Rotation problem with rotary phase converter

[jwearing]

So you say, if I run the mill during the lathe operation, it will help with the start-stop-reverse situation.

I'm not saying it's a permanent solution, just use it as a test to confirm that your RPC is undersized. If it works then it's time to upgrade.

 

[eKretz]

Your thread title is awful, and does not meet posting guidelines. Since I am pretty sure you're past the editing deadline, I've altered it for you. Try a little harder to make it informative next time, please. Also - moving this to VFD section.

 

[Mr.JAJA]

Your thread title is awful, and does not meet posting guidelines. Since I am pretty sure you're past the editing deadline, I've altered it for you. Try a little harder to make it informative next time, please. Also - moving this to VFD section.

Don't know how to make it "un awful" must be my German original.
But thanks, I got some good information and will give it a try.

 

[eKretz]

Don't know how to make it "un awful" must be my German original.
But thanks, I got some good information and will give it a try.

You need to make the title reflect the discussion subject better so that people understand the topic before they have to click and read. How to make it "unawful" is just to add more info to the title. This is necessary to help anyone searching in the future and to save anyone who has no input or curiosity on the subject some time. Your original title "Rotation Problem" didn't give much info. What is rotating? What is the problem? I should have also added "motor" in front, but now it's @JST 's problem.

Guidelines:

PRACTICAL MACHINIST RULES AND GUIDELINES

OFF TOPIC GUIDELINES OT new threads should be for asking advice or presently advice on a non machining related subject, but, ideally, of a technical nature. Non technically related questions, such as health related are ok if you're desperate or have valuable insights on same to offer. But it...

www.practicalmachinist.com

Specifically:

TOPIC TITLES NEED TO BE DESCRIPTIVE

The worst topic titles are things like-

1. Please Help !
2. Hey everybody, look at this !

But even more specific titles like-

1. Need info on D31
2. G53

Are meaningless or nearly so to many, so will be locked as meaningless. My theories on the reasons for meaningless titles are-

1. The "surprise" aspect... open the package to find out
2. Laziness
3. Shyness (newbies are especially prone to meaningless topic titles)

Bottom line is we need members to simply tell us in advance enough info to at least get a hint of what the topic is about.

This for two reasons-

1. Many of us are in a hurry and don't have time to open posts we then find out are of no interest

2. Search engines concentrate on topic titles more so than content, so with proper title your topic is more likely to be found by others with similar issues later.

 

[Mr.JAJA]

Thanks for your clarification. I learned to be short on the talks.
Now, today I went through the wiring to make sure the created leg is not switched when changing rotation direction on the lathe. Sure the leg was one of the two. Now this is corrected the motor current switch (FI) still kicks out. I tried to run the 1HP motor from the mill at the same time on the 5 HP RPC, problem is still the same. Now I will spend the $$$ for a VFD. This will leave the RPC for the mill only.

 

[JST]

For stopping, and not for the reverse problem, if you want to do it electrically, a VFD with DC injection (not all have it) is going to stop the motor quickly. How quickly depends on the DC current used/allowed by the VFD for the stop.

This is not problem-free. When you do a DC stop, the energy of the stop goes into heating the rotor portion of the motor. * That can build up over a number of stops.

You mentioned threading of coarse threads. That can be an issue for the DC stop, because you will be repeating the stop fairly often. The only ways the rotor can get rid of heat is, first; radiation into the stator metal. That is not particularly efficient. Second; if the rotor has fins cast into it to form a fan, then those will dissipate heat well while the motor is running, but not much while it is stopped.

So, one solution is just to add another idler motor. For reversing, or using reverse to stop, you will want the total idler capacity to not be less than 2x the connected motor. More than that may improve reversing.

You have a 5 HP (actual motor HP, I assume) idler, and run a 5 HP motor with that. OK, that works. But direct reversing is a much heavier load, and needs more idler current capacity. You would need to connect another at least 5 HP motor as an added idler.

Doing that will not load down the RPC as far as the generated leg. But it will load the power source, doubling the current draw from the electric service vs what the 5 HP idler draws.

A 5HP motor at full load will draw about 15A 3 phase. It will likely draw about 6 or 7 amps at no load. Used as an idler, it may draw a bit more than 7. It may draw perhaps 10 or 12A when driving a load motor, because it only supplies 1/3 of the load motor power (the rest comes from the two direct legs) So another idler will draw another 10 or 12A, and the load 5 HP motor may draw 15A per wire at full power.

So you would have a total current draw of maybe 35A from the power source when running the lathe at the full 5 HP with the extra idler connected. If you were not going to do reversing, you would not need the extra idler to be connected, and draw would be about 25A (estimate) max. At most times current draw might be under 20A when the lathe is not at full power.

