What is the difference between rated voltage and full load voltage of a transformer? Are the ratings given on the nameplate at full load? Suppose if transformer is at full load, then are the voltages on primary & secondary side equal to their rated values?
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\$\begingroup\$ In general or for a specific transformer? It's all down to the leakage inductance and resistance of your transformer. \$\endgroup\$– winnyCommented Sep 17, 2016 at 9:09
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\$\begingroup\$ Can you explain it for a general transformer? \$\endgroup\$– Sumit KumarCommented Sep 17, 2016 at 9:11
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\$\begingroup\$ Sure thing! Take a look at this transformer equivalent: homepages.uel.ac.uk/W.N.Bailey/BEng3_Projects/Images/…. Now, the leakage inductance at your mains frequency will present an impedance Zl in series. Let's just say that Zl=Rleak and say it's a resistance. Add your series resistance and your total Rtot=Rseries+Relax and your output voltage will drop with ohms law over this lumped resistance. Your new equivalent will just be an AC source with your unloaded voltage and that resistance in series. \$\endgroup\$– winnyCommented Sep 17, 2016 at 9:14
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\$\begingroup\$ I couldn't get it. I know this equivalent circuit, but how can i relate it to rated & full load voltages? \$\endgroup\$– Sumit KumarCommented Sep 17, 2016 at 9:17
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\$\begingroup\$ Check again. I pressed post too soon there. \$\endgroup\$– winnyCommented Sep 17, 2016 at 9:19
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2 Answers
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Take this typical small power transformer datasheet as an example.
The rated voltage is the nominal input voltage. For example, 115VAC (50/60Hz) for the primaries in parallel (or 230VAC for the primaries in series). This is something you apply to the transformer.
The nameplate output voltage is the nominal RMS output voltage under full load (and the load is assumed to be a pure resistive load).
The no-load secondary voltage will be higher. In this case, the manufacturer says it is "typically" 25% higher.
If you are specifying components to be connected to the transformer, note that the 25% does not include tolerance of the transformer windings (probably small) nor does it include the fact that the applied mains voltage could be higher than nominal. If you allow an additional 10-12% for those factors you need to allow for about 40% higher output voltage.
So if your nominal output voltage is 12.6V, the peak voltage (which determines your capacitor rating) could be as high as 25V (about double). Different transformers will have different numbers for the 'regulation', generally larger transformers will have less difference than small transformers.
answered Jan 19, 2017 at 17:48
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What is the difference between rated voltage and full load voltage of a transformer? Are the ratings given on the nameplate at full load?
The rated voltage will apply to the primary. The primary voltage will be equal to the voltage of the supply.
The full load voltage will apply to the secondary at full load when the transformer is supplied with the rated primary voltage. You can expect the voltage to rise when the transformer is partially loaded or open-circuit.
Suppose if transformer is at full load, then are the voltages on primary & secondary side equal to their rated values?
The primary voltage will be the mains voltage no matter what it says on the rating plate.
The secondary voltage will be the primary voltage divided by the transformer turns ratio.
Example:
- Rating primary voltage: 240 V.
- Rating secondary voltage: 120 V.
- Turns ratio is N:n = 240:120 = 2:1, where N = primary and n = secondary turns.
- Actual primary voltage: 220 V
Output at full load should be \$ V_{OUT} = \frac {n}{N}V_{IN} = \frac{120}{240} 220 = 110~V \$.
answered Sep 17, 2016 at 9:29
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\$\begingroup\$ Suppose we are not given the actual primary voltage but it is given that the transformer is operating at full load. Can we determine the secondary voltage? Can we take it as the rated voltage on the secondary side? \$\endgroup\$ Commented Sep 17, 2016 at 9:39
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\$\begingroup\$ @SumitKumar You need the resistance and leakage inductance in order to calculate it. Alternatively measure it at no load and full load. Or possibly if the turns ratio is given and the secondary voltage rating is at full load. \$\endgroup\$– winnyCommented Sep 17, 2016 at 9:42
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1\$\begingroup\$ You have changed your question. I've wasted time on questions where the poster did not ask properly and then drip feeds details so I don't want to do this on your question. Now your question is "I don't know the primary voltage. I don't know the secondary voltage. Can we just take it as the rated voltage on the secondary side." This is clearly not a reasonable approach. \$\endgroup\$ Commented Sep 17, 2016 at 9:45