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Determining the direction a motor will spin

beansdiesel

Plastic
Joined
Dec 5, 2013
Location
Woodbury, TN
Trying to get a pony motor hooked up to my massive 75hp RPC. The 75hp motor doesnt have a shaft that sticks out on either side but I have adapted a pulley to one side of it for the pony to turn.

I can bump the breaker (pops instantly of course) and it will slightly spin. The problem I have is I have seen it try and go both directions. I thought I had it figured out, and after many tries finally got the 75hp to run without blowing the breaker. My pony was too small so I got a bigger one and now when I try to throw the big breaker the whole RPC will jump like its trying to spin the other way and the breaker pop is much much more violent. So it worked one time, now it doesnt, I'm so confused LOL.

Is there such thing as spinning the motor too fast? The pulley on my pony is a little bigger so if its actually getting to full speed the RPC will be going faster then its 1725rpms it runs at idle.

My next thing to try is to reverse my pony motor and try it all again that way and see if it takes.
 
Trying to get a pony motor hooked up to my massive 75hp RPC. The 75hp motor doesnt have a shaft that sticks out on either side but I have adapted a pulley to one side of it for the pony to turn.

I can bump the breaker (pops instantly of course) and it will slightly spin. The problem I have is I have seen it try and go both directions. I thought I had it figured out, and after many tries finally got the 75hp to run without blowing the breaker. My pony was too small so I got a bigger one and now when I try to throw the big breaker the whole RPC will jump like its trying to spin the other way and the breaker pop is much much more violent. So it worked one time, now it doesnt, I'm so confused LOL.

Is there such thing as spinning the motor too fast? The pulley on my pony is a little bigger so if its actually getting to full speed the RPC will be going faster then its 1725rpms it runs at idle.

My next thing to try is to reverse my pony motor and try it all again that way and see if it takes.

"Both Directions" is because it hasn't got all three phases that normally tell a 3-P motor which way to rotate. Start cap, however modest, even that still needs a pony motor, can help 'steer' it to make the same choice, each time and not 'fight' the pony. Keeping out-off-phase power OFF it until the pony has got it up to speed should work also.

But there is still an issue as to the kick-out and dramatic theatricals...

I think I've already mentioned this, but the most-likely issue is that the physical pole positions of the spun-up RPC idler are not a match to where the incoming power peaks and valleys would WANT them to be at the time the main is thrown-over to put the juice to it.

It then throws its physical momentum into a brief 'fight' with the mains power as they try to sync-up by slowing-down or speeding up the rotor a partial turn. There's a lot of mass and energy involved. Sometimes they are close, even very close, other times about as far out of phase-match as can be, and it is a dice-roll which one.

50+ years ago, when syncing large 3-Phase Diesel gen sets run in groups of three identical, we had neon indicator lights on each. We'd get the blinking or 'beat' frequency of those to match by tap-dancing on the hand-throttles before cutting-over the transfer switches. Speaking of 3 X 300 kW to 3 X 1 mW AKA serious Mike Foxtrot Diesels.

Fast-forward fifty and more years, there is probably a ten-dollar LED gadget out there that could do the same for your needs so you threw the switch over at a time of lower mis-match.

Someone will know of such goods - or how to build yer own. I've not had the need since 1969, so I am not he.

I'd actually do it differently.

I'd be 'leaking' mains power at reduced but increasing levels into the idler as it approached full speed so that it could have gradually picked up its own phase match BEFORE full power was applied. Goal would be near-as-dammit ZERO mis-match, lower stress on all players, Utility Company included.

No need to watch lights, but that's otherwise more complex and expensive to do.

Someone may sell THAT sort of device off-the-shelf - or publish plans to build yer own - as well.

Again no need here, so I haven't looked. Others will have to help with that, too.

No fear. A solution - or several - IS 'out there' that you can use.
 
