Opinions on pushing an 120v PT to 240v
redruM69, Fri Feb 03 2017, 08:22PMDoes anybody have an experience running a potential transformer beyond, or even double its rated voltage? I have this beast in my possession. Its 175:1 21kv. 120v output. I have no doubt the secondary can handle 42kv, as the BIL is rated up to 150kv, but what about the primary @ 240v instead of 120v? I know these things are grossly overbuilt, but they ARE designed for measurement purposes.
Who has pushed the limits on these things?
Re: Opinions on pushing an 120v PT to 240v
Hydron, Sat Feb 04 2017, 11:27AM
Could just try it i guess? Crank it up on the variac while monitoring the current (preferably with an oscilloscope), see if it begins to saturate as the voltage goes up. 2x rating is quite a bit though!
Hydron, Sat Feb 04 2017, 11:27AM
Could just try it i guess? Crank it up on the variac while monitoring the current (preferably with an oscilloscope), see if it begins to saturate as the voltage goes up. 2x rating is quite a bit though!
Re: Opinions on pushing an 120v PT to 240v
Mads Barnkob, Mon Mar 06 2017, 09:24AM
I agree with hydron, you will have to test this by measurements, we can only guess on the core size and thus we can not calculate the flux density in its normal operation.
Doubling the input voltage will double the flux density.
Mads Barnkob, Mon Mar 06 2017, 09:24AM
I agree with hydron, you will have to test this by measurements, we can only guess on the core size and thus we can not calculate the flux density in its normal operation.
Doubling the input voltage will double the flux density.
Re: Opinions on pushing an 120v PT to 240v
Dr. Slack, Mon Mar 06 2017, 10:59AM
The ratings of a transformer bite in different ways. For instance, you can over-current any transformer as long as you keep the temperature down.
Coil voltage has two limits. There's a fairly hard limit where the core goes into saturation. If this was a power transformer, I could categorically say that no, you cannot over-volt by a factor of 2, as the core will have been designed to run quite close to the saturation limit, for economy. There is also a limit where the insulation is at risk of breakdown.
This is a PT, and might have been designed to run the core at very low flux, perhaps for accuracy, perhaps for power consumption. So it's worth a try. Run the 120v winding from a variac and a series filament bulb as a current limiter and current indicator. Crank it up and measure the current. There will be a knee where the magnetising current will increase rapidly with increasing voltage, choose what magnetising current is acceptable to you.
From the BIL, it looks like you'll have no problem with only a factor of 2 overvoltage.
Dr. Slack, Mon Mar 06 2017, 10:59AM
The ratings of a transformer bite in different ways. For instance, you can over-current any transformer as long as you keep the temperature down.
Coil voltage has two limits. There's a fairly hard limit where the core goes into saturation. If this was a power transformer, I could categorically say that no, you cannot over-volt by a factor of 2, as the core will have been designed to run quite close to the saturation limit, for economy. There is also a limit where the insulation is at risk of breakdown.
This is a PT, and might have been designed to run the core at very low flux, perhaps for accuracy, perhaps for power consumption. So it's worth a try. Run the 120v winding from a variac and a series filament bulb as a current limiter and current indicator. Crank it up and measure the current. There will be a knee where the magnetising current will increase rapidly with increasing voltage, choose what magnetising current is acceptable to you.
From the BIL, it looks like you'll have no problem with only a factor of 2 overvoltage.
Re: Opinions on pushing an 120v PT to 240v
Hazmatt_(The Underdog), Fri Mar 10 2017, 02:46AM
A good solid transformer is going to handle 125% of its nameplate rating, and possibly 150% for short duration, but you're asking for 200% and I wouldn't go there.
The low voltage winding is the secondary and its going to get very hot very quickly. The losses are going to increase dramatically as the core saturates, and doubling the primary current is going to cause a lot of heating due to IIR.
Another trouble is if you melt one of the low voltage connections from the excessive current draw, it's impossible to repair without de-potting, and why ruin such a nice transformer?
It's 2KVA, asking for 4KVA is not going to happen, the core won't support it, nor will with windings, it's not built for it.
