Transformers used in conjunction with measuring instruments for the measurement of electrical quantities like current,
voltage, power, frequency, and power factor are called instrument transformers. Instrument transformers are used to provide isolation between the power circuit and the control circuit. They are also used in conjunction with protective relays.
Instrument transformers are of two types. They are,
1. Current transformer – measures high current
2. Potential transformer – measures high voltage
In this article, the differences between the current transformer and the potential transformer are presented.
Current Transformer (CT)
Potential Transformer (PT)
The current transformer is used for measuring high current.
The potential transformer is used for measuring high voltage.
It is a step-up transformer. It steps down a high current into a low current.
It is a step-down transformer. It steps down high voltage into low voltage.
The primary winding of CT is of few turns (sometimes, a single turn) to carry the current to be measured.
The primary winding of PT has a large number of turns of fine wire and is connected to the voltage to be measured.
The primary winding carries a full line current.
The primary winding is impressed with full line voltage.
A small voltage exists across its input terminals as it is connected in series with the supply.
Full line voltage appears across its input terminals.
The primary current is independent of secondary circuit conditions.
The primary current depends on the secondary circuit conditions.
The secondary winding consists of more turns of thinner wire and is connected across the ammeter of the lower range.
The secondary winding consists of a few numbers of turns of thick wire and is connected across the low-range voltmeter.
The secondary CT must never be operated on open circuit conditions.
The secondary PT is nearly used under open circuit conditions.
The secondary of CT must always be shorted to avoid the induction of high voltage in the multiturn secondary.
The secondary winding should be completely insulated from the high-voltage primary and should be grounded for safety purposes.
CT can be treated as a series transformer under short circuit conditions.
PT can be treated as a parallel transformer under open circuit conditions.
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