Top 10 Powerful Insights into Open and Short Circuit Test of Transformer pdf.
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Sep 25, 2025
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About This Presentation
To understand the performance of a transformer, we need to determine its equivalent circuit parameters. These parameters help us know how efficient the transformer is and how its voltage regulation will behave under different load conditions.
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Top 10 Powerful Insights into Open and
Short Circuit Test of Transformer
Table of Contents
Introduction
Working Principle: open and short circuit test of transformer
Working Principle of Open-Circuit Test
Working Principle of Short-Circuit Test
Purpose of Tests
Test Setup & Diagrams: open and short circuit test of transformer
1. Open Circuit Test
Short Circuit Test
Calculations & Formulas (Solved Examples)
Formulas: open and short circuit test of transformer
Quick Revision Sheet —open and short circuit test of transformer.
Open-Circuit Test — Quick Table
Short-Circuit (SC) Test — Quick Table
Derived Parameters & Their Uses
1. Equivalent Circuit of Transformer
2. Efficiency of Transformer
3. Voltage Regulation
Phasor Diagrams: open and short circuit test of transformer
1. Open Circuit Test Phasor Diagram
2. Short Circuit Test Phasor Diagram
Key Differences: OC Test vs SC Test (Phasor Perspective)
Comparison Table: open and short circuit test of transformer
Comparison of Transformer Tests: Direct Loading vs OC/SC vs Sumpner’s
Safety & Precautions: open and short circuit test of transformer
FAQ
Introduction
To understand the performance of a transformer, we need to determine its equivalent circuit
parameters. These parameters help us know how efficient the transformer is and how its
voltage regulation will behave under different load conditions.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
https://elentechpk.com/open-and-short-circuit-test-of-transformer/ 2/29
Short Circuit Test of Transformer
Table of Contents
Introduction
Working Principle: open and short circuit test of transformer
Working Principle of Open-Circuit Test
Working Principle of Short-Circuit Test
Purpose of Tests
Test Setup & Diagrams: open and short circuit test of transformer
1. Open Circuit Test
Short Circuit Test
Calculations & Formulas (Solved Examples)
Formulas: open and short circuit test of transformer
Quick Revision Sheet —open and short circuit test of transformer.
Open-Circuit Test — Quick Table
Short-Circuit (SC) Test — Quick Table
Derived Parameters & Their Uses
1. Equivalent Circuit of Transformer
2. Efficiency of Transformer
3. Voltage Regulation
Phasor Diagrams: open and short circuit test of transformer
1. Open Circuit Test Phasor Diagram
2. Short Circuit Test Phasor Diagram
Key Differences: OC Test vs SC Test (Phasor Perspective)
Comparison Table: open and short circuit test of transformer
Comparison of Transformer Tests: Direct Loading vs OC/SC vs Sumpner’s
Safety & Precautions: open and short circuit test of transformer
FAQ
Introduction
To understand the performance of a transformer, we need to determine its equivalent circuit
parameters. These parameters help us know how efficient the transformer is and how its
voltage regulation will behave under different load conditions.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
https://elentechpk.com/open-and-short-circuit-test-of-transformer/ 2/29
There are two main ways to obtain these parameters:
1. Direct Loading Method – In this method, the transformer is tested by connecting it
to an actual load.
?????? The Direct Loading Method gives slightly higher accuracy, but the biggest drawback is
that it wastes a large amount of power equal to the transformer’s rating. For example, if
a transformer is rated at 100 kVA, nearly that much power will be consumed during the test.
2. open and short circuit test of transformer – In this method, the transformer is tested
without applying any real load.
Open Circuit Test (OC Test)
Short Circuit Test (SC Test)
?????? These two tests are easy to perform and do not require the transformer to be operated on
an actual load. That is why they are called indirect loading tests.
The OC Test provides information about the transformer’s core (iron) losses and no-
load parameters.
The SC Test gives the copper losses and the equivalent resistance and reactance of
the transformer.
By using the results of these two tests, we can determine the equivalent circuit, efficiency,
and voltage regulation of a transformer—without ever running it on full load.
✅ Advantages of open and short circuit test of transformer over Direct Loading
It provides the no-load current (Io), iron loss (Pi), full-load copper loss (Pcu), and
the complete equivalent circuit of the transformer.
It enables us to predict the performance of the transformer at any load and power
factor without requiring a real load connection.
It is cost-effective, since the power consumed is only a tiny percentage of the
transformer’s rating.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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1. Direct Loading Method – In this method, the transformer is tested by connecting it
to an actual load.
?????? The Direct Loading Method gives slightly higher accuracy, but the biggest drawback is
that it wastes a large amount of power equal to the transformer’s rating. For example, if
a transformer is rated at 100 kVA, nearly that much power will be consumed during the test.
2. open and short circuit test of transformer – In this method, the transformer is tested
without applying any real load.
Open Circuit Test (OC Test)
Short Circuit Test (SC Test)
?????? These two tests are easy to perform and do not require the transformer to be operated on
an actual load. That is why they are called indirect loading tests.
The OC Test provides information about the transformer’s core (iron) losses and no-
load parameters.
The SC Test gives the copper losses and the equivalent resistance and reactance of
the transformer.
By using the results of these two tests, we can determine the equivalent circuit, efficiency,
and voltage regulation of a transformer—without ever running it on full load.
✅ Advantages of open and short circuit test of transformer over Direct Loading
It provides the no-load current (Io), iron loss (Pi), full-load copper loss (Pcu), and
the complete equivalent circuit of the transformer.
It enables us to predict the performance of the transformer at any load and power
factor without requiring a real load connection.