The low voltage winding is probably 12 AWG, and you're asking for it to handle possibly 40A into a .5 ohm winding, I don't think that's reasonable.
Hazmatt_(The Underdog), Fri Mar 10 2017, 02:46AM
A good solid transformer is going to handle 125% of its nameplate rating, and possibly 150% for short duration, but you're asking for 200% and I wouldn't go there.
The low voltage winding is the secondary and its going to get very hot very quickly. The losses are going to increase dramatically as the core saturates, and doubling the primary current is going to cause a lot of heating due to IIR.
Another trouble is if you melt one of the low voltage connections from the excessive current draw, it's impossible to repair without de-potting, and why ruin such a nice transformer?
It's 2KVA, asking for 4KVA is not going to happen, the core won't support it, nor will with windings, it's not built for it.
The low voltage winding is probably 12 AWG, and you're asking for it to handle possibly 40A into a .5 ohm winding, I don't think that's reasonable.
Re: Opinions on pushing an 120v PT to 240v
Dr. Slack, Fri Mar 10 2017, 09:23AM
@Hazmatt You seem to be confusing over voltage with over current. Testing a transformer for what voltage it will handle, with a filament bulb in series for over-current limiting and indication, is only an insulation over-voltage breakdown hazard, not an overcurrent links melting hazard. As this is a PT with a huge BIL, we can dismiss the likelyhood of the first with only 200% voltage rated voltage. When the core saturates, the lamp will light, nothing more exciting than that.
The plated VA rating, 2kVA at 30C, will be made up of two components, core loss due to the applied voltage, and copper loss due to the rated current. Once the saturation voltage is has been measured, then the consumption off load will need to be measured at both the rated and the higher voltage to see whether there's still any margin between the increased core loss and heat generation it can handle.
Dr. Slack, Fri Mar 10 2017, 09:23AM
@Hazmatt You seem to be confusing over voltage with over current. Testing a transformer for what voltage it will handle, with a filament bulb in series for over-current limiting and indication, is only an insulation over-voltage breakdown hazard, not an overcurrent links melting hazard. As this is a PT with a huge BIL, we can dismiss the likelyhood of the first with only 200% voltage rated voltage. When the core saturates, the lamp will light, nothing more exciting than that.
The plated VA rating, 2kVA at 30C, will be made up of two components, core loss due to the applied voltage, and copper loss due to the rated current. Once the saturation voltage is has been measured, then the consumption off load will need to be measured at both the rated and the higher voltage to see whether there's still any margin between the increased core loss and heat generation it can handle.
Re: Opinions on pushing an 120v PT to 240v
Hazmatt_(The Underdog), Fri Mar 10 2017, 04:51PM
He wants to put 240V into a 120V winding = blown winding.
That's what he said in the first post, I'm not confused about that.
Go ahead and do it, blow out the winding, and make a really nice doorstop. Probably cost over $2K to replace it too.
Just remember I said it was a bad idea, so its not my fault.
For my PT, 100:1 1500VA transformer I have a step-down transformer 5kw 240:120. I can't really show it off though because I have a 400 lb. Lathe in the way right now and its a total bitch to move by myself.
If I didn't have a step-down I would use a "ballast" and I don't see anywhere he mentions a ballast, he's just gonna put 240 right in and blow out the transformer. there was no mention of sliding inductors, fixed inductors, capacitors, lamps, stove heating elements, or power resistors in the first post as a ballast, just 240 right in.
Hazmatt_(The Underdog), Fri Mar 10 2017, 04:51PM
He wants to put 240V into a 120V winding = blown winding.
That's what he said in the first post, I'm not confused about that.
Go ahead and do it, blow out the winding, and make a really nice doorstop. Probably cost over $2K to replace it too.
Just remember I said it was a bad idea, so its not my fault.
For my PT, 100:1 1500VA transformer I have a step-down transformer 5kw 240:120. I can't really show it off though because I have a 400 lb. Lathe in the way right now and its a total bitch to move by myself.
If I didn't have a step-down I would use a "ballast" and I don't see anywhere he mentions a ballast, he's just gonna put 240 right in and blow out the transformer. there was no mention of sliding inductors, fixed inductors, capacitors, lamps, stove heating elements, or power resistors in the first post as a ballast, just 240 right in.