Actually, the motor on single phase input (which is what it will have) can go either direction. The single phase doesn't have a specific direction to go, you can reverse it by the direction you start it, same as you do with a single phase motor.

Pick one. Stick with that direction and don't worry about it
 
Going to try and figure out a way to detect the shaft rpms and see where its at then. See how far its really off one way or the other.

RPM helps only to the extent you have 'enough revs'. Even a perfect RPM match to nameplate won't tell you if they are in phase-match. Adding a 'resolver' and comparator to see where mains phase state is in reference to idler phase state is way over the top.

A dual-trace or quad-trace scope could show that easily - or even a single-trace as Lissajous. I have one of each. But that is ALSO over the top for starting an RPC one or more times a day.

What you want is a simpler gadget that monitors one leg of the idler as it is acting like a weak generator. Something fast enough to display the peaks so the phasing is obvious, and really looking actually at the DIFFERENCE between utility-mains peak and idler peak. Slower the blink, darker the lamp, closer they are.

Incandescent lamps have too much thermal inertia, won't display the changes as well. LEDs' or neons do work fast enough.

BOTH need circuitry to protect them from the high voltages involved and to display only the peaks, not just stay on all the time.

A commercial device meant to do the job is recommended.

FWIW - the OEM maker's 'start' or 'start run' caps - either one - basically bypass any such need.

Part of why they aren't such a bad spend, after all. Punch the button, go make chips - even have a bit of Warranty coverage. No need to re-invent anything, then-also have to maintain it for many years with no help.
 
Whoa, there......

An INDUCTION motor has no phase matching if you are talking about checking whether it is up to speed. The stator just "writes" poles onto the rotor as and where it is. The phase just happens, but NOTHING happens until there is power on the stator, unless I have missed something and he has a synchronous motor there.....

But the whole deal is not really an issue. If you get the idler up to anything over perhaps 80% of full speed, it will accel up to speed, so long as there is no load on the shaft, or on the generated leg. It should at considerably lower speeds, but there is a larger current spike the slower you go. At 80% you are pretty likely to be on the "good side" of the torque curve.

There should be NO need to match speeds, or the like. People rope start smaller motors, and all that does is basically "bias" the rotation direction, it's not as if they get the thing going at anything very close to full rpm.

Don't make this more complicated than it needs to be. There are issues, but not in that direction.

Ther may be an issue with the inrush current as the power is applied to the big motor. No telling what that does, but the first effect will be something similar to DC braking, as a big pulse of current goes into the motor.. That is likely to be what you are seeing. HOW big depends on how fast you get the motor running with the pony.

If the motor does not "go" after that transient is over with, then there is a different issue. If the motor just pops the breaker, which is what it seems like, OK, that is what can be cured.

It is not usual to just "drop" a big motor on the line. Much more common to have a reduced voltage start. The simplest way to do that is with some big, but low ohms resistors in series. Cuts the current, and does some of the "leaking current in" deal. the usual goal is to allow between 50 and 60 % of the normal start current and about the same proportion of line voltage. When the power is applied, the only thing limiting current flow is DC wiring resistance, unless you add some external resistance.

The good news is that with single phase input, you may get by with a single resistor, or at least a single group.

Since you are starting with NO LOAD (right?), the motor could actually work with lower voltage and current as the first step. So the resistor could be larger in ohms value than it seems.

I know you probably told us earlier, but to make it easy on us, what is the full load current of that motor? Seems like start current was something like 600A? maybe 800?
 
Is there such thing as spinning the motor too fast? The pulley on my pony is a little bigger so if its actually getting to full speed the RPC will be going faster then its 1725rpms it runs at idle.

On the pre-start, I would not spin the idler motor faster than 1725.
 
Are you attempting to power the pony motor and the RPC both immediately at startup?

Is the 75hp motor actually wound for 1725rpm? Its not a low speed motor is it?

Is the 75hp RPC wired for 240VAC? How big is the breaker that you are tripping?

