilliantly, though the shim thing was a lot of faffing about and I just used Loctite on the second one I built.

     

    I used 0.75 kW (1 HP), 3-phase, 4-pole motors because I had a number of different VFDs available. If I was doing it again now, I would use a 1.1 kW (1.5 HP) 2-pole motor and a Sensorless Vector VFD.

     

    I mounted the third attachment to a really cheap, nasty 1/2 HP bench grinder. It beats the wotsits out of having no belt grinder, but is definitely underpowered. I had a try of one that someone had put on a 900-Watt bench grinder and that was a properly useful tool in a fab shop.

     

    The MT kits we had were for a 3/4" shaft with reducing bushes supplied to fit smaller sizes. I gather there have been other kits that will only go to a maximum 5/8" shaft, so cannot be readily used on an 80-frame motor. It's worth checking maximum shaft size on the one you are considering.

     

    In your shoes, I'd get the Multitool, mount it to a cheap/available bench grinder, then save for a motor and VFD. Have a think about making an adaptor plate in the meantime. That way, the only thing that you'd need to buy that wasn't on the direct upgrade path would be the cheap bench grinder.

     

    Even on a 1/2 HP bench grinder (a little under 3000 RPM, assuming you are on 50 Hz mains), it's able to stock-remove for knifemaking faster than most beginners can do with files. It does mean that you get to develop grinding skills, rather than filing skills.

  9. As mentioned, some systems have a built-in flow-limiting valve. It is broadly similar to the "air fuse" sometimes fitted to pneumatic systems before a hose. The idea being that if the hose gets cut, the air fuse will close and limit the airflow, so you are not having to deal with several yards of angry hose thrashing about and trying to beat you to death. It will limit the flow to some (fairly small) fraction of the maximum design flow and will automatically reset when the flow drops to zero (or near-zero). With a gas system, the potential is for several yards of angry hose with several feet of even angrier flame issuing from the end, so it's a pretty good safety feature really. 

     

    If you have such a system in your cylinder, it will be designed to limit the flow once it gets over the maximum design flow for the cylinder. The supplier should be able to tell you what the maximum design flow is. You can then calculate what your burner takes at different pressures using the BTU calculator on Ron Reils site (other calculators are available) and see whether or not it'll do what you need. It is always a good idea to back off the regulator before closing the cylinder valve and to open the cylinder valve before increasing the pressure with the regulator. Opening the cylinder valve with the regulator turned up can cause an initial flow surge that may be enough to trigger such a safety system.

     

     

  10. The "Inverter duty and TENV motors" curve in Cody's second graph will almost certainly be for Non-Ventilated motors and for fan-cooled motors with a separately-powered cooling fan: effectively a mains-powered, fixed-speed fan which maintains the cooling airflow regardless of the motor speed.

  11. The VFD will effectively vary the Voltage and the Frequency together from minimum speed up to the rated speed of the motor. The maximum Current (in Amps) is whatever the motor is rated at, regardless of speed. Power can be thought of (in slighly oversimplified terms) as Volts times Amps. As the Volts go up, and the Amps stay the same, the power goes up. This is the "constant torque" part of the torque/speed/power diagram: Power is also Torque times RPM.

     

    Once the rated motor speed is reached, the rated Voltage is also reached. The VFD cannot increase Voltage beyond this, but can still increase the frequency. Power is again Volts times Amps. This is the "constant power" part of the torque/speed/power diagram. Power is Torque times RPM, so as the RPM increases, the torque must reduce.

     

    In real terms, the 1750 RPM 2 HP motor and the 3500 RPM 2 HP motor will both be giving 2 HP at 3500 RPM. Both will be giving the same torque because power is torque times RPM.

     

    Most of the heat generated within a motor is down to the Current (Amps) being drawn. At full Current (Amps), the heat generated will be broadly similar, whether the motor is running at maximum speed or at minimum speed.