Re: Opinions on pushing an 120v PT to 240v
Dr. Slack, Fri Mar 10 2017, 04:56PM
@redruM69 looks like Hazmatt and I are saying almost the same thing.
He says don't slam 240v into a winding rated at 120v, no way, no sir, no how.
I say don't slam 240v into a winding rated at 120v, wind it up carefully with a Variac and a filament lamp as a series current limiting resistor measuring the magnetisation current to see what the saturation voltage is. You may be lucky, but if not, nothing will go bang, and you will know what voltage you can go to.
You choose.
Dr. Slack, Fri Mar 10 2017, 04:56PM
@redruM69 looks like Hazmatt and I are saying almost the same thing.
He says don't slam 240v into a winding rated at 120v, no way, no sir, no how.
I say don't slam 240v into a winding rated at 120v, wind it up carefully with a Variac and a filament lamp as a series current limiting resistor measuring the magnetisation current to see what the saturation voltage is. You may be lucky, but if not, nothing will go bang, and you will know what voltage you can go to.
You choose.
Re: Opinions on pushing an 120v PT to 240v
Sulaiman, Fri Mar 10 2017, 06:26PM
Just connect directly to 240 Vac via a 240 V filament lamp and measure the voltages.
OK other than the e.h.t. danger.
Sulaiman, Fri Mar 10 2017, 06:26PM
Just connect directly to 240 Vac via a 240 V filament lamp and measure the voltages.
OK other than the e.h.t. danger.
Re: Opinions on pushing an 120v PT to 240v
Hazmatt_(The Underdog), Fri Mar 10 2017, 07:48PM
My prediction with a 240V lamp in series with the transformer:
24V across the transformer, the remainder voltage across the bulb.
I'd like to know what is measured, I am curious.
The transformer will look like a dead short on the low side.
Hazmatt_(The Underdog), Fri Mar 10 2017, 07:48PM
My prediction with a 240V lamp in series with the transformer:
24V across the transformer, the remainder voltage across the bulb.
I'd like to know what is measured, I am curious.
The transformer will look like a dead short on the low side.
Re: Opinions on pushing an 120v PT to 240v
Dr. Slack, Sat Mar 11 2017, 05:34AM
Just curious as why you predict a voltage of less than 120v across the low side. Neglecting the magnetising current, a transformer with no load on the secondary is open circuit up to its rated voltage, Sulaiman's comment shows he gets this.
Now there are some unstated assumptions.
a) I don't think 'no load' has been mentioned yet, but when testing a transformer for magnetising current, the no load condition is obvious.
b) Similarly, we haven't discussed the rated power of the filament bulb. When you use such a bulb for protecting a device under test from excess current, it's obvious we want it to remain cold for 'normal' operating current, and get hot for fault currents. This means the bulb should be rated to pass the magnetising current with low drop. We don't know what the magnetising current is yet, so we have to measure it. We *do* know that whatever we measure up to 120v across the low side is *within the rating* of the transformer and therefore OK. I would guess that the magnetising current would fully turn on a 5w 'slumberlight', but would probably pass through a 500w halogen flood with little drop, but that's what measurement is for.
Dr. Slack, Sat Mar 11 2017, 05:34AM
Hazmatt_(The Underdog) wrote ...
My prediction with a 240V lamp in series with the transformer:
24V across the transformer, the remainder voltage across the bulb.
I'd like to know what is measured, I am curious.
The transformer will look like a dead short on the low side.
My prediction with a 240V lamp in series with the transformer:
24V across the transformer, the remainder voltage across the bulb.
I'd like to know what is measured, I am curious.
The transformer will look like a dead short on the low side.
Just curious as why you predict a voltage of less than 120v across the low side. Neglecting the magnetising current, a transformer with no load on the secondary is open circuit up to its rated voltage, Sulaiman's comment shows he gets this.
