What is a control power transformer?

A Control Power Transformers refers to a two-winding transformer that converts an electrical voltage from one value to another using the concept of mutual inductance. The Control Power Transformers help to control/switch the main circuit components. Therefore, the transformer that provides a lower or higher voltage supply to the industrial controls for control circuits like motor starters, controllers, etc., refers to as Control Power Transformers.

Overview of the control transformer

A control transformer is a stationary machine that changes low AC voltage to high AC or high AC voltage to low AC voltage. A control transformer is based on Faraday’s law of mutual induction principle.

Consider a coil as shown in the figure above; if an alternating voltage is applied to coil 1, the current through coil 1 changes. This changing current changes the flux associated with the coil and an electromotive force is generated across the coil. This electromotive force is also called the back electromotive force since it opposes the cause responsible for its own production.

This variation in magnetic flux is associated with coil 2, as shown in the figure above. Now, as the flux in coil two changes, there is a back EMF generated in coil 2. This is the theory of mutual inductance. Mutual inductance indicates if the magnetic flux associated with one coil changes, then the flux associated with the other coil also changes, leading to the generation of back EMF in the other coil.

What are the types of control transformers?

There are the following two types of control transformers:

• Step-up transformer
• Step-down transformer

Step-up transformer

A step-up transformer is employed to raise the alternating voltage. It is based on Faraday’s principle of electromagnetic induction law through the process of mutual induction. This transformer transfers electrical energy from one coil to another (primary to secondary) without changing the total power or frequency. There is no electrical connection among these two coils, and they are wrapped commonly throughout a magnetic strip of the soft iron designated core.

That core is built of thin, rectangular structures constructed of soft iron, located one above the other and insulated by paint or varnish. That core is built up of soft iron to enhance the magnetic field. In a step-up transformer, the number of turns in the secondary coil is higher than the number of turns in the primary coil.

Here are some valuable details about step-up transformers:

• The number of turns in the primary coils and secondary coils in a step-up transformer is provided by Np and Ns, respectively.
• These transformers can't be practiced with DC as we require a changing magnetic field to produce EMF in the secondary coil.
• In an isolation transformer, the number of turns in the primary coil is equivalent to the number of turns in the secondary coil.
• In the case of a large step-up transformer, insulating oil is utilized as a coolant so that the coolant can consume the heat generated in the core due to various kinds of energy losses. Otherwise, this heat could destroy the transformer. As the boiling point of water is 100 degrees Celsius and it evaporates at that temperature; therefore, the water cannot be adopted as a coolant.

Working of a step-up transformer

Whenever an alternating current of low voltage is supplied to the primary coil, producing a changing current to pass through it, it creates a changing magnetic field in the transformer's core; its magnetic field lines connect with the secondary coil change and produce EMF in the secondary coil. The frequency of the induced EMF is the same as that of the applied voltage in the primary coil. In this way, raised voltage is obtained across the secondary coil.

Let, N_p is the number of coils in the primary coil, N_s is the number of coils in the secondary coil, V_p is the voltage in the primary coil, and V_s is the voltage in the secondary coil.

In the step-up transformer, N_s is greater than N_p, therefore V_s is greater than V_p and more current flows through the primary winding of a step-up transformer. For this, the primary winding is built up of thick, insulated copper wire, and that wire decreases energy loss due to heating in the coil, usually identified as a copper loss. Copper loss is the most common loss which is produced in the winding of a transformer. Other losses include iron loss, eddy current loss, hysteresis loss, etc.

Applications of the step-up transformer

• It is used in microwave oven and in other electrical equipment to increase the input voltage level.
• It is used in the electrical circuit, which supplies energy to give the flashlight in cameras.
• It is used to amplify the energy of sound waves in public address systems.
• It is used to increase the voltage of a cell from 12 V to 20000 V to 40000 V in car ignition coil circuit.
• It is used to increase the speed of electrons falling on the screen in television picture tubes.
• It is used in power transmission lines of power-houses. The electrical energy produced at the power station at low voltage is supplied to a step-up transformer to raise its voltage up to 40000 V. It is done to minimize the loss of power in the transmission wires. At the distribution end, the voltage is decreased with the help of a step-down transformer.

Step-down transformer

If we want to reduce the voltage across the secondary coil, the number of turns of the secondary coil should reduce compared to the primary coils. Due to a reduction in the number of turns of the secondary coil, the current across the secondary coil rises. A transformer in which the number of turns of the secondary coil is less than the number of primary coils is called a step-down transformer.