     

    For TEFC motor, the cooling airflow moved by the cooling fan will vary with the speed of the shaft.

     

    At full rated speed, there is plenty of airflow and the motor can run at full-load continuously. As the speed reduces, the airflow reduces and the ability of the motor to cool itself reduces, such that the motor will need to be derated at lower speeds.

     

    Grinders tend not to be run at constant maximum torque, even at high speed, but particularly not at low speeds for a number of reasons: usually the operator has some degree of mechanical sympathy, workpieces get hot and need to be cooled down (during which the motor is under minimal load), belts get changed, etc, etc.  This means that in "our" application, the derating usually happens without anyone needing to think about it.

     

     

  12. I think the consensus view on the internet (insofar as such a thing is possible) is 7 times the burner port area as the minimum for the combined openings. In most cases, the need to have an opening big enough to get the workpiece in and out will mean that the area of the openings comfortably exceeds this.

     

    Taking a worst-case scenario with a 3/4" burner: the thinnest wall, largest inside-diameter pipe anyone is likely to use is Schedule 5, which has 1.050" OD and .065" wall thickness for an ID of 0.92" and an area of 0.66 sq in. Multiply by 7 to get 4.7 sq in. A 2 1/2" x 2" opening will be enough, even with a 5/8" rebar handle in it. 

     

    From the photo, it looks like the front hole you already have may be big enough, even if you close up the back hole when doing short stuff, and I'm guessing it'll need to be enlarged for workpiece access anyway?

     

  13. What sort of burner(s) are you thinking of? 

     

    Naturally Aspirated, I'd go one (bigger) burner every day. It is much easier (and cheaper) to build a single adequate burner than it is to build two identical burners. 

     

    I don't know what size your forge will be (I'm in the UK so "propane tank" is not a size I am necessarily familiar with), but for bladesmithing I'd guess you are looking at two 3/4" burners or one 1" burner.

     

    If you are going for blown, either one or two ports will work fine, so long as you keep everything downstream of the mixing point perfectly symmetrical.

     

    Ribbon burners are very good at keeping down the flame length. Whilst many swear by them, they are not exactly easy to get right (they look dead simple, but there is actually a lot going on and the opportunities to louse things up are much greater than most folk realise). I'd recommend starting with (a) more conventional burner(s) and only move onto a ribbon burner if your first 2 or 3 forges show you that you would benefit from a ribbon burner in your application.

     

    If you can get BSP fittings in your location, an Amal atmospheric injector from Burlen in the UK will form the basis of a very good burner indeed. The ones jetted for Butane work better than the ones jetted for Propane in "our" application.

     

    If you are building a burner to plans, make sure you follow the plans EXACTLY. If there are different jet sizes suggested for a NA burner, always start with the smallest: it will almost always get hotter and use less gas than the bigger one.

  14. When you “increased the pressure a little bit”, what was the proportion?  It will need a big change in pressure to make a significant change in flow/mixture speed.

     

    If you poke out the gas jet, be very careful. The diameter, shape and surface finish of the jet are all important. If you change anything by poking it through with something inappropriate, you will alter the air:fuel ratio of the burner. I usually end up using Copper wire to poke out gas jets because it is soft enough not to scratch the bore but stiff enough to work. 

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  15. If you get the same with no wind, one thing you might try is increasing the gas pressure. Burnback is usually because the flamefront travels through the mixture faster than the mixture is moving through the burner tube in the opposite direction. Flame speed depends on several things: air:fuel ratio, pressure, temperature. Pressure in the burner tube is about atmospheric and you don't want to be messing with the air:fuel ratio if it's right. Initially, things are cold and the flame speed is slow. As the forge heats up, the flame speed increases. If the flame speed gets fast enough, it will travel back until it runs out of mixture and will go out. Fresh mixture will flow and will ignite when it reaches the hot forge. When the flame travels back along the tube, it heats the tube a little. Often there will be a burnback, followed a few seconds later by another, followed by more at reducing intervals as the tube heats, until the burn is just a rapid succession of burnback cycles (or sometimes the flame stabilizes within the burner tube). Increasing the gas pressure at the first sign of burnback might get the mixture speed up enough to prevent the problem. I would aim to double the pressure if tying this. Gas flow through a jet varies as the Square Root of the pressure difference across it, so doubling the pressure will give about a 41% increase in gas flow and therefore mixture flow.  