It is cost-effective, since the power consumed is only a tiny percentage of the
transformer’s rating.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Working Principle: open and short circuit test of
transformer
Working Principle of Open-Circuit Test
Since the secondary winding is kept open, the current in the secondary winding is: I₂
= 0
As a result, the output power is zero.
However, the wattmeter on the primary side still records some input power (Wₒ). This
happens because the transformer needs energy for two purposes:
1. Magnetizing the core – to produce alternating magnetic flux.
→ Represented by the magnetizing current component (Iµ).
2. Overcoming core losses – hysteresis loss and eddy current loss in the
transformer’s core.
→ Represented by the active current component (Iw).
Therefore, the total input power during the open-circuit test is used only to supply
iron/core losses.
⚡ Key Point: No-load copper loss is negligible (since Io is only 1–5% of full-load current).
Thus, the wattmeter reading (Wₒ) is practically equal to the transformer’s iron loss (Pᵢ) at
rated voltage.
Working Principle of Short-Circuit Test
The short-circuit test is performed to determine the equivalent resistance (Rsc), equivalent
leakage reactance (Xsc), and the rated copper loss (Pcu(FL)) of a transformer.
1. Test Setup
The secondary winding is short-circuited.
The primary winding is supplied with a reduced voltage (Vsc), just sufficient to
circulate the rated full-load current (Isc) in both windings.
Measuring instruments (ammeter, voltmeter, wattmeter) are connected on the
primary (HV) side.
2. Measurements Taken
Vsc = Applied short-circuit voltage
Isc = Short-circuit current (equal to rated current)
Wsc = Wattmeter reading
3. Working Principle
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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transformer
Working Principle of Open-Circuit Test
Since the secondary winding is kept open, the current in the secondary winding is: I₂
= 0
As a result, the output power is zero.
However, the wattmeter on the primary side still records some input power (Wₒ). This
happens because the transformer needs energy for two purposes:
1. Magnetizing the core – to produce alternating magnetic flux.
→ Represented by the magnetizing current component (Iµ).
2. Overcoming core losses – hysteresis loss and eddy current loss in the
transformer’s core.
→ Represented by the active current component (Iw).
Therefore, the total input power during the open-circuit test is used only to supply
iron/core losses.
⚡ Key Point: No-load copper loss is negligible (since Io is only 1–5% of full-load current).
Thus, the wattmeter reading (Wₒ) is practically equal to the transformer’s iron loss (Pᵢ) at
rated voltage.
Working Principle of Short-Circuit Test
The short-circuit test is performed to determine the equivalent resistance (Rsc), equivalent
leakage reactance (Xsc), and the rated copper loss (Pcu(FL)) of a transformer.
1. Test Setup
The secondary winding is short-circuited.
The primary winding is supplied with a reduced voltage (Vsc), just sufficient to
circulate the rated full-load current (Isc) in both windings.
Measuring instruments (ammeter, voltmeter, wattmeter) are connected on the
primary (HV) side.
2. Measurements Taken
Vsc = Applied short-circuit voltage
Isc = Short-circuit current (equal to rated current)
Wsc = Wattmeter reading
3. Working Principle
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Since the secondary is shorted, the terminal voltage is zero and the output
power = 0.
However, the wattmeter shows some power (Wsc) consumed from the supply.
The applied input voltage (Vsc) is very small compared to the rated voltage →
therefore, iron/core loss is negligible.
The entire wattmeter reading (Wsc) represents the full-load copper loss
(Pcu(FL)).
Purpose of Tests
The Open Circuit Test (OC Test) and Short Circuit Test (SC Test) are performed to find
out important losses and parameters of a transformer without applying a real load.
Open Circuit Test (OC Test) → Iron/Core Loss
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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power = 0.
However, the wattmeter shows some power (Wsc) consumed from the supply.
The applied input voltage (Vsc) is very small compared to the rated voltage →
therefore, iron/core loss is negligible.
The entire wattmeter reading (Wsc) represents the full-load copper loss
(Pcu(FL)).
Purpose of Tests
The Open Circuit Test (OC Test) and Short Circuit Test (SC Test) are performed to find
out important losses and parameters of a transformer without applying a real load.
Open Circuit Test (OC Test) → Iron/Core Loss
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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In this test, the secondary winding of the transformer is kept open.
The test measures the iron loss (core loss) that occurs due to hysteresis and eddy
currents in the transformer core.
It also gives us the no-load parameters, such as magnetizing current and shunt
branch values of the equivalent circuit.
SC Test → Copper Loss
In this test, the secondary winding of the transformer is short-circuited.
The test measures the copper loss, which happens in the windings due to the
resistance of the conductors when current flows.
It also provides the series resistance and leakage reactance of the transformer’s
equivalent circuit.
Direct Loading Method
The Direct Loading Method is one of the ways to determine the efficiency and voltage
regulation of a transformer. In this method, an actual load is connected to the transformer’s
secondary winding, and measurements are taken on both the input and output sides.
Key Points:
The transformer is loaded with its rated load or different fractions of load.
Input power, output power, and losses are directly measured.
Efficiency is calculated by comparing output power to input power.
Voltage regulation is determined by measuring the difference in secondary voltage
from no-load to full-load conditions.
Advantages:
Provides accurate and reliable results.
Directly shows how the transformer performs under actual working conditions.
Disadvantages:
Requires a large load bank for high-capacity transformers.
Consumes power equal to the transformer’s rating, making it inefficient and costly.
Not practical for large power transformers due to high energy wastage.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
https://elentechpk.com/open-and-short-circuit-test-of-transformer/ 6/29
The test measures the iron loss (core loss) that occurs due to hysteresis and eddy
currents in the transformer core.
It also gives us the no-load parameters, such as magnetizing current and shunt
branch values of the equivalent circuit.
