Another RPC question

I will make this as simple as I can. I have an Anderson 7 1/2 hp RPC. I use it to run an older 40s lathe with a 7 1/2 hp motor,huge compared to the RPC motor. It starts the motor fine within a few seconds of bringing it to speed.
Before I start the lathe motor, the RPC is somewhat noisy,almost annoying. Start the lathe and it is smooth and quiet. Very pleasant.
I recently got my Bridgeport with a 1 1/2 hp motor, back together and wired in. I started the RPC, then the Bridgeport motor and the RPC didn't quiet down a bit. The RPC is located about 5 feet away from the mill, so that doesn't help any.
I was at a point of either running off single phase to a static Phase a Matic, which the previous owner used, or going VFD.
Today I decided to start the lathe motor first, like an idler, and try the Bridgeport. What a difference. It starts the BP motor and the RPC runs very quiet.
I have read that the additional motor on the first machine started is a boost to the RPC.
I am not fully certain that this is entirely the case. All I know is once the lathe motor is running, the shop sure is a lot quieter. The lathe has a clutch by the way.
Without the lathe motor running, the RPC seems like it takes more of a load when starting the BP motor. It sounds that way anyhow. With the lathe motor on the BP motor starts with barely hearing any start up load on the RPC.
Is this typical, and am I good to go running like this?

I run my mill off a RPC. I usually start the table feed motor and the suds pump before the two speed spindle motor.
For all the reasons that may be explored from the content of your post.

Bottom line, the more 3ph induction motors on line, the truer the 3ph generated leg.
Think about this. Which motor is the "converter", when more than one motor is energized?

Pretty much exactly what I was thinking. I have definitely heard this before, but since all of my equipment and the powering of it is a fairly new reality to me, I have been very cautious.
Thanks.

conant said:

Is this typical, and am I good to go running like this?

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Yes. 'Supplementary idler' effect. Expect that the clutch engagement/dis-engagement mechanism will see some wear?

Next time you trip across an affordable used-but-good (they usually are..) 3-P motor of 3 to 7.5 HP, consider adding it a-purpose so you can leave the lathe idle if you choose.

Main reason is that your are lucky already that a 7.5 HP idler starts and runs a 7.5 HP load as well as it does. 50% extra is a more common 'sweet spot'.

Your 7.5 + an add-on 5 HP, for example, will make starting the lathe even easier, and will also be better if/as/when you really need to load it up hogging.

Is the noise a high pitched whine?

Bill

9100 said:

Is the noise a high pitched whine?

Bill

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No. A humming like sound, not high pitched, rather mid range sound like a loud electric motor.

Monarchist said:

Yes. 'Supplementary idler' effect. Expect that the clutch engagement/dis-engagement mechanism will see some wear?

Next time you trip across an affordable used-but-good (they usually are..) 3-P motor of 3 to 7.5 HP, consider adding it a-purpose so you can leave the lathe idle if you choose.

Main reason is that your are lucky already that a 7.5 HP idler starts and runs a 7.5 HP load as well as it does. 50% extra is a more common 'sweet spot'.

Your 7.5 + an add-on 5 HP, for example, will make starting the lathe even easier, and will also be better if/as/when you really need to load it up hogging.

Click to expand...

Sorry I left this out, I have a 5 hp motor on the floor next to the RPC with a fused disconnect switch. I start the RPC, then start the 5 hp. Next I start the lathe. The idea behind that was to boost the RPC for starting the lathe. After I start the lathe I turn off the RPC. Today I will try the RPC with the 5hp Idler, lathe off and see how that works for the Bridgeport.
I also don't like the lathe motor running with no clutch engagement for an extended time because the oil pump for the headstock isn't engaged and most likely the clutch unit is not getting necessary oil.

I will post later today on how the RPC, 5hp idler and Bridgeport combination works out.

The RPC running the Bridgeport with the 5hp sub idler added is quieter than with only the 7 1/2 hp RPC running by itself, but not as quiet as with the lathe motor running.
I did check today and looking through the lathe headstock sight glass for the oil pump, there is no oil stream unless the clutch is engaged. So the lathe motor is no longer any consideration for a sub idler.
Thanks for the info