  16. Rather than use gas, which as Alan says is relatively difficult to control because the temperatures needed are much lower than the flame temperature and the exhaust gases need to be able to get out, would you not be better off using Mineral Insulated Rod-type electrical elements and a PID controller? 

     

    I'm pretty sure I could do it either way if I needed to, but I'd certainly be inclined to do it electrically because it's so much easier. The only reason I can think of to do it with gas would be because mains power is unavailable in the location. 

     

    I would not try to partition the forge/oven for smaller items because the additional variables will be a pain to deal with. 

     

    I have built electric HT ovens and gas HT "forges" for Austenitizing. I rather like the idea of using gas forges with a seriously reducing atmosphere for HT of Carbon steels (I'm pretty sure it can all-but-eliminate decarb), but it is not without its problems. The most serious of these is probably the massive levels of Carbon Monoxide produced: A perfectly tempered blade is good, but it tends to be better if the maker survives to do the other things that are needed to produce a finished article. I only ever use a gas HT forge outdoors. 

     

    Search ebay for "6802 thermometer" and put the money saved towards a good thermocouple. The thermocouples supplied with the 6802s are often not much good for us at all. Sometimes you'll get a couple of PVC-insulated thermocouples which are only good to about boiling water temperatures. Sometimes you'll get glassfiber-insulated thermocouples that are good to about 400 degC/750 degF: useful for checking the temperature of a tempering setup. The ones pictured in the HF link look like the glassfiber ones.

     

    I would recommend an Omega KHXL-14U-RSC-24 thermocouple assembly. https://www.omega.com/en-us/sensors-and-sensing-equipment/temperature/sensors/thermocouple-probes/khxl-nhxl/p/KHXL-14U-RSC-24?searchterm=KHXL-14U-RSC-24

     

    It is not cheap, but it's not very much more expensive than the HF pyrometer in the HF link. Because it comes with a fitted cable and plug, there is no chance of wiring it wrong.

     

    It is well worth making the time to talk to the technical sales guys at Omega and order by phone. They know their stuff and are helpful/patient, IME. If you need a bespoke thermocouple, they can supply and costs are very reasonable.

  17. The best hydraulic system I have ever seen for a limited-power forging press is the Anyang design, which uses a swash-plate pump with variable displacement. At low pressure, the pump is at full stroke and the ram moves fast. As the resistance increases, the pump stroke decreases, the ram speed decreases and the pressure developed increases. This means that the system is able to use the full power of the motor throughout the cycle.

     

    I think the 25-tonne Anyang press uses a motor that is either 4 HP (3 kW) or 4 kW. It is pretty impressive and I get the feeling it probably out-performs most 10 HP presses and many 15 HP forging presses (though I have to confess to pathetically little actual forging experience myself).

     

     

    All of the fixed-displacement-pump systems that I have seen move at a fixed speed and only use a fraction of the available motor power until the resistance is very close to maximum.

     

    2HP is pitifully little power for a forging press.

     

    I would be looking for ways to increase the available power, rather than trying to build something that will work poorly, at best, on the power that you already have available. If a bigger electrical supply is out of the question, could you use a gas- or diesel-engined power pack for instance? 

     

    Work is Force times Distance. Power is work per unit time. I struggle a little with US units, so we'll convert to units I understand.