SC Test → Copper Loss
In this test, the secondary winding of the transformer is short-circuited.
The test measures the copper loss, which happens in the windings due to the
resistance of the conductors when current flows.
It also provides the series resistance and leakage reactance of the transformer’s
equivalent circuit.
Direct Loading Method
The Direct Loading Method is one of the ways to determine the efficiency and voltage
regulation of a transformer. In this method, an actual load is connected to the transformer’s
secondary winding, and measurements are taken on both the input and output sides.
Key Points:
The transformer is loaded with its rated load or different fractions of load.
Input power, output power, and losses are directly measured.
Efficiency is calculated by comparing output power to input power.
Voltage regulation is determined by measuring the difference in secondary voltage
from no-load to full-load conditions.
Advantages:
Provides accurate and reliable results.
Directly shows how the transformer performs under actual working conditions.
Disadvantages:
Requires a large load bank for high-capacity transformers.
Consumes power equal to the transformer’s rating, making it inefficient and costly.
Not practical for large power transformers due to high energy wastage.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
https://elentechpk.com/open-and-short-circuit-test-of-transformer/ 6/29
Thus, while the Direct Loading Method is straightforward and accurate, it is rarely used in
practice for large transformers. Instead, open and short circuit test of transformer are
preferred because they are indirect, cost-effective, and safer.
Test Setup & Diagrams: open and short circuit
test of transformer
1. Open Circuit Test
Purpose: To determine the core (iron) losses, no-load current, and shunt branch parameters
of the transformer equivalent circuit.
Setup:
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practice for large transformers. Instead, open and short circuit test of transformer are
preferred because they are indirect, cost-effective, and safer.
Test Setup & Diagrams: open and short circuit
test of transformer
1. Open Circuit Test
Purpose: To determine the core (iron) losses, no-load current, and shunt branch parameters
of the transformer equivalent circuit.
Setup:
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The secondary winding is left open (no load connected).
The rated voltage is applied to the low-voltage (LV) side.
Instruments connected:
A voltmeter across the LV winding.
An ammeter in series with the LV supply to measure the no-load current.
Wattmeter to record input power (iron/core loss).
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The rated voltage is applied to the low-voltage (LV) side.
Instruments connected:
A voltmeter across the LV winding.
An ammeter in series with the LV supply to measure the no-load current.
Wattmeter to record input power (iron/core loss).
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Short Circuit Test
Purpose: To determine the copper losses, equivalent resistance, and leakage reactance of the
transformer.
Setup:
The secondary winding is short-circuited with thick copper wire.
A low voltage is applied to the high-voltage (HV) side, just enough to circulate full-
load current.
Instruments connected:
Voltmeter across HV winding.
An ammeter in series with the HV supply to measure current.
Wattmeter to record input power (copper loss).
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Purpose: To determine the copper losses, equivalent resistance, and leakage reactance of the
transformer.
Setup:
The secondary winding is short-circuited with thick copper wire.
A low voltage is applied to the high-voltage (HV) side, just enough to circulate full-
load current.
Instruments connected:
Voltmeter across HV winding.
An ammeter in series with the HV supply to measure current.
Wattmeter to record input power (copper loss).
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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~AC Source── Ammeter ─────── HV Winding ───── Voltmeter
│
Wattmeter
│
LV Winding (Shorted)
Calculations & Formulas (Solved Examples)
Below are two clean, step-by-step solved problems—one for the Open-Circuit (OC) Test
and one for the Short-Circuit (SC) Test—with all formulas written in bold.
Example 1: Open-Circuit (OC) Test
Given (measured):
Applied voltage, Vo = 230 V
No-load current, Io = 2.0 A
Wattmeter reading, Wo = 120 W
Find: No-load power factor (cosφ₀), current components (Iw, Iµ), and shunt parameters (Ro,
Xo).
Step 1: No-load Power Factor
cosφ₀ = Wo / (Vo × Io) = 120 / (230 × 2.0) = 120 / 460 = 0.261
Step 2: Current Components
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│
Wattmeter
│
LV Winding (Shorted)
Calculations & Formulas (Solved Examples)
Below are two clean, step-by-step solved problems—one for the Open-Circuit (OC) Test
and one for the Short-Circuit (SC) Test—with all formulas written in bold.
Example 1: Open-Circuit (OC) Test
Given (measured):
Applied voltage, Vo = 230 V
No-load current, Io = 2.0 A
Wattmeter reading, Wo = 120 W
Find: No-load power factor (cosφ₀), current components (Iw, Iµ), and shunt parameters (Ro,
Xo).
Step 1: No-load Power Factor
cosφ₀ = Wo / (Vo × Io) = 120 / (230 × 2.0) = 120 / 460 = 0.261
Step 2: Current Components
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Active (loss) component:
Iw = Io × cosφ₀ = 2.0 × 0.261 = 0.522 A
Magnetizing component (by geometry):
Iµ = √(Io² – Iw²) = √(2.0² – 0.522²) = √(4.000 – 0.272) = 1.93 A
(You may also use Iµ = Io × sinφ₀ .)
Step 3: Shunt Branch Parameters
No-load resistance:
Ro = Vo / Iw = 230 / 0.522 = 440 Ω
Magnetizing reactance:
Xo = Vo / Iµ = 230 / 1.93 = 119 Ω
Results (OC Test):
cosφ₀ ≈ 0.261, Iw ≈ 0.522 A, Iµ ≈ 1.93 A, Ro ≈ 440 Ω, Xo ≈ 119 Ω.
(Iron loss at rated voltage: Pi = Wo = 120 W.)
