Boost-Buck Switch

[Joethemechanic]

I had this idea, but I'm thinking it might be a problem. This is for a piece of test equipment. 120V in with a 32 volt boost.

The reason I wanted to put the switch there is because it is a low current portion of the circuit

What I'm worried about is , it the switch is thrown under load, the primary windings could end up with a voltage of almost 4 times the 120 volt line imposed on them. Possibly damaging the transformer and requiring a 480 V rated switch.

Do you think I'm worrying needlessly?

 

[augie47]

Appears to me to be a violation of 404.2(B)

 

[kwired]

I had this idea, but I'm thinking it might be a problem. This is for a piece of test equipment. 120V in with a 32 volt boost.

The reason I wanted to put the switch there is because it is a low current portion of the circuit

What I'm worried about is , it the switch is thrown under load, the primary windings could end up with a voltage of almost 4 times the 120 volt line imposed on them. Possibly damaging the transformer and requiring a 480 V rated switch.

Do you think I'm worrying needlessly?

I don't follow the logic of the placement of the switch.

To me I see the auto transformer in the circuit as normal in one position and when switched the other direction (and it could be a single throw switch and still do the same thing) it puts the aux winding in series with the load.

After looking at it some more if you did use a double throw switch you would put the aux winding in series with the load and at same time short circuit the main winding which will have a voltage induced on it by the aux winding.

 

[jim dungar]

I had this idea, but I'm thinking it might be a problem. This is for a piece of test equipment. 120V in with a 32 volt boost.

Looks like a DIY hack job, by somebody too cheap to do it correctly.

Have you considered the issue of putting 120V across a 32V primary winding and then short circuiting the secondary?

 

[Joethemechanic]

Have you considered the issue of putting 120V across a 32V primary winding and then short circuiting the secondary?

Yeah that is exactly the issue that raised my concern.

A whole lot can happen while those switch contacts are moving.

the switch is thrown under load, the primary windings could end up with a voltage of almost 4 times the 120 volt line imposed on them. Possibly damaging the transformer and requiring a 480 V rated switch.

And the code thing,,,,,, What goes on on my test bench is up to me. My 120 volt cord with the alligator clips on the end violates code. Oh well,,,,,,,,,,,

 

[iwire]

And the code thing,,,,,, What goes on on my test bench is up to me. My 120 volt cord with the alligator clips on the end violates code. Oh well,,,,,,,,,,,

Yet you ask these questions at a code forum.

 

[Joethemechanic]

I thought this was the "Electrical Calculations/Engineering" forum.

Have any of you guys ever even been inside an electric motor shop?

 

[Joethemechanic]

Yet you ask these questions at a code forum.

Are you telling me, that you don't have a 120 volt cord with alligator clips for testing?

 

[__dan]

I had this idea, but I'm thinking it might be a problem. This is for a piece of test equipment. 120V in with a 32 volt boost.

The reason I wanted to put the switch there is because it is a low current portion of the circuit

What I'm worried about is , it the switch is thrown under load, the primary windings could end up with a voltage of almost 4 times the 120 volt line imposed on them. Possibly damaging the transformer and requiring a 480 V rated switch.

Do you think I'm worrying needlessly?

At first I thought it was a SPST switch and saw the 480 volt open circuit voltage, missed the shorted winding. It looks like the switch open to the line gives you 120 volt with the 32 volt winding in series with the load. Winding is a load with a shorted secondary, basically a current limiter but not a 152 volt source. Switch closed to the line could be a 152 volt source.

Image attached, is this what you wanted ? Did you want a voltage source or a current limiting effect ? apologies for the drawing. first time with MS paint

 

[Joethemechanic]

At first I thought it was a SPST switch and saw the 480 volt open circuit voltage, missed the shorted winding. It looks like the switch open to the line gives you 120 volt with the 32 volt winding in series with the load. Winding is a load with a shorted secondary, basically a current limiter but not a 152 volt source. Switch closed to the line could be a 152 volt source.

Image attached, is this what you wanted ? Did you want a voltage source or a current limiting effect ? apologies for the drawing. first time with MS paint

Yeah kinda, that is what I was going to do at first. But the box already has a SPDT on the face of the panel, but it's too light to take the 20 amps carried by the secondary. So I was thinking, primary doesn't carry much current, and if I dead short the primary the secondary will be fairly low impedance too.

But then my head went "

"This is gonna be like an open circuit CT while those switch contacts are moving and I know this switch ain't fast enough to beat the voltage rise"

Just had to make a little more room in the box, or maybe I should say add a box. I bolted a 1900 deep on the side. If I got to make room I might as well add a switch to select boost or buck mode.

Just the way it was going to work out if I had been able to use the existing panel switch would have been nice. It was funny, even the wires from the switch with their spade terminals were the perfect length.

The panel even has a perfect spot for the ammeter I put in it. It bolted right in. All standard stuff, but all surplus stuff I had on the shelf.

You ever have one of those projects that kinda wants to fall together?

Although most times you hit a snag:lol:


I can tell you one night I knew we had enough parts to build a M38A1,,,,,,,,,,,,, I woulda bet my life,,,,,,,,,,,, Who would have ever known that there were so many internal variations of a T-90 trans:happysad:

 

[jim dungar]

"This is gonna be like an open circuit CT while those switch contacts are moving and I know this switch ain't fast enough to beat the voltage rise"

You are wrong.
It is a voltage transformer, it will behave like one.

 

[Joethemechanic]

You are wrong.
It is a voltage transformer, it will behave like one.

Open circuit = high unintended voltage due to current in the other winding.

That is enough like a CT to raise a red flag in my head.

 

[jim dungar]

Open circuit = high unintended voltage due to current in the other winding.