Now there are some unstated assumptions.
a) I don't think 'no load' has been mentioned yet, but when testing a transformer for magnetising current, the no load condition is obvious.
b) Similarly, we haven't discussed the rated power of the filament bulb. When you use such a bulb for protecting a device under test from excess current, it's obvious we want it to remain cold for 'normal' operating current, and get hot for fault currents. This means the bulb should be rated to pass the magnetising current with low drop. We don't know what the magnetising current is yet, so we have to measure it. We *do* know that whatever we measure up to 120v across the low side is *within the rating* of the transformer and therefore OK. I would guess that the magnetising current would fully turn on a 5w 'slumberlight', but would probably pass through a 500w halogen flood with little drop, but that's what measurement is for.
Re: Opinions on pushing an 120v PT to 240v
Sulaiman, Sat Mar 11 2017, 08:44AM
The nameplate ratings are clear:
25 kV rated system, 21 kV nominal, rated for 50 kV for 1 minute
... so YES you can use the primary (what was the secondary) DIRECTLY connected to 240 Vac
but it will be at the top end of its ratings,
and how long it can be used, at what power output before overheating, I do not know
so when you are confident that the transformer is ok on 240Vac,
in series with the primary, add something like this
attached to the transformer.
My only concern with directly connecting to 240 Vac is that this is a fairly large transformer,
quite likely that the mains breaker will trip occasionally - depending upon where in the ac cycle the transformer is powered on,
zero-crossing is the worst case.
and as above, a series filament lamp would need to be high wattage.
but in this specific case it is not required, other than for preventing occasional mcb trips.
CORRECTION: My main concern is your safety ...
I suspect that you have negligible experience with transformers like this,
if you have an eht short-circuit the transformer will provide very little current limiting,
so almost 1/175 th of whatever the mains can supply before disconnect is available to kill or destroy,
e.g. 13 A fuse, 6000 A rating , so less than 30 A at your eht secondary.
less than 1% of that can kill you! ... You would be lucky to survive ONE mistake.
Consider it as MOTs, with the shunts removed ... and on steroids.
it even looks scary, and ss a general rule, I prefer live members to dead ones :)
I want one !
Sulaiman, Sat Mar 11 2017, 08:44AM
The nameplate ratings are clear:
25 kV rated system, 21 kV nominal, rated for 50 kV for 1 minute
... so YES you can use the primary (what was the secondary) DIRECTLY connected to 240 Vac
but it will be at the top end of its ratings,
and how long it can be used, at what power output before overheating, I do not know
so when you are confident that the transformer is ok on 240Vac,
in series with the primary, add something like this
attached to the transformer.My only concern with directly connecting to 240 Vac is that this is a fairly large transformer,
quite likely that the mains breaker will trip occasionally - depending upon where in the ac cycle the transformer is powered on,
zero-crossing is the worst case.
and as above, a series filament lamp would need to be high wattage.
but in this specific case it is not required, other than for preventing occasional mcb trips.
CORRECTION: My main concern is your safety ...
I suspect that you have negligible experience with transformers like this,
if you have an eht short-circuit the transformer will provide very little current limiting,
so almost 1/175 th of whatever the mains can supply before disconnect is available to kill or destroy,
e.g. 13 A fuse, 6000 A rating
, so less than 30 A at your eht secondary.less than 1% of that can kill you! ... You would be lucky to survive ONE mistake.
Consider it as MOTs, with the shunts removed ... and on steroids.
it even looks scary, and ss a general rule, I prefer live members to dead ones :)
I want one !
Re: Opinions on pushing an 120v PT to 240v
Dr. Slack, Sat Mar 11 2017, 10:57AM
I missed the '50kV for 1 minute' bit, well spotted.
The fact that it's time limited presumably means it would be drawing heavy magnetising current, and so dissipating hard, and well into the saturation knee.
It's 50kV at 60Hz, so that would be only 41.7kV at 50Hz. Where is redruM69?
Dr. Slack, Sat Mar 11 2017, 10:57AM
I missed the '50kV for 1 minute' bit, well spotted.
The fact that it's time limited presumably means it would be drawing heavy magnetising current, and so dissipating hard, and well into the saturation knee.
It's 50kV at 60Hz, so that would be only 41.7kV at 50Hz. Where is redruM69?
