What is transformer excitation current? When normal voltage is applied to the terminals of a transformer with the secondary circuit open, a small current will flow in the primary. This current is called as transformer excitation current and flows all the time during the operation of the transformer. Excitation current is required by transformer to sustain a magnetic field inside the core and is largely independent of the secondary load. There are two components to this current:
* Core loss current component
*Magnetizing current component
Core loss current
The core loss current can be considered the resistive loss in the core and is in phase with the applied voltage. Core loss current determines the no-load losses of the transformer. Core loss current represents the no-load losses of the transformer and includes iron losses, small dielectric losses and copper losses caused by flow of excitation current. Of these only the iron losses caused by eddy currents are significant. Losses depend on frequency, maximum flux density and characteristics of magnetic circuit. Core loss values are usually provided from transformer factory at the time of product delivery.
Magnetizing current
The magnetizing current component lags the applied voltage by 90 degree and its magnitude depends on the number of turns in the primary winding, shape of transformer saturation curve and maximum flux density at which the transformer was designed. Magnetizing current is a purely reactive current and hence do not contribute directly to no-load losses. Reactive magnetizing power (VARS) is required by a transformer to operate and this current has to be supplied by the source.
How to calculate reactive power of a transformer?
Magnetizing current component has an interesting shape to it. Below is the waveform capture of a 30kVA 480V/208V transformer under no-load condition.
Why does magnetizing component of transformer excitation current has this shape?
It is the interaction between the magnetic flux produced in the core and the core steel ‘B-H’ curve that produces this shape. The equation for the primary circuit of the transformer with secondary open can be written as:
If we plot the resultant flux ø (green trace below) on the B-H curve (red trace) of the transformer core steel, we can obtain the magnetization current shape which is ‘peaky’ and have this peculiar shape. Note that the total current (excitation current) is the vectoral sum of magnetization current and core loss current. However, since the core loss component is very small it does not affect the shape of the total current. Transformers are designed to operate near the peak of B-H curve near the curved portion. As a result, during every peak of the flux waveform, exciting current gets an unusual shape as can be seen from the animation below.
Note that core loss current (Ic) is largely in phase with the supply voltage (Voc) while the magnetization current (Im) lags voltage by 900. The total excitation current Iex can be calculated to be:
Typical core loss current (Ic) is around 1% of the full load current. Typically magnetizing current (Im) can vary from about 0.25% to about 5% of full load current (0.05 pu) and can be as high as 10% in some special transformers.
Below is a summary of actual exciting current and no-load losses test results for modern dry type transformers that can be used as a reference. Note that currents are in percentage of the rated full load current of the transformer and system frequency is 60 Hz.
Sample Transformer Excitation Current Test Results
| Specimen# | kVA | Primary Voltage (V) | Secondary Voltage (V) | Excitation Current (%) | No-Load Loss (Watts) |
|---|---|---|---|---|---|
| 1 | 4,000 | 13,800 | 4,160 | 0.263 | 7,790 |
| 2 | 4,000 | 13,800 | 4,160 | 0.265 | 7,743 |
| 3 | 2,000 | 13,800 | 480 | 0.258 | 3,833 |
| 4 | 2,000 | 13,800 | 480 | 0.247 | 3,797 |
| 5 | 2,000 | 13,800 | 480 | 0.254 | 3,795 |
| 6 | 2,000 | 13,800 | 480 | 0.244 | 3,712 |
Excitation current harmonics in transformers
A Fourier analysis of the excitation current waveform will reveal that the current is rich in third harmonics, followed by smaller percentages of fifth and seventh and ninth. Third harmonic component is caused almost entirely due to the magnetization current and very little due to the core loss current.
Third harmonic current is approximately around 50% of the fundamental, fifth around 15% and seventh around 1-2%.
It should be noted that the third harmonic current flow and third harmonic voltage in the transformer winding terminals depends on the type of transformer connections and the type of core construction. This topic is discussed in the article below.
How transformer connections affect third harmonic voltages and currents?
Exciting Current and Loss Test
Exciting current test in transformer is an open circuit test done by applying rated voltage at rated frequency to one of the windings with the other winding open circuited. For reasons of safety and convenience, measurements are usually made on the low voltage side leaving the high voltage side open circuited. This test is used primarily to evaluate:
*Health of magnetic circuit of the transformer
*Turn to turn winding insulation
*Condition of tap changer
*Find the transformer core parameters (resistance and impedance)
Current and losses in watts are measured during the test. The resistive core loss current (Ic) is a measure of amount of energy consumed in the process of establishing magnetic flux in the core. This energy is the no-load loss of the transformer (Watts= E* Ic).
Any core problem will increase the reluctance of the magnetic circuit and consequently higher primary current. A turn to turn short will create additional current flow in the transformer that will translate in to higher than expected exciting current. Higher than expected/published exciting current will indicate problems with the transformer that may need additional inspection.
Exciting current and no-load loss measurements are recommended as routine diagnostic tests and during acceptance testing or when the transformer is subjected to extreme physical stress.
How to find magnetizing inductance of transformer?