TRANSFORMER DATA
A simple transformer consists of two electrical conductors called the primary winding and secondary winding, and a steel core that magnetically links them together.
These two windings can be considered as a pair of mutually coupled coils. Energy is coupled between the windings by the magnetic field that links both primary and secondary windings.
How to convert VA to Amps.
Apparent power in volt-amps (VA) to electrical current in amps (A)
You can calculate amps from volt-amps and volts, but you can't convert volt-amps to amps since volt-amps and amps units do not measure the same quantity.
Single phase VA to Amps calculation formula.
The current I in amps is equal to the apparent power S in volt-amps (VA), divided by the RMS voltage V in volts (V):
I(A) = S(VA) / V(V)
So amps are equal to volt-amps divided by volts.
amps = volts-amps / volts or A = VA / V
EXAMPLE:
What is the current in amps when the apparent power is 100VA and the voltage supply is 12 volts.
Solution:
I = 100VA / 12V = 8.3 Amps
We know that a transformer generates its current output with the help of two windings, namely Primary and Secondary windings. The primary coil of the transformer is always connected to the alternating power supply, as it is the only method of suppling power to the transformer, by connecting the power supply in parallel with the two free ends of the primary windings. The current produced is then transferred to the secondary windings by Faraday's Law of mutual Induction.
Since a transformer can have more than one primary windings or secondary windings as well, so if two or more coils exist at any terminal, then they can be connected with each other in two basic ways. These two ways for connecting the two or more windings with each other are:
1. Series Connection:
2. Parallel Connection:
3. Centre Tap Connectiopn:
1. Series Connection of Windings
The secondary windings of a transformer, connected in series are shown below.
As we know that the voltage is divided in series whereas the current remains the same. So if there are more than one primary windings, then the supply voltage gets divided equally in all the windings at the input terminal but the current remains the same.
Now at the secondary terminal, if we are using two windings rated at 12v each, as shown in the figure above, then we get a total of 12v + 12v = 24v at the output terminal, which means that the voltage has been added up. The same amount of current will flow through each of the windings, which is 1A for each in this case, and hence the total curent at the output is also equal to 1A
From this we can conclude that, if we want to get twice the voltage at the output, we can connect two secondary windings in series and so on, provided that the current remain constant.
2. Parallel Connection of Windings.
The secondary windings of a transformer, connected in parallel are shown below.
As we know that the current in a parallel combination divides or adds up, so in this case as well, if the primary windings would also have been connected in a parallel combination , then the current supplied by the source would have been divided, but at the output terminal, aswe see that both the secondary windings which have been connected in parallel are rated at 1A each, so the current will add up here, and as there are two windings, so it add up like
1A + 1A =2A total.
Since the voltage remains the same, so the voltage drop across the output terminal will be the same as that on each of the windings of the transformer and the output above will be rated as 12v, 2A transformers.
If we use a dual voltage transformer, then only the readings of the current and voltage will be changed accordingly, but the principle that the voltage remains constant in parallel combination and the current adds up at the output, remains the same.
Moreover, other than these two configurations, other types of connections are also available, like driving two independent outputs by two individual secondary coils of a transformer, such that both of them have no inter connection.
One thing that should betaken care of is that while connecting two windings with each other, their phase relationships should be kept in mind and the connections should be made accordingly. If the terminals magnetic flux, and hence we wont get any output. So in order to get the desired output, only the terminals with the same phase relationship should be connected with each other.
3. Center Tapped Transformers
A center-tap transformer is desined to provide two separate secondary voltages, V(a) and V(b) with a common connection. This type of transformer configuration produces a two-phase, 3-wire supply.
The secondary voltages are the same and proportional to the supply voltage, V(p), therefore power in each windings is the same. The voltages produced across each of the secondary winding is determined by the turns ratio as shown.
Above shows a typical center-tap transformer.The tappinmg point is in the exact ceter of the secondary winding providing a common connection for two equal but opposite secondary voltages. With the center-tap grounded, the output v(A) will be positive in nature with respect to the ground, while the voltage at the other secondary, V(B) will be negative and opposite in nature, that is they are 180' electrical degrees out-of-phase with each other.
However, there is one disadvantage of using an ungrounded center tapped transformer and that is it can produce unbalanced voltages in the two secondary windings due to unsymmetrical currents flowing in the common third connection because of unbalanced loads.
We can also produce a center-tap transformer using the dual voltage transformer from the above. By connecting the secondary windings in series, we can use the center link as the tap as shown. If the output from each secondary is V, the total output voltage for the secondary winding will be equal to 2v as shown.
4. Center-Tap Transformer using a Dual Voltage Transformer.
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