Re: Opinions on pushing an 120v PT to 240v
Hazmatt_(The Underdog), Sat Mar 11 2017, 05:38PM
Guys, this is a STEP DOWN transformer.
50 KV for one minute is the input surge that the transformer will handle for one minute without going tits up.
21kv is its continuous input.
Just because it can tolerate 50kv into its primary for short duration, does not mean it can handle 240V into it's secondary.
If I have to, I will pull my PT out and measure the primary, it's probably .5 ohms.
It's impedance might be 2 ohms, I can measure that with my impedance meter, it doesn't matter to me.
The impedance must, MUST be very low, these are designed as small power sources for metering and instruments on the pole.
If I have to I can do a full workup on my transformer, short circuit and open circuit parameters, IF I can find the time, it is time consuming.
Honestly you guys, you should realize that a low impedance secondary (which should look like 0 ohms as a source ideally) to any load presented to it, is basically a short circuit.
Let's guess a 240V bulb is 250W, or say 1 ohm. (approximations, I will get something wrong, but this is not life or death so I don't really care,
this is just a pissing contest at this point.)
the transformer probably as a .85 power factor, so P = IV * .85, 2000VA = I * 120 * .85, so I = 20A roughly
Now I = E/Z secondary, or Z secondary = E/I = 120 / 20 = 6 ohms (so I thought it was going to be much much lower than that)
The 240V lamp, on the 240V line will restrict current to 1 Amp drawn (approximations)
So with a primary current of 1 amp, and impedance of 6 ohms, E = IZ = 6 VAC on the secondary terminals ( I wagered 24 V, but these are just
"napkin" calculations)
21000:120 is 175:1, so 6VAC * 175 = 1050 VAC on the primary side terminals.
I am curious, let's get it measured and see what we get.
Hazmatt_(The Underdog), Sat Mar 11 2017, 05:38PM
Guys, this is a STEP DOWN transformer.
50 KV for one minute is the input surge that the transformer will handle for one minute without going tits up.
21kv is its continuous input.
Just because it can tolerate 50kv into its primary for short duration, does not mean it can handle 240V into it's secondary.
If I have to, I will pull my PT out and measure the primary, it's probably .5 ohms.
It's impedance might be 2 ohms, I can measure that with my impedance meter, it doesn't matter to me.
The impedance must, MUST be very low, these are designed as small power sources for metering and instruments on the pole.
If I have to I can do a full workup on my transformer, short circuit and open circuit parameters, IF I can find the time, it is time consuming.
Honestly you guys, you should realize that a low impedance secondary (which should look like 0 ohms as a source ideally) to any load presented to it, is basically a short circuit.
Let's guess a 240V bulb is 250W, or say 1 ohm. (approximations, I will get something wrong, but this is not life or death so I don't really care,
this is just a pissing contest at this point.)
the transformer probably as a .85 power factor, so P = IV * .85, 2000VA = I * 120 * .85, so I = 20A roughly
Now I = E/Z secondary, or Z secondary = E/I = 120 / 20 = 6 ohms (so I thought it was going to be much much lower than that)
The 240V lamp, on the 240V line will restrict current to 1 Amp drawn (approximations)
So with a primary current of 1 amp, and impedance of 6 ohms, E = IZ = 6 VAC on the secondary terminals ( I wagered 24 V, but these are just
"napkin" calculations)
21000:120 is 175:1, so 6VAC * 175 = 1050 VAC on the primary side terminals.
I am curious, let's get it measured and see what we get.
Re: Opinions on pushing an 120v PT to 240v
Sulaiman, Sat Mar 11 2017, 07:46PM
You need to revise your theory of transformers, and resistance vs. impedance.
Sulaiman, Sat Mar 11 2017, 07:46PM
You need to revise your theory of transformers, and resistance vs. impedance.
Re: Opinions on pushing an 120v PT to 240v
Dr. Slack, Sat Mar 11 2017, 08:13PM
The first transformer I wound, as a callow schoolboy, who knew just enough to get himself into trouble but not much more, used resistance wire as the primary, because 'I didn't want too much current to flow through the near zero resistance of copper wire'. Would you believe it, it was an abject failure.