Example 2: Short-Circuit (SC) Test
Given (measured):
Applied short-circuit voltage, Vsc = 18 V
Short-circuit current (rated), Isc = 10 A
Wattmeter reading, Wsc = 120 W
Find: Equivalent impedance (Zsc), equivalent resistance (Rsc), equivalent reactance (Xsc).
(Optional: %Z if rated voltage is known.)
Step 1: Equivalent Impedance
Zsc = Vsc / Isc = 18 / 10 = 1.80 Ω
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Iw = Io × cosφ₀ = 2.0 × 0.261 = 0.522 A
Magnetizing component (by geometry):
Iµ = √(Io² – Iw²) = √(2.0² – 0.522²) = √(4.000 – 0.272) = 1.93 A
(You may also use Iµ = Io × sinφ₀ .)
Step 3: Shunt Branch Parameters
No-load resistance:
Ro = Vo / Iw = 230 / 0.522 = 440 Ω
Magnetizing reactance:
Xo = Vo / Iµ = 230 / 1.93 = 119 Ω
Results (OC Test):
cosφ₀ ≈ 0.261, Iw ≈ 0.522 A, Iµ ≈ 1.93 A, Ro ≈ 440 Ω, Xo ≈ 119 Ω.
(Iron loss at rated voltage: Pi = Wo = 120 W.)
Example 2: Short-Circuit (SC) Test
Given (measured):
Applied short-circuit voltage, Vsc = 18 V
Short-circuit current (rated), Isc = 10 A
Wattmeter reading, Wsc = 120 W
Find: Equivalent impedance (Zsc), equivalent resistance (Rsc), equivalent reactance (Xsc).
(Optional: %Z if rated voltage is known.)
Step 1: Equivalent Impedance
Zsc = Vsc / Isc = 18 / 10 = 1.80 Ω
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Step 2: Equivalent Resistance (from power)
Rsc = Wsc / Isc² = 120 / (10)² = 120 / 100 = 1.20 Ω
Step 3: Equivalent Reactance
Xsc = √(Zsc² – Rsc²) = √(1.80² – 1.20²) = √(3.24 – 1.44) = √1.80 = 1.34 Ω
Results (SC Test):
Zsc ≈ 1.80 Ω, Rsc ≈ 1.20 Ω, Xsc ≈ 1.34 Ω.
Optional: % Impedance (if rated voltage Vrated is known)
For example, if Vrated = 400 V (referred side):
%Z = (Vsc / Vrated) × 100 = (18 / 400) × 100 = 4.5%
Formulas: open and short circuit test of transformer
Important Formulas for Transformer Testing
Open-Circuit Test Formulas (with Explanation):
cosφ₀ = Wo / (Vo × Io) → Power factor at no-load, ratio of wattmeter power to
apparent input.
Iw = Io × cosφ₀ → Active (working) component of no-load current responsible for
iron losses.
Iµ = Io × sinφ₀ → Magnetizing component of no-load current that produces flux.
Xo = Vo / Iµ → No-load reactance of transformer, linked to magnetizing branch.
Pi = Wo → Input power at no-load equals iron (core) losses only.
Ro = Vo / Iw → Equivalent resistance representing core losses.
Iµ = √(Io² – Iw²) → Derived magnetizing current using Pythagoras’ relation.
Short-Circuit Test Formulas (with Explanation):
Zsc = Vsc / Isc → Equivalent impedance per phase referred to primary.
Rsc = Wsc / Isc² → Equivalent resistance representing copper (winding) losses.
Xsc = √(Zsc² – Rsc²) → Equivalent leakage reactance of transformer.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Rsc = Wsc / Isc² = 120 / (10)² = 120 / 100 = 1.20 Ω
Step 3: Equivalent Reactance
Xsc = √(Zsc² – Rsc²) = √(1.80² – 1.20²) = √(3.24 – 1.44) = √1.80 = 1.34 Ω
Results (SC Test):
Zsc ≈ 1.80 Ω, Rsc ≈ 1.20 Ω, Xsc ≈ 1.34 Ω.
Optional: % Impedance (if rated voltage Vrated is known)
For example, if Vrated = 400 V (referred side):
%Z = (Vsc / Vrated) × 100 = (18 / 400) × 100 = 4.5%
Formulas: open and short circuit test of transformer
Important Formulas for Transformer Testing
Open-Circuit Test Formulas (with Explanation):
cosφ₀ = Wo / (Vo × Io) → Power factor at no-load, ratio of wattmeter power to
apparent input.
Iw = Io × cosφ₀ → Active (working) component of no-load current responsible for
iron losses.
Iµ = Io × sinφ₀ → Magnetizing component of no-load current that produces flux.
Xo = Vo / Iµ → No-load reactance of transformer, linked to magnetizing branch.
Pi = Wo → Input power at no-load equals iron (core) losses only.
Ro = Vo / Iw → Equivalent resistance representing core losses.
Iµ = √(Io² – Iw²) → Derived magnetizing current using Pythagoras’ relation.
Short-Circuit Test Formulas (with Explanation):
Zsc = Vsc / Isc → Equivalent impedance per phase referred to primary.
Rsc = Wsc / Isc² → Equivalent resistance representing copper (winding) losses.
Xsc = √(Zsc² – Rsc²) → Equivalent leakage reactance of transformer.
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%Z = (Vsc / Vrated) × 100 → Percentage impedance, important for fault level and
voltage regulation calculations.
Quick Revision Sheet —open and short circuit
test of transformer.
Open-Circuit Test — Quick Table
Inputs (measured) Calculations (formulas) Outputs (results)
Vo, Io, Wo cosφ₀ = Wo / (Vo·Io) No-load power factor cosφ₀
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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voltage regulation calculations.
Quick Revision Sheet —open and short circuit
test of transformer.
