Current Transformer Types & Current Transformers

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A Current Transformer (CT) detects electric currents in circuits to generate signals proportional to that current and the signals can be displayed, measured or stored for further analysis. They will measure AC and DC current – Current Transformers are of two main types, open or closed loop. The open-loop types measure AC & DC currents giving electrical isolation between the circuit being measured and the output of the CT. The primary current is measured without electrical contact with the primary circuit and this gives galvanic isolation. Open loop current transformers are usually preferred in battery-powered circuits for their low power needs and small footprint features. Open-loop current transformers are often cheaper than closed loop CT’s.

  • Magnetoresistive Current Transformer – The property of certain materials changes the value of its resistance when a current flows through the plate. When currents flow for a longer distance, resistance is increased and decreased at the shorter distance.
  • Hall Effect Current Transformer – These devices have a core, Hall Effect device and signal conditioning circuitry. When the Hall device is exposed to magnetic field from the core it produces a potential difference  which can be measured and amplified. Current transformer work when a current conductor passes through a magnetically permeable core that concentrates the conductors magnetic field.
  • Inductive Current Transformer – This type uses coils which current-carrying wires pass. Making power that is proportional to the current flow in the coil. The current flow then can be measured or transformed.

Current Transformer

Current transformers change the voltage of an alternating circuit by connecting two circuits at a magnetic core. What defines how the energy-input circuit to the energy-output circuit the voltage changes is the ratio of windings the two circuits make around the core. Current Transformers use can be placed into two broad categories: signal matching and power supply.

Step-up Transformer – The step-up current transformer increases voltage while reducing current as it has a greater number of windings on the side it outputs energy. Take the cathode ray tube screen as an example, it requires thousands of volts, and runs off a 110V wall socket.

Step-down Transformer – A step-down transformer is used to reverse the winding ratio and this is shown when a battery-operated device can also be plugged into a wall socket. Meaning the radio may run on 12V batteries and also be able to run on 11V adapters with a step-down transformer.

Isolation Transformer – Isolation transformers can step up and down voltage but they don’t usually. Isolation current transformers can serve a number of purposes. They break a circuit into a primary and a secondary – This disallows direct-current noise through and they can also prevent capacitance buildup between the primary and secondary, this will cause a high-frequency noise. They stop accidental ground connections from the primary and secondary. It can prevent shock and inadvertent grounding from high voltage discharge by isolating the secondary circuit from the primary’s current.

Variable Auto-Transformer – A variable auto-transformer will change the voltage to the secondary (energy-output) circuit and the primary and secondary’s number of windings vary with a dial. The proximity of the primary and secondary windings are used with low voltages as this will prevent arcing alike the Current Transformer.

Signal Matching – Signal current transformers are used to convey frequencies from one circuit to a different one. As communication electrical like signal matching transformers use low power level this means power loss is a concern. Signal transformers are selected or tuned based on the impedance of other components in the two circuits to achieve maximum impedance matching,

current transformer

Open Loop Transformers – Open loop current transformers have a Hall sensor mounted in the air gap of a magnetic core and conductors give a magnetic field that is comparable to the current. The magnetic field is measured by the Hall sensor current transformer and concentrated by the core. The signal from the Hall generator is amplified as it is low, this amplified signal becomes the current CT output. Normally a open-loop current transformer has circuitry that provides temperature compensation and calibrated high-level voltage output. While these CT’s are cheaper than their counterparts, they can be prone to saturation and temperature drift. However by injecting a positive coefficient in the control current to reduce the drift in sensitivity over temperature the drift can be minimised.

Closed loop Transformers – A Closed-loop current transformers provide electrical isolation, measure AC and DC currents and they ideal for their fast response, low temperature drift and high linearity. The closed-loop current output is quite immune to electrical noise and as Hall-Effect transformer feeds back an opposing current into a secondary coil, the Closed- Loop CT is sometimes called a ‘Zero-Flux’ sensor. The secondary coil of the current transformer is wound on the magnetic core to zero the flux made in the magnetic core by the primary current. When high accuracy is priority, closed-loop current transformers are a good choice of CT.


Current Transformer Winding

As the primary winding is in series with the current carrying conductor supplying a load the current transformer is also known as a “series transformer”. However the secondary winding may be wound on a laminated core of low-loss magnetic material however and have more coil turns. So that the density of the magnetic flux created is low using smaller cross-sectional area wire, the core has a large cross-sectional area. This is dependent upon how much the current needs to be stepped down as it attempts to output a constant current which is independent of the connected load of the current transformer.

