# Electrical ower service question

Discussion in 'Electronic Design' started by John E., Dec 15, 2005.

1. ### John E.Guest

Service to building is 150A, 115/220 single-phase. Proprietor wants to run a
3-phase machine using rotary converter.

How efficient are these converters?

What is a safe maximum running load for a building with 150A service?
Obviously the machine won't be pulling maximum rated current all the time. Is
it safe to run 120A load? 140A load? Machine is mostly motor, running less
than 2 hours at a time, typically (printing press).

Building has only a half-dozen fluorescent lights, nothing else (no
computers, etc.).

Thanks.

2. ### Tim WescottGuest

I don't know the answers to your questions.

But

Many of the folks on rec.crafts.metalworking use 3-phase machinery in
residential or light industrial buildings with no 3-phase power. So the
interest level in rotary converters is high, there are a number of folks
there with practical experience and even a few who seem to really know
their stuff in this area.

I'm going to cross-post this, even though there's already three groups

3. ### Jon ElsonGuest

I don't think you'll get away with it. The big problem is the reactive
current,
which is a substantial part of the real current draw. If the machine
draws 120 A
of real current, and the reactive current is 60 A, the total current will be
Sqrt(120^2 + 60^2) = 134 A. Also, if the 3-phase load is rated at 120 A
per phase, the single-phase equivalent current is 207 A, without considering
the reactive component. The 120 A at 240 V indicates a motor around
35 Hp. The starting current will be in the neighborhood of 1000 A for
several seconds, given the high source impedance for such a heavy load.

Unless this press has a slow-start feature, you will blow the 150 A service
entry breaker, and possibly the breaker on the pole transformer, when you
start the press. Probably everybody's lights will dim so far that computers
will crash, air conditioners, etc. will pop breakers, and so on for several
blocks around.

Jon

4. ### Ignoramus8558Guest

I am highly confused by the question. 120A of "mostly motor" drawing
220V amounts to 35 horsepower. When I think of printing presses (like
at a factory that was liquidating where I picked some stuff), it is
hard to imagine a 35 horsepower motor on a printing press.

I think that it would be good to clarify the needs of the owner, as
everything hinges on it.

A 5 HP easy starting motor (no large inertial loads or loaded
compressors) can run on 120 amps. A 35 hard starting motor, probably
not. Everything else is in between.

So, the first thing that is in order is to find out the power
requirements. That includes answering how much inductive load would
there be, whether all motors start at once, how much reactive load and
what degradation of 3rd leg quality his 3 phase equipment would
tolerate.

Second, not all electric services are equal. Some would happily
produce 120 amps and some woould not, as utilities sometimes install
undersized pole transformers. Again, that should be found out before
investing too much money.

There are experts here who could help further when you clarify your
power needs. I am not one of them, even though I built a couple of
phase converters, a 10 HP one and a 17.5 HP one.

http://igor.chudov.com/projects/17.5-Phase-Converter/

i

5. ### CWattersGuest

Check out the table ..
http://www.elect-spec.com/rotary.htm
It has figures for the single phase current need for a particular converter
at max load. It suggests you need >160A if I've read it right. They give a
phone number for advice.

6. ### SQLitGuest

Exceed the 150 amps and the overcurrent should open. Therotically you
should be able to run 150 amps through a 150 amp service. Typically NO ONE
DOES. The last 150 amp service I had was on a house, its total draw was

Pay real close attention to the other posters suggestions about reactive
current.

I used to partner with a steel welding company. They had a 200 amp 3 phase
service on their shop building. Owner went out and bought a stud welder that
they used infrequently. They made some type L copper fuses for their
service and replaced the fuses every time they needed to use the stud
welder. Nothing ever melted down. They were diligent about replacing the
fuses. Eventually because of the demand the utility got wise and REQUIRED a
service change.

The service change is some thing that you should consider. No converter ever
necessary. Also you had better look into the rate structure.

