RPC output

I recently read that "all" rotary phase converters produce a DELTA output. Is this true for a WYE wound converter motor? I never thought about it before.

Had one customer with a CNC mill (do not remember the make) that required "balanced voltages to ground", which meant a RPC could not be used because of the high leg (without an isolation transformer). Are there any concerns with running a WYE wound motor on DELTA power, or a DELTA wound motor on WYE power?

Howard

"Are there any concerns with running a WYE wound motor on DELTA power, or a DELTA wound motor on WYE power?"

No.

As long as there are no connections to ground within the load, the load cannot tell if the source is Y or is ∆.

For practical reasons, most motors are designed as Ys.

Yes, there is indeed an internal "star point", which otherwise would be a neutral, but this point is seldom, if ever, brought out to the motor's connection box. For, if it was, it would be the junction of what would be T10, T11 and T12.

(In rare cases, motors are supplied as "12 wire", with T10, T11 and T12 brought out, but most are supplied as "9 wire", with T10, T11 and T12 as an internally connected point, and not brought out).

When one changes a 208-240/480 motor from the higher voltage to the lower, T4, T5 and T6 are connected, and are insulated from all the others, and T1, T2 and T3 are connected to T7, T8 and T9, respectively.

The source is then applied to T1, T2 and T3, as is usual.

There remains, then, two "star points", namely, T4-T5-T6 and what would be T10-T11-T12, both effectively isolated from one another.

The motors are designed with this in mind, and it is not possible, nor is it required that T4-T5-T6 be connected to T10-T11-T12.

Polyphase systems have for more than a century been designed with ∆ and Y being interchangeable, through the ∆-Y transformation.

For practical reasons, most primaries are ∆, while many secondaries are Y, but a lot are still ∆.

It really makes no difference, for as long as there are no grounds, ∆ and Y are interchangeable.

The ubiquitous North American 120/240 ∆ residential power distribution scheme presents certain problems, as there is an implicit ground.

When one constructs an RPC, there becomes a virtual neutral, N', at the "star point" of the idler motor, this virtual neutral being elevated from the real neutral, N, by 69 volts.

Customer has a CNC lathe and mill which require 208v Y input. I plan to build a 40ph RPC and feed a 30 kva 240/208-120 (DELTA/Y) transformer. Do these transformers have a Y ground tap (I do not have it and have not seen it yet)? If so, is the Y ground the same as the service enterance ground? If not, do I use the service entrance ground as the machine ground?

Is there a "transformers-for-dummies" book avaiable?

Boy...

I wish there was, Howard, but there's as many transformers as they're are dummies!

If I were in your shoes (and I suspect Peter would do the same) is look very closely at the machine's power and control wiring diagram.

More often than not, machines that I've worked on were DELTA only. If the machine requires Y, it's because they're using the neutral lead to derive lower-voltage power for control circuitry.

Deriving low-voltage power using the neutral of Y is not considered good practice, both in machine power design, and in industrial electrical power distribution. In simplest terms, it opens a whole avenue for electrical faults and dissymetry in the load, shock hazards and grounding concerns.

IF the machine actually derives power this way, better thing to do, is to determine HOW they've done it. Most likely, they're using the neutral to pick 125v from ONE of the hot legs. If this is the case, figure out how MUCH power they're expecting, and install a dry-type transformer that provides enough 125v current to power those loads (usually lights, control circuitry, perhaps some electronics), and wire the PRIMARY of the transformer ACROSS two legs, so it steps down 208 to 120v for your control circuits.

By doing this, you've eliminated the need for a neutral, and also eliminated the expense and shop-space consumption of a huge transformer, while at the same time reducing shock hazards from 'fake' neutral of the RPC and other wonderful gremlins.

These CNC's are Fedals (sp?) and data plates specify 208 Y. This is what the guy had in his shop in Calif. He has moved to a somewhat remote area in Tenn, and 3ph is not readily avaiable (affordable).

I assume the machines need 120 for the computer, lights, controls, etc. A seperate 240/120 stepdown could provide the needed 120v, but what about a machine ground? If the 240/208 transformer has a neutral tap, this would be the 120v source, but can service entrance ground be used for machine grounding?