A 150 kVA transformer with a primary rating of 9000 V would be additive or subtractive?

• A. Additive
• B. Subtractive
• C. Cannot determine
• D. Both

Answer: (B)

In an autotransformer, whether voltages add or subtract depends on how the windings are connected in series, which is determined by their polarity. If you connect the windings so their voltages aid each other, you get an additive connection—the total voltage across the series combination is the sum of the two winding voltages. If you connect them so the voltages oppose each other, you get a subtractive connection—the net voltage is the difference between the two.

With a primary rating of 9 kV, using the transformer in a configuration where you want to deliver a lower voltage without overstressing the windings typically requires subtractive polarity. For example, if the secondary winding is 3 kV, an additive connection would give 12 kV across the series pair, while a subtractive connection would give 6 kV. To stay within the winding ratings and provide a practical output, the connection is arranged subtractively.

The 150 kVA rating describes how much apparent power the device can handle, but it doesn’t dictate whether the connection is additive or subtractive. The subtractive arrangement is about the winding polarity and how the voltages combine in series.