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  1. #101
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    Quote Originally Posted by 9100 View Post
    Now you have it. With a free wheeling motor, the sort of phase relationship I diagrammed will start the motor turning and once it does, the situation will keep improving until it gets up to speed. This case seems to have large starting friction, which keeps it from starting to turn, Forget the idler. At this point it is only along for the ride. View the operation as just starting a three phase motor off the line with a large starting capacitor. I said 1,000 mfd or more. Actually, my back of he envelope calculation is more like 2,000. You could have the idler in the circuit and switch the large capacitor in to start, but it would probably be simpler with less chance of frying something to simply have the idler running but not connected and start on the capacitor alone. Once it is moving, switch to the idler for running.

    Bill
    I don't buy ALL of that. More to it. 2,000 MFD is a "high inrush starting load" in its own right.

    If it was that easy STATIC converters would start loads better than RPC's. Ordinarily, they do not. A "hybrid" would want some critical timing.

    Starting under load is such a big deal, I'd just bring it up off its knees with a Dee Cee motor as "launcher". Or even sole and only.


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    I havenít seen any mention of this yet what is your main panel amps into the building? If itís a 100 amp garage setup you donít have enough power for the rpc to work in this scenario. Iím guessing this is a automotive belt sander/ surfacer. If Iím wrong sorry Iíll shoot myself in the foot for the rest of you. If it has a aggressive belt going to start way harder. When I started my shop I had same issues with starting machines with big motors. Donít have the 3p stuff weíre Iím at cost prohibitive to bring it in. Long ramble short I upgraded to 400 amp single phase, 60 hp rpc problems are gone run 3 CNCs, can run my Sidney lathe with its big ass starting load no problems. Oh ya if your power company doesnít have a big enough tub on the pole quit now wonít work. Residential commonly has several homes running off one tubĒ maybe 15-20kw split 3-4 times.


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    Figuring the inrush current, the motor has about 1.2 ohms resistive/reactive. You need to swing the vector marked 85 on the diagram up to 90 degrees, straight up, and 208 volts. That takes about the same capacitive reactance, which comes out in the neighborhood of 2,000 mfd. Somewhat less if you have the idler contributing. Switch capacitors while burning incense to Terman. AC circuits 101.

    A capacitor start will kick a motor off as well as an RPC. The RPC allows you to leave it in that configuration longer as the motor picks up speed.

    There was a guy in the neighborhood who had a Bridgeport running on single phase. He had a starting capacitor, which started it off fine, but he didn't change after getting up to speed. In that state, motors run slower, overheat, and make a peculiar death scream. He blithely went on running that way.

    Bill

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    Quote Originally Posted by Turbowerks View Post
    I haven’t seen any mention of this yet what is your main panel amps into the building? If it’s a 100 amp garage setup you don’t have enough power for the rpc to work in this scenario. I’m guessing this is a automotive belt sander/ surfacer. If I’m wrong sorry I’ll shoot myself in the foot for the rest of you. If it has a aggressive belt going to start way harder. When I started my shop I had same issues with starting machines with big motors. Don’t have the 3p stuff we’re I’m at cost prohibitive to bring it in. Long ramble short I upgraded to 400 amp single phase, 60 hp rpc problems are gone run 3 CNCs, can run my Sidney lathe with its big ass starting load no problems. Oh ya if your power company doesn’t have a big enough tub on the pole quit now won’t work. Residential commonly has several homes running off one tub” maybe 15-20kw split 3-4 times.
    it is an automotive belt sander/surfacer - commonly called platen grinder. i have [email protected] service. could have been 400 but i only run one thing at a time these days.
    don't have any tubs or poles though - just a big green thing in the middle of the property :-)

    9100:

    even though i always sat in the first chair during my 1968 electronics class it was a long time ago and i didn't pursue that career. don't remember discussing terman but transistors were new on the market (per my retired army colonel teacher)

    any specifics about exactly what rating and where this capacitor goes? i would think i could engage it with the foot switch along with a timer
    thanks

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    There is a very easy, likely very cheap, solution.
    Get a suitable vfd and a motor to match new.
    Forget the old stuff, ebay will get you 1-200$ for it.

    New stuff is very cheap, and cheaper relatively as the hp goes up.

    A 10 hp setup might cost 5-600$, minus 1-200$ that you flog off.
    Mucking around is a loss of time, intelligence, and effort.