Working of a step-down transformer

Whenever a specific high electrical voltage is applied on the primary coil of a specific step-down transformer, then there would be a low voltage obtained; as a result, output. A step-down transformer gives just opposite to a spet-up transformer. The frequency of the induced EMF is the same as that of the applied voltage in the primary coil. In this way, a decreased voltage is obtained across the secondary coil.

Applications of the step-up transformer

At power stations, the output voltage is generated at 11 kV. We need to transmit the electricity produced at a high voltage (without stepping down) to minimize power loss (as power is equal to I squared multiplied by R, and high voltage transmission means a lower current). The output voltage is stepped down at the receiving end according to the voltage required by the apparatus or appliances in use. A Step-down control transformer is used in televisions, radios, mobile phone chargers, musical keyboards, fans, doorbells, invertors, UPS devices, welding mechanisms, and power distribution substations.

Energy losses in control transformer

Magnetic field link-loss

There may be energy loss if all the magnetic field lines generated due to the current supply in the primary coil are not linked with the current supply in the secondary coil. This loss of energy is reduced by taking a closed core so that the magnetic field lines generated by the primary coil are entirely linked with the secondary coil.

Hysteresis loss

The magnetization and demagnetization of the core occur due to the varying supply of current in the primary coil. In each cycle of magnetization and demagnetization, there is some loss of energy called hysteresis loss. Hysteresis loss is reduced by making the core in a laminated form in place of a single-block form. Moreover, the soft iron core being ferromagnetic concentrates the magnetic field lines and reduces the hysteresis loss.

Copper loss

Primary and secondary coils get heated due to the supply of current. For this, the primary coil of a step-up transformer is made up of thick wires.

Eddy currents in the core

As the varying current flows in the primary coil, the magnetic flux through the soft iron core changes, inducing eddy currents that supply in the core. There is some loss in energy in the form of heat.

Common Mistakes

• Students sometimes get confused about the major difference between the transformers, like step-up and step-down. However, the number of turns is more on the secondary coil than the primary in the step-up transformer, while in the step-down transformer, there are fewer turns on the secondary coil than the primary coil.

• Students also get confused between the distribution transformer and the power transformer. However, the distribution transformer distributes the electrical power at low voltage, whereas the power transformer helps to supply electrical power at high voltage.

Context and Applications

The topic of control transformer is very much significant in the several professional exams and courses for undergraduate, diploma level, graduate, and postgraduate. For example:

• Bachelor of Technology in Electrical Engineering
• Master of Technology in Electrical Engineering
• Diploma in Instrumentation
• Diploma in Electrical Engineering

Related Concepts

• Electrometer
• Electrical safety
• Induction coil
• Root mean square current
• Root mean square voltage

Practice Problems

Q1: Among the four options, which of the following quantity doesn’t vary in transformer?

(a) Frequency

(b) Voltage

(c) Current

(d) None of these

Correct option: (a)

Explanation: In a transformer, the frequency remains unchanged because it relies on the magnet's speed, and the transformer is not utilized to generate electricity. Hence, frequency is the correct answer.

Q2: In a control transformer, the core is laminated due to:

(a) Decrease copper losses

(b) Decrease eddy current losses

(c) Decrease hysteresis losses

(d) None of these

Correct option: (b)

Explanation: In a transformer, eddy current loss takes place in the core as well as the conductor of the machine. In order to minimize the eddy current losses, the core is laminated because the eddy current loss is directly related to the sheet's thickness.

Q3: The magnetic flux’s path in a control transformer should have:

(a) Low reluctance

(b) High resistance

(c) Low resistance

(d) High reluctance

Correct option: (a)

Explanation: In a control transformer, the path of the magnetic flux is required to have low reluctance for the easier movement of magnetic flux from the element's core.

Q4: Among the four options, which of the following is not a part of transformer installation?

(a) Buchholz relay

(b) Exciter

(c) Breather

(d) Conservator

Correct option: (b)

Explanation: The exciter is not a component of the transformer installation because the exciter transformer is considered as an isolation transformer. Also, the exciter is applied to maintain the alternator's output.

Q5: A control transformer is used to transform:

(a) Frequency

(b) Voltage

(c) Current

(d) Power

Correct option: (b)

Explanation: The control transformer is one of the types of isolation transformer applied to transform the voltage to deliver power to the control devices.