RPC Theory

[strantor]

the electrician said to be careful when adding a circuit and not get the "wild leg." So it must have been a Delta transformer supply. The center tap would have been neutral and "bonded" (attached to earth ground.) True??

Yes. He was warning you against connecting a 120V load to the 208V leg.

When I moved to a newer location I got 208 Volt 3 phase and had to install transformers to get higher voltages (460+-.) needed by other machines.

120V/208V is typically a lighting circuit in large facilities. It's a WYE topology where you have 120V from any leg to neutral, and 208V between any two legs. the geometry looks like this:

Having 3 legs of 120V is convenient for lots of small 120V loads and helps keep the load more balanced on the utility side. Not useful for much else though, hence the transformers you had to install I assume.

I had machines that used a shift between wye and Delta motor windings to improve acceleration (?) and reduce inrush current (?) 6 wire motors.
How does that work? How does it reduce in rush current? There was a timer that was set to make the switch after a few seconds.

You refer to a WYE/Delta motor starter. A normal WYE motor has one end of each phase joined together inside in a WYE configuration that you can't change, like this:

and a normal delta motor has both ends of each phase joined to the other phases inside that you also can't change, like this:

But a 6 wire motor has both ends of all 3 coils brought out to the peckerhead so you can wire it either way.
Each phase coil is rated for 240V but if you wire them in Wye and apply 240V then they don't see the whole 240V. Instead each coil only sees about 139V:


With only 139V across each phase winding of the motor it won't develop full torque but also it won't draw gobs of amps when starting. So when ampacity is limited and/or you're starting really big motors, a WYE/Delta starter can cut down on the huge current surges by starting the motor in a WYE configuration and once it gets up to speed (after a few seconds, hence the timer you mentioned) a contactor will disengage and a different will engage, and those contactors change the wiring scheme from WYE to Delta. After it's made the switch-over you can develop full torque.

 

[rwm]

I am reviving this thread to ask a different theoretical question of all y'all experts!
So long term planning... I will have multiple 3 phase machines some of which may have multiple motors. What is the best solution for a whole shop system where motors may go on and off line randomly?
It is my understanding that VFDs do not like this. Are RPCs better? What about a large digital phase converter.
Something like:

Phase Perfect Enterprise | Single to Three Phase Power Converter | Variable Frequency Drives and Digital Phase Converters | Phase Technologies

www.phasetechnologies.com

 

[Eddyde]

From what you are describing, an RPC sounds like it might the best way to go, or a mix of the two as I have in my shop. Yes, VFD's can only drive one motor so you would need one for each motor in each machine. (Eg I have 2 on my mill, one for the spindle and one to raise/lower the head). The VFD must be wired directly to the motor no switches or anything else in between, so you must either use the VFD control panel to run the machine or wire the machines original controls to the aux inputs on the VFD. Of course the advantage is you have more control over the motor like speed control, soft start, braking, toque limiting and so on. So if you have a machine that would benefit from those features it might be worth going that route. On the other hand the RPC is plug and play, uses all the machines original controls and multiple motors are not a problem. I have 2 lathes, a hydraulic press and a cloths washer on an RPC. I also have an air compressor, a band saw and a drill press on VFDs.
My RPC is 7.5 hp and easily starts the 5 hp motor in my lathe, I can also run the machines attached to it at the same time.

 

[markba633csi]

The 480v test might still give you a shock- the bottoms of your feet and the shoes you are wearing form a capacitor with respect to the earth. Even if they are rubber shoes, you might still feel something
Capacitors pass ac current
That's the reason for the tingly, buzzy feeling when you touch some metal appliances that are not well grounded
If you watch those helicopter power line service guys attach their safety wire, they always draw an arc. The heli has some capacitive reactance with respect to the earth.

 

[Larry$]

I agree that a RPC makes the most sense. Sized to start the largest motor or motor under the greatest starting load. After that motor is running you can run multiple motors . You can buy or make a RPC. If you can come by a used 3 phase motor that is big enough/correct size all it takes is a way to spin it up on starting with: big electrolytic caps or a single phase motor, even a bicycle would work. I made one many years ago before I had access to 3 phase.