I'm sure that it would work, but I'm curious about the limitations or caveats involved.
I'm looking at reducing transformer output by applying 120V to a 240V primary configuration. I have a few transformers that have been recovered from old amplifiers that would be nice to recycle but have a secondary voltage that is too high. They have dual primaries (at least one has dual primaries tapped for 110V and 120V) and got to thinking about running them configured for 220V or 240V to make them more useful to me.
I'd have to think that this won't pose any danger, but what other effects, limitations or caveats may be involved that I am simply not aware of?
I'm looking at reducing transformer output by applying 120V to a 240V primary configuration. I have a few transformers that have been recovered from old amplifiers that would be nice to recycle but have a secondary voltage that is too high. They have dual primaries (at least one has dual primaries tapped for 110V and 120V) and got to thinking about running them configured for 220V or 240V to make them more useful to me.
I'd have to think that this won't pose any danger, but what other effects, limitations or caveats may be involved that I am simply not aware of?
Using the 240Vac transformer on a 120Vac supply still limits the secondaries to the same output current, i.e. you get half the VA.
In return you get VERY low quiescent current and VERY cool running and VERY low electricity bills.
In return you get VERY low quiescent current and VERY cool running and VERY low electricity bills.
You are not magnetising the core to the same level with half the primary voltage so the output power will be reduced.
Isn't there something with the frequencies too?
If I remember well, you can safely run a 50Hz-"rated" transformer at 60Hz, but you have to be careful the other way around, ie. using it at a frequency lower than "rated").
Sorry, I forgot the details, and I am too lazy to look them up...
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If I remember well, you can safely run a 50Hz-"rated" transformer at 60Hz, but you have to be careful the other way around, ie. using it at a frequency lower than "rated").
Sorry, I forgot the details, and I am too lazy to look them up...
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It depends on the design of the transformer. Our Oriental friends are very good at pushing a transformer core to its absolute limits in order to reduce costs.
In that case a 50Hz transformer will normally be OK at 60Hz.
If you buy a bespoke transformer from a recognised supplier then it will work quite happily at 50Hz or 60Hz.
It all comes down to the maximum flux that the core can withstand. For long term reliability a transformer will have a BT of about 320mT. For less reliability you can go much higher, the cheapie transformers are sometimes as high as 1000mT.
As BT has a frequency connection, a 1000mT core will be overstretched if the frequency is too low.
In that case a 50Hz transformer will normally be OK at 60Hz.
If you buy a bespoke transformer from a recognised supplier then it will work quite happily at 50Hz or 60Hz.
It all comes down to the maximum flux that the core can withstand. For long term reliability a transformer will have a BT of about 320mT. For less reliability you can go much higher, the cheapie transformers are sometimes as high as 1000mT.
As BT has a frequency connection, a 1000mT core will be overstretched if the frequency is too low.
but you have plenty of margin for reduced frequency operation if you are using series 240pri with 110/120 since the flux is porportional to the waveform V*T product and you've halved the V
Quite correct. But as you are putting less power into the core, the regulation may be poorer too.
OK, basically halving the VA rating of the transformer should not be an issue for me - the one I'm specifically talking about came out of an old HH V800 (rated 2x 400W into 4Ω). This will be gross overkill for my purposes but I have it and no use for it as it sits. Not too sure what its VA rating is but I'd be curious if someone familiar with the art of transformer design could give a rough estimate. Here are the critical dimensions as I understand them:
Total weight 10.77kg
Total dimensions of the laminations are W=134mm x D=96mm x H=112mm
The centre post cross section is W=50mm x D=96mm
Primary wound with 1.7mm diameter wire, secondary wound with 2.2mm diameter wire and I wasn't able to get a meaningful resistance measurement, but suffice it to say 1/2 ohm or less for any winding.
It has a single centre tapped secondary and dual 110/120V primaries. I haven't measured its open circuit voltage but the schematic says it is 60-0-60 VAC out.
Total weight 10.77kg
Total dimensions of the laminations are W=134mm x D=96mm x H=112mm
The centre post cross section is W=50mm x D=96mm
Primary wound with 1.7mm diameter wire, secondary wound with 2.2mm diameter wire and I wasn't able to get a meaningful resistance measurement, but suffice it to say 1/2 ohm or less for any winding.
It has a single centre tapped secondary and dual 110/120V primaries. I haven't measured its open circuit voltage but the schematic says it is 60-0-60 VAC out.