    Quote Originally Posted by cg285 View Post
    it is an automotive belt sander/surfacer - commonly called platen grinder. i have [email protected] service. could have been 400 but i only run one thing at a time these days.
    don't have any tubs or poles though - just a big green thing in the middle of the property :-)

    9100:

    even though i always sat in the first chair during my 1968 electronics class it was a long time ago and i didn't pursue that career. don't remember discussing terman but transistors were new on the market (per my retired army colonel teacher)

    any specifics about exactly what rating and where this capacitor goes? i would think i could engage it with the foot switch along with a timer
    thanks

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    When I redid my shop power I set it up for a 60 hp rpc ended up using a Phoenix phase panel and a toshiba 60 hp idler. When talking to Phoenix first thing he asked was how big is you transformer on your service? I know I know just should have bought a American rotary like all the sponsored u tubers have should have worked great right? But.... after hearing why you need a bigger transformer and larger rotary than your max hp amps it made perfect sense, so if you have a 20 kw transformer. ( big green box) thatís only roughly enough to start you rpc if it is a 20 hp unit with no other load on the system. 200 amps in a hous with a 200 amp shop/ garage no problem until you start the rpc and throw another 20 hp at it. Many sights have not been updated from the 60 amp 1960s power requirement. But speaking from experience my situation improved 10 fold when I went from a 30 hp rpc to a 60 hp rpc the power company had to switch out the green box but my power bill is no more maybe less. Itís only money canít take it with you and your wife will just blow it.


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    [QUOTE=cg285;
    even though i always sat in the first chair during my 1968 electronics class it was a long time ago and i didn't pursue that career. don't remember discussing terman but transistors were new on the market (per my retired army colonel teacher)

    any specifics about exactly what rating and where this capacitor goes? i would think i could engage it with the foot switch along with a timer
    thanks[/QUOTE]

    The Terman I referred to was Fredrick Terman, who taught at Stanford. He wrote the standard electrical engineer's textbook and encouraged his two star pupils to start a business. They did, in a garage, a little rinkydink one called Hewlett Packard.

    Re the circuit, connect the two incoming lines to two of the inputs to the motor and a capacitor from one to the third input. You should recall that when you put AC on a capacitor the current through it will lead voltage. That produces the third phase. If you get the capacitor matched to the current drawn by the motor, it is a good simulation of a standard three phase service. Since the motor draws more current at startup, you need more capacity, which then needs to be reduced after it gets running. You should feed the power through a contactor capable of breaking the circuit when it is several hundred amps. You don't want a contact welded. I suggest that you start with a capacity that will start the machine without a load, say 100-200 mfd, then add scrap iron on the belt until it won't start, then add more capacity to make it start, increase the load, etc. until you reach the heaviest load you anticipate.

    That will establish the starting circuit, then you need to switch to an amount of capacity that will give good running with parts on the belt. You can switch in the idler at that point, which will give you a wider range of load tolerance.

    The capacitors need to be AC type. They come in two flavors, electrolytic and oil filled. You can use electrolytics for starting but they don't like continuous running. For running, use oil filled. oil filled can also be used for starting but they are larger for a given capacity and cost a lot more. The voltage rating has to be higher than they will be used. Higher doesn't matter.

    Do not try to use DC capacitors such as are used in DC power supplies. They will blow up and spray noxious electrolyte all over you.

    You can scrounge them from HVAC shops. They throw them away when they replace air conditioners. You can also find them in scrapyards.

    There are all sorts of designs in the first thread of the stickys beginning this forum and countless entries in the general discussion.

    Bill

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    That is likely to work.

    It is also likely to draw a bunch-o-current on startup. If that is an issue for the source, another method may be better.

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    It drew a lot of current in the previous installation. Remember the comment that it made lights dim? Any direct drive system is going to have to cope with the start up friction. Any other solution will require some sort of geared down motor to break it loose or a clutch to allow the motor to spin up and use its inertia to get things moving. In short, a mechanical modification Any sort of soft start or ramp up like a VFD is going to ramp up to the torque and then it will start to turn if the supply is good enough. None of those systems multiply starting torque.

    TANSTAAFL

    Bill

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    Quote Originally Posted by 9100 View Post
    It drew a lot of current in the previous installation. Remember the comment that it made lights dim? Any direct drive system is going to have to cope with the start up friction. Any other solution will require some sort of geared down motor to break it loose or a clutch to allow the motor to spin up and use its inertia to get things moving. In short, a mechanical modification Any sort of soft start or ramp up like a VFD is going to ramp up to the torque and then it will start to turn if the supply is good enough. None of those systems multiply starting torque.

    TANSTAAFL

    Bill
    Yah but... the OP is in the wheels biz. "We" actually "dig" clutches and torque conversion. By mechanical means.

