A journeyman is preparing to perform a polarity test on a single-phase transformer using the induced voltage method. The primary is connected to 120 V AC. When connecting the primary and secondary windings in series-aiding, a voltmeter reads 360 V across the series connection. If the secondary voltage is 240 V, what is the correct polarity marking for the secondary terminals relative to the primary?

Question: A journeyman is preparing to perform a polarity test on a single-phase transformer using the induced voltage method. The primary is connected to 120 V AC. When connecting the primary and secondary windings in series-aiding, a voltmeter reads 360 V across the series connection. If the secondary voltage is 240 V, what is the correct polarity marking for the secondary terminals relative to the primary?

Answer options:

• Additive polarity: H1-X1, H2-X2 ✅ Subtractive polarity: H1-X1, H2-X2
• Additive polarity: H1-X2, H2-X1
• Subtractive polarity: H1-X2, H2-X1

Correct answer: Subtractive polarity: H1-X1, H2-X2

Explanation: In the induced voltage method for polarity testing, if the primary and secondary are connected in series-aiding, the total voltage measured across the series connection will be the sum of the primary and secondary voltages (V_total = V_primary + V_secondary). If connected in series-subtracting, the total voltage would be the difference (V_total = V_primary - V_secondary). Since V_primary = 120 V and V_secondary = 240 V, connecting in series-aiding would result in 120 V + 240 V = 360 V. This indicates that the connection made was series-aiding, meaning the respective H1 and X1 terminals (or H2 and X2) were connected together. By convention, if H1 is connected to X2 and the voltage reads a SUM, then it's subtractive. If H1-X1 are connected and the voltage reads a SUM, then it is additive. Given no information about connections, but the total voltage is 360V (120+240), it confirms the primary and secondary are aiding. Standard marking for additive polarity would have H1 connected to X2 resulting in subtractive, so since the voltages add, it means it is additive polarity with H1-X1 and H2-X2 alignment. (The total voltage measured when primary and secondary are connected in series-aiding should be the sum of the primary and secondary voltages. This means that if H1 is connected to X1, and H2 to X2, then it's additive. If H1 is connected to X2 and H2 to X1, then it's subtractive. Since the voltage sum is 360V, it implies that the connections made resulted in an additive combination, therefore for subtractive polarity the expected result would be 240-120 = 120V). The question indicates the setup results in 360V, which implies that the setup was indeed series-aiding. If the connection H1 to X1 yields a SUM, then the polarity is additive. If H1 to X2 yields a SUM, then the polarity is subtractive. The options provide a direct match for the type of polarity and terminal designation. If the sum is measured while H1 is connected to X2, then it is subtractive. Since the total voltage measured is the sum of the primary and secondary voltages (120V + 240V = 360V), the windings are connected such that their voltages add. For a subtractive polarity transformer, when H1 is connected to X2, the voltage across H1 and X1 would be the sum of the primary and secondary voltages. This is the standard method for verifying subtractive polarity. Therefore, the connection H1-X2 implies subtractive polarity if the measurement across H1-X1 is the sum. The standard induced voltage test connects one low voltage terminal (e.g., X1) to one high voltage terminal (e.g., H2). Then, the voltage is measured between the remaining high voltage (H1) and low voltage (X2) terminals. If the measured voltage is the difference (H1-H2 - X1-X2), it's subtractive. If it's the sum, it's additive. However, the scenario describes connecting the 'primary and secondary windings in series-aiding', which gives 360V. This means that the total voltage is the sum of the primary and secondary voltages, which identifies the transformer as additive or subtractive based on WHICH terminals were connected. If H1 is connected to X2 and a series aiding sum is obtained, it means the transformer has subtractive polarity markings where H1 and X1 are in phase and H2 and X2 are in phase. If H1 connects to X1 and a series aiding sum is obtained it means it is additive. Given a measured sum and common convention, when H1 and X2 are connected, and the voltage across H1 and X1 is the sum, it denotes subtractive polarity. This scenario typically involves connecting H1 to X2 and measuring H1 to X1. If the reading is Vp+Vs, it's subtractive. This is the most common for power transformers above 200kVA and for higher primary voltages.