Open-Circuit Test — Quick Table
Inputs (measured) Calculations (formulas) Outputs (results)
Vo, Io, Wo cosφ₀ = Wo / (Vo·Io) No-load power factor cosφ₀
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Inputs (measured) Calculations (formulas) Outputs (results)
Iw = Io·cosφ₀
Active component Iw (iron-
loss current)
Iμ = √(Io² − Iw²) or Io·sinφ₀Magnetizing component Iμ
Ro = Vo / Iw No-load resistance Ro
Xo = Vo / Iμ Magnetizing reactance Xo
Pi = Wo Iron loss Pi (rated)
Short-Circuit (SC) Test — Quick Table
Inputs (measured) Calculations (formulas) Outputs (results)
Vsc, Isc, Wsc Zsc = Vsc / Isc Equivalent impedance Zsc
Rsc = Wsc / Isc²
Equivalent resistance Rsc
(from copper loss)
Xsc = √(Zsc² − Rsc²)
Equivalent leakage
reactance Xsc
%Z = (Vsc / Vrated) × 100
Percentage impedance (if
Vrated known)
Pcu(FL) = Wsc
Full-load copper loss Pcu
(treated as Wsc)
Quick Tips
OC test: performed on LV side; measures iron loss; Io ≈ 1–5% of IFL.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Iw = Io·cosφ₀
Active component Iw (iron-
loss current)
Iμ = √(Io² − Iw²) or Io·sinφ₀Magnetizing component Iμ
Ro = Vo / Iw No-load resistance Ro
Xo = Vo / Iμ Magnetizing reactance Xo
Pi = Wo Iron loss Pi (rated)
Short-Circuit (SC) Test — Quick Table
Inputs (measured) Calculations (formulas) Outputs (results)
Vsc, Isc, Wsc Zsc = Vsc / Isc Equivalent impedance Zsc
Rsc = Wsc / Isc²
Equivalent resistance Rsc
(from copper loss)
Xsc = √(Zsc² − Rsc²)
Equivalent leakage
reactance Xsc
%Z = (Vsc / Vrated) × 100
Percentage impedance (if
Vrated known)
Pcu(FL) = Wsc
Full-load copper loss Pcu
(treated as Wsc)
Quick Tips
OC test: performed on LV side; measures iron loss; Io ≈ 1–5% of IFL.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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SC test: performed on HV side with LV shorted; measures copper loss; Vsc ≈ 5–10%
of Vrated.
Use OC results (Ro, Xo) + SC results (Rsc, Xsc) to build the equivalent circuit and
compute efficiency & regulation for any load and power factor.
Derived Parameters & Their Uses
The Open -Circuit Test and Short Circuit Test of a transformer provide practical values for
equivalent circuit parameters. These parameters are not just theoretical; they form the
foundation for performance analysis, design validation, condition monitoring, and
operational planning of transformers. Engineers rely on them to predict behavior under
different loading conditions, ensure reliability, and improve efficiency.
1. Equivalent Circuit of Transformer
The equivalent circuit represents the electrical model of a real transformer. It allows
engineers to replace the physical transformer with an analytical circuit model for calculations.
From OC Test (performed on LV side usually):
Ro = V₀ / Iw → Core loss resistance (represents iron loss)
Xo = V₀ / Iµ → Magnetizing reactance (represents magnetization requirement)
From SC Test (performed on HV side usually):
Rsc = Wsc / Isc² → Equivalent resistance (represents total copper losses of both
windings)
Xsc = √(Zsc² – Rsc²) → Equivalent leakage reactance (represents leakage flux)
Thus, the per-phase equivalent circuit consists of:
Shunt branch: Ro, Xo (core loss and magnetizing components)
Series branch: Rsc, Xsc (winding resistance and leakage reactance)
?????? Importance: The equivalent circuit is widely used for load flow studies, designing
protective relays, determining transformer performance, and simulating system stability.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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of Vrated.
Use OC results (Ro, Xo) + SC results (Rsc, Xsc) to build the equivalent circuit and
compute efficiency & regulation for any load and power factor.
Derived Parameters & Their Uses
The Open -Circuit Test and Short Circuit Test of a transformer provide practical values for
equivalent circuit parameters. These parameters are not just theoretical; they form the
foundation for performance analysis, design validation, condition monitoring, and
operational planning of transformers. Engineers rely on them to predict behavior under
different loading conditions, ensure reliability, and improve efficiency.
1. Equivalent Circuit of Transformer
The equivalent circuit represents the electrical model of a real transformer. It allows
engineers to replace the physical transformer with an analytical circuit model for calculations.
From OC Test (performed on LV side usually):
Ro = V₀ / Iw → Core loss resistance (represents iron loss)
Xo = V₀ / Iµ → Magnetizing reactance (represents magnetization requirement)
From SC Test (performed on HV side usually):
Rsc = Wsc / Isc² → Equivalent resistance (represents total copper losses of both
windings)
Xsc = √(Zsc² – Rsc²) → Equivalent leakage reactance (represents leakage flux)
Thus, the per-phase equivalent circuit consists of:
Shunt branch: Ro, Xo (core loss and magnetizing components)
Series branch: Rsc, Xsc (winding resistance and leakage reactance)
?????? Importance: The equivalent circuit is widely used for load flow studies, designing
protective relays, determining transformer performance, and simulating system stability.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
https://elentechpk.com/open-and-short-circuit-test-of-transformer/ 15/29
2. Efficiency of Transformer
Efficiency indicates how effectively the transformer converts input power into output power.
