There is a common misconception that current transformers (CTs) and voltage transformers (VTs) are the same. Both devices are used to monitor and measure electrical current, they each have unique capabilities and purposes. In this article, we’ll explore the key differences between CTs and VTs so you can decide which device is suitable for your needs.
A current transformer is defined as “an instrument transformer in which the secondary current is generally proportional to the primary current (under normal conditions of operation) and differs from it by an angle that is approximately zero for an appropriate direction of the connection of the secondary winding for the primary winding.” Therefore, it acts like a turn-ratio transformer, but the secondary winding has a much lower impedance than an ordinary transformer. The CT is used to change high levels to low secondary currents while maintaining a close relationship between the two. This is done while keeping the voltages relatively constant.
A voltage transformer is an instrument transformer in which the secondary winding voltage is (nearly) proportional to the primary winding voltage, regardless of the current in either winding. The transformer operates on the principle that when there is a change in the voltage (potential difference) across the primary coil, there will be an induced electromotive force (EMF) in the secondary coil due to electromagnetic induction.
Differences Between Current And Potential Transformer
- Transformation Ratio: The current transformer has a much lower transformation ratio than the voltage transformer. The typical range of the current transformer is 1 to 1000, while that of the potential transformer is 1 to 10.
- Primary and Secondary Winding: The primary winding is connected in series with the line, while the secondary winding is connected to the measuring instrument in the current transformer. While in the potential transformer, both windings are either isolated or parallel.
- Purpose: The purpose of a current transformer is to change the heavy current in the primary circuit to a small current in the secondary course, while the definition of a potential transformer is to change the high voltage in the primary winding to a low voltage in the secondary winding. Current transformers (CTs) and potential transformers (PTs) are instrument transformers used for measuring or sensing currents and voltages, respectively, in electrical circuits.
- Primary Current: A current transformer can carry the primary current while a potential transformer cannot.
- Secondary Side: The secondary side of a current transformer is always kept open, while the secondary side of a potential transformer is either empty or short-circuited.
- Number of Phases: A potential transformer can be single-phase or three-phase, while a current transformer is always three-phase.
- Input Value: The input value for a current transformer is always in the form of alternating current (AC), while the input value for a potential transformer can be either alternating current (AC) or direct current (DC).
- Secondary Winding Range: The secondary winding of a voltage transformer is always designed to produce a safe, specified voltage under all conditions within the specified range of transformer operation. The secondary winding of a current transformer is designed to carry the total rated current of the primary winding at all times, regardless of the voltage applied to the secondary circuit.
Current Transformer Connections
Current transformer connections are made in such a way that the primary is always connected in series with the load or voltage source while the secondary is always connected to the ammeter. The turn ratio is generally kept at 1:5 for convenience. This means that there are five turns of the secondary winding for every turn of the primary winding.
On the other hand, voltage transformer connections are made so that the primary is permanently connected across the load while the secondary is always connected to the voltmeter. The turn ratio is also generally kept at 1:5 for convenience. This means that there are five turns of the secondary winding for every turn of the primary winding.
The main difference between current transformers and voltage transformers is that current transformers are used to measure currents, while voltage transformers are used to measure voltages. Both devices have advantages and disadvantages, so choosing the right one for the application is essential. In general, current transformers are more accurate than voltage transformers but are also more expensive. Voltage transformers are less accurate, but they are cheaper.