Control Transformer

What is a Control Transformer?

A control transformer is the power supply for circuits which include controlling and switching components. Control circuits are part of the auxiliary circuits. Auxiliary circuits are galvanically isolated from the main circuits by the control transformer. The characteristics of a control transformer as well as its areas of application are defined in a number of standards.

In Europe the standard EN 61558 is valid for transformers with a maximum input voltage of 1100 V. The standard includes two parts. The first part includes general requirements. The second part includes several standards of which each describes a type of a transformer. The special requirements for control transformers are found in DIN EN 61558-2-2. The VDE guideline for control transformers is VDE 0570 Part 2-2.

Control Transformer 400v 230v

Electric components and devices in a circuit have to be supplied with a certain voltage. The control transformer has an output voltage which has to match the required voltage of the control circuit. A control transformer 24V has an output voltage of 24V. Depending on the type a control transformer can support a different output voltage which is common in industrial applications. This includes one phase control transformer 24V AC, one phase control transformer 115V AC and one phase control transformer 230V AC. An example for output voltage on the three phase control transformer 3x400 V AC. The control transformer 400V 230V is available with an input voltage of 400V or 230V AC.

Inside of the control transformer the coils of the primary circuit and secondary circuit have to be seperated. An autotransformer therefore can not be used as a control transformer.

When do you have to use a Control Transformer?

The electric components of machines and plants require a power supply. The requirements to the power supply are standardized for several applications. DIN VDE 0113 part 1 defines cases which require the use of a control transformer. If machines require to be equipped with more than 5 contactors a control transformer should be used for power supply in the control circuit.

Control Transformer Tasks

In addition to the requirements to use a control transformer mentioned in standards further requirements may be of importance when deciding if a control transformer shall be used. If a control transformer is used a specific control voltage can be set which is an advantage for the configuration of a machine since it permits the use of proven and known components in the control circuit.

Control Transformer protects Control Units

If an electric plant uses different voltages, a control transformer simplifies adjustments.
The cost of investment for a control unit (SPS) for a machine can be very high. A control transformer helps to protect this investment.

Control Transformer 24V

If an electric engine requires high voltages it may be too high for safe operation of components in the control circuit. The protection of people is simplified when the voltage in the control circuit is lowered by using a control transformer. For example if the motor voltage in the main is 600V it is possible to lower it to 24V for the SPS in the control circuit.

By seperating main circuit and control circuit with a control transformer the components of the control circuit are protected when a voltage test in accordance to VDE 0570/2-2 has to be conducted.

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