η = Output Power / Input Power = (V₂I₂cosφ) / (V₂I₂cosφ + Pi + Pcu)
Where:
Pi = Iron/Core loss (from OC test, Wo) → Constant loss (independent of load)
Pcu = Copper loss (from SC test, Wsc) → Variable loss (depends on load current)
At any load ‘x’ (fraction of full-load):
Pcu(x) = x² × Pcu(FL)
η(x) = (x × V₂I₂cosφ) / (x × V₂I₂cosφ + Pi + x²Pcu(FL))
?????? Importance: Efficiency analysis helps in economic operation, deciding transformer
loading, and energy-saving optimization.
3. Voltage Regulation
Voltage regulation shows the ability of the transformer to maintain a constant secondary
voltage as the load varies.
%Regulation = [(E₂ – V₂) / V₂] × 100
Where:
E₂ = No-load secondary voltage (theoretical EMF)
V₂ = Full-load secondary voltage
Using equivalent parameters:
%Regulation ≈ [(I₂Rsccosφ ± I₂Xscsinφ) / V₂] × 100
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Efficiency indicates how effectively the transformer converts input power into output power.
η = Output Power / Input Power = (V₂I₂cosφ) / (V₂I₂cosφ + Pi + Pcu)
Where:
Pi = Iron/Core loss (from OC test, Wo) → Constant loss (independent of load)
Pcu = Copper loss (from SC test, Wsc) → Variable loss (depends on load current)
At any load ‘x’ (fraction of full-load):
Pcu(x) = x² × Pcu(FL)
η(x) = (x × V₂I₂cosφ) / (x × V₂I₂cosφ + Pi + x²Pcu(FL))
?????? Importance: Efficiency analysis helps in economic operation, deciding transformer
loading, and energy-saving optimization.
3. Voltage Regulation
Voltage regulation shows the ability of the transformer to maintain a constant secondary
voltage as the load varies.
%Regulation = [(E₂ – V₂) / V₂] × 100
Where:
E₂ = No-load secondary voltage (theoretical EMF)
V₂ = Full-load secondary voltage
Using equivalent parameters:
%Regulation ≈ [(I₂Rsccosφ ± I₂Xscsinφ) / V₂] × 100
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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‘+’ sign → Lagging power factor (inductive load)
‘–’ sign → Leading power factor (capacitive load)
?????? Importance: Voltage regulation is critical for ensuring stable supply quality in power
systems. Poor regulation causes voltage drops, reduced performance of equipment, and
potential system instability.
Phasor Diagrams: open and short circuit test of
transformer
1. Open Circuit Test Phasor Diagram
Test Condition: Secondary winding is open, primary is supplied with rated voltage.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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‘–’ sign → Leading power factor (capacitive load)
?????? Importance: Voltage regulation is critical for ensuring stable supply quality in power
systems. Poor regulation causes voltage drops, reduced performance of equipment, and
potential system instability.
Phasor Diagrams: open and short circuit test of
transformer
1. Open Circuit Test Phasor Diagram
Test Condition: Secondary winding is open, primary is supplied with rated voltage.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Current: Only small no-load current (Io) flows.
Components of Io:
Iw → In-phase with applied voltage (V1) → Represents core (iron) loss.
Iµ → Lags voltage by 90° → Magnetizing component.
Input Power: Entirely iron loss, as copper loss is negligible (since Io is very small).
Phasor Relations:
The reference is V1 (applied voltage).
Iw is in phase with V1.
Iµ lags V1 by 90°.
Io is the vector sum of Iw and Iµ.
Core flux (Φ) is proportional to Iµ, lags V1 by nearly 90°.
Diagram (OC Test):
Iµ
↑
|
|
Iw ----->|----> V1 (Reference)
\
\
Io (Resultant)
2. Short Circuit Test Phasor Diagram
Test Condition: Secondary winding shorted, primary supplied with reduced voltage
(Vsc) just enough to circulate full-load current.
Current: Rated short-circuit current (Isc) flows.
Losses: Copper losses are present at the rated value; iron losses are negligible because
the applied voltage (Vsc) is very small.
Phasor Relations:
Reference is I1 (= Isc).
Voltage drop across equivalent resistance (Isc × Rsc) is in phase with current.
Voltage drop across leakage reactance (Isc × Xsc) leads current by 90°.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Components of Io:
Iw → In-phase with applied voltage (V1) → Represents core (iron) loss.
Iµ → Lags voltage by 90° → Magnetizing component.
Input Power: Entirely iron loss, as copper loss is negligible (since Io is very small).
Phasor Relations:
The reference is V1 (applied voltage).
Iw is in phase with V1.
Iµ lags V1 by 90°.
Io is the vector sum of Iw and Iµ.
Core flux (Φ) is proportional to Iµ, lags V1 by nearly 90°.
Diagram (OC Test):
Iµ
↑
|
|
Iw ----->|----> V1 (Reference)
\
\
Io (Resultant)
2. Short Circuit Test Phasor Diagram
Test Condition: Secondary winding shorted, primary supplied with reduced voltage
(Vsc) just enough to circulate full-load current.
Current: Rated short-circuit current (Isc) flows.
Losses: Copper losses are present at the rated value; iron losses are negligible because
the applied voltage (Vsc) is very small.
Phasor Relations:
Reference is I1 (= Isc).
Voltage drop across equivalent resistance (Isc × Rsc) is in phase with current.
Voltage drop across leakage reactance (Isc × Xsc) leads current by 90°.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Vsc is the vector sum of these two drops:
Vsc = Isc × (Rsc + jXsc)
Diagram (SC Test):
jIscXsc (90° lead)
↑
|
|
IscRsc --->|----> Vsc (Resultant)
Key Differences: OC Test vs SC Test (Phasor Perspective)
In the OC test, the applied voltage is rated → current is small → focus on iron loss.
In the SC test, applied voltage is small → current is rated → focus on copper loss.
OC test phasor shows core-loss and magnetizing components of current.
SC test phasor shows equivalent series impedance drops.
Comparison Table: open and short circuit test
of transformer
Feature / Parameter Open Circuit (OC) Test Short Circuit (SC) Test
Purpose
To determine core/iron
losses (Pi), magnetizing
current, and no-load
parameters.
To determine copper losses
(Pc), equivalent resistance
(R), and equivalent
reactance (X).
Connection
Performed on the low-
voltage (LV) side, keeping
the high-voltage (HV) side
open.
Performed on the high-
voltage (HV) side, keeping
the low-voltage (LV) side
shorted.
Applied Voltage
Rated voltage is applied to
the LV side.
A small fraction (5–10%) of
rated voltage is applied to
the HV side to circulate full-
load current.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Vsc = Isc × (Rsc + jXsc)
Diagram (SC Test):
jIscXsc (90° lead)
↑
|
|
IscRsc --->|----> Vsc (Resultant)
Key Differences: OC Test vs SC Test (Phasor Perspective)
In the OC test, the applied voltage is rated → current is small → focus on iron loss.
In the SC test, applied voltage is small → current is rated → focus on copper loss.
OC test phasor shows core-loss and magnetizing components of current.
SC test phasor shows equivalent series impedance drops.
Comparison Table: open and short circuit test
of transformer
Feature / Parameter Open Circuit (OC) Test Short Circuit (SC) Test
Purpose
To determine core/iron
losses (Pi), magnetizing
current, and no-load
parameters.
To determine copper losses
(Pc), equivalent resistance
(R), and equivalent
reactance (X).
Connection
Performed on the low-
voltage (LV) side, keeping
the high-voltage (HV) side
open.
Performed on the high-
voltage (HV) side, keeping
the low-voltage (LV) side
shorted.
Applied Voltage
Rated voltage is applied to
the LV side.
A small fraction (5–10%) of
rated voltage is applied to
the HV side to circulate full-
load current.
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Feature / Parameter Open Circuit (OC) Test Short Circuit (SC) Test
Measurements Taken
Input voltage (V), no-load
current (I0), and no-load
power (W0).
Applied voltage (Vsc),
short-circuit current (Isc),
and power (Wsc).
Type of Loss Measured
Core/iron losses (constant
losses).
Copper losses (variable
losses).
Power Factor of Input
Current
Negligible, as the current is
small.
Approximately unity (since
resistance dominates
reactance under SC).
Equivalent Circuit
Parameters
Determines shunt branch
parameters: R0 (core loss
resistance), X0
(magnetizing reactance).
Determines series branch
parameters: R (equivalent
resistance), X (equivalent
leakage reactance).
Wattmeter Reading Shows iron loss (Wi).
Shows copper loss at full-
load current (Wc).
Current Magnitude
Very small (no-load current
≈ 2–5% of full load).
Full-load current flows
through the windings.
Heat Generation
Conducted for a short
duration to avoid
overheating.
Significant heating due to
full-load current.
Safety
Very safe, since voltage and
current are low.
Requires caution due to
high currents in the
windings.
Duration of Test
It can be conducted for a
long duration without
damage.
Conducted for short
duration to avoid
overheating.
Comparison of Transformer Tests: Direct
Loading vs OC/SC vs Sumpner’s
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Measurements Taken
Input voltage (V), no-load
current (I0), and no-load
power (W0).
Applied voltage (Vsc),
short-circuit current (Isc),
and power (Wsc).
Type of Loss Measured
Core/iron losses (constant
losses).
Copper losses (variable
losses).
Power Factor of Input
Current
Negligible, as the current is
small.
Approximately unity (since
resistance dominates
reactance under SC).
Equivalent Circuit
Parameters
Determines shunt branch
parameters: R0 (core loss
resistance), X0
(magnetizing reactance).
Determines series branch
parameters: R (equivalent
resistance), X (equivalent
leakage reactance).
Wattmeter Reading Shows iron loss (Wi).
Shows copper loss at full-
load current (Wc).
Current Magnitude
Very small (no-load current
≈ 2–5% of full load).
Full-load current flows
through the windings.
Heat Generation
Conducted for a short
duration to avoid
overheating.
Significant heating due to
full-load current.
Safety
Very safe, since voltage and
current are low.
Requires caution due to
high currents in the
windings.
Duration of Test
It can be conducted for a
long duration without
damage.
Conducted for short
duration to avoid
overheating.
Comparison of Transformer Tests: Direct
Loading vs OC/SC vs Sumpner’s
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Test Type Purpose / Features Advantages Disadvantages
Direct Loading
Test
Two identical
transformers are
tested
simultaneously
under full load,
measuring both iron
and copper losses.
Very accurate
results; true
representation of
performance.
Costly, energy-
wasting, not suitable
for large
transformers.
Open Circuit Test
Determines
iron/core losses and
shunt parameters at
rated voltage.
Simple, requires
little power; safe to
perform.
Does not give
copper losses.
Short Circuit Test
Determines copper
losses and series
parameters at
reduced voltage but
full-load current.
Requires very low
voltage;
economical; safe for
loss separation.
Cannot measure
core losses.
Sumpner’s Test
(Back-to-Back
Test)
Two identical
transformers tested
simultaneously
under full load,
measuring both iron
and copper losses.
Most efficient;
accurate
measurement of
efficiency and
regulation under
load.
Requires two
identical
transformers; not
always feasible.
Safety & Precautions: open and short circuit
test of transformer
✅ Do’s
Ensure proper earthing/grounding of the transformer before starting any test.
Use properly calibrated instruments (wattmeter, ammeter, voltmeter) rated for the
test conditions.
