Does a Rotary Phase Converter Actually Make REAL 3-Phase Power? (120 Degrees)

Does a rotary phase converter produce real 3-phase power? Real 3-phase power has its three phases offset by 120 degrees, but a rotary phase converter passes through the two 180-degree input phases, so how can it possibly make real 3-phase power? In this video, we'll throw a scope on the output of my American Rotary AD-5 and confirm that yes, it does, and we'll talk about how.

Tools used in this video:
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Further reading:

Licensed under Creative Commons: By Attribution 3.0 License

00:00 Intro
01:31 Voltage references
02:31 What is 'phase'?
06:25 Split-phase power
08:06 Three-phase wye with neutral
10:23 3-Phase delta (no neutral)
11:28 High-leg (wild-leg) delta
17:03 Measuring it for real
19:58 American Rotary FAQ
21:42 Conclusions

american rotary, high leg delta, bastard-leg delta, rotary phase converter

[billrotundo]

As an Electrical Engineer, you did a great job explaining.

[uncledansworkshop]

I’m a EE myself, and I wish I could have had James explain this subject to me 40 years ago when I was at Penn State learning it for the first time. Very straight forward and clear. Thanks for making a mind twisting subject simple!

[keithburk]

Man I could have used this video about 4 years ago. I built a rotary Phase converter from scratch very challenging but a great project. I purchased a 3 phase 40 taper mill and needed the power to run it in my shop. After buying 20 hp slave motor, another panel, a box a bunch of capacitors, contactors and a few other bits it was mostly education. I found it very interesting and rewarding to get mine build and running. every time I start it up it makes me smile. This was a very good video thank you!

[swp]

The image shown in the 11:40 segment is wrong. The 240V leg just to the left of the 208V leg should be between phase A and C, not A and neutral as shown (that would be 208V as is correctly shown right next to it).

[BernardinoCampos]

I decided to do something I had never done before, to congratulate someone for an exceptional video. When I started watching I thought it would be a thankless task, but the end result was perfect. This subject is something that many professionals in the field still do not understand!

[Ariccio]

This is an *excellent* video. That said, the first thing that comes to my mind to answer the question of "how can it create 3 phase power from split phase power" is, "because it's rotating". If it's rotating, you can generate 3 phases, exactly the same way they make 3 phases at the power plant! 😉

[TechOne]

As an electrical engineer who understands this stuff your explanation is superb. I wish I had videos like this 30 years ago, they didn’t explain it well back in the day.

[MartinKrafft]

good video.
i'm glad that in germany we have a three-phase network that reaches almost all apartments and houses.
but it is interesting for an electrician to see the difficulties that arise when one does not have such a luxury.

[davekellogg]

James, Last week I asked for this explanation of the “phantom phase”. You succeeded far beyond my expectations. I feel somewhat guilty for having caused you to buy the fancy probes. But I don’t feel **too** guilty, because now you have another cool tool. 😊

Excellent explanation. Detailed without fluff. The point about reference voltage was the insight I needed.

Thanks for all your YouTube work. I get lots of ideas from you (which causes *me* to spend $$ on tool tools).

— Retired embedded software engineer.

[phoolb]

James, After last week when you replied "only when referred to neutral" I replied "yea Duh" and walked away thinking "how is he going to explain this to everybody else that is not an engineer?" As an electronics engineer I am biased, but I must say that I think that you did an incredible job in describing what is going on with just the right amount of math so as hopefully not to overwhelm most people. And gathering the information, scripting, shooting/editing the video in less than a week in itself if quite an accomplishment. Have a Great Weekend and try to stay warm. By the way, How do you keep your shop/garage warm in the winter?

[rowanjones]

And for anyone not familiar with RMS, it’s simply the equivalent DC voltage and current that would deliver the same power into a resistive load.

For 120V AC the peak is 120 * sqrt(2) = 169.71V, assuming an idealised, time invariant sinusoidal waveform with no DC offset.

In reality, the AC V/I amplitudes vary, and their shapes somewhat distorted due to the presence of harmonic content at frequencies above the fundamental 50/60Hz (120, 180, 240….). A true RMS meter will calculate integrate its V/I measurements over a time window (either fixed or moving window) to calculate the RMS value for that window period 8:48

[realtimjimmy]

I've been an industrial electrician for five years, and this video FINALLY helped me really grasp how rotary phase converters function.

[firstmkb]

I wondered about how that third leg fit into 3 phase as well! Thanks for putting the scope on it, because I never got around to it!

[I_ammm_mojojojo]

Just wanted to take the time to say that I am not an electrical engineer.. and I fully understood your explanation. You are an excellent teacher

[josepheccles]

My hat is off to you. You are the first guy I have even seen to explain this correctly. I have been building rotary phase converters from scrap motors for many years. While I have always known the answer to the question, I have never seen anyone other than you explain it from the reference of a high leg delta configuration. The hardest part is balancing the voltages on the phases. I do this with capacitors. But you get trouble with matching loads. A friend of mine showed me a cool trick. He uses balancing capacitors at each load. This maintains the most consistent voltage over various load conditions.

[bruceadler-]

There's a mistake in Figure 3 at +11:45. The arrows for two voltages on the both point to the same legs (A and Neutral) but the first one says 240V and the second says 208V. The one one that says 240V should point to the A and C legs, not the A and Neutral legs.

[JohnBare]

Yep! That explanation cleared up my confusion that I have had for over 70 years. It is obvious to me now that whoever I asked before had no idea how it worked. Thanks! It is surprising how FACTS clear up confusion in a blink of an eye.

[squelchstuff]

Nice job explaining that James.

Y/wye 3 phase connection is more commonly called a Star connection here in the UK.
Neutral taken from the centre point of the star, and is ~0v

As most may already know, the direction of a motor can be changed by transposing two of the phases. Also, a 3-phase motor can run on just two phases if for some reason one leg is disrupted (blown fuse, broken connection), but this will cause the motor to overheat with reduced power output. It's always worthwhile to know how warm to the touch the motor runs in normal use, and check periodically. Reduced output can go unnoticed.

[Sparky-wwre]

Probably the best and most throughly explained video on rotary phase converters. I am a residential electrician but am familiar with three phase power. In rural settings I have seen three phase power distributed to farms in the form of open high leg delta (one large and one small transformer) with the meter on a pole in the barnyard between the house and outbuildings, 120/240 single phase ran to house and 120/240 3 phase ran to outbuildings that require 3 phase for the milker, auger/conveyer, grain dryer or other large motor loads..

[MyJp]

Electrician here, you did such a great job with this one! I learned a lot. I guess it was a misconception of mine that the sine wave would look so good, I thought that the manufactured phase would be lagging, at least before the power factor capacitors, but of good enough quality for motors, the oscilloscope convinced me otherwise. Thank you for a great video.