Open-circuit and short-circuit test on a single-phase transformer
Open-circuit and short-circuit test on a single-phase transformer: circuit connections, procedure, equivalent-circuit calculations, efficiency and regulation, precautions and viva questions with answers.
Aim
To conduct open-circuit and short-circuit tests on a single-phase transformer, determine its equivalent circuit parameters, and predetermine efficiency and regulation at any load and power factor.
Apparatus required
| Apparatus | Specification | Qty |
|---|---|---|
| Single-phase transformer | 1 kVA, 230/115 V | 1 |
| Single-phase autotransformer (variac) | 230 V, 0–270 V, 8 A | 1 |
| Voltmeter (MI) | 0–150 V and 0–30 V | 1 each |
| Ammeter (MI) | 0–2 A and 0–10 A | 1 each |
| Wattmeter (LPF) | 150 V, 2 A, low power factor | 1 |
| Wattmeter (UPF) | 150 V, 10 A, unity power factor | 1 |
| Connecting wires | Adequate rating | As required |
Theory
A transformer has two kinds of loss. Core loss depends on the flux, which is set by the applied voltage and does not change with load. Copper loss depends on the square of the winding current, and so changes with everything. Two cheap tests separate them.
In the open-circuit test the rated voltage is applied to one winding with the other left open. Full flux is established, so core loss is at its normal value, but the current drawn is only the small magnetising current — a few percent of rated — so the copper loss it produces is negligible. The wattmeter therefore reads core loss alone, and the same readings give the shunt branch R₀ and X₀.
In the short-circuit test the other winding is shorted and the applied voltage is raised only until rated current flows — typically 2 to 15 % of rated voltage. The flux is proportionally tiny, so core loss is negligible, and the wattmeter reads full-load copper loss. The same readings give the series branch R₀₁ and X₀₁.
The OC test is done on the LV side because applying its rated voltage is easier and the meters are cheaper; the SC test is done on the HV side because rated current is smaller there and the low voltage needed is easier to set precisely on a variac. The equivalent circuit that results is referred to whichever side the SC test was performed on.
With both losses known, efficiency and regulation at any load can be predetermined without ever loading the transformer — which is the whole point, since loading a large transformer means dissipating its full output as heat.
Circuit connections
Check every point below against your board before switching on. There is no diagram here on purpose — a wrong diagram is worse than none, and this is the list a demonstrator actually walks through with you.
- OC test: variac output to the LV winding through the ammeter (series) and the LPF wattmeter's current coil. Voltmeter across the LV terminals, in parallel. HV winding left completely open — do not let its terminals touch anything, because rated flux induces full HV voltage across them.
- Wattmeter pressure coil across the supply on the load side of the current coil, with M and L terminals as marked.
- SC test: variac output to the HV winding through the ammeter and the UPF wattmeter's current coil. The LV winding is shorted with a thick, low-resistance link — a thin wire will drop voltage and spoil the reading.
- Start with the variac at zero in both tests, before the supply is switched on.
Procedure
- 1Make the open-circuit connections and have them checked. Set the variac to zero.
- 2Switch on and raise the variac slowly until the voltmeter reads the rated LV voltage.
- 3Record V₀, I₀ and W₀. Note the wattmeter's multiplying factor — the LPF meter's factor is the one students most often forget.
- 4Bring the variac back to zero and switch off. Rewire for the short-circuit test.
- 5With the LV winding solidly shorted, switch on and raise the variac very slowly. The current rises steeply for a small change in voltage — stop the moment the ammeter reads rated HV current.
- 6Record V_sc, I_sc and W_sc, then reduce the variac to zero and switch off.
- 7Compute the equivalent circuit parameters, then efficiency and regulation at the load and power factor asked for.
Work out your readings
Type in the numbers off the meters. This fills the tabular column, works the calculation through step by step, plots the characteristic — and tells you when a reading cannot physically be right, which is the part a manual can't do. Everything stays on this device, and it works with the network off.
Nameplate and machine data
0 to 1
Fill in the machine data to see the results, the worked calculation and the curve.
Precautions
- The variac must be at zero before every switch-on. In the SC test, rated current is reached at a small fraction of rated voltage, and starting anywhere above zero will destroy the meters.
- Never touch or short the open HV winding during the OC test — it is at full rated voltage.
- Use a low-power-factor wattmeter for the OC test. A UPF wattmeter on a 0.2 pf load deflects only a fifth of scale, and the reading is mostly error.
- Short the LV winding with a proper link. A thin wire acts as an added resistance and inflates the copper loss.
- Apply the multiplying factor to every wattmeter reading before doing anything with it.
Sources of error
Every record asks for these, and every record gets the same three lines copied from the one before. These are the errors this particular experiment actually has.
- Instrument loading: the wattmeter's own pressure-coil loss is included in the OC reading. On a small transformer it is a real fraction of the core loss.
- The SC test is normally done cold, but the winding resistance rises with temperature, so the copper loss in service is higher than measured.
- The synchronous impedance and equivalent circuit assume linearity. Around the knee of the magnetisation curve the transformer is not linear, so an OC test run above rated voltage gives a misleading core loss.
- Reading either meter while the variac is still being adjusted, rather than after the reading settles.
Viva questions with answers
Why is the open-circuit test conducted on the low-voltage side?
Because the rated voltage of the LV winding is easier and safer to apply from a standard variac, and the instruments needed are lower-range and cheaper. The core loss measured is the same whichever side it is measured from — it depends on the flux, not on which winding is energised.
Why is the short-circuit test conducted on the high-voltage side?
Rated current is lower on the HV side, so a smaller ammeter and wattmeter current coil will do. Also, the voltage needed to circulate rated current is only a few percent of rated, and setting that small voltage accurately is easier on the higher-voltage winding.
Why is the copper loss negligible in the OC test and the core loss negligible in the SC test?
In the OC test the current is 2–8 % of rated, and copper loss goes as I², so it is well under 1 % of the full-load value. In the SC test the applied voltage is 2–15 % of rated, so the flux — and core loss, which depends on the flux — is a similarly small fraction.
At what load does a transformer have maximum efficiency?
When the variable loss equals the constant loss, i.e. x²·W_cu = W_i, so x = √(W_i / W_cu). Distribution transformers are deliberately designed so this falls near the average load rather than full load, because they stay energised all day.
What is voltage regulation, and why can it be negative?
The drop in secondary terminal voltage from no load to full load, as a percentage of the no-load value. It is negative on a leading power factor: the capacitive load current produces a magnetising effect that makes the terminal voltage rise on load instead of falling.
The OC wattmeter pointer reads backwards. What do you do?
Reverse the pressure-coil connections (interchange the M and L terminals), not the current coil. Then take the reading as usual — the direction only tells you which way the coil was connected relative to the current.
Why is the efficiency found this way called predetermined?
Because the transformer is never actually loaded. The two tests measure the losses, and efficiency at any load is calculated from them. It is also called the indirect method, and it is the only practical route for large transformers.
What would happen if you applied rated voltage during the short-circuit test?
A short-circuit current of the order of 20 times rated would flow, limited only by the leakage impedance. The windings and instruments would be destroyed in a fraction of a second.