Different to the voltage transformer, the primary current of a current transformer is controlled by an external load and is not dependent of the secondary load current. The rating of the secondary current is usually at a standard 1 Ampere or 5 Amperes for larger primary current ratings. Accurate and small instruments can be used with CT’s being insulated away from any high-voltage power lines because Current transformers can reduce or “step-down” current levels from thousands of amperes down to a standard output of a ratio to 5 Amps or 1 Amp for normal operation.

The Current Transformer is different to the voltage and power transformer as its primary winding has only one or few turns. This primary winding can be:

  • A coil of heavy duty wire wrapped around the core
  • A conductor or bus bar placed through a central hole or
  • A single flat turn, or just as shown.
current transformers

Customer Support

Our team have years of experience in dealing with our products and are happy to give advice on current transformers and their uses so call or email anytime to speak to our specialists.

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Accurate Current Transformers

The current transformers we stock are built for accuracy and effective current reading. You can trust our products to be affordable and optimal in any current sensor task.

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Quick Reading Transformers

Current transformers from Manutech do not waste time as they are built to perform and read current in fast time scales giving you the current transformer reading you need when you need it.

Usually the current transformers secondary winding connects to its burden which is closed at all times. The current flows through the primary and secondary winding and amperes turns of each winding are opposite and equal. The secondary turns of a current transformer is always 1% and 2% less than the turns of the primary winding and the difference is used within magnetising core. This means that if the secondary winding is opened and the current flows through the primary windings, due to the secondary current there not be a de-magnetizing flux. The value of the load connected across the secondary transformer defines the burden of a current transformer and this is shown as the output in volt-amperes (VA). The burden on the nameplate of the Current transformer defines the value of the rated burden and it is the product of the voltage and current on the secondary when the current transformer supplies the instrument.

We know the secondary of a current transformer connects across ammeter, that has a small resistance and the voltage drop of the secondary winding is 1.0 volts. Removing the ammeter basically makes the secondary winding open-circuited making the transformer act as a step up current transformer. This is because of the secondary core’s very large increase in magnetising flux as the secondary induced voltage is influence by the leakage reactance. This would be prevented with an opposing current in the secondary winding.

The outcome is a very high voltage induced within the secondary winding with a ratio reading:  Vp(Ns/Np) which is developed across the CT secondary Winding. This is as the volts per turns ratio is near constant in both primary and secondary windings. This is why when the main primary current is flowing through it a current transformer should not be left open-circuited or operated when no-load is attached. Alike how a voltage transformer should not operate into a short circuit. To take away the risk of shock the ammeter (or load) should be removed and a short-circuit should be placed across the secondary terminalsprior.

The secondary being open-circuited while iron core of the transformer operates at a high degree of saturation and with nothing to stop it causes the high voltage. It makes a large secondary voltage and this high secondary voltage can cause electric shock if the CT’s terminals are touched and the Current transformer can be damaged.

High Voltage Current Transformer

Current transformers are used in high-voltage electrical substations and electrical grids for protection, control and measuring. Current transformers are commonly free-standing and outdoor current transformers but can be installed inside in apparatus bushings or switchgear. CT’s have a substantial part of their enclosure energized at the line voltage In a switchyard and they have to be mounted on insulators. Dead Tank current transformers work by isolating the circuit that is measured from the enclosure. The Live tank Current transformer is useful as the primary conductor is short making better stability and making the higher short-circuit rating of current. The primary of the winding can be evenly given out around the magnetic core and this also gives better overload and transient performance. If the live-tank current transformers major insulation is not exposed primary conductors heat then the insulation life and thermal stability is greater.

A high-voltage current transformer can have several cores that all have a secondary winding, and for different reasons(whether metering circuits, protection or control). Neutral current transformers are used for earth fault protection measuring the neutral point of a current transformer.

Current Transformer

ACT-0140 Current Transformer is a split core CT for measuring current and the input ratings vary from 1500 and 2400 Amps. The corresponding output is 5A. The transformer has been made using high quality, durable materials and installs perfect on an electric wire as it snaps around the conductor carrying the current to be measured.

Current transformer Input up to 2,400 Amps
Output 5A at rated current
UL Registered, CE and RoHS compliant

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