I know these are elementary questions but since the RPC is not behaving in a normal fashion, that tells me that something is probably not wired correctly or rated for the assumed characteristics.

How you are stating some of your questions also leads me to believe that you probably do not understand as much about setting the RPC up as we are assuming. Nothing wrong with asking questions but if the assumption is that you understand the RPC more than you actually do, we will not be understanding the problem correctly.

As JST said, this is a lot of motor to do an across the mains start, especially for a RPC application. Using a low voltage starter with starting caps would seem to be a much cleaner approach.
 
"An INDUCTION motor has no phase matching if you are talking about checking whether it is up to speed. The stator just "writes" poles onto the rotor as and where it is. The phase just happens, but NOTHING happens until there is power on the stator, unless I have missed something and he has a synchronous motor there....."

+1

I don't think a syncroscope is the best approach here.

Suggest you get a strobe-O-tach and see what the idler speed really is.
Thoughts are that the inrush is lowest if the idler is at about syncronous speed,
which is somewhat above nameplate speed. Nameplate speed is lower to
account for slip.

1725 motor, for example, might work best if spun to 1800 rpm.

That size idler, I'd expect to need a 1 or 1.5 hp single phase motor to get it
moving fast enough.
 
My old Phase Converter I had a button and a switch, button to spin pony, switch to fire idler.

Don't recall it being all that fussy about speed matching

wasn't 75 hp tho
 
Are you attempting to power the pony motor and the RPC both immediately at startup? No, Pony up to speed then throwing the breaker on the RPC

Is the 75hp motor actually wound for 1725rpm? Its not a low speed motor is it? I assume so since that is the RPM spec from the RPC manufacture

Is the 75hp RPC wired for 240VAC? How big is the breaker that you are tripping? 240 on L1 and L2 in the RPC box

I know these are elementary questions but since the RPC is not behaving in a normal fashion, that tells me that something is probably not wired correctly or rated for the assumed characteristics.

How you are stating some of your questions also leads me to believe that you probably do not understand as much about setting the RPC up as we are assuming. Nothing wrong with asking questions but if the assumption is that you understand the RPC more than you actually do, we will not be understanding the problem correctly.

As JST said, this is a lot of motor to do an across the mains start, especially for a RPC application. Using a low voltage starter with starting caps would seem to be a much cleaner approach.



Answers to other questions, currently using a 100 amp breaker on the RPC circuit.

RPC Amp specs are 700A start and 13.6 Idle.

Pony motor is a 3hp and it grunts to get up to speed and will pop a 40 amp breaker if I try and hold it at speed for too long. The RPC is really easy to turn, but the weight of it and once the fan really starts producing drag at rpm the pony motors really hate life.

I am headed out of town today, will be back on Monday to work on getting this thing up and going. Thanks to all who are helping, it's giving me some things to go on!
 
An induction motor by definition does not have fixed poles in the armature. Otherwise, it could not slip, as it does when turning 1725 RPM. When slipping, the armature poles are constantly rotating back.

This sounds like a soft start job. First look at the field leads. If the fields are separate, connecting two of them in wye (you don't need the third connected) and switching to the normal delta after getting up to speed would help. That is assuming it is a delta connected motor in the first place. If it is a typical 9 wire motor, it is already in wye, so no help there, but a 9 wire can be connected for the higher voltage for starting. If that isn't enough, a line transformer with a few taps or a resistor would do.

I don't think I have ever seen a line start 75 hp motor. Most of the ones in that range I tangle with are wye/delta start.

Bill
 
...currently using a 100 amp breaker on the RPC circuit. RPC Amp specs are 700A start and 13.6 Idle.
Brief inrush of 8:1 not uncommon. You are not far-off what it can handle. 150A would be better if you can even fit it.

But that is all AS a 'breaker'.

A safety device, not a heavy-load start/stop SWITCH or contactor.

