Power Down Buck Transformer when not in use ?

I am installing two buck transformers, one for my compressor, and one for a New CNC Mill.

These machines are at home, and don't get used everyday, could go a week or two without using them sometimes.
Should I have a power disconnect up line of the buck transformers? Do they draw any current on their own?

They feed off of a sub panel in the garage, but don't want to use a breaker to continually turn them on and off.

On another note, why are fused and non-fused disconnects so expensive once you get over 30 amps

Insert said:

I am installing two buck transformers, one for my compressor, and one for a New CNC Mill.

These machines are at home, and don't get used everyday, could go a week or two without using them sometimes.
Should I have a power disconnect up line of the buck transformers? Do they draw any current on their own?

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"Full isolation" transformers draw mild power for their primary. even if no load on the secondary.

Buck/boost wired as "autotransformers" should draw none. Clamp ammeter should tell yah that pretty easily.

On another note, why are fused and non-fused disconnects so expensive once you get over 30 amps

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They are not, actually. Makers have costs and liability to meet codes and stand ready to defend against stoopid lawsuits. Costs have to be covered for all of that, spread over all products.

Try buying an empty but NEMA "rated", NFPA 70-compliant enclosure same size for mounting a VFD, relays , semiconductor fuses and such, even if not gen-u-whine "Hoffmann".

It can be cheaper to buy a used major-maker fused disconnect and strip the guts!



Keep your search open for NOS remaindered off large commercial projects or out of distributors rebalancing stale inventory, "remaindermen" scarfing up failed biz, etc, and the prices aren't so bad. Even so, your local Big Box or commercial distributor may not be any higher for brand-new goods, clean box, no rust nor buggered paint, nothing missing, and with warranty and a local service desk. Do yer research.

Also check the part numbers so you don't get box-only, cover-panel-only, guts-only, or goods no longer compliant with up-revised code, etc.

Photos don't always match the description - a level of expertise in that P/N lookup skill is "on you", and presumed 'coz full-time sparkies already know that stuff by memory and number de-code habit, and/or where to find it fast.

2CW

Just got part numbers from Eatons website, and found a bunch on eBay that are listed by part number only and not description. Found some brand new ones for under $30

For that price might as well just add them.

I'd turn them off. They make waste heat just idling. Cycling them isn't going to stress connections and so on, like it does for electronics. I suppose buck-boosts don't waste as much heat as, say, a 5 kVA 240-600V transformer. I have two of those in open delta arrangement, and my PhasePerfect's panel meter says they pull 3A just idling. That's 720W. (Or is there a factor of SQRT(3) involved?)

rklopp said:

I'd turn them off. They make waste heat just idling. Cycling them isn't going to stress connections and so on, like it does for electronics. I suppose buck-boosts don't waste as much heat as, say, a 5 kVA 240-600V transformer. I have two of those in open delta arrangement, and my PhasePerfect's panel meter says they pull 3A just idling. That's 720W. (Or is there a factor of SQRT(3) involved?)

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AFAICS, he IS "turning them off". Just load side, not line side. So no current flow atall, and no waste heat.

Yours are "ON", BOTH sides if placed line-side of the Phase-Perfect and it is alive enough to tell you anything at all, even if it has no loads active, output side.

Insert said:

Just got part numbers from Eatons website, and found a bunch on eBay that are listed by part number only and not description. Found some brand new ones for under $30

For that price might as well just add them.

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I found a "bargain" on NOS Eaton/Cutler-Hammer 3-PH load centers, main breaker capable, but not so equipped. After catching-up on their breakers - plentiful, but gone obsolescent arredy - I decided I didn't want to downgrade from my usual Square-D "QO" after all!

Bought cheaply enough, so they'll be stripped of buss-bar & c. for "project" boxen.

thermite said:

AFAICS, he IS "turning them off". Just load side, not line side. So no current flow atall, and no waste heat.

Yours are "ON", BOTH sides if placed line-side of the Phase-Perfect and it is alive enough to tell you anything at all, even if it has no loads active, output side.

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If he turns off the machine, but not the supply side for the buck transformers, the primaries will still be energized across the line, and they will draw magnetizing current.

A disconnect switch would allow cutting off power to the transformers.

Using a breaker is not a good idea unless the breaker is rated for switch duty.

JST said:

If he turns off the machine, but not the supply side for the buck transformers, the primaries will still be energized across the line, and they will draw magnetizing current.

A disconnect switch would allow cutting off power to the transformers.