     

    I think a US ton is 2000 Lb (as opposed to the 2240 Lb of the British ton or the 1000 kg of the Metric Tonne) 16 US tons is therefore 32000 Lb

     

    32000 Lb is 14,545 kg and each kg of mass exerts a downward force of 9.8 Newtons at the surface of the Earth.

     

    32000 lb (force) is therefore (14545 x 9.8) = 142545 Newtons

     

    2 HP is 1500 Watts (as close as makes no difference).

     

    Power(Watts) is Work done(Joules) per unit time(seconds), so 2 HP(1500 Watts) is 1500 Joules/second.

     

    Distance is Work/force, so the distance over which 2 HP can exert 16 tons of force in one second is 1500/142545N = 0.0105 metres.

     

    That gives a speed of 10.5 mm/sec for a 16-ton ram driven by a PERFECTLY EFFICIENT system: about 0.4 inches/second.

     

    Perfectly efficient is unattainable.

     

    It's over 30 years since I played with Hydraulics as part of my job, but I don't ever recall them being particularly efficient. Even allowing for 30 years of development, I don't think there would be much chance of exceeding 1/4" per second with a real-world 2HP, 16-ton system.

     

    Hydraulics are usually used because they offer a convenient way to achieve a numerically large mechanical advantage.

     

    Levers are usually very efficient compared to Hydraulics, but trying to combine the two seems like adding a lot of complication. If you want to be able to keep the stroke short with different thicknesses of workpiece, you could use adjustable limit switches to limit the return stroke or, if you are using a very basic power pack with just a relief valve, threaded adjustable stops on the return stroke. 

     

     

     

     

  18. First question is whether the motor is wired correctly? Over here, most small 3-phase motors are wound to be run at 230V in Delta or 400V in star (wye).

     

    Connecting a motor over here to 230V in Wye gives a similar problem to yours, easily sorted by reconnecting in Delta. I am in the UK and strongly advise you to avail yourself of local knowledge.

     

    I'm not sure whether the high Voltage being double the low Voltage on your nameplate means there are 2 sets of windings, to be wired in parallel for 230V or series for 460V. Check your wiring diagram.

     

    Are the PBxx settings correct for your motor?

     

     

     

  19. That looks old. I would not try running it on a VFD unless a Sine-Wave filter was used in the circuit.

     

    What follows is "as I understand things": somewhat oversimplified and perhaps not technically accurate, but close enough for me to get my head around. YMMV and it is worth researching things further if you are intending to go that way. 

     

    Old motors often used a brittle Shellac-based impregnation material for the windings. I think some of the early synthetic impregnation materials were also rather brittle. Later polymeric resins are much "tougher". My rather limited experience is that post-1980-ish motors are no problem with VFDs. Pre-1960-ish motors are a problem and between 1960 and 1980, things are unclear.
     

    On a true sine wave supply (mains), the voltage rise is quite slow. As the current rises, the magnetic fields around each of the coils rise and interact with the other coils in the winding. I think they cause the windings to move towards each other, compressing the insulation, with each peak (positive and negative), causing the characteristic transformer hum tone. 

     

    VFDs use PWM to switch a DC Voltage on and off very fast (most modern VFDs switch at 4 kHz and up), giving an almost instantaneous Voltage (and current) rise time. The force exerted is dependent on the rate-of-change-of-Current, so the mechanical forces on the insulation are very much higher than when run from "real" mains power. On a motor with brittle insulation, the forces are higher than the insulation can withstand and tiny cracks form.

     

    The upshot is that old motors tend to fail very quickly when run from VFDs. There are some other factors that come into play as well, but I can get my head around the one I've described and it has been sufficient to keep me from going down that particular rabbit-hole.

     

    A Sine-Wave filter can be fitted between the VFD and the motor to smooth out the high-frequency stuff and leave a sinusoidal waveform very similar to "true" mains (though still with the Variable Frequency that we want for speed control). These will often let you run an old motor on a VFD. Sinusoidal filters typically seem to cost "about" as much as the VFD with which they are used, so they are not cheap. 