Pony motor is a 3hp and it grunts to get up to speed and will pop a 40 amp breaker if I try and hold it at speed for too long. The RPC is really easy to turn, but the weight of it and once the fan really starts producing drag at rpm the pony motors really hate life.
Simplify that part one more stage than you already have done. A 5 HP shouldn't be that hard to find, used, and one more marginal player is reduced as problematic.

For your other needs at getting this thing cleaned-up to safer use, there happen to be quite a few major electrical gear recyclers within 200 miles, if not half or less of that from Woodbury.

I've bought from some of them, had goods shipped a lot further to Metro DC fast and cheaply enough. Not a lot of risk to that - they don't traffic in failed or smoked junk, only the undamaged stuff with lots of useful life left in it.

Helpful if you can get local expertise to eyeball it all. A master electrician who has dealt routinely with big pump motors for a water or sewage authority rather than piddly residential loads is your man, and most Counties do have that sort of facility. If he needs help? He'll already have contacts. Won't be his first rodeo with edge-case challenges.
 
If the breaker is a 100A, and the motor takes 700 for starting, that is not necessarily a guaranteed fail, but it is close if he RPC takes some time to come up to speed from wherever it is spun up to by the pony. We have ALL been suggesting a soft-start can help with the inrush.

IF the RPC can be spun up close to the synchronous speed, or over, then it should just "pick up" without much drama. That would indeed be the speed at which it draw the least, because there is no power needed to accelerate it.*

So, the SIMPLEST thing to do now, is to find out what speed you are getting the RPC to go when you spin it up. Try for 1800 rpm, as anything less than that will draw accelerating current.

The cooling fan may be an issue. It could absorb some serious power

There is also a serious issue contained in the following statement by the OP:

"Pony motor is a 3hp and it grunts to get up to speed and will pop a 40 amp breaker if I try and hold it at speed for too long. The RPC is really easy to turn, but the weight of it and once the fan really starts producing drag at rpm the pony motors really hate life. "

A 3 HP motor on 230V single phase SHOULD draw a maximum full load current of 17A or so. If your pony motor is popping a 40A breaker, then it is SEVERELY overloaded (or the breaker is bad). That suggests the pony is too small to get the motor up to speed, or the pulley ratio is messed up, or both. You may not be getting anywhere on rpm for the RPC.

The immediate thing may be to get the pony motor set up with a pulley ratio that lets it at least spin up to full speed. That should get the RPC going considerably faster than it is now, and that "might" be good enough to get it started.

Bottom line is that a 75 HP motor on a 100A circuit is "not optimal". Full load current for 75 HP is 190A on 3 phase, (although you won't draw that ever). I do not have a full load amp listing for a 75 HP single phase motor, but that would be around 325 A.

As for the fan power. The RPC is stated to pull 13.6A at idle. That is remarkably low, and must reflect power factor correction, so that would be the power in "losses". If so, 13.6A at a good high corrected power factor is a little over 3 kW, which would be about 4 HP. Without electrical losses, some small amount less than that. That is an overload for a 3 HP motor, but perhaps not too bad. probably not 40A bad....

The issue is likely that the pony cannot get the RPC up to speed fast enough, and the breaker blows because the 3 HP is pulling start current for a long time, which could be 100A or so. Breakers respond to the current AND time. A large short overload, or a long smaller overload will pop the breaker. if you pull 80A or more on a 40A breaker for many seconds, it will likely open.



I expect you are going to need a breaker bigger than 100A. Life would be far simpler for you if you had either a smaller RPC, or a larger electrical supply.

Please also triple check the motor wiring and be sure that there is no issue with wrong connections. If the motor has only 3 leads, as it might, that's not an issue, but if it has more, there is a chance of wrong polarity etc, which will prevent it working no matter what the breaker size.



