Hard starting three Phase Lathe

Hi can someone explain why turning on multiple machines and letting them run with no load can help with starting a more powerful machine. Seems like that wouldbe the opisite of what I should do? As you can tell I'm not a electrical engineer just the basic (enough to get me in trouble sometimes)
Thanks, Glenn

Glenn,

Here's a low tech answer that may help, from a low tech person.

I assume you are referring to a circuit in which you are running three phase motors from a single phase supply and using a rotary phase converter instead of a static converter. If this assumption is wrong, quit reading, I'm full of beans.

To understand why the statement is true, first you have to understand what the rotary phase converter (RPC) is doing, and what its limitations are.

A rotary phase converter is essentially a plain three phase motor that is using two windings (legs) to turn the motor and the third winding as a generator to create the missing third phase. There's more to it than that, and by making such a simplistic statement I am placing myself at risk of being pilloried by the more technically conversant among us, perhaps rightly so. But consider the RPC in that regard, at least for now.

One RPC shortcoming is that the motor was originally designed as a motor and optimized for that use. It wasn't intended to be used as a motor/generator. As a result the third, synthesized leg's output isn't as strong as that of the other two legs. The RPC's performance can be improved with the addition of capacitors to help balance the output, but that has nothing to do with your question.

Adding more "idle" (unloaded) motors to the circuit improves the quality of the third leg because each additional motor's third leg circuit also acts in a small way as a generator, and improves the quality of that phase. You might say that the extra motor, by running at idle, is consuming electricity from the circuit but its third leg is contributing more to the balance between the legs than the motor is consuming, for an overall net improvement.

How'd I do?

Steve

Would there be any advantage to using two RPC "motors" to achieve the desired output rather than one "larger" idler? 2+2=4 etc.

Generally, larger motors are are more effecient than smaller devices. It would make sense that one 10 hp RPC would provide "better" 3ph starting/running than two 5hp units, but....
That is the question.

Would this be a test making use of volt meters, Amp clamps and stop watches ?

TIA

CalG

CalG,

I do not have a hard and fast technical answer for you. From an absolute power standpoint I don't think you will find much difference between a single 10HP RPC or two 5HP RPC's in parallel. Bear in mind that this is an OPINION. But there are factors to be considered.

1. The 10HP single will be one less thing to trip over and to wire up. Simpler is better.
2. The two 5HP units will start (sequentially) with much less inrush current than the 10HP single, if that is an issue.
3. The two unit approach is scalable - no need to run both if you are only using a 1/2hp Drill press.
4. Other factors will probably affect actual output and performance more than the one/two motor question. For example, if the 10HP motor you are using to build your RPC (I assume you are building your own, what the heck they're pretty simple) is a late model lightweight and the two 5's are old pre-NEMA monsters with plenty of iron in 'em, expect the two 5's to outperform the single 10. I expect that the opposite will also hold true - a beastly old 10HP will yield better results than two shiny new tinny motors.

But at the end of the day, the best performing motor will be the one you already have, forlorn and gathering dust under the workbench. As the old hot rodders used to say, "Run what ya brung!"

Steve

Steve

You have hit the exact "real questions" in your response.

Yep! I have a hunk of iron 5 HP that has been serving just fine .....except for starting the big lathe in high gear)
Sitting a second motor on the floor would be fine, and all the present wiring is on the side of overkill. Adding a switch box would be easy. A cap or two might not hurt...

I like the "turn it on as you need" flexibility.

It's all good!

CalG

Thanks Steve and Calg, I think I will talk to my electician about setting up a second RPC for two of my larger machines. Most of my machines are in are in the 3 to 5 hp range and I never had an issue with any of them and can use just the one RPC.

GalG: I have several customers with dual RPC set-ups, usually a 20 + 30 hp. or a 30 +20hp combination (one is a 40 + 30). The reason is that he can run most of his shop equipment with the first converter, but may have a larger machine (maybe with a 25 hp motor) that is only run occassionally. When he needs to use the larger machine, he starts the second, to provide the necessary power. This way, he is not always running a 50hp converter.