Using a breaker is not a good idea unless the breaker is rated for switch duty.

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What JST says. Otherwise, why would my (or practically any) transformers be warm even with no load on the output.

rklopp said:

What JST says. Otherwise, why would my (or practically any) transformers be warm even with no load on the output.

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Quick! Cover it up, don't let anybody see it. You have just discovered how transformers work. When they get warm, they generate electricity and store it in the magnetic field for when you want some joules.

Tom

TDegenhart said:

Quick! Cover it up, don't let anybody see it. You have just discovered how transformers work. When they get warm, they generate electricity and store it in the magnetic field for when you want some joules.

Tom

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Curses! Foiled again!

Inadvertently offen'ed the incandescents that are the primary heat in my "shop" and the bugger was down to 43 F this morning.

Figured I'd heat it to 55 F or so just off getting JST fired up, and an easy 65 F if I could get Jim Rozen to pitch in some heat... the routers, WiFi and GigE switches sit out there, y'see'



Now you too-reasonable low-BTU fuel-value lot have gone and given away the game!



Yah. Put a clamp-ammeter on it. Or check the temp of the case.

It won't ordinarily waste enough to fund a proper upstream disconnect.

Even so, a "whole shop" or "zoned shop" set of disconnects is not a bad addition.

Or just upgrade a few of yer breakers to those as ARE rated for switching duty.

That's the goods as earn UL 489 "SWD" label (<20A), or the "HID" one (<50A).

Code Q&A: Using Circuit Breakers as a Switch | Electrical Construction & Maintenance (EC&M) Magazine

Examples:

Which Square D or Schneider Electric circuit breakers have SWD (Switch-Duty) ratings?

http://trcsupport.eaton.com/Residen...d-HID-rating-signify-for-circuit-breakers.htm

rklopp said:

I'd turn them off. They make waste heat just idling. Cycling them isn't going to stress connections and so on, like it does for electronics. I suppose buck-boosts don't waste as much heat as, say, a 5 kVA 240-600V transformer. I have two of those in open delta arrangement, and my PhasePerfect's panel meter says they pull 3A just idling. That's 720W. (Or is there a factor of SQRT(3) involved?)

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How much power they actually waste is a bit complex. There are core magnetization losses as Jerry has pointed out and there is a lot of 90 degree out of phase current, so called bad power factor, which does not theoretically waste power but it is flowing in the windings so there are some resistive losses there. Transformers are typically designed to less than optimum idle characteristics to save cost and weight. In an earlier incarnation I worked on development of an instrument for measuring specific gravities of liquids, basically a scale that weighed a bob in the liquid. We had problems with the transformer warming the unit, causing zero drift. Ogden Coil and Transformer was making the transformers and they added some laminations to the stack which brought the heat down to a usable level. Idling losses are pretty much what the designer decides to make them.

If the input is energized, it doesn't matter whether it is an autotransformer or one with two windings. Either way, you are magnetizing the core.

Bill

When I have been involved in the use of transformers feeding a machine one of 2 possibilities exist.

1. Attach transformer to machine and wire transformer after machine main breaker/disconnect.

2. This method is normally used for isolation transformers. Attach a fused disconnect on/near machine, install transformers near ceiling or from building steel away from the floor. The disconnect is used for the main breaker not the machine breaker. Nothing like having a 100-200 KVA transformer next to you pumping out heat on a 90° day!

Bill

Thermite's initial response is correct. In an autotransformer, there is no "primary" or secondary winding, there is only ONE winding. No current flow to the load means no current flow through the transformer; no current flow means no magnetic coupling, no magnetic coupling means no losses. There is no "primary" so there is no magnetizing energy until AFTER current begins to flow through it. There ARE lower losses (compared to an isolation transformer) through an autotransformer when supplying power to a load, but not when the load is turned off.

The only possible danger of not turning off the line side of an autotransformer is that when only the load side is open circuited, the load side of the autotransformer voltage potential becomes equal to the line side voltage potential. But when using it to buck or boost just a few volts, that's not a big deal.

Jraef said:

Thermite's initial response is correct. In an autotransformer, there is no "primary" or secondary winding, there is only ONE winding. No current flow to the load means no current flow through the transformer; no current flow means no magnetic coupling, no magnetic coupling means no losses. There is no "primary" so there is no magnetizing energy until AFTER current begins to flow through it. There ARE lower losses (compared to an isolation transformer) through an autotransformer when supplying power to a load, but not when the load is turned off.