A real transformer will have a very low resistance to its windings. An ideal transformer would have zero resistance, and would work slightly better than a real one.
Any transformer winding is an inductor. Inductors have a finite impedance at the operating frequency of the transformer. In a well designed transformer, operated within its ratings, that impedance is large enough to limit the no load current to a small fraction of the rated current. This small current is called the magnetising current, as it's what's required to generate the H field that pushes a B field through the core, whose changes generate the back EMF by Faraday's Law that opposes the input voltage.
What happens if you operate a transformer at too high a voltage? The B field must still change fast enough to oppose the input voltage. However, now it's got to swing to a value higher than the saturation level, which means the permeability falls, which means an H field one or more orders of magnitude is now required to create the B field. An H field orders of magnitude greater means an input current orders of magnitude greater. That increase of magnetising current is why you shouldn't operate a transformer in saturation.
With a turns ratio of 175:1, that 50kV 1 minute input becomes nominally 285v at the secondary. Which appears to allow some scope for driving the secondary at 240v, just from the ratings plate. I would still feel more comfortable measuring the magnetising current at voltages between 120v and 240v inclusive.
Hazmat, you state that the stepdown output must be very low impedance, because it's designed to drive stuff. That is true, it is. However, it only is that when the primary is being driven from a low impedance. If we look what happens when we drive the secondary with 120v and the primary is open circuit, it will see a high impedance because it's only 'seeing' the secondary inductance. Now, if you were to short circuit the 21kV winding, this short *would* be reflected back through the transformer as a very low impedance, that's what transformers do.
Dr. Slack, Sat Mar 11 2017, 08:13PM
The first transformer I wound, as a callow schoolboy, who knew just enough to get himself into trouble but not much more, used resistance wire as the primary, because 'I didn't want too much current to flow through the near zero resistance of copper wire'. Would you believe it, it was an abject failure.
A real transformer will have a very low resistance to its windings. An ideal transformer would have zero resistance, and would work slightly better than a real one.
Any transformer winding is an inductor. Inductors have a finite impedance at the operating frequency of the transformer. In a well designed transformer, operated within its ratings, that impedance is large enough to limit the no load current to a small fraction of the rated current. This small current is called the magnetising current, as it's what's required to generate the H field that pushes a B field through the core, whose changes generate the back EMF by Faraday's Law that opposes the input voltage.
What happens if you operate a transformer at too high a voltage? The B field must still change fast enough to oppose the input voltage. However, now it's got to swing to a value higher than the saturation level, which means the permeability falls, which means an H field one or more orders of magnitude is now required to create the B field. An H field orders of magnitude greater means an input current orders of magnitude greater. That increase of magnetising current is why you shouldn't operate a transformer in saturation.
With a turns ratio of 175:1, that 50kV 1 minute input becomes nominally 285v at the secondary. Which appears to allow some scope for driving the secondary at 240v, just from the ratings plate. I would still feel more comfortable measuring the magnetising current at voltages between 120v and 240v inclusive.
Hazmat, you state that the stepdown output must be very low impedance, because it's designed to drive stuff. That is true, it is. However, it only is that when the primary is being driven from a low impedance. If we look what happens when we drive the secondary with 120v and the primary is open circuit, it will see a high impedance because it's only 'seeing' the secondary inductance. Now, if you were to short circuit the 21kV winding, this short *would* be reflected back through the transformer as a very low impedance, that's what transformers do.
Re: Opinions on pushing an 120v PT to 240v
Sulaiman, Sat Mar 11 2017, 08:32PM
to continue is pointless,
redruM69 either has lost interest - or was not careful.
Sulaiman, Sat Mar 11 2017, 08:32PM
to continue is pointless,
redruM69 either has lost interest - or was not careful.
Re: Opinions on pushing an 120v PT to 240v
Hazmatt_(The Underdog), Sat Mar 11 2017, 10:06PM
I'll measure my PT when I get around to it.
Yes I understand open and short circuit, and 1/(Z primary)^2 reflected back to the primary by the turns ratio.
