Effect of flywheel on an RPC idler.

Miguels244 · Apr 11, 2018

What is the effect of a flywheel on an RPC idler.
It seem to me that having a flywheel would store energy and help the idler with starting loads.

JST · Apr 11, 2018

Miguels244 said:
What is the effect of a flywheel on an RPC idler.
It seem to me that having a flywheel would store energy and help the idler with starting loads.

Possibly it could.

The usual problem with starting loads is not the availability of mechanical power, but more the problem that pulling that much current from the generated leg produces a large voltage drop due to the impedance of the idler motor and the current of several times normal.

No amount of stored energy can fix that problem, that is a problem of getting the energy out of the idler and to the load. The actual idler is likely to be only somewhat mildly overloaded, not enough to slow to the point of torque breakdown. Maybe enough to reduce the voltage generated.

There could be a benefit momentarily, because the motor would not slow as rapidly, since it would not need to draw all the load power from the input lines. However, you would need to calculate the stored power vs the load requirement to see how big a flywheel is needed to keep from slowing more than "X" amount, where "X" corresponds to an acceptable output voltage. It might be pretty substantial.

The benefit would be keeping the generated voltage up closer to the mains input voltage. The motor impedance would still be present, so the net benefit might be somewhat limited vs just using a substantially larger idler.

thermite · Apr 11, 2018

JST said:
the net benefit might be somewhat limited vs just using a substantially larger idler.

No free lunch, or it would be common as hobby "gunsmiths" now are.

Supplementary idler, cut-in prior to need, either dropped out after startup of main load, ELSE left onlne, is far the better approach.

A "dutch roll" damped oscillation from kinetics and electricals chasing each other's latency tails, increased gyroscopic precession effects, we do not gain from, here.

Prolly why it is not common in general or as a shade-tree wooden RPC trick. Also seems to have had some careful Engineering input and test where it exists at all - a small part of those "built for RPC duty" bespoke, no external shaft, idlers.

DDoug · Apr 12, 2018

Boy-O-Boy do I want to find a synchronous motor, so I don't have to play with balance caps anymore.....
https://www.youtube.com/watch?v=ih1siAJFdmM

Miguels244 · Apr 12, 2018

If you spin up a 30 hp idler you still only have the power available from the single phase supply.
I suppose you are relying on the increased energy stored in the inductance of the idler to provide starting energy.
The mechanical difference would be easily replicated by a flywheel.

As noted...if it worked every one would do it.

thermite · Apr 12, 2018

Miguels244 said:
relying on the increased energy stored in the inductance of the idler to provide starting energy.

Very much so. Starting is brutal, but mercifully brief as well. Usually.

Not always so brief, though.

Some commercial RPC makers publsh tables of recomended upsizing for various loads ranked by typical assessment as to "hard' starting.

"We" usually figure 150%, idler vs the heaviest of perhaps several load motors as fine. Even where a no-clutch machine tool - such as a drillpress or bandsaw - is the load, it doesn't begin operation right out of the box with a drill or blade already buried in the work.

Some other loads, the recommendations are for idler HP 15 and more times that of the load motor.
Which is impractical and costly. Silly, even.

Anything much over double, an RPC is progressively less wise a choice for the tasking, even with a supplementary idler already online.

Miguels244 · Apr 12, 2018

It never occurred to me to have an auxiliary idler.
Would it need it’s own caps?

thermite · Apr 12, 2018

Miguels244 said:
It never occurred to me to have an auxiliary idler.
Would it need it’s own caps?

No. Nothng extra. "Idler", not load. Stiil a "starting" load, though. The "primary" is already online and delivering 3-P. Only needs a suitable contactor. Manually operated, ELSE staggered start, so one COULD add a TD relay or any of several other "sensorish" means.

CAN it use caps? "Tuning" can still improve performance of the net-larger RPC "coalition", but ordinarily only if it is to be used for the same load, majority of the time.

Not to forget you still have that net TOTAL load applied to the 1-P supply, so starting a 1 HP or 2HP, , then dropping four more onto line in succession isn't going to gain much.

That IS close to what happnes if instead of a besoke supplementary idler, one just goes 'round switching ON, but NOT loading-up every 3-P motor on that idler's output, though.

Makes more sense at 10+10, or 10+7.5, or 10+5, 7.5+7.5, 5+5. etc.

Probably fire-up that SAG right smartly with a 10 HP primary idler and control for it, drop another ten online, then drop it back off once the lathe is uo "on step", if the work of the day is light or medium. Leave it in only when pushing hard and furious-like.

JST · Apr 12, 2018

Miguels244 said:
If you spin up a 30 hp idler you still only have the power available from the single phase supply.
I suppose you are relying on the increased energy stored in the inductance of the idler to provide starting energy.
.....

Not so as to inductance storage. That is so tiny as not to be an issue.

ALL the power comes from the single phase supply, SURPRISE! There is no reason for that to be a problem or Phase Perfect would not work any better.

Miguels244 · Apr 12, 2018

thermite said:
No. Nothng extra. "Idler", not load. Stiil a "starting" load, though. The "primary" is already online and delivering 3-P. Only needs a suitable contactor. Manually operated, ELSE staggered start, so one COULD add a TD relay or any of several other "sensorish" means.

CAN it use caps? "Tuning" can still improve performance of the net-larger RPC "coalition", but ordinarily only if it is to be used for the same load, majority of the time.

Not to forget you still have that net TOTAL load applied to the 1-P supply, so starting a 1 HP or 2HP, , then dropping four more onto line in succession isn't going to gain much.

That IS close to what happnes if instead of a besoke supplementary idler, one just goes 'round switching ON, but NOT loading-up every 3-P motor on that idler's output, though.

Makes more sense at 10+10, or 10+7.5, or 10+5, 7.5+7.5, 5+5. etc.

Probably fire-up that SAG right smartly with a 10 HP primary idler and control for it, drop another ten online, then drop it back off once the lathe is uo "on step", if the work of the day is light or medium. Leave it in only when pushing hard and furious-like.

I have a 7.5 high e Baldor I was planning on using.
Pony start I think.

Why not just throw the largest motor you can find on Craig’s list in there as an idler.
Not a lot of parasitic loss.

thermite · Apr 12, 2018

Miguels244 said:
Why not just throw the largest motor you can find on Craig’s list in there as an idler.
Not a lot of parasitic loss.

Two reasons:

A) I also have to LIVE here. My "shop" was a 2-plus car garage plus annex. The residence - 3 -level split - is attached and parially over it. In those premises, I have more electronic gadgetry, wired and wireless, than the average bear. I'd prefer minimal disruption all that when the Mike Foxtrot RPC has to be STARTED.

B) I don't want to unduly annoy my adjacent neighbours who share the PowerCo transformer, either. RPC's produce pure sne-wave outputs, but they pump harmonic distortion back UP the grid as a byproduct of how they make the 1-P to 3-P happen. Bigger they are, the nastier they are as neighbours UPline.

Supplementary idler approach lets one have a heavy capabilty for load starting, needed only briefly, then fall back to a less bothersome range for running the far lighter actual loads over long hours.

Miguels244 · Apr 12, 2018

One would think that after all these years there would be an engineering solution to these things instead of a bunch of recipes scattered around the internet.

For instance...you say there’s harmonics pushed back up...
What order and what magnitude?

Miguels244 · Apr 12, 2018

thermite said:
Two reasons:

A) I also have to LIVE here. My "shop" was a 2-plus car garage plus annex. The residence - 3 -level split - is attached and parially over it. In those premises, I have more electronic gadgetry, wired and wireless, than the average bear. I'd prefer minimal disruption all that when the Mike Foxtrot RPC has to be STARTED.

B) I don't want to unduly annoy my adjacent neighbours who share the PowerCo transformer, either. RPC's produce pure sne-wave outputs, but they pump harmonic distortion back UP the grid as a byproduct of how they make the 1-P to 3-P happen. Bigger they are, the nastier they are as neighbours UPline.

Supplementary idler approach lets one have a heavy capabilty for load starting, needed only briefly, then fall back to a less bothersome range for running the far lighter actual loads over long hours.

I’m going to pony start mine because of starting loads.

thermite · Apr 12, 2018

Miguels244 said:
I’m going to pony start mine because of starting loads.

I'm a believer in pony start. But most of the value being in 10 HP or ABOVE, I don't yet need it, so use a wire-and-forget Phase-Craft. Well... at some future date, as with any other gadget having capacitors, it should be gifted a new set.

