# Electrical ower service question

• posted

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.

• posted

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 on your list...

• posted

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

• posted

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.

i
• posted

Check out the table ..

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.

• posted

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

• posted

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

• posted

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

• posted

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

• posted

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

• posted

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

• posted

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
• posted

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

GWE

• posted

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

Thanks, Rich

• posted

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
• posted

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

• posted

According to :

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.

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