    Even so, it would probably be easier to just swap the AC motor for a salvaged RPM III Dee Cee motor ... with a shaft reducer and extender. Doubt there is physical space inside the grinder.

    "Funny thing" but a FIVE HP RPM III has the same shaft diameter as a TEN HP 3-Phase motor. You don't suppose that coming off zero RPM / locked-rotor with several times as much Torque has anything to do with that do yah?

    OTOH, as you said "TANSTAAFL".

    That 5 HP RPM III weighs several times as much in 180 VDC winding, takes twice the space of a 3-P AC motor as well!

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    Well, the good news with a VFD (which I do not willy-nilly recommend as a fix-all), is that it will apply at least full current and torque at low frequency, without pulling huge currents from the source.

    So it will be a soft-start, with the potential for larger currents during start..

    The "hard start kit" DOES get stuff started, but pulls big current in the process. Soft start kits generally do not have the torque, as mentioned earlier. There's your TANSTAAFL.

    The VFD has an unfair advantage, in that it will change the frequency, and GET the full current into the motor at low speed and reasonable input current. If the VFD is oversized, as may be the case with a single phase source, then the OUTPUT current in that condition can be more than normally allowed.

    Most VFDs have a 150% current rating for maybe a minute. I'd hope the thing would start in a minute, that's a long time. A double rated (50% derate) VFD can therefore supply 300% of FLA for that minute or so. That has a good chance of getting the motor going.

    And, because of the way VFDs operate (like a "buck regulator"), the INPUT current at low output voltage is not the same as the output current. So the input current drawn from the mains is going to be substantially less than pretty much ANY other approach.

    You can change the accel rate to keep the output currents under control, and you can find VFDs that will hold at the max current set in a parameter, so they do not just fault out.

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    Quote Originally Posted by JST View Post
    Well, the good news with a VFD (which I do not willy-nilly recommend as a fix-all), is that it will apply at least full current and torque at low frequency, without pulling huge currents from the source.

    So it will be a soft-start, with the potential for larger currents during start..

    The "hard start kit" DOES get stuff started, but pulls big current in the process. Soft start kits generally do not have the torque, as mentioned earlier. There's your TANSTAAFL.

    The VFD has an unfair advantage, in that it will change the frequency, and GET the full current into the motor at low speed and reasonable input current. If the VFD is oversized, as may be the case with a single phase source, then the OUTPUT current in that condition can be more than normally allowed.

    Most VFDs have a 150% current rating for maybe a minute. I'd hope the thing would start in a minute, that's a long time. A double rated (50% derate) VFD can therefore supply 300% of FLA for that minute or so. That has a good chance of getting the motor going.

    And, because of the way VFDs operate (like a "buck regulator"), the INPUT current at low output voltage is not the same as the output current. So the input current drawn from the mains is going to be substantially less than pretty much ANY other approach.

    You can change the accel rate to keep the output currents under control, and you can find VFDs that will hold at the max current set in a parameter, so they do not just fault out.
    WTF.. And all these years I wasn't even aware a VFD came with its own mini-nuke or tap on leis force or black hole energy bursts inside of it to deliver more power out than it can pull IN from the line.

    No damned WONDER they are treated like a magical new religion!

    Wonder if I can heat my home and shop for-cheap off that independent power source next winter?


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    Quote Originally Posted by thermite View Post
    WTF.. And all these years I wasn't even aware a VFD came with its own mini-nuke or tap on leis force or black hole energy bursts inside of it to deliver more power out than it can pull IN from the line.

    No damned WONDER they are treated like a magical new religion!

    Wonder if I can heat my home and shop for-cheap off that independent power source next winter?


    The exact point you make is THE point....... Betcha didn't realize that It's the magic of SMPS'.

    SMPS and VFDs, basically transform power. Input power always equals output power plus losses.

    The VFD is at low frequency... the V/Hz curve has that at a low voltage to the motor. But the input power comes from the 230V (or whatever).

    So the VFD puts out say, 10A and is at 20V output. That's not much "power" , but it's 10A worth of torque. So what is the input current? It sure as heck is not 10A, that would be 2300VA, and it is NOT pulling that.

    Nope, the input is pulses, just like the output, but they are short pulses of current that average out to a low INPUT current. The input power is equal to the output power plus losses.

    No "magic manufactured power".... the difference is because the incoming power is at a high voltage (same power is a low current), and the output power is at a low voltage (same power is a higher current) when the VFD is starting to accelerate the motor.

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    Quote Originally Posted by JST View Post
    The exact point you make is THE point....... Betcha didn't realize that It's the magic of SMPS'.

