Had my first dealings with 2 phase. Was disconnecting a machine and wanting make sure all the leads are identified, I started going over everything. At first I thought the 4 pole disconnect may have had a nutreal being disconnected. But wait there is no 110v on this machine. readings to ground were all over, and there was 2 sets of 240v. Aaaa wow! 2 phase. I thought it was a urban legend! Sure as could be a 3 phase to 2 phase 50kw trans. What I have learned to be a Scot Trans. comments welcome:)

Ob

The only and last place I saw 2 phas was in Philadelphia, many, many years ago. Working with one of the 'old timers', and I don't remember much of it today.

John,

The strangest voltage system I ever saw was in Jersey City. There were 3 phase legs going to a machine. You got 240 volts between each phase leg. On 2 of the phase legs you got 120 volt and on the third leg you got zero voltage to ground. There was no neutral for these machines. I was told that it was a very old style system.

I thought that true 2 phase was there were 2 phase legs and 2 neutrals. 1 neutral for each phase leg.

Harold, are you sure that wasn't 240v to ground on the other 2 legs?

That would be corner grounded delta.

Harold,
Thats about right, except voltage to ground was more like 135v, 120v, 180v, 0v. approx voltages? I got on the phone asap when I knew I wasn't in Kansas anymore.



Ob

Greg,

No, I believe it was 120-125 volts to ground and 240 between legs. The 3rd leg was zero volts to ground. Even my old boss found this wiring system to be real weird. He used to work for Bendix and they made airplane parts here in NJ. They use to work with all sorts of wacky voltage and freq. in their factory. They even had one system that was 400 Mz I believe. It was for satellites or something.

Ob,

As I said to Greg, I believe it was around 125 volts. It as many years ago and my memory isn't what it used to be. I just started as an apprentice with the old boss, so that would have been around 1977. Plus I only saw this type of system once in all the years I have been doing electrical work. I was more a resi-com type of electric, not much into heavy industrial.

I am not sure how you can have 240 volts from phase to phase, one phase that looks be grounded and not have 240 to ground. It may be impedance grounding on that corner and a low impedance measuring tool (wiggy?)

Greg,

Like I said, it was a long time ago. I maybe mistaken. If I see my old boss again, I will ask him if he remembers. He still works around here but not as much as he used to.

Impedance grounded corner delta might give the result you saw if you were using a wiggy. I really don't understand all I know about those systems and I never actually saw one.
We do have some corner delta around here tho, usually in something like a lift station where the whole load is 3p L/L, basically just the motor and the controller.

I suspect that would be 400hz, which is very common in aerospace. I believe that it became a standard from about 1940s onwards, because it was the most efficient for weight/iron loss in transformers, long before power conversion electronics became available.
However, I've heard of 400hz gear in military naval roles, where weight presumably isn't an issue. I've wondered why.
The systems I worked with were always 200v 3 phase/115v to ground.

We used a lot of 400 hz in the big iron computer days. They either use big MG sets or the Liebert UPS inverters that produced 400 hz directly.

In the Navy they used 400 hz in servo systems to point guns. That was for the same reason. The magnetic elements could be smaller. You also got better precision.
Since it was available, other folks may have used it but I was in the pointing the gun business. That is all I know about.

On the aircraft carrier, we had 400 Hz systems for airplanes. It was my understanding that the Soviet aircraft used 300 Hz for same reasons. We typically ran the ships service generators at slightly over or under 60 Hz so the fifth harmonic did not overpower the passive SONAR systems listening for 300 Hz.

I was told that the 400Hz was chosen because it wasn't a harmonic of 60Hz AC. The Soviet is probably the same thing but calculated for 50Hz power.