Apply rated voltage carefully in the OC test (on the LV side).
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Direct Loading
Test
Two identical
transformers are
tested
simultaneously
under full load,
measuring both iron
and copper losses.
Very accurate
results; true
representation of
performance.
Costly, energy-
wasting, not suitable
for large
transformers.
Open Circuit Test
Determines
iron/core losses and
shunt parameters at
rated voltage.
Simple, requires
little power; safe to
perform.
Does not give
copper losses.
Short Circuit Test
Determines copper
losses and series
parameters at
reduced voltage but
full-load current.
Requires very low
voltage;
economical; safe for
loss separation.
Cannot measure
core losses.
Sumpner’s Test
(Back-to-Back
Test)
Two identical
transformers tested
simultaneously
under full load,
measuring both iron
and copper losses.
Most efficient;
accurate
measurement of
efficiency and
regulation under
load.
Requires two
identical
transformers; not
always feasible.
Safety & Precautions: open and short circuit
test of transformer
✅ Do’s
Ensure proper earthing/grounding of the transformer before starting any test.
Use properly calibrated instruments (wattmeter, ammeter, voltmeter) rated for the
test conditions.
Apply rated voltage carefully in the OC test (on the LV side).
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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In the SC test, apply only a small fraction of the rated voltage to circulate full-load
current.
Keep protective devices (fuses, circuit breakers) in the circuit during testing.
Continuously monitor instruments during the test.
Keep the SC test duration short to prevent overheating.
Ensure the test is performed by trained personnel under supervision.
❌ Don’ts
Do not touch transformer terminals or connections while energized.
Do not apply full rated voltage in SC test (can damage windings).
Do not exceed the rated current during testing.
Do not allow excessive heating of windings in the SC test.
Do not use damaged or uncalibrated instruments.
Do not perform the test without proper isolation and safety barriers.
Do not leave the transformer energized unattended during the test.
Do not conduct tests in wet or unsafe environments.
FAQ
What is the purpose of the Open-Circuit (OC) Test in a transformer?
Answer: The OC test is used to determine the iron/core losses and the parameters of
the shunt branch (magnetizing reactance and core loss resistance) of the transformer.
What is the purpose of the Short-Circuit (SC) Test in a transformer?
Answer: The SC test is performed to determine the copper losses at full load and the
equivalent series parameters (equivalent resistance Rsc and leakage reactance Xsc) of
the transformer.
Why is the OC test carried out on the LV side of the transformer?
Answer: The OC test is done on the Low Voltage (LV) side because rated voltage
can be applied easily and safely, while the High Voltage (HV) side remains open.
Why is the SC test carried out on the HV side of the transformer?
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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current.
Keep protective devices (fuses, circuit breakers) in the circuit during testing.
Continuously monitor instruments during the test.
Keep the SC test duration short to prevent overheating.
Ensure the test is performed by trained personnel under supervision.
❌ Don’ts
Do not touch transformer terminals or connections while energized.
Do not apply full rated voltage in SC test (can damage windings).
Do not exceed the rated current during testing.
Do not allow excessive heating of windings in the SC test.
Do not use damaged or uncalibrated instruments.
Do not perform the test without proper isolation and safety barriers.
Do not leave the transformer energized unattended during the test.
Do not conduct tests in wet or unsafe environments.
FAQ
What is the purpose of the Open-Circuit (OC) Test in a transformer?
Answer: The OC test is used to determine the iron/core losses and the parameters of
the shunt branch (magnetizing reactance and core loss resistance) of the transformer.
What is the purpose of the Short-Circuit (SC) Test in a transformer?
Answer: The SC test is performed to determine the copper losses at full load and the
equivalent series parameters (equivalent resistance Rsc and leakage reactance Xsc) of
the transformer.
Why is the OC test carried out on the LV side of the transformer?
Answer: The OC test is done on the Low Voltage (LV) side because rated voltage
can be applied easily and safely, while the High Voltage (HV) side remains open.
Why is the SC test carried out on the HV side of the transformer?
9/25/25, 4:48 PM Top 10 Powerful Insights into Open and Short Circuit Test of Transformer - Elentechpk
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Answer: The SC test is conducted on the High Voltage (HV) side because only a
small fraction of rated voltage is required to circulate the full-load current, making it
safer and more practical.
What losses are measured in the OC test?
Answer: Only iron losses (hysteresis and eddy current losses) are measured, as
copper loss is negligible due to very small no-load current.
What losses are measured in the SC test?
Answer: Only copper losses are measured, as iron losses are negligible because the
applied voltage is very small compared to the rated voltage.
. What is the significance of Vsc in SC test?
Answer: The applied short-circuit voltage (Vsc) helps in calculating the equivalent
resistance (Rsc) and equivalent reactance (Xsc) referred to the test side of the
transformer.
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small fraction of rated voltage is required to circulate the full-load current, making it
safer and more practical.
What losses are measured in the OC test?
Answer: Only iron losses (hysteresis and eddy current losses) are measured, as
copper loss is negligible due to very small no-load current.
What losses are measured in the SC test?
Answer: Only copper losses are measured, as iron losses are negligible because the
applied voltage is very small compared to the rated voltage.
. What is the significance of Vsc in SC test?
Answer: The applied short-circuit voltage (Vsc) helps in calculating the equivalent
resistance (Rsc) and equivalent reactance (Xsc) referred to the test side of the
transformer.
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Website
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ElentechPK is a dedicated platform for
electricians,
AC motor
basic electricity
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Gejet
Indian daily news
International news
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Other motor
physics
Transformer
Uncategorized
About us
Contact
Disclaimer
Privacy Policy
Services
Turm and condition
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