Polarity Test of Single Phase Transformers

The polarity test of a single phase transformer is a fundamental electrical test performed during transformer commissioning and maintenance to determine the relationship between primary and secondary windings. This test is essential for ensuring safe parallel operation of transformers, correct three-phase bank connections (three single phase transformers), and proper functioning of protective relaying and metering systems.

In this blog post, we will explore what transformer polarity is, why it matters, the two types of polarity (additive and subtractive), detailed testing procedures, safety precautions, and practical considerations for commissioning and testing applications.

What is Transformer Polarity?

Transformer polarity refers to the instantaneous phase relationship between the primary and secondary windings of a transformer at any given moment. It describes which terminals on the primary and secondary sides have the same instantaneous polarity (positive or negative) at a particular instant in time.

In alternating current (AC) systems, the polarity of transformer terminals constantly changes. However, at any specific instant, one terminal is positive and the other is negative. The polarity test identifies which terminals on the primary and secondary sides have matching instantaneous polarities.

Terminal Identification and Marking

Transformer terminals follow standardized naming conventions established by ANSI (American National Standards Institute) and other international standards:

Primary Winding Terminals: Labeled as H1 and H2 (High Voltage terminals)
Secondary Winding Terminals: Labeled as X1 and X2 (Low Voltage terminals)
Even-numbered terminals (H2, X2) have the same instantaneous polarity
Odd-numbered terminals (H1, X1) have the same instantaneous polarity

Many transformer manufacturers also use dot markings or dot convention to indicate polarity in schematic diagrams. When both dots are placed at the same relative position (e.g., both at the top), it indicates that current entering the dotted terminal of the primary winding will induce a voltage with the same polarity at the dotted terminal of the secondary winding.

Types of Transformer Polarity

1. Additive Polarity

Additive polarity occurs when the primary and secondary terminals with the same instantaneous polarity are positioned on the same side of the transformer (when viewing from the front). In this configuration, when terminals on the same side are connected together, the primary and secondary voltages add together.

Characteristic Equation for Additive Polarity:

\(V_{Total}=V_{1}+V_{2}\)

Where:

\(V_1\) = Primary winding voltage
\(V_2\) = Secondary winding voltage
\(V_{Total}\) = Voltage measured between opposite terminals

Example: If a 230V/115V transformer with additive polarity has the primary terminal connected to the secondary terminal on the same side, and you measure the voltage between the other terminals (H2 to X2), you would read approximately 345V (230V + 115V).

Terminal Configuration: H1 and X1 are on the left; H2 and X2 are on the right (when facing the low-voltage side).

2. Subtractive Polarity

Subtractive polarity occurs when the primary and secondary terminals with the same instantaneous polarity are positioned on opposite sides of the transformer. In this configuration, the primary and secondary voltages subtract from each other.

Characteristic Equation for Subtractive Polarity:

\(V_{Total}=V_1−V_2\)

Where:

\(V_1\) = Primary winding voltage
\(V_2\) = Secondary winding voltage
\(V_{Total}\) = Voltage measured between opposite terminals

Example: If the same 230V/115V transformer has subtractive polarity, measuring the voltage between H2 and X2 would give approximately 115V (230V – 115V).

Terminal Configuration: H1 and X2 are on the same side (left); H2 and X1 are on the opposite side (right).

For step-down transformers in subtractive polarity, \(V_{Total}=V_1−V_2\). For step-up transformers, the calculation becomes \(V_{Total}=V_2−V_1\)

Why Polarity Test is Important

1. Parallel Operation of Transformers

When two or more transformers are connected in parallel to share electrical load, matching polarities are mandatory. If transformers with opposite polarities are incorrectly connected in parallel:

The secondary windings develop voltages that are 180° out of phase
A massive circulating current flows between the transformers
This creates a virtual dead short circuit between the transformer secondaries
The circulating current causes extreme overheating, insulation breakdown, and transformer burnout
System protective devices (circuit breakers, fuses) may trip, causing power interruption

2. Three-Phase Bank Connection

When combining three single-phase transformers to create a three-phase system, correct polarity of each transformer is essential for:

Proper phase-angle relationships
Correct voltage and current magnitudes at the three-phase output
Prevention of unbalanced voltages and currents
Avoiding resonance conditions that could damage equipment

3. Protective Relaying and Metering

In metering and protective relay systems:

Current transformers (CTs) and potential transformers (PTs) must have correct polarity
Incorrect polarity causes protective relays to malfunction or not operate during faults
Energy meters and power factor meters will give incorrect readings

4. System Safety and Equipment Protection

Correct polarity ensures:

Safe parallel operation without excessive circulating currents
Prevention of short circuits caused by reversed connections
Proper operation of the electrical system at design voltage levels
Extended transformer lifespan and reduced maintenance costs
Personnel safety by preventing unexpected high currents and equipment damage

Polarity Test Procedure for Single Phase Transformer

Equipment and Apparatus Required

Equipment	Specification	Quantity
Single Phase Transformer	Rated transformer under test	1
Autotransformer (Variac)	0-250V AC, rated for test current	1
Voltmeter 1 (V1)	0-300V AC, preferably digital	1
Voltmeter 2 (V2)	0-300V AC, preferably digital	1
Voltmeter 3 (V3)	0-1000V AC or higher if additive polarity expected	1
Connecting Wires	Properly insulated PVC wires	As required
Safety Equipment	Gloves, goggles, mat	1 set

Detailed Step-by-Step Procedure

Step 1: Initial Preparation and De-energization

Ensure the transformer is completely de-energized and isolated from all power sources
Use a voltage tester to verify that no residual voltage remains in the transformer
Check all insulation on connecting wires and test equipment
Ensure proper grounding of all test equipment
Wear appropriate personal protective equipment (PPE) including safety gloves and goggles

Step 2: Circuit Connection for Polarity Test

Single Phase Transformer Polarity Test Practical Connection Diagram

Connection of Voltmeters:

Connect Voltmeter V1 (0-300V AC) across the primary winding terminals (H1 to H2)
Connect Voltmeter V2 (0-300V AC) across the secondary winding terminals (X1 to X2)
Connect Voltmeter V3 (high range 0-1000V) between primary and secondary terminals:
- Positive terminal of V3 → Connected to H1 (primary)
- Negative terminal of V3 → Connected to X1 (secondary)

The polarity of V3 connection is critical. Make sure the positive probe connects to the primary terminal and the negative probe to the secondary terminal as specified.

Step 3: Apply Reduced Voltage to Primary Winding

Connect the autotransformer (Variac) to the AC supply (230V, 50Hz or 60Hz depending on location)
Set the autotransformer to ZERO position before switching on the supply
Connect the primary winding terminals (H1 and H2) to the autotransformer output
Gradually increase the autotransformer voltage to apply approximately 50-120 volts to the primary winding
DO NOT apply full rated voltage to the primary—this could damage secondary insulation if polarity is tested with high voltage

Step 4: Record Voltage Readings

Once the test voltage is stabilized, carefully record all three voltmeter readings:

V1: Voltage across primary winding (H1 to H2)
V2: Voltage across secondary winding (X1 to X2)
V3: Voltage between primary and secondary terminals (H1 to X1)

Example Values:

V1 = 100V (primary voltage)
V2 = 50V (secondary voltage)
V3 = ? (depends on polarity)

Step 5: Determine Polarity Based on V3 Reading

Analysis of Results:

If V3 = V1 + V2 (approximately 150V in our example)
→ The transformer has ADDITIVE POLARITY
→ H1 and X1 have the same instantaneous polarity

If V3 = V1 – V2 (approximately 50V in our example)
→ The transformer has SUBTRACTIVE POLARITY
→ H1 and X2 have the same instantaneous polarity

Tolerance for Results: Allow ±2-3% tolerance in voltage readings due to instrument accuracy and transformer impedance effects.

Step 6: Mark Terminal Polarity

After determining polarity type:

Mark the transformer terminals with permanent markers or stickers indicating polarity
Use dot markings (●) to indicate same-polarity terminals
For example, if additive: mark H1 and X1 with dots
Document findings in the transformer test logbook or commissioning report

Step 7: De-energize and Disconnect

Gradually reduce autotransformer voltage to ZERO before switching off supply
Wait a few moments to allow any residual charge to dissipate
Disconnect all test equipment and connecting wires
Remove transformer from test setup safely

Alternative Testing Methods

DC Kick Test Method

The DC kick test (also called battery kick test) is a quick method to verify polarity without applying full AC voltage:

Procedure:

Connect a low-voltage DC source (9V battery) momentarily to the primary terminals
Connect an analog meter to the secondary terminals with specified polarity
Close the battery circuit momentarily (for less than 1 second)
Observe meter needle deflection:
- Positive deflection = Correct polarity assumption
- Negative deflection = Incorrect polarity (reversed)

Advantages:

Quick and non-destructive
Requires minimal equipment
Safer than AC voltage testing
Can be used on rewound or damaged transformers

Disadvantages:

Cannot determine polarity type (additive vs. subtractive) precisely
Requires experience in interpreting needle movement
May not work reliably on transformers with very high impedance

Jumper Connection Method

This method involves temporarily connecting specific terminals: