Transformer Voltage Regulation

Transformer voltage regulation is the ratio or percentage value at which the output terminal voltage of a transformer changes up or down from the unburied value as a result of changes in the connected load current.

In the transformer series, when the primary winding of a transformer is given power, the transformer will produce a secondary voltage and current in an amount determined by the rotation rate (TR), so if a single-phase transformer has a down rotation rate of 2:1 and 240V is applied to the high-voltage primary winding, we expect to see an output terminal voltage of 120 VAC in the secondary winding because we assume this situation is ideal and lossless.

However, in the real world this is not always true, since it is a winding magnetic circuit, all transformers suffer from losses of I 2 R copper and losses of magnetic core losses that will reduce this ideal secondary value by several percentages, such as 117 VAC.However, there is another value associated with transformers (and electrical machines), which affects this secondary voltage value when the transformer provides full power, which is called "regulation".

Transformer Voltage Regulation

Voltage Regulationof single-phase transformers is a percentage change in secondary terminal voltage (or per unit value) compared to the original load-free voltage under changing secondary load conditions.In other words, regulation determines the change in secondary terminal voltage occurring within the transformer as a result of changes in the load to which the transformer is connected, affecting its performance and efficiency if these losses are high and the secondary voltage is too low.

When connected to the transformer without load, the secondary winding, i.e. output terminals, are open circuits, there is no closed circuit state, so there is no output load current (I L = 0) and the transformer acts as one. single bandage with high self-induced.Keep in mind that the load-free secondary voltage is a result of the constant primary voltage and the rotational rate of the transformer.

Loading the secondary winding with a simple load impedance causes a secondary current to flow along the inner winding of the transformer in any power factor.Thus, due to the internal resistance of the windings, the voltage decreases and the leakage recess causes the output terminal voltage to change.

Voltage regulation of a transformer varies from a loadless state to a fully loaded secondary terminal voltage when I L = I MAX (maximum current) is given I L = I MAX (maximum current) for a constant primary voltage:

Transformer Voltage Regulation as Fractional Change

Transformer Voltage Regulation

Note that when this voltage regulation is expressed as a fraction or unit change of the load-free terminal voltage, it can be defined in one of two ways: voltage regulation-down , (Reg down ) and voltage regulation-up , (Reg up).This means that when the load is connected to the secondary output terminal, the terminal voltage decreases or the secondary terminal voltage increases when the load is removed.Thus, the regulation of the transformer will depend on which voltage value is used as the reference voltage, load or unburied value.

We can also refer to transformer voltage regulation as the percentage change between load-free condition and full load conditions as follows:

Transformer Voltage Regulation as Percentage Change

Transformer Voltage Regulation

For example, if the open-circuit load-free terminal voltage of a single-phase transformer is 100 volts and the same terminal voltage drops to 95 volts in the application of a connected load, the transformer voltage regulation will therefore be 0.05 or 5. %, ((100 – 95)/100)*100%).Therefore, a transformer voltage regulation can be expressed as a unit change value or the percentage change value of the loadless voltage.

Transformer Voltage Regulation Question Sample 1

The primary winding of the 500VA, 10:1 single-phase drop transformer is fed from a fixed 240Vrms source.Calculate the percentage regulation of the transformer when connected to the 1.1Ω impedance

Data: VA = 500, en = 10: 1, V , P = 240V, Z, S = 1.1Ω,% Reg.

Transformer Voltage Regulation

Therefore, VS(no-load) = 24 Volts

Transformer Voltage Regulation

Therefore, V S(full-load) = 23.45 Volts

Transformer Voltage Regulation

The percentage regulation calculated for the transformer is then given as follows: 2.29%.

Transformer Voltage Regulation Question Sample 2

A single-phase transformer with a voltage regulation of 4% has a secondary terminal voltage of 115.4 volts at full load current.Calculate the load-free terminal voltage when the load is removed.

Transformer Voltage Regulation

Next, we can see that a change in the connected load creates a change in the transformer terminal voltage between the "unburied" voltage and the "full load" voltage, making the transformer voltage regulation a function other than the transformer.Therefore, the lower the percentage of voltage regulation, the more stable the secondary terminal voltage of the transformer, regardless of the load current value.If the connected load is completely resistant, the voltage drop will be smaller.Thus, an ideal transformer will have zero voltage regulation, that is, V S (full-load) is equal to V S (no-load) since it will be zero loss.

We now know that the voltage regulation of a transformer is the difference between the full load voltage and the non-load voltage, the maximum rated secondary current, which can be expressed as a ratio or percentage (%).But why does the secondary voltage change or decrease with changes in load current?

Installed Transformers

While the secondary winding of a transformer feeds a load, there are magnetic iron losses in the laminated core and copper losses due to the resistance of its windings, and this applies to both primary and secondary windings.

Transformer Voltage Regulation

These losses produce a reassurance and resistance in transformer windings, which provides an impedance path through which the secondary output current (IS) must flow as shown.

Since secondary winding consists of both resistance and reactance, as stated by the Ohm Act, an internal voltage drop should occur in the windings of the transformer depending on the active impedance and the load current provided: V = I*Z .

Then, as the secondary load current increases, we can see that the voltage falling in the transformer windings should also increase, and the secondary output voltage should decrease for a constant primary supply voltage.

The impedance (Z) of secondary winding is the phaser sum of both its resistance (R) and leakage reagenance (X) with a different voltage drop produced in each component.Then we can define the secondary impedance and load-free and full load voltages as follows:

Transformer Voltage Regulation

Thus, the loadless voltage of secondary windings is defined as follows:

V S(unburied) = E S

and the full load voltage is defined as follows:

V S(full load) = E S – I S R – I S X

or V S(full load) = E S – I S (R+jX)

∴ V S(full load) = E S – I S *Z

Clearly then we can see that the transformer winding consists of a serial reassurance with a resistance that the load current is common to both.Since the voltage and current for a resistance are in the same phase, the voltage drop along the resistance given as I S R, therefore the secondary current should be in the "same phase" as the I S.

However, in a pure inductor with inductive colorance, X L , current 90 is so regent, so the voltage drop along the reassurance given as I S X directs the current up to the angle of Φ L as an inductive load.

Since the impedance of secondary winding is Z, resistance and phaser sum of recess, their individual phase angles are given as follows:

Transformer Voltage Regulation

In V = I*Z, the voltage drop along the secondary impedance is given as follows:

V drop = I S (RcosΦ + XcosΦ)

and V S(full load) = V S(no load) – V as a drop, percentage arrangement can be given as follows:

Delayed Power Factor Expression

Transformer Voltage Regulation

For a positive regulatory expression between cos(Φ) and sin(Φ), the transformer secondary terminal voltage will decrease (decrease), indicating a delayed power factor (inductive load). For a negative regulation expression between cos(Φ) and sin(Φ), the secondary terminal voltage of the transformer will increase (increase) by indicating a leading power factor (capacitive load).Therefore, the expression of a transformer arrangement is the same for both leading and delayed loads, it is only a changing sign to indicate a voltage rise or fall.

Leading Power Factor Expression

transformer leading power factor

Therefore, a positive regulation condition produces a voltage drop (drop) in secondary winding, while a negative regulation condition produces a voltage increase (ascent) in the winding.Although leading power factor loads are not as common as inductive loads (coils, solenoids or coils), a transformer that feeds a light load with low currents may experience a capacitive state that causes the terminal voltage to rise.

Transformer Voltage Regulation Question Sample 3

A 10KVA single-phase transformer provides a load-free secondary voltage of 110 volts.If the equivalent secondary winding resistance is 0.015Ω and the total reagenance is 0.04Ω, determine the voltage regulation when the 0.85 power factor feeds a delayed load.

Data supplied: VA = 10000, V S(unloaded) = 110V, R = 0.015Ω, X = 0.04Ω, Find %Reg.

if cosΦ = 0.85, Φ = cos -1 (0.85) = 31.8 o ∴ sinΦ = 0.527

Secondary current is defined as follows:

I S = VA/V = 10000/110 = 90.9 Amps

Percentage voltage regulation is given as follows:

Transformer Voltage Regulation


In this tutorial on Transformer Voltage Regulation, we found that when the secondary winding of a transformer is loaded, the output voltage may change, and this voltage change can be expressed as a ratio or more commonly as a percentage value.There is no secondary current when connected without load, which means that the secondary voltage is at its maximum value.

However, when fully loaded, secondary currents flow and produce core losses and copper losses in the winding.Core loss is a constant loss from the transformer magnetic circuit produced by the primary winding voltage, while secondary copper loss is a variable loss related to the demand for load current due to secondary winding.

Subsequent changes in load current will cause changes in losses affecting regulation.The smaller the voltage regulation of the transformer, the less the change in secondary terminal voltage with changes in load, and this is very useful in regulated power supply circuits.

We also said that the secondary terminal voltage for a delayed power factor (inductive load) will decrease.If the transformer provides a very low delayed power factor, large secondary currents will flow and result in poor voltage regulation due to larger voltage drops in the winding.

A leading power factor (capacitive load), the output terminal voltage will increase.Therefore, positive regulation produces a voltage drop in the winding, while a negative arrangement produces a voltage increase in the winding.Although it is not possible to have a zero voltage regulation condition (ideal transformers only), minimum regulation and therefore maximum efficiency usually occur when the core losses and copper losses of the windings are approximately equal.