* reason for the speed issue is that the motor draws current due to the difference between its "generated voltage" (back EMF) and the line volts. When you start from a stop, the motor is not turning, and has no "back EMF", so it pulls a lot of current. The back EMF is proportional to rpm, and the faster it spins, the larger the back EMF. If t slows, back EMF drops, and more current flows.

A large motor does not accelerate fast. So it may take many seconds for it to come up to full speed, and all that time it is pulling high current.

If you can start it at synchronous speed, "theoretically" it will draw very little current once the large short inrush that establishes the magnetic field is over with. How that works out in practice needs to be seen, but if you can get it to 1800 RPM, or a bit over that, you should have the best chance of getting it going on your limited supply current.

I do not think you can do better with any conventional start circuit.
 
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The OP has a 40A circuit. The 5 HP is not going to pull LESS amperage than the 3 HP. The only hope there is that the extra HP will accelerate the RPC fast enough to get below 40A before the breaker opens. Of course it will pull MORE current to do that.

Changing the pulley is easy and relatively cheap, especially if we knew what he has for a ratio now..... And with his limited power, may be the best idea At least that will pull LESS current than what he has now.

If the full speed friction and windage losses are 4 HP, which is "kinda reasonable" with that being about 10% of the 75HP.... even the 5 HP is going to work hard. FLA on the 5 HP is around 28A, and start current 150A or so. It may not get the acceleration to be fast enough without pulling even MORE current, and opening the breaker faster.

It's a pretty thin justification for obtaining the 5 HP unless I missed where he said he had one already. If he does, may as well try it, of course, but the shaft size may be different, so another pulley may still be needed.......

I assume he can start the 3 HP by itself OK. If it pulls 40+ A by itself, there is another problem.

EVERYONE HERE already knows that the real problem is either a power problem, or a motor wiring problem. He's basically trying to pull his 40' yacht and trailer with an old VW bug.... If he had decent power, the RPC would probably start as-designed just as we see it. It's POSSIBLE that the motor has been connected with a coil backward or some such issue that would prevent a start. That needs checked if there is any chance a wiring issue could have occurred.

The OP (IIRC) has a nominal 400A service, but it is broken up into 2 separate 200A subpanels, and even there he has only 100A for this RPC. He may never USE more power IF he can get the thing started, but there is the problem of starting current, which is huge.

AND IIRC, the power company has him on a 15 kVA transformer (is that the OP that said it in a different thread?) I believe if that was him, the powerco was going to upgrade him, but.... Did they? and if so, what TO?

NOTHING is going to help if the power is being pulled down to 170VAC by the load..... ANOTHER pice of info that I don;t recall seeing anywhere.... what the volts are as he is trying to start the thing.

>>>>>>>>>>>>>>>>>>>>>>>>
Frankly, he may be better off, if he cannot do anything about the electric service, to push that tanker-anchor into a corner and get another RPC, or at least another motor to use as one which is better suited to the power he has.
<<<<<<<<<<<<<<<<<<<<<<<<<<<


Oh, yeah... the rope start thing.... All a rope start does is to get the motor rolling over . It "biases" the motor so it can get going. If you were thinking it was intended to get the motor up to 80% of full speed, or anything like that, you got another think or two coming....

And, he would STILL need to have the basic ampacity to supply start current (as much as 700A !!!!) to the motor for as long as it takes to get up to speed. He does NOT seem to have that, so it's a moot point.
 
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Couple of points.

The motor is a twelve lead designed for normal wye delta start.
But is currently wired for DOL start, 230V, along with a healthy cap bank. Motor and cap leads terminated on multitap 3 pole block.

I take the cap bank to be the large section for phase shift. 800MFD
The smaller bank to be PFC, for the low idle value 13.6A it lists. 400MFD

Idler Specs NEC values 75HP 230V 192A FLC 55KVA, Listed as 713A starting current on the RPC label.

Utility Power supply 230V 1Φ, 62A FLC 15KVA

2P 100A 240V Breaker for starting means.