    SMPS and VFDs, basically transform power. Input power always equals output power plus losses.

    The VFD is at low frequency... the V/Hz curve has that at a low voltage to the motor. But the input power comes from the 230V (or whatever).

    So the VFD puts out say, 10A and is at 20V output. That's not much "power" , but it's 10A worth of torque. So what is the input current? It sure as heck is not 10A, that would be 2300VA, and it is NOT pulling that.

    Nope, the input is pulses, just like the output, but they are short pulses of current that average out to a low INPUT current. The input power is equal to the output power plus losses.

    No "magic manufactured power".... the difference is because the incoming power is at a high voltage (same power is a low current), and the output power is at a low voltage (same power is a higher current) when the VFD is starting to accelerate the motor.
    Sorry. No. Not biting on that one! "Starting to SLOW COOK the motor", perhaps?



    Platen surfacer. Load on it. Not MEANT to start unloaded.

    Think "brakes." Set. Hard.

    No free lunch to see there when the motor isn't free to move.

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    What I was taught in sparkie school is that with reduced voltage mains frequency starters (primary resistance, autotransformer, star/delta, electronic etc.), your starting torque is proportional to the square of winding voltage. Winding current is directly proportional to winding voltage.

    So a resistor or soft starter sized for 50% voltage will give you 50% of starting current and 25% of starting torque.

    Note that starting torque is typically ~180% of full load torque, so 25% of starting torque gets you 45% of FLT. Likewise, starting torque is e.g. 6x full load current, so you get 1.5x full load current.

    Autotransformers and star-delta are a special case, as they multiply the current through transformer action. So at 1/1.73 (57%) tap (your only option for star/delta), you get 0.57^2 = 33% torque, 57% current through the motor windings, and 33% drawn from the supply.

    The above all refers to balanced three phase. I don't know what RPCs do to it. Probably bad things.


    On the other hand, VFDs can alter the frequency, and act as a transformer. Starting torque and current are the same as full load torque and current and I think directly proportional, but at a small percentage of full load voltage.

    So a VFD sized for 'high overload' (generally 180% FLA for 3 seconds, low overload is 150% for 3 seconds with most brands) might deliver 180% winding full load output current, 180% full load torque (same as DOL starting torque) and this at less than 10% full load voltage - so drawing ~18% of full load line current.

    Double the VFD size - remember, when we derate a VFD for single phase input, it's only the input stage. So the output stage can still deliver its full current, at low output voltages.

    Now your VFD can deliver 360% of full load torque for 3 seconds and 300% torque for 1 minute (if sized for high overload) 3.6x full load torque, 3.6x full load output current (still half of what the motor sees DOL), and drawing around half the full load line current. Just find out what the rated shock loading on the motor shaft is - you might snap it...


    VFDs are magic.

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    Quote Originally Posted by SomeoneSomewhere View Post

    Now your VFD can deliver 360% of full load torque for 3 seconds and 300% torque for 1 minute (if sized for high overload) 3.6x full load torque, 3.6x full load output current (still half of what the motor sees DOL), and drawing around half the full load line current. Just find out what the rated shock loading on the motor shaft is - you might snap it...

    VFDs are magic.
    It's sorry-SMALL magic, 300% to 360%, if-even.

    Per a Reliance Electric & Engineering "white paper", thoroughly tested in their labs, not pulled out of some part of my "personal" anatomy... Dee Cee motors start with an inherent 400% off-the-starting blocks or locked-rotor Torque multiple... simply by their nature.

    And can then deliver SIX to NINE times "nameplate" for 90 seconds.. by which point the commutators bars are subject to so much heat they can rise up out of their intended position off the back of thermal expansion of the Copper. "Wedging" it's called. We've even seen at least one example in fotos, "Right here, on PM", so theoretical it was never.

    No more "magic" in that than comparing a Dexter horse to a Morgan horse. Both can be saddled and ridden ... or genteely pull a one-hoss shay to Sunday go to meeting, all dressed-up. The family. Not the horse.

    But the Dexter has a set of HAMS on the aft-end akin to a Miss teen-age Armenia reality TV star.

    During the WORK week, back on the small farm, a Dexter could pull a plow as would put a HURT on the nicely balanced Morgan.

    "Horses for courses". Literally.

    See also "D9 Cat" vs "F-150". The pickup's HP number vs mass of the ass would appear to give it an edge. Surely the F-150 can outrun the D9 down a paved road by one hell of a speed advantage.

    Now chain them back-to-back.... take up the slack for a cold start at a tug of war...and see which one prevails!