7. ### Grant ErwinGuest

Yes, it can also run on 100 amps, 50 amps, or probably even 30 amps! 220V power
at 30 amps is nearly 9 hp worth of watts. Sure it would draw more at startup,
but it would run on a 30 amp circuit fine.

GWE

8. ### Wayne CookGuest

First question is what kind of drive does this press have? I must
admit that I've only worked on one printing press but it's got a DC
motor driven by a electronic controller for variable speed (as far as
I can see this is pretty much a necessity for a printing press). Thus
my question of what kind of drive.

Actually that's a pretty small press to just have a 35hp motor. The
press I work on from time to time has a 75hp DC motor on it and that's
a smaller motor than was on the press originally. It's a press for
printing newspapers and it's fairly small as far as news paper presses
go (the owner talks about installing and running much larger presses).
It's got 8 units (though only 7 are in service at the moment) and each
unit is capable of printing both sides of the page. But 4 units are
needed to print the color pages. Thus they have the capability of
printing a max of 16 pages per section currently.
Definitely.

Wayne Cook
Shamrock, TX
http://members.dslextreme.com/users/waynecook/index.htm

9. ### Guest

Thats true if youve got the right type of overcurrent protective
devices all along the line, but on a domestic supply this wont normally
be the case, and they wont sit through the startup without tripping. If
you can control the motor so as to startup gradually you should get
round this, but whether that can be done depends on what the mechanical
load is, whether its connected during startup, and what type of
controller the motor has, if any.

NT

10. ### Grant ErwinGuest

What do you base this on? I have a 7.5hp motor I regularly start and it's wired
to a normal 30A breaker in my panel. It's the idler motor on my phase converter.

GWE

11. ### Eric R SnowGuest

I have 400 amps available to my shop though at this time am only using
a 200 amp service. Just like any residential service. Which makes
sense because I'm in a rural area with only single phase residential
service offered. I make my own three phase like Grant does. Except my
phase converter uses a 15 HP motor. It is wired through a 60 amp
breaker. It has never been a problem.
ERS

12. ### Rich GriseGuest

The keyword here is "idler". It's a 7.5 hp motor, sure, but I seriously
doubt that it needs anything like the startup torque would be if it had
an actual 7.5 hp load.

Thanks,
Rich

13. ### Ignoramus20852Guest

It all depends on what kind of load. For example, a blower is a nice
easy starting load (develops resistance with speed), a compressor
without unloaders is a hard load, etc.

i

14. ### Grant ErwinGuest

Correct, Rich, but the OP did say "easy starting 5hp" ..

GWE

15. ### Rich GriseGuest

But, isn't the idler's job just to spin up the main unit, which then
bootstraps itself?

Thanks,
Rich

16. ### Ignoramus4758Guest

Not exactly. It is true that a 3 phase motor, once spun up, could
continue to run, however, it would develop only 2/3 of horsepower (and
suffer from unbalanced power that it was not designed for).

With a phase converter, a loaded motor could develop 100% of its
horsepower, provided that the capacity of phase converter is
sufficient.

i

17. ### Guest

A motor draws well above run current during start-up. How large this
current is, and how quickly it diminishes depends on the motor type,
how its controlled, and what its mechnical load is.

A fuse or breaker will tolerate overcurrent of a limited amount and
duration only before it trips. Again, what exactly it tolerates depends
on the characteristics of the breaker/fuse.

So whether your motor/breaker combination will run or not depends on a
lot more than just the ratio of motor run power to breaker capacity.
Some combos will run ok, some will never get off the ground, and some
will sometimes start, sometimes not.

Note also that breaker characteristics vary a lot between eg US and UK.
Our domestic breakers are normally type B, whereas US breakers are
closer to our type D, which are a lot more tolerant of overload than
type B.

NT

18. ### DoN. NicholsGuest

The typical US breaker tolerates quite a bit of *short-term*
overload, but for long-term, the overload limit is a lot closer. I don't
know how these compare with your Type-D and Type-B breakers.

I believe that the short-term overload trip is magnetic, while
the long-term overload trip is thermal.

Enjoy,
DoN.