If you want to turn this into whose a better engineer that's fine, I don't have the energy for that, I could care less.
Rough estimates and ballpark figures get us close to actuals without getting wrapped around the axle.
I have wound my own transformers before with just ballpark figures and they work fine, loaded.
Here's my 100:1 1500VA PT, stuck behind my Atlas Lathe.
Hazmatt_(The Underdog), Sat Mar 11 2017, 10:06PM
I'll measure my PT when I get around to it.
Yes I understand open and short circuit, and 1/(Z primary)^2 reflected back to the primary by the turns ratio.
If you want to turn this into whose a better engineer that's fine, I don't have the energy for that, I could care less.
Rough estimates and ballpark figures get us close to actuals without getting wrapped around the axle.
I have wound my own transformers before with just ballpark figures and they work fine, loaded.
Here's my 100:1 1500VA PT, stuck behind my Atlas Lathe.
Re: Opinions on pushing an 120v PT to 240v
hen918, Sun Mar 12 2017, 12:08PM
Just to expand on Sulaiman's rather inflammatory post, where you went wrong is the 6 ohms. Yes the impedance of the secondary would be 6 ohms if 20 A was the short circuit current, but it isn't. 20 A is the maximum it can supply, the short circuit current will be much greater than that. 6 ohms is the load you can put on the the secondary when it is working at 120 V.
A transformer like this can be modelled with an inductance in series with the primary (leakage inductance) and an inductance in parallel with the primary (magnetising inductance). When there is no load on the transformer the current flowing will only be the (low) reactive current through the (high) magnetising inductance. If the secondary is shorted out the only current limiting will be leakage inductance which will be much lower, therefore the current will be much higher (I = V/(j*2*pi*f*L)).
There is also a series resistance which represents the resistance of the windings. This also acts to limit the maximum current.
If the transformer primary now becomes the secondary and vice versa, the transformer can be drawn with the leakage and magnetising inductance on the secondary instead, but for convention we transform them through the transformer and put them with the appropriate values on the new primary.
In short, a transformer works the same forwards as well as backwards.
hen918, Sun Mar 12 2017, 12:08PM
Just to expand on Sulaiman's rather inflammatory post, where you went wrong is the 6 ohms. Yes the impedance of the secondary would be 6 ohms if 20 A was the short circuit current, but it isn't. 20 A is the maximum it can supply, the short circuit current will be much greater than that. 6 ohms is the load you can put on the the secondary when it is working at 120 V.
A transformer like this can be modelled with an inductance in series with the primary (leakage inductance) and an inductance in parallel with the primary (magnetising inductance). When there is no load on the transformer the current flowing will only be the (low) reactive current through the (high) magnetising inductance. If the secondary is shorted out the only current limiting will be leakage inductance which will be much lower, therefore the current will be much higher (I = V/(j*2*pi*f*L)).
There is also a series resistance which represents the resistance of the windings. This also acts to limit the maximum current.
If the transformer primary now becomes the secondary and vice versa, the transformer can be drawn with the leakage and magnetising inductance on the secondary instead, but for convention we transform them through the transformer and put them with the appropriate values on the new primary.
In short, a transformer works the same forwards as well as backwards.
Re: Opinions on pushing an 120v PT to 240v
Hazmatt_(The Underdog), Sun Mar 12 2017, 05:02PM
They work the same forward and backward ONLY when the windings are equally distributed, which is called interleaving.
If these transformers were interleaved, the reluctance plays a lesser role and yes they would be easier to energize. But the windings on these guys is primary and secondary, which makes energizing them "backward" difficult and current hungry, again a short circuit.
I had a 3180 to 120 interleaved transformer, in pieces now to make use of its core someday, if you need proof I still have one of its 2 winding cores.
It will be some time before I can do the measurements, I have all the stuff, but I have to jerry rig the power cord and variac, and 1256D's are heavy.
I gave an approximation, that's what was meant and explained.
MY PT will likely need 2000VA input in step up to deliver the same power at the HT terminals, as it will require 1500VA in step down to deliver at the LT terminals, so no it's not the same, but I will measure it.
What's your approximation then?
Assume the secondary inductance is 8H, since that's the puzzle piece you need.
