Part 4 – Transformer Testing
Transformer Lost Measurement
A series of tests are performed on a new transformer to determine:
- Equivalent resistance, impedance, and reactance of the windings
- Iron and copper losses are to be expected when the transformer is on load
The tests that are performed for transformer lost measurement are the short-circuit test and the open-circuit test.
Short-Circuit Test
The short-circuit test is conducted to determine the copper losses of a transformer. It involves simulating a full-load condition by short-circuiting the secondary winding. During this test, the primary current is adjusted using a rheostat until the ammeter reads the rated full load current.
Short-circuit Testing on a Single-phase Transformer
Click on the image hotspots to explore the functions of short-circuit testing on a single-phase transformer.
If the measurements taken when the rated current is flowing in the primary winding are as follows:
- Input power = P watts
- Primary current = I1 amps
- Applied volts = E1 volts
The following calculation can be made:
| Input power | = I12R |
| R | = P/I1 2
= Resistance of winding (ohms) |
| Z | = E1/I1
=Impedance of winding (ohms) |
| X | = Z2 – R2
= Reactance of winding (ohms) |
Open-Circuit Test
The open-circuit test is a process that is used to determine the value of the impedance in the excitation branch of a real transformer. Firstly, the secondary winding of the transformer is left open-circuited. Then, a full-rated line voltage is applied to the primary winding of the transformer. Subsequently, measurements are taken for input voltage, primary current, and power. When the secondary circuit is open, there can be no secondary current flow, no demagnetizing of the transformer core, and the magnetic flux is maximum, giving maximum primary inductance XL.
Open-circuit Testing on a Single-phase Transformer
Click on the image hotspots to explore the functions of open-circuit testing on a single-phase transformer.
For a open-circuit test, the no-load current, flowing only in the primary, has two components:
- One producing the flux in the core
- The other taking care of the copper losses and the hysteresis-plus eddy current-losses in the core
Iron losses are calculated as follows:
- Wattmeter reading minus the copper losses
- Copper losses = I2R
| I = | Current flow during open circuit test |
| R = | Resistance of primary winding as determined by applying DC to the primary winding |