That is enough like a CT to raise a red flag in my head.

It appears you have little idea of what you are talking about.
If the voltage is being stepped up, what happens to the current?

Have you ever open circuited the secondary of a power transformer before?

I would be much more concerned with your blatant disregard for voltage ratings, by applying 120V to a 32V winding.

 

[kwired]

Open circuit = high unintended voltage due to current in the other winding.

That is enough like a CT to raise a red flag in my head.

Have you ever open circuited the secondary of a power transformer before?

Open circuiting the winding is no different than throwing the main breaker of a service - the transformer still has same voltage on output terminals. It is not a current transformer.

Voltage on the 32 volt coil will not necessarily be 120 volts. It will depend on impedance of winding and impedance of the load as they are in series when this switch is in position where it shorts the 120 volt winding. Of course the shorted 120 volt winding will complicate calculating the impedance of the 32 volt winding.

Actual applied voltage will need to be known also.

 

[templdl]

I agree with Dan's wiring diagram (Post #9) to select simply between a straight 120v bypassing the boost transformer or to connect through the Boost transformer. Simple and safe.
The only time that I could see an issue if one were to place of fuse in the shunt winding circuit (in series with the 120v transformer winding). If you open up that cicuit thats when the voltage on the output would go sky high. Bottom line is that you never want to open the shunt winding.

 

[Joethemechanic]

It appears you have little idea of what you are talking about.
If the voltage is being stepped up, what happens to the current?

Have you ever open circuited the secondary of a power transformer before?

I would be much more concerned with your blatant disregard for voltage ratings, by applying 120V to a 32V winding.

I'm not sure you are comprehending at all.

It's a boost buck, the secondary is in series with the load,

That is the way a boost buck is connected

If I open the primary under load it goes overvoltage just like when you open up a CT circuit

The conductor passing through a CT is a winding, even though it is really only one turn.

Just as the 32 volt secondary in a b-b trans is a winding

One winding, or a 32V winding connected in series with the load next to a higher number of turns is going to create an an unintentional high voltage on the high number of turns side if you open circuit it.

What part of what I am telling you isn't clear?

You never hooked up a boost buck? The low voltage secondary is always in series with the load. That is not a disregard of voltage ratings.

 

[Joethemechanic]

I had this idea, but I'm thinking it might be a problem. This is for a piece of test equipment. 120V in with a 32 volt boost.

The reason I wanted to put the switch there is because it is a low current portion of the circuit

What I'm worried about is , if the switch is thrown under load, the primary windings could end up with a voltage of almost 4 times the 120 volt line imposed on them. Possibly damaging the transformer and requiring a 480 V rated switch.

Do you think I'm worrying needlessly?

This is mu ORIGINAL POST

DOES IT LOOK LIKE I AM IGNORING ANY FACTORS?

 

[kwired]

I'm not sure you are comprehending at all.

It's a boost buck, the secondary is in series with the load,

That is the way a boost buck is connected

If I open the primary under load it goes overvoltage just like when you open up a CT circuit

The conductor passing through a CT is a winding, even though it is really only one turn.

Just as the 32 volt secondary in a b-b trans is a winding

One winding, or a 32V winding connected in series with the load next to a higher number of turns is going to create an an unintentional high voltage on the high number of turns side if you open circuit it.

What part of what I am telling you isn't clear?

You never hooked up a boost buck? The low voltage secondary is always in series with the load. That is not a disregard of voltage ratings.

The main coil is not necessarily going to see nearly 480 volts, unless the aux coil has nearly 120 volts across it. This will depend entirely on the impedance of the load. If it has 120 volts across it the switch in question probably lasts longer than the transformer. Once you open that switch the 32 volt coil is no longer a boost coil it is just an inductor in the circuit.

If this is all test equipment made of extra components - why not play with it a little and find out what is going on?

 

[Joethemechanic]

The main coil is not necessarily going to see nearly 480 volts, unless the aux coil has nearly 120 volts across it. This will depend entirely on the impedance of the load. If it has 120 volts across it the switch in question probably lasts longer than the transformer. Once you open that switch the 32 volt coil is no longer a boost coil it is just an inductor in the circuit.

If this is all test equipment made of extra components - why not play with it a little and find out what is going on?

It's been done and in operation for hours

I bolted a 1900 on it to make room for the larger switches. I used a 3 way to take the 32 volt winding in and out of the secondary, and I used a 4 way to be able to reverse the secondary for when I want to operate in buck mode.

 

[Joethemechanic]

Yeah kinda, that is what I was going to do at first. But the box already has a SPDT on the face of the panel, but it's too light to take the 20 amps carried by the secondary. So I was thinking, primary doesn't carry much current, and if I dead short the primary the secondary will be fairly low impedance too.

But then my head went "

"This is gonna be like an open circuit CT while those switch contacts are moving and I know this switch ain't fast enough to beat the voltage rise"

Just had to make a little more room in the box, or maybe I should say add a box. I bolted a 1900 deep on the side. If I got to make room I might as well add a switch to select boost or buck mode.

Just the way it was going to work out if I had been able to use the existing panel switch would have been nice. It was funny, even the wires from the switch with their spade terminals were the perfect length.

The panel even has a perfect spot for the ammeter I put in it. It bolted right in. All standard stuff, but all surplus stuff I had on the shelf.

You ever have one of those projects that kinda wants to fall together?

Although most times you hit a snag:lol:


I can tell you one night I knew we had enough parts to build a M38A1,,,,,,,,,,,,, I woulda bet my life,,,,,,,,,,,, Who would have ever known that there were so many internal variations of a T-90 trans:happysad:

I just can't get how many people don't read entire posts before they start making comments.