Using a VFD is possible, but may require changes to the machine controls, and seems like extra work, when you have an RPC and may be able to add an idler at relatively low cost. A used motor would be fine if it works.

I do not know (maybe you said?) what the supply rating is. If you cannot get enough current, then a VFD may be your only reasonable choice, since rewiring will probably cost more than the VFD.

*
Rotor heating:
The constant magnetic field from the DC is going through the steel of the rotor. The rotor has "windings", the rotor bars and the shorting rings on the ends of the bars. Those rotor bars will then have a pretty heavy current induced in them, by generator action, and will then have losses due to the bar resistance.

In extreme cases, with the aluminum casting alloy used in the bars, the heat may actually melt the relatively low-melting aluminum, which obviously destroys the motor. You probably would not get to that point, but it can and has happened.

 

[SomeoneSomewhere]

I am pretty sure that the majority of the braking energy from a plug reverse also goes into the motor, along with the heat of a direct-on-line start from stationary to the new direction. I am not sure how much of that braking energy goes into the rotor vs stator. The heat from a direct-on-line start would not occur with a VFD due to lower slip.

Motors for machine tools are generally somewhat oversized relative to the actual expected load, to handle frequent starting/stopping/plug-reversing.

When you plug reverse a 3~ motor, it and the grid fight each each other. Either one changes direction, a breaker blows, or things go bang. You're essentially dropping a clutch between two shafts operating in opposite directions.
Usually, the grid wins. Sometimes things go bang, where the load doesn't like to suddenly reverse.

Because your idler motor is relatively small and has nothing attached, it has very low inertia. The spinning lathe with an equally large motor probably has much higher inertia. The lathe is winning because it's bigger than the RPC.

Consider adding more idlers, but also adding flywheel mass to the idlers to make them harder to reverse.

Bear in mind that the RPC is still going to be a fairly soft supply (unlike a proper 3~ grid), and therefore reversing will still be slower than on a solid grid connection.

 

[JST]

If the RPC is sufficiently oversized, it will always "win".

I can "plug reverse" a lathe here on the RPC, and it is effectively instant. It may be slower than grid power, but it is not detectable without measuring the delay somehow.

Any motor can produce only a certain maximum torque (the iron saturates, and the motor current is limited by resistance when saturated, as well as by inductive impedance when below saturation ). If the source provides enough current to achieve that torque, you will have the same reversing performance as with the grid.

Plug reversing dumps energy into the coils, same as during running. The rotor current is no more than with a normal start, due to the torque limit as above. Some of the energy of the reverse is also put back into the grid by generator action (similar to energy going into the VFD DC bus from a controlled stop).

With the DC braking, all the energy is put into the rotor, because there is no generation path back to the source.

 

[Mr.JAJA]

For stopping, and not for the reverse problem, if you want to do it electrically, a VFD with DC injection (not all have it) is going to stop the motor quickly. How quickly depends on the DC current used/allowed by the VFD for the stop.

This is not problem-free. When you do a DC stop, the energy of the stop goes into heating the rotor portion of the motor. * That can build up over a number of stops.

You mentioned threading of coarse threads. That can be an issue for the DC stop, because you will be repeating the stop fairly often. The only ways the rotor can get rid of heat is, first; radiation into the stator metal. That is not particularly efficient. Second; if the rotor has fins cast into it to form a fan, then those will dissipate heat well while the motor is running, but not much while it is stopped.

So, one solution is just to add another idler motor. For reversing, or using reverse to stop, you will want the total idler capacity to not be less than 2x the connected motor. More than that may improve reversing.

You have a 5 HP (actual motor HP, I assume) idler, and run a 5 HP motor with that. OK, that works. But direct reversing is a much heavier load, and needs more idler current capacity. You would need to connect another at least 5 HP motor as an added idler.

Doing that will not load down the RPC as far as the generated leg. But it will load the power source, doubling the current draw from the electric service vs what the 5 HP idler draws.

A 5HP motor at full load will draw about 15A 3 phase. It will likely draw about 6 or 7 amps at no load. Used as an idler, it may draw a bit more than 7. It may draw perhaps 10 or 12A when driving a load motor, because it only supplies 1/3 of the load motor power (the rest comes from the two direct legs) So another idler will draw another 10 or 12A, and the load 5 HP motor may draw 15A per wire at full power.

So you would have a total current draw of maybe 35A from the power source when running the lathe at the full 5 HP with the extra idler connected. If you were not going to do reversing, you would not need the extra idler to be connected, and draw would be about 25A (estimate) max. At most times current draw might be under 20A when the lathe is not at full power.

Using a VFD is possible, but may require changes to the machine controls, and seems like extra work, when you have an RPC and may be able to add an idler at relatively low cost. A used motor would be fine if it works.

I do not know (maybe you said?) what the supply rating is. If you cannot get enough current, then a VFD may be your only reasonable choice, since rewiring will probably cost more than the VFD.

*
Rotor heating:
The constant magnetic field from the DC is going through the steel of the rotor. The rotor has "windings", the rotor bars and the shorting rings on the ends of the bars. Those rotor bars will then have a pretty heavy current induced in them, by generator action, and will then have losses due to the bar resistance.

In extreme cases, with the aluminum casting alloy used in the bars, the heat may actually melt the relatively low-melting aluminum, which obviously destroys the motor. You probably would not get to that point, but it can and has happened.

Thank you for this explanation. I can now understand the concept and mechanic function of a RPC. All the amps makes sense now. The draw per leg is than 15 Amp, with additional idler pulling around 5-7 Amp.
So my 50 Amp service Fuses are good. A new motor protector is on order. (10-16A) might get a 16-25A. And I will search for a second 5Hp idler.
Can I install the second idler, with a disconnect switch? Only run with the Lathe. The second machine is a 1 HP mini mill.
All this equipment is used maybe 1-2 times a month for no more than 1 hours.
The acme thread was a one of and it would have be cheaper for me, to buy 2 feed on Amazon for $28.
But that's the challenge and entertainment I took and found this function problems on the lathe.
I will retrofit the RPC and report back.
By the way, I use to do this kinde of electric work 45-50 years ago in Germany. Just never with this RPC. We have 3 legs 220V to every Hause as a standard. There might be some backcountry farms without, but I have never seen any.

Thanks for the help.

 

[Mr.JAJA]

If the RPC is sufficiently oversized, it will always "win".

I can "plug reverse" a lathe here on the RPC, and it is effectively instant. It may be slower than grid power, but it is not detectable without measuring the delay somehow.

Any motor can produce only a certain maximum torque (the iron saturates, and the motor current is limited by resistance when saturated, as well as by inductive impedance when below saturation ). If the source provides enough current to achieve that torque, you will have the same reversing performance as with the grid.

Plug reversing dumps energy into the coils, same as during running. The rotor current is no more than with a normal start, due to the torque limit as above. Some of the energy of the reverse is also put back into the grid by generator action (similar to energy going into the VFD DC bus from a controlled stop).

With the DC braking, all the energy is put into the rotor, because there is no generation path back to the source.

Ok, I did check the Voltage with the lathe running. L1-L2 240V, L1-L3 240V, L2-L3 235V. Current L1 82A max 5.2A running, L2 20 max 4.6 run, L3 71A max 5.0 run. The overload protection is set at 13A and has not tripped.
I will run it like this and see if I have to change something in the future. Next, I will find a 3-5HP motor and ad this to the idler with a switch.

Thanks for all the help. I learned (refreshed) a lot.

 

[johansen]

Ok, I did check the Voltage with the lathe running. L1-L2 240V, L1-L3 240V, L2-L3 235V. Current L1 82A max 5.2A running, L2 20 max 4.6 run, L3 71A max 5.0 run. The overload protection is set at 13A and has not tripped.
I will run it like this and see if I have to change something in the future. Next, I will find a 3-5HP motor and ad this to the idler with a switch.

Thanks for all the help. I learned (refreshed) a lot.

Whats the amps on the three legs when the lathe is plug reversed but continues running the same direction?

 

[Mr.JAJA]

Whats the amps on the three legs when the lathe is plug reversed but continues running the same direction?

I would have to check that. So I would have to switch L1-L2 and than L1-L3. but than I get the X (created leg) leg into the switching process again. I will give it a try and will report back. Now I have the diagram all documented.

 

[johansen]

I would have to check that. So I would have to switch L1-L2 and than L1-L3. but than I get the X (created leg) leg into the switching process again. I will give it a try and will report back. Now I have the diagram all documented.

Lets go back to what you said.

Like I said, it runs on a RPC. When stopping or reversing the rotation the machine keeps going in the cutting direction. This only happens when I switch the lever and touch the reverse direction before the machine comes to a full stop.

When the machine is running in this condition, what are the amps..

 

[Mr.JAJA]

Lets go back to what you said.



When the machine is running in this condition, what are the amps..

I will check it this week and report back.

 

[Mr.JAJA]

I can get a 15 HP motor, can I keep the electric box and only run the 15HP as idler motor. Will this improve the RPC function?

 

[johansen]

I can get a 15 HP motor, can I keep the electric box and only run the 15HP as idler motor. Will this improve the RPC function?

Yes substantially.

A 15hp motor is probably equivalent to 4 or 5, 5hp motors.

For a few hundred$ you can buy a 15hp rpc starter