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# Maximum possible secondary current for a transformer

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#### prasannarao

##### Newbie level 3
transformer secondary current calculation

Hello,

I am curious to know if we can estimate the maximum possible secondary current for a transformer with some given VA rating. This can be useful in estimating the approximate time for the rectifier-filter circuit to charge-up to the peak rated voltage under no-load condition. To the best of my knowledge it is a function of primary and secondary coil resistances and also to an extent their leakage reactances.

Note that the reservoir capacitor in the filter becomes a near short circuit at startup resulting in a very large secondary current.

So I am interested in finding a way to manually measure this maximum possible peak current, or even better, roughly estimate it for a transformer with some given specifications.

Thanks,
Prasanna

secondary current

If you know the voltage, you can easilly divide:
Max current1 = VA/V_secondary

Max current2 = less then a wire that have same thickness as internal secondary windings.
This problem only occurs where input voltage is much lover than rated.

So max current will be the least value of max current1 and max current2.

I didn't understand what you meant with "filter". Can you post a scematic?

transformer max current

The transformer's current is high when it first charges the main filter capacitor through rectifier diodes and the current is almost as high for each half-cycle of the mains frequency when the recifier current replenishes the charge in the main filter capacitor.

calculating transformer secondary current

In your case, it probably won't matter, but the max current of the shorted secondary is it's VA current divided by it's % impedance.

For example, a 2Kva transformer rated at a 240v secondary and a 2.5% impedance, have a secondary short circuit current of (2000/240)/.025 = 333.33 amps.

The transformers %impedance is the percent ratio of the rated primary voltage under the primary voltage that is required to generate the rated secondary current while shorted.

secondary current calculation

GetDeviceInfo said:
In your case, it probably won't matter, but the max current of the shorted secondary is it's VA current divided by it's % impedance.

For example, a 2Kva transformer rated at a 240v secondary and a 2.5% impedance, have a secondary short circuit current of (2000/240)/.025 = 333.33 amps.

Thanks for the info. So I guess the bridge definitely draws a large peak current at start up because the filter capacitor effectively shorts the secondary loop. The magnitude of this current being dependent on the transformer impedance.

I have attached the schematic.

Here is the schematic.

Best,
Prasanna

calculate transformer max current

I can help calculating max theoretical current, but I need to know both input/output reactance and resistance. Also I need to know n wich is primary turns / sec turns. voltage can also be used to get the n.

rectifier transformer calculate impedance

As the inrush current isn't sinusoidal, you must know the resistive and reactive component of the said % impedance. The resistive part can be determined with an ohmmeter (4 terminal measurement is required with low ohmic windings), but for the resistive part, an AC measurement or an LCR meter is needed.

Also the moment of switch on plays a role. As an additional issue in calculating (or simulating) the exact current waveform, the transformer core gets usually saturated for several periods when switch on moment is near to voltage zero crossing.

short the secondary of the transformer

FvM said:
The resistive part can be determined with an ohmmeter (4 terminal measurement is required with low ohmic windings), but for the resistive part, an AC measurement or an LCR meter is needed.
If input voltage is sinusoidal, this isn't a good way to measure. Low resistance values is difficult to measure exact. Also LCR meters is not commonly avaiable and most transformators have big H values that is often out of range of the LCR meter.

A better way is to short the secondary output. Then slowly increase voltage from zero volts until input current reach maximum rated (or what you get from VA/V).
Then you must measure current, voltage and angle of the current so you're able to calculate impedance (resistance and reactance separately).

Now you have a combination of input impedance and output impedance, and this is values that is printed on bigger transformers.

After that, you'll treat the transformer as ideal with this resistor and inductor in series with the input.
Now, if you know the input/output turns or voltage you have the value of n.
A theorem says that you can move this impedance from one side to another by multiplying the values by n when moving from secondary to primary. You need to divide by n.

Now, you know the voltage from the ideal transformer. Output current from it goes through the impedance mentioned above and two diodes in series/antiparrallel (1,2V loss).

calculate current draw transformer

If input voltage is sinusoidal, this isn't a good way to measure.
Then you must measure current, voltage and angle of the current.
Of course, you are applying an sinusoidal input voltage...

calculate transformer secondary current

@Prototyp_V1.0:

Thank you. I understand your logic.

Prototyp_V1.0 said:
A better way is to short the secondary output. Then slowly increase voltage from zero volts until input current reach maximum rated (or what you get from VA/V).
Then you must measure current, voltage and angle of the current so you're able to calculate impedance (resistance and reactance separately).

Most rectifier transformers that I have seen, do not print or specify the impedance figure. So perhaps, the best way would be to use an auto-transformer(variac) to drive the primary current (with secondary shorted) to the rated VA/V, and then measure the voltage, current and phase across it, as you mentioned. Just that it would get a little costly as I'll have to buy a variac.

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calculate rated current output of a transformer

Assuming linearity of transformers leak inductance, it's not necessary to supply the rated current for the % impedance measurement, although it's the specified method to get exact values. But practically, you get meaningful results between e.g. 5% and 100% of rated current.

If you don't have instruments to measure the current angle directly, it's a simple way to supply the current through a power resistor and calculate the transformer impedance from three voltage measurements.

transformer current related to filter capacitor

FvM said:
If you don't have instruments to measure the current angle directly, it's a simple way to supply the current through a power resistor and calculate the transformer impedance from three voltage measurements.

Could you please elaborate it further?

resistance vs secondary current in a transformer

Assume a 230V / 150 VA transformer with 2% resistive and 4% inductive impedance (just a quantitative example). The total impedance is √(2²+4²) = 4.47%, you get 0.0447 * 230 V = 10.3 V at the primary with shorted secondary and nominal current of 0.65 A. If you have 10 ohm resistor in series with the transformer, the voltage drop is 6.5 V at this current.

The 10.3 V at the transformer and 6.5 V at the resistor are summing vectorial, the resistor voltage drop is in phase with the transformer resistive impedance component and orthogonal to the inductive part. In this case, the sum is 14.4 V, which is also the output voltage of the supplying transformer.

With unknown transformer impedance, the calculation goes the other way: You have the input voltage, the resistor voltage drop and the transformer voltage. You can draw a triangle and solve the problem graphically or use a pocket calculator (or a slide rule).

how to measure transformer maximum current

Be ginger and fast with this testing. It's a good way to let the smoke out of the tranformer.

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