Current Transformer vs. Voltage Transformer: Key Differences

This article explains the basic differences between current transformers (CTs) and voltage transformers (also known as potential transformers or PTs). Instrument transformers are essential in AC systems for measuring current, voltage, power, and energy. They’re also vital in protection circuits, triggering relays of various types. The transformation ratio represents the ratio of the primary phasor’s magnitude to the secondary phasor’s magnitude.

Current Transformer (CT)

• The primary winding of a CT is connected in series with the load.
• It carries the load current that needs to be measured.
• The primary winding typically has very few turns, while the secondary winding has more turns.
• The measuring instrument is directly connected across the secondary winding.

Formulas:

• Transformation Ratio for C.T. = (Primary Winding Current) / (Secondary winding current)
• Turns Ratio for C.T. = (Number of turns of secondary winding) / (Number of turns of primary winding)

Voltage Transformer (Potential Transformer - PT)

• These are used to measure high voltages using lower-range voltmeters.
• The primary winding of a PT is connected across the line whose voltage is to be measured.
• The voltage circuit (voltmeter) is connected across the secondary winding.

Formulas:

• Transformation Ratio for voltage transformer = (Primary Winding voltage) / (Secondary winding voltage)
• Turns Ratio for P.T. = (Number of turns of primary winding) / (Number of turns of secondary winding)

Key Differences Summarized