     

    If an old motor has been rewound, it will have been done with the materials used at the time, so a 1935 motor rewound in 1995 can be treated as a 1995 motor (unless it was specifically rewound with 1935 materials as a museum piece, for example). 

  20. 2 hours ago, Tristan T said:

    Also, any suggestions on how to enlarge the size of the hole which the propane goes through?

     

    It's not entirely clear what you mean here. Do you mean the size of the hole through your "refractory" and into which you stick the torch, or the size of the gas jet in the torch?

     

    I once made a 2-brick forge and tapered the hole for the torch to go in. By moving the torch in and out of the tapered hole (wider at the outside), it was possible to vary the amount of air that was drawn in and thereby vary the temperature. (tip: in most cases, more air = hotter)

     

    The problem with using a torch, which is designed to work in open air and draw secondary air, is that it usually can't get enough secondary air to burn with the gas when used in a forge. If you open out the gas jet, you will almost certainly make things worse: even more gas and no more air producing an even richer burn and lower temperature.

  21. On a Venturi burner, knowing the pressure is useful, rather than essential, mainly to allow the operator to start the forge, set it to where it was last time it did the same job, and get on with something else while it gets to temperature.

    I always used to fit a gauge at the mixer (and still do when I want other folk to see what is happening with the burner), but now use plugged welding regulators on my forges. These have graduations marked on the body so that the skirt of the adjusting knob shows the pressure they are set to. They have the advantage of being built "industrial", with a nice big adjusting knob intended for use by someone wearing heavy welding gloves. 

    It sounds like a pretty crude way of knowing the pressure, but there are not many gauges that provide significantly more accuracy/repeatability at reasonable cost. 

    I like to keep all of my controls at the gas cylinder end: it's the direction I head if things start going wrong and I don't want to introduce the need to make an extra decision while the midden is hitting the windmill. I'm also not keen on adding weight to the end of the burner. 

    There are various different gauges available. As said, very few will offer good accuracy at anything like reasonable cost. I tend to use glycerine-filled gauges with 1/4" bottom connections. These have stainless steel cases with sufficient structural integrity to contain the glycerine and the 1/4" connection is a lot harder to break than a 1/8" one. I have seen 1/8" back-entry gauges, salvaged from the regulators of old compressors, fitted on burners and try to keep my distance.

    If I *really* want to know what the pressure is doing, I use a 4-20 mA pressure transmitter and a datalogger, but I'm probably a little more anal than most smiths.

    It is worth pointing out that gas flow through a jet does not vary linearly with pressure, but as the square root of the pressure. If you want to double the gas flow, you need 4 times the pressure. Doubling the pressure will give 1.41 times the gas flow and halving it will give 0.707 times the gas flow. 

    If you want to spend money to find out what is happening with your forge, you will almost certainly get more bang for your buck by investing it in a decent thermocouple and pyrometer.

  22. Wherever you buy your thermocouples should be able to supply them. Otherwise search for "thermowell". I'm the wrong side of the pond to recommend suppliers.

    Although they are used a lot in industry, they are not often the best solution in "our" applications.

    Thermowells tend to slow thermocouple response and tend to be very spendy in anything more exotic than 316 SS. Mineral Insulated thermocouple asssemblies are often a better option. 

    I strongly recommend spending a few minutes scrutinising and understanding the process you are dealing with and the temperature you need to measure, before sitting down with a cup of coffee, notebook, pen and calling the technical sales folks at Omega to ask their advice. 

    It is worth mentioning that bespoke thermocouples are pretty normal in industry, so the premium for non-standard is usually surprisingly small. It does mean dealing with a specialist temperature control business, rather than a box-shifter, but this gives you access to their technical knowledge and experience. 

    • Like 1
  23. 230V single phase at 50A is 11500 VA: There will be a power factor to consider, so probably not enough to run a 15 HP (11,250 W) motor via a VFD, but more than adequate for any motor/VFD you might fit to a 2" wide belt grinder.

     

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