Original Thread -http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/advice-needed-wiring-up-80-hp-rpc-400amp-service-333795/

My take is there is a few combined issues. Utility supply of 62A FLA. Then the trip current curve of the 100A 2P breaker, shown below. Then is the issue of the supply voltage dropping like a stone under the startup load, acting as a built in reduced voltage starting, prolonging the start times, which then affects the breaker trip curves timing ratings.

A Sq D QO 2P Breaker trip curve for reference. We don't know what brand he is using, but I would suspect the bi-metal element being out of calibration from his repeated attempts to use it as a starter for a DOL 75HP motor.
100A QO Curve.jpg
7x rated 100A = 700A = 0.6sec trip time
4.25x rated 100A = 425A = 2.0sec trip time
3.25x rated 100A = 325A = 6.0sec trip time

My feeling is if he had some means to reduce the starting voltage, to get from 713A starting surge to below the 325A shown above, that would increase his starting time from 0.6 sec to 6.0 sec, which would allow for the stone dropping voltage to recover before the breaker tripped out.

A larger breaker is not likely in his future with a standard loadcenter panel, or without a service rework.

The other thing I see adding to the start problem is the cap bank permanently connected. It surely adds additional current inrush to the motor currents.

I have done some 3Φ motor testing in the past to record some starting/running currents of 9 lead 10HP 460/230V Y/YY wound units, that were unloaded.

Testing was done from rest with reduced voltage with 1/2 voltage applied to a YY connection, the current was reduced by a factor of 2.4, starting time was slightly extended but normal, about 1 sec

A standard Y connection (460V), fed with 240V, so that would be a Y start, YY run on 230V, reduced the current buy a factor of 5. Starting time was extended to about 1/2 sec, but normal.

The best current reduction was obtained with 1/4 applied voltage on start. With a Y connection (460V) and YY (230V)run connection. 110V 3Φ was used as the source. Current was reduced by a factor of 9. It took about 2 sec starting time but was smooth.

Most of the builders I looked at, that provide these large units use a Y/Delta or Y/YY starting setup on all of the larger units I looked at. In the case of a 75HP unit that would require a Size 4 Nema starter for the Y/Delta Mode.

If this were my conundrum to play with, and the start circuit is 1Φ, using a pony motor. I would consider using a 1Φ autotransformer in one leg for reduction taps, and using a contactor to shunt it out once at speed.

The OP got a quote from the vendor for such a setup with a new cap bank, but decided it was a little to spendy for him.

Even if he manages to get it started on a 100A 2P breaker that wont leave much headroom for attaching any loads to it. Especially his 73A 3Φ load.

SAF Ω
 
If this were my conundrum to play with, and the start circuit is 1Φ, using a pony motor. I would consider using a 1Φ autotransformer in one leg for reduction taps, and using a contactor to shunt it out once at speed.
A bit of jackleg ingineering, something common that could handle the current might be ID'ed that could be connected-as an autotransformer to do that.
The OP got a quote from the vendor for such a setup with a new cap bank, but decided it was a little to spendy for him.
Add one more contactor to the list of goods he is already short of, and DIY might still not be cheaper. It surely would not have the OEM's experience and blessing coming in the hall along with it. That does have real-world value, what with Inspectors, Fire Marshalls, Insurance folks, and the basic "Prudent Man" test as to personal safety of owner and staff, even if none of those other Agencies ever existed.
Even if he manages to get it started on a 100A 2P breaker that wont leave much headroom for attaching any loads to it. Especially his 73A 3Φ load.
I don't see much in the way of "quitting" on the menu, so that may be the point where it hits the wall unless he's at least had a serious mains power upgrade to get around that 15 kVA pole-pig AND can source a panel that supports a larger breaker.

Using the main breaker of a service-entrance grade panel for this task alone as sole load the easiest and cheapest if 'legal'. I'm not current on that part.