The only possible danger of not turning off the line side of an autotransformer is that when only the load side is open circuited, the load side of the autotransformer voltage potential becomes equal to the line side voltage potential. But when using it to buck or boost just a few volts, that's not a big deal.

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Wait a minute. If the two line side connections are still made, there will be magnetizing current.

Tom

You cant have a fixed buck boost ratio without a line side coil. If that coil is upstream of the equipment disconnect you are going to have parasitic losses.

I would estimate 50 watts no load, 150 watts full load loss for a 150C temperature rise 1kw transformer. Probably 20 pounds mass.


A 1kw 16vac tx is good for 60 amps, or 30 at 32 amps, so such a 50watt parasitic load has to be balanced against a 6600 watt load, at 8 hours a day that'ss a 2 to 4 thousand dollar electric bill per year. So how does 30$ compare to the time required to remember to power it down everyday?

If they are not in use for periods of time add a disconnect.

They are not that expensive and surplus magnetic one would allow simple light switch to control them

Simple

Sent from my SAMSUNG-SM-G930A using Tapatalk

Jraef, Tom is correct. If there is no load, it will be as if the boost winding wasn't there and the transformer looks like a core with a primary energized by the line. Any other windings will not be visible to the primary if they are not connected to a load.

An auto transformer is more efficient in this service because only the buck or boost winding has to carry the full current. The current in the primary winding is the load current divided by the ratio.

You understand transformers and are almost always right, but this time you have had a logic slip. It happens to the best of us.

Bill

TDegenhart said:

Wait a minute. If the two line side connections are still made, there will be magnetizing current.

Tom

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See prior post and "measure it with your clamp Ammeter".

Some autotransformers MAY have their one and only coil across the line. See also "variac".

Others do not.

WHEN they do not, the circuit is only completed when the LOAD is connected. ergo no significant current flows (there is ALWAYS leakage of SOME kind ...if you have gear good enough to measure such..).

WHEN you have a coil always across the line, it might be less contentious to consider it an autotransformer arranged special case of an isolation transformer.

What would have been a fully-isolated secondary has been wired in series, out-of-phase so as to buck - or in-phase so as to boost - the line Voltage presented to the primary.

Look at the maker's schematics for the "raw" device(s). Then at the several ways of connecting them, depending on tasking.

MOST commercially-offered "buck/boost" ship with distinct and separate primary and secondary windings. They are also suited for full-isolation use for lower-voltage loads.

These include "grounds lighting", and at anywhere from 6 to 48 Volts, quite commonly 12/24 or 16/32, much less often 24/48 or 6/12.

The PHYSICS are precise enough.

Pesky humans want lots of choices, get them, then don't all make the SAME choices when they arrange the taps for their specific application.

I try, and try HARD - so far with 100% success EXCEPT FOR my several Variacs and one multi-tapped bench test equivalent - to not use Autotransformers AT ALL.

Nought to do with efficiency, mass, space, nor even value-for-money.

I simply have a residence full of fussy electronics gear adjacent a shop full of NASTY, NOISY electrical gear.

Drive isolation transformers help.. would you believe "isolate"... that trash. Rather well, too for the actual environment I have to deal with. SCR-class DC drives yes, VFD's no longer.

Other than kitchen and laundry, anyway.

THOSE "inverter Drive" AKA VFD buggers are on their own. The makers have FAR better labs, test facilities, motivated staff, and deeper pockets with more MONEY in them than I do. They do rather well even when the products are mass-produced to slender margins. (LG, Panasonic, Whirlpool, GE, etc..)

thermite said:

See prior post and "measure it with your clamp Ammeter".

Some autotransformers MAY have their one and only coil across the line. See also "variac".

Others do not.

WHEN they do not, the circuit is only completed when the LOAD is connected. ergo no significant current flows (there is ALWAYS leakage of SOME kind ...if you have gear good enough to measure such..).

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Show us a diagram of a transformer that does not have the primary energized when there is no load. That is, with the transformer connected to the line and the switch is on the load side.

A clamp on ammeter will show the total current but does not separate watts and VARs. The reading does not show power lost or heat generated. You need a wattmeter for that.

Bill

I run auto Transformers. My small one (6 kva) has 0.5 A, 0.6 A, 0.6 A, draw at 238V with no load on output. When I throw the input disconnect it comes on with a small hummm, even though the output disconnect has not been thrown. There is even a small surge.