    Hard to move initial load? Two things can beat a Dee Cee motor, every time.

    Clever Hydraulics.

    Dumb Explosives.


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    Yeah, but that generally requires drawing the corresponding 4x full load current from the supply as well.

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    Quote Originally Posted by SomeoneSomewhere View Post
    Yeah, but that generally requires drawing the corresponding 4x full load current from the supply as well.
    More, actually.

    I don't do any extreme starting WHATSOEVER with the 10EE, for example.

    And yet.. the nominal 12 point-something-or-other FLA is on a DC Drive set to trip only after 90 seconds at 24 A draw. At over-boosted Armature (270 VDC vs 230 VDC, nominal) AND boosted Field power (140 VDC vs 115 VDC Nameplate.

    Reserves for enhanced stability under load, not raw chip-rip. Got a 14" lathe with more than double the HP for that, and even that one is about finesse, not BFBI.

    There's no "free lunch" on Dinosaur Current any more than there is with the allegedly "magical" VFD.

    There are just motors and drives with different attributes. Some better suited to certain challenges than others.

    Type T straight shunt DC motor on the 10EE only hits about 73% efficiency at "base RPM", for example. Much less when run slower. Part of the price one pays for smooth, wide power band with gobs of "reserve" Torque.

    It could be series-wound Dee Cee as best-suited to bringing a high-friction starting load up off its knees the "simple, low-tech" way. See the history of electric "traction" use, over the rail.

    Fast forward to the last 30 years - underground and surface commuter rail with a starting "whine" that goes rapidly up until clear off the upper scale of human hearing?

    "Electronics involved". Plus insanely high RPM and costly gearing. No longer simple-cheap. That's about bean-counters and spend for the INPUT power.

    OTOH, these public transport network goods aren't being funded off a smallholder's shop capex budget to keep over-age-in-grade machine-tools serving nearly as over-age-in-grade owners.


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    Quote Originally Posted by thermite View Post
    Sorry. No. Not biting on that one! "Starting to SLOW COOK the motor", perhaps?



    Platen surfacer. Load on it. Not MEANT to start unloaded.

    Think "brakes." Set. Hard.

    No free lunch to see there when the motor isn't free to move.
    Not sure what the heck you said....

    Point being that the VFD does do the same job with less draw of CURRENT from the source. The closest competitor on input current is probably the transformer type soft start, but that is a reduced voltage start at full frequency, so is not going to produce good torque.

    The VFD "cheats" by changing the frequency to go along with voltage, so it has constant current capability at all speeds/voltages. During accel, if you have a short term overload capacity, which essentially all VFDs do have, you can provide starting current up to 150% with a 1:1 rated VFD, or 300% with a 2:1 rated (50% derate), generally for a minute, which should be enough..

    The other soft start methods just cannot do that, they either fall off on motor current, or they do not reduce the line current significantly.

    As for the "slow cook" comment, that, and all the other conditions, apply just as much to DC motors. Either they cannot provide the torque, due to current reduction (soft start) devices, or they do not reduce line current, but produce lots of torque.

    Only the VFD can provide a combination of good torque, in excess of normal torque, and also have a soft start (reduced line current) at the same time. It happens specifically due to the "buck" topology of the system, which provides "power transformation" instead of just voltage transformation. The VFD acts electrically a bit like the "back gears" of a lathe do mechanically.

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    Quote Originally Posted by JST View Post
    Not sure what the heck you said....
    That the VFD "free lunch" is only in your theoretical head.

    BECAUSE ...you are comparing an AC motor WITHOUT one to an AC motor WITH one and crowing about the "improvement".

    It is still the wrong TYPE of motor for starting heavy loads from stand still.

    Putting an automatic transmission with a torque converter's multiplication into a pick-up truck does NOT make it the equal of a bulldozer for low-end grunt.

    They are just different, pickups and bulldozers. As are AC induction motors and DC motors.

    "Commercial puffery", nor optimism, can't change the physics.

    No matter HOW you sugar-coat it, "advertise" it, rationalize it? You can only convince those who have never had them BOTH to compare, "REAL world".

    I am not one of those people. AC where it works well. DC where it works well. Other means where NEITHER work, unaided.

    Pragmatic?

    Or maybe I'm just too dammed lazy to argue with the universe that something that doesn't work well SHOULD do because "they say" a VFD is magical?

    Some of y'all "VFD CULT" are sounding like drug addicts or religious zealots about those one-true-answer-and-no-other "magical" solutions.

    There IS NO "one true answer" in power or its transmission. Wishing won't change that.


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