Since the transformer has split primaries, I would parallel the primaries. The transformer will produce more current and will certainly have less 'sag' under load.
Frank,
I'm not sure I get your suggestion. For 120V operation the primaries would be in parallel. I'm suggesting to get lower secondary output voltage that I configure the primaries in series as if it were to operate off 240V, but actually run it on 120V. Basically what I want to do is take a 60-0-60 output transformer and get something more like 30-0-30 out. Maybe I am not seeing your suggestion clearly.
Jason
Sorry. For some reason, I focused on the two primaries. You can safely operate the transformer at half voltage. You'll get half the output voltage at the transformer's rated output current.
Do you feel up to the task of splitting the 60Vac windings into four 30Vac windings?
You can tap into the winding on the opposite side from the outlet leads and measure the tapped secondary voltage. when you have found the midpoint snip and add wiring fot two lead outs.
Then find the midpoint of the other winding and repeat.
While the outer insulation is off you can separate the centre tap to create four windings.
This allows the full VA rating to be used
This also allows isolation between two channels, enabling dual mono from a common Primary.
You can tap into the winding on the opposite side from the outlet leads and measure the tapped secondary voltage. when you have found the midpoint snip and add wiring fot two lead outs.
Then find the midpoint of the other winding and repeat.
While the outer insulation is off you can separate the centre tap to create four windings.
This allows the full VA rating to be used
This also allows isolation between two channels, enabling dual mono from a common Primary.
No. That's not the case. Regulation may be poorer because there's more turns than a 120V transformer would have and higher copper loss. "Power into the core" has nothing to do with it.
I see the benefit if I needed to maximize the available VA rating - better core utilization and lower copper losses - but I'm not too sure about modifying this beast of a transformer, or if I'd even realize notable improvement.
It is one of the EI type cores and appears to have been fixed / set with a dip in varnish, so probably difficult to pull it apart to be able to accurately split the windings (or outright rewind). I'm also not sure if it is a split bobbin arrangement or if the secondary is on top of the primary. I'd be more inclined to modify it if it is a split bobbin since there is less opportunity to disturb / damage the primary.
I had posted some dimensions in an earlier post in the hopes someone who knows more than I could give a guesstimate on the likely VA range this was originally designed to be.
Sorry,
I jumped to assume it was a toroid.
EI will be a complete secondary rewind. Not easy.
If you could get it apart.
I jumped to assume it was a toroid.
EI will be a complete secondary rewind. Not easy.
If you could get it apart.
This definitely seems to be true. I tried hooking up this transformer last evening to check its open circuit output and to verify which taps were which on the primaries. My mains varies very little and was ~117VAC at the time.
With the primaries in parallel and feeding the 120V taps there was non-negligable primary draw, as evidenced by my bulb limiter. Off the limiter my open circuit voltage was 54-0-54 on the secondary.
With the primaries in series, again the 120V taps, there was no indication of any quiescent current via the bulb limiter. Of course there was some primary current flowing but it was just much lower than with the primaries in parallel. This makes AndrewT's assertion true. The off limiter open circuit secondary voltage was 28-0-28, which is close to my target. I retried using the 110V taps in series and got the 30-0-30 I was after.
Now, and mainly just for interests sake, I just need to determine how much current this can deliver. I know there are a few possibilities, such as loading until a certain about of sag occurs, long term temperature rise (ideally at the core), and calculating from some critical parameters of the transformer itself, which have been posted. I'm open to suggestions.
This will be used for audio so I'd be inclined to go for a simple regulation test, but to what value? It's an EI type, so won't be quite as efficient as a toroid. I know big toroids are typically rated to about 3% regulation, but I'm not sure what is typical for large-ish EI types.
Using the EI at half voltage keeps the flux low.
That results in the copper giving you a regulation value.
This is quite different from the temperature determined regulation value.
You may find that the EI at half voltage is more efficient than a half sized toroid.
It's because you are NOT forcing the core to high flux values.
You can measure the open circuit output voltage and compare it to the resistor loaded output voltage. You may get a surprise.
That results in the copper giving you a regulation value.
This is quite different from the temperature determined regulation value.
You may find that the EI at half voltage is more efficient than a half sized toroid.
It's because you are NOT forcing the core to high flux values.
You can measure the open circuit output voltage and compare it to the resistor loaded output voltage. You may get a surprise.
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