Hazmatt_(The Underdog), Sun Mar 12 2017, 05:02PM
They work the same forward and backward ONLY when the windings are equally distributed, which is called interleaving.
If these transformers were interleaved, the reluctance plays a lesser role and yes they would be easier to energize. But the windings on these guys is primary and secondary, which makes energizing them "backward" difficult and current hungry, again a short circuit.
I had a 3180 to 120 interleaved transformer, in pieces now to make use of its core someday, if you need proof I still have one of its 2 winding cores.
It will be some time before I can do the measurements, I have all the stuff, but I have to jerry rig the power cord and variac, and 1256D's are heavy.
I gave an approximation, that's what was meant and explained.
MY PT will likely need 2000VA input in step up to deliver the same power at the HT terminals, as it will require 1500VA in step down to deliver at the LT terminals, so no it's not the same, but I will measure it.
What's your approximation then?
Assume the secondary inductance is 8H, since that's the puzzle piece you need.
Re: Opinions on pushing an 120v PT to 240v
Uspring, Mon Mar 13 2017, 09:38AM
The input current of my 230V 2kVA variac is about 60mA (measured) under no load. That amounts to an inductance of about 12H. A 120V winding would have about half the number of turns and thus 1/4 of the inductance, i.e. 3H. The secondary inductance of the OPs PT will be similar.
The impedance of this inductance is about 1.1k. Connected in series with a 240V 1A bulb (240ohms), the bulb will be quite dim. Most of the voltage drop will be over the winding. That assumes, that there is no load on the high voltage side of the PT and that it won't go into saturation. It probably will saturate to some extent. Measuring the voltage drops or better scoping them can be a good indication of saturation.
Uspring, Mon Mar 13 2017, 09:38AM
The input current of my 230V 2kVA variac is about 60mA (measured) under no load. That amounts to an inductance of about 12H. A 120V winding would have about half the number of turns and thus 1/4 of the inductance, i.e. 3H. The secondary inductance of the OPs PT will be similar.
The impedance of this inductance is about 1.1k. Connected in series with a 240V 1A bulb (240ohms), the bulb will be quite dim. Most of the voltage drop will be over the winding. That assumes, that there is no load on the high voltage side of the PT and that it won't go into saturation. It probably will saturate to some extent. Measuring the voltage drops or better scoping them can be a good indication of saturation.
Re: Opinions on pushing an 120v PT to 240v
Finn Hammer, Sun Mar 19 2017, 06:55PM
All,
It is a pity that redruM69 left the thread, but perhaps others will benefit, because the obvious solution is to drive the transformer with a VFD.
A VFD does not put out full voltage before it has reached the maximum frequency that it will produce, and at that frequency, the primary will be able to support many more volts per turn than it will at mains frequency.
I have here before me a 2kW 7.5kV transformer with a 110V primary. Driven by a 220V VFD, it puts out 15kV at 400hz.
Cheers, Finn Hammer
Finn Hammer, Sun Mar 19 2017, 06:55PM
All,
It is a pity that redruM69 left the thread, but perhaps others will benefit, because the obvious solution is to drive the transformer with a VFD.
A VFD does not put out full voltage before it has reached the maximum frequency that it will produce, and at that frequency, the primary will be able to support many more volts per turn than it will at mains frequency.
I have here before me a 2kW 7.5kV transformer with a 110V primary. Driven by a 220V VFD, it puts out 15kV at 400hz.
Cheers, Finn Hammer
Re: Opinions on pushing an 120v PT to 240v
Kolas, Thu May 11 2017, 10:06PM
these encapsulated transformers are increasingly rare. They are wonderful power supplies when powered in the manner for which they were designed.
It would be a terrible shame to see this device ruined by powering it at 2x it's voltage without increasing it's drive frequency as well.
Get another transformer.
Kolas, Thu May 11 2017, 10:06PM
these encapsulated transformers are increasingly rare. They are wonderful power supplies when powered in the manner for which they were designed.
It would be a terrible shame to see this device ruined by powering it at 2x it's voltage without increasing it's drive frequency as well.
Get another transformer.
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