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Need Help with Transformer

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Frosty_47

New Member
I pulled out a pair of transformers from APC "SMART UPS-3000" power back-up unit. Transformer part number search did not yield any results so I need your help in estimating it's VA ratting:

Part #: LEI-6 E154515

EI Core Size:
L = 4.5"
W= 3.5"
H = 3.75"

It's got 4 wires on primary (120V input) White, Yellow, Blue, Black (Fused). Secondary only two wires. One of the secondary wire is attached directly to batteries internally, the other one goes to electronic circuitry. It is EI type transformer. Please help in estimating it's VA ratting.

APC Link: APC Smart-UPS 3000VA 120V
 
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Sceadwian

Banned
You answered your own post... 3000VA.
 
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Frosty_47

New Member
You answered your own post... 3000VA.

No way each unit is 1500VA. I mean how can 59Cubic inches
EI core = 1.5KVA ?

Does anyone know of any method to estimate the VA ratting according to Core size ?

I don't want to risk overloading the transformer and cause core saturation.
 
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kchriste

New Member
Forum Supporter
That transformer won't be operating at 60Hz. Probably more like 100Khz or more. That's why it is relatively small.
 

Frosty_47

New Member
That transformer won't be operating at 60Hz. Probably more like 100Khz or more. That's why it is relatively small.

Um no, the transformer is operated on 50/60Hz because the 48V is pulsed at 50/60Hz into the transformer that outputs 120V at 50/60Hz. This is not a switching power supply. Can you imagine what would happen to your household equipment if all of a sudden it would be connected to 100Khz instead of 60Hz ?

The ratted output of the unit clearly says 50/60Hz.
 

jrz126

Active Member
Its very common for high frequency switching to be used. The pulse width is varied to make a nice smooth AC waveform.
8960-03230.png

The output and input labels are reversed.
the 100Khz out of the transformer is put through a low pass filter and it becomes the 60hz ac.
 

audioguru

Well-Known Member
Most Helpful Member
I think you said the UPS has two transformers (in parallel to double their power?).
A 50Hz-60Hz transformer has a laminated steel core and would be huge at 1500VA.
A 100kHz transformer is made of ferrite and is fairly small for 1500VA.
 

Sceadwian

Banned
Frosty... it's a 3000VA UPS, would you care to explain how it could be using transformer(s) rated at less than 3000VA, seeing as how all the current goes through them? It begin an inverter it is by definition a switching power supply, just as in compact fluorescent ballasts are smaller than line frequency ballasts because of the increase in frequency the transformer size is much smaller. Filtering from the natural inductance of the coil, and likely some other added filtering make the output appear as 50/60hz.
 
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Frosty_47

New Member
Its very common for high frequency switching to be used. The pulse width is varied to make a nice smooth AC waveform.
8963-03230.png

The output and input labels are reversed.
the 100Khz out of the transformer is put through a low pass filter and it becomes the 60hz ac.

Yes but wouldn't most of power be lost after LPF ? The LPF must be nudge in order to handle so much current. I will try and investigate the electronic circuitry inside this darn UPS unit to see if I can find any 100Khz oscillators. The transformer is not that small, around 30lb but looks to small to be 1.5KVA.
 

Frosty_47

New Member
Frosty... it's a 3000VA UPS, would you care to explain how it could be using transformer(s) rated at less than 3000VA, seeing as how all the current goes through them? It begin an inverter it is by definition a switching power supply, just as in compact fluorescent ballasts are smaller than line frequency ballasts because of the increase in frequency the transformer size is much smaller. Filtering from the natural inductance of the coil, and likely some other added filtering make the output appear as 50/60hz.

Wouldn't a lot of power be lost due to filtering ? And how will you get the 60Hz component if it's not originally present ? Sub-harmonics of 100Khz ?
Sorry I never dealt with switching PSU.

thanks
 
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Frosty_47

New Member
I think you said the UPS has two transformers (in parallel to double their power?).
A 50Hz-60Hz transformer has a laminated steel core and would be huge at 1500VA.
A 100kHz transformer is made of ferrite and is fairly small for 1500VA.

Yes, two transformers in ||
Their weight is around 30lB each.
The laminated steel core is around 59 Cubic inches
 
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Sceadwian

Banned
An inductor and capacitor will filter power just fine with very little loss at high frequency. The only losses are in the real world resistance, if the filter is capacitance and inductance (such as in carefully chosen transformer properties perhaps with a few other filter elements) the only loss is the overall ohmic resistance of the wire and core loss, the rest of the power is transferred. Basically that transformer is meant to be fed a PWM modulated high frequency carrier, and output a slightly filtered 50/60hz wave.
 
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audioguru

Well-Known Member
Most Helpful Member
The PWM filter removes only the very high 100kHz frequency and has no effect at 50Hz and 60Hz.
 
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