Back to the real-word load and how many hours of how many days it has to be operated in any given week or calendar quarter. Doubt it is anywhere near even one full shift a day, all week, let alone three, and all-month.

And that "Beansdiesel" is not just a PM-handle - Diesels actually ARE are he, plain or exotic.

For part-time use, refurb of a medium 3-Phase Diesel gen set could solve the whole problem in a manner he is already better equipped to support than the average bear. If trailer-mounted, such critters can be all-around useful for lots of other things, too.

RPC no longer required at all.

Except for keeping an old GE Telecron wall-clock dead-accurate - and who still even needs THAT? - they are as good as utility mains.

Just sayin'

"I got mine." Flip a switch, my smaller MEP-803A becomes a 3-phase source, minimal hassle, 15 kVA peak, 12 kVA sustained. The 10 kVA continuous MIL-spec rating was at 8,000 feet above sea-level, after all, not my mere 350 feet-minus.
 
When he gets back, let's see a voltmeter reading of the line when the pony is turning it, and then when he hits the power for the idler itself. That will pretty much tell the story.

If he has been upgraded to a 25 kVA that may not be enough. 50 kVA would be a bit over 200A actual service capability, getting to be credible.

With 15 kVA, at a 5% impedance, about 12V drop at 62A. At 620A, that would be more in the 120V drop category, and nothing is going to work that way. Been there, seen it. Now that I have done this estimate, I see what the issue iis with the motor slowing when power is applied. Depending on what rpm he gets to, he may still have a drop of 50 to 100 V out of 230 when he hits the switch (breaker). No wonder it slows down.


With 50 kVA, about 12V at 200A. so more like 35V drop at 600A, or 15%. That has a chance of working straight up if the breaker is upgraded. If he was only boosted to 25 kVA, then it's not as nice, I'd expect the drop to be in the 70 volt area, and that is somewhat more of a problem.

Converting to a wye delta is possible, but seems like a good bit of work, and the OP may not be up for that. It might get the 100A breaker to hold, but whether it will work on 15 kVA seems doubtful. About 58% of normal start current.

He does have a pony start operating, although not working right nor starting the motor. A pony start is very likely to be the lowest current approach he can do. But right now his pulley ratio is overloading the 3 HP motor. If he changes that to something that the 3 HP can actually do, then he very possibly may get it to run.

The pony start is a form of reduced-voltage start. With the motor already spinning, the voltage is reduced by back EMF as soon as the initial short magnetizing surge is over. So if the motor is brought up to 50% speed, the effective voltage reduction is about 50%. If brought to 60%, the reduction in effective voltage is 60%, meaning the effective voltage is 40% of line voltage, and current will be a bit less than 40% of normal start current.

With voltage reduced, current is reduced as well. So long as the RPC has no load, it should "go" at that point, and very possibly will not pop the breaker, since most of the high current part of the start is avoided. The motor essentially gets a "head start".

Dunno if the 100A will hold it, but seems possible if he can get the pony to run it up reasonably close to 1725 RPM. Whether it will hold with the lathe in operation is another story that depends entirely on the machining power required.


SAF:

What di you think of that spec of 13.6A idle current for the RPC? If so, that is the losses, and I would tend to believe in 5% mechanical losses and iron losses as at least credible (about 4 HP out of 75).

Without any correction, I would expect a motor with 190A FLA to pull around 60A at idle, most of which is reactive current. Then the power factor caps need to compensate out 12 kVA of reactive current, and the caps which you think (and I agree) are the PFC caps appear to be 8 or 9 kVA worth... Reasonably possible, I'm just guesstimating at the idle reactive current..
 
When he gets back,
Good post. No argument. One suggestion.

First thing back should probably start with confirming (or not) that he has been given the upgrade from 15 kVA, or a planned install date, and if so to WHAT?

Can't really 'get there from here' if not, no matter how well planned or executed, load-side.
 








 
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