By then I shall probably have ceased messing with Old Iro............THUNK

Miguels244 · Apr 12, 2018

thermite said:
I'm a believer in pony start. But most of the value being in 10 HP or ABOVE, I don't yet need it, so use a wire-and-forget Phase-Craft. Well... at some future date, as with any other gadget having capacitors, it should be gifted a new set.

By then I shall probably have ceased messing with Old Iro............THUNK

I had a friend that had a pony start unit he spun up with a rope.

JST · Apr 12, 2018

Miguels244 said:
Why not just throw the largest motor you can find on Craig’s list in there as an idler.
Not a lot of parasitic loss.

That is actually the best plan. You can tune out the power factor and so develop less line loss (and voltage drop)

The larger idler has lower impedance, and will hold up the generated leg better than the "usually recommended" size. In fact, many of the RPCs for "hard starting duty" are more-or-less just a larger than normal idler.

There are things you can do to help out, but eventually it comes out to "what is the series resistance and inductive reactance?". The larger the idler, the lower both of those are.

As for starting, pony motor, reduced voltage start, etc, all will help minimize any disruption caused by starting a big motor. Normally the powerco requires some such method, by limiting the allowed disturbance on the line.

Miguels244 · Apr 12, 2018

JST said:
That is actually the best plan. You can tune out the power factor and so develop less line loss (and voltage drop)

The larger idler has lower impedance, and will hold up the generated leg better than the "usually recommended" size. In fact, many of the RPCs for "hard starting duty" are more-or-less just a larger than normal idler.

There are things you can do to help out, but eventually it comes out to "what is the series resistance and inductive reactance?". The larger the idler, the lower both of those are.

As for starting, pony motor, reduced voltage start, etc, all will help minimize any disruption caused by starting a big motor. Normally the powerco requires some such method, by limiting the allowed disturbance on the line.

Well, my 7.5 is going to have to do for now.

JST · Apr 13, 2018

Miguels244 said:
Well, my 7.5 is going to have to do for now.

So you were asking as relating to a particular application?

I used to consider that a flywheel might do good, but after doing some simple analysis, I do not think it is worthwhile to do. The larger motor may also have a heavier rotor, but the main point about a larger motor is lower series impedance, so it acts more like the power company as far as current capability at a given voltage..

For maximum capability wit a given idler, you might consider running the RPC on an autotransformer at a higher voltage than the supply lines that go on to the load. Then do NOT use much in the way of balance capacitors, rely mostly on the boost to raise voltage.

By starting with a higher voltage, the lowest voltage it drops to should be higher as well, which then would improve starting. Not as good as a bigger idler, but may make enough difference in cases where your idler is a little weaker than you need.

Miguels244 · Apr 13, 2018

JST said:
So you were asking as relating to a particular application?

I used to consider that a flywheel might do good, but after doing some simple analysis, I do not think it is worthwhile to do. The larger motor may also have a heavier rotor, but the main point about a larger motor is lower series impedance, so it acts more like the power company as far as current capability at a given voltage..

For maximum capability wit a given idler, you might consider running the RPC on an autotransformer at a higher voltage than the supply lines that go on to the load. Then do NOT use much in the way of balance capacitors, rely mostly on the boost to raise voltage.

By starting with a higher voltage, the lowest voltage it drops to should be higher as well, which then would improve starting. Not as good as a bigger idler, but may make enough difference in cases where your idler is a little weaker than you need.

Not really a particular application...more ‘idle’ thought.

jim rozen · Apr 13, 2018

JST said:
So you were asking as relating to a particular application?

I used to consider that a flywheel might do good, but after doing some simple analysis, I do not think it is worthwhile to do. The larger motor may also have a heavier rotor, but the main point about a larger motor is lower series impedance, so it acts more like the power company as far as current capability at a given voltage..

For maximum capability wit a given idler, you might consider running the RPC on an autotransformer at a higher voltage than the supply lines that go on to the load. Then do NOT use much in the way of balance capacitors, rely mostly on the boost to raise voltage.

By starting with a higher voltage, the lowest voltage it drops to should be higher as well, which then would improve starting. Not as good as a bigger idler, but may make enough difference in cases where your idler is a little weaker than you need.

Back in the day, Fitch Williams convinced me that an extra flywheel on a converter is a detriment, NOT an asset.

Theory behind this I seem to recall is that idlers deliver power to the manufactured leg by slipping, and that a flywheel inhibits slip.

Effect of flywheel on an RPC idler.

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