A step-down transformer is an electrical device that reduces the voltage of an alternating current (AC) power supply.
The transformer includes:
1. Primary winding
2. Secondary winding
3. Iron core
4. Bobbin Materials list:
1. Copper wire with insolation
2. Core: Design a device with low frequencies as 50Hz and 240Hz using lamination steel.
For core type, we have EE type, Toroid type, UU Type…etc.
3. Bobbin: normally used plastic such as Nylon 66, phenolic, PBT…etc.
4. Isolation materials such as mylar tape, paper…etc.
Power source alternating current (AC) power supply, when an input voltage is applied to the primary winding, it creates a magnetic field in the iron core.
5. Varnish
The magnetic material and quality factors of the core will affect the temperature rise and magnetic saturation.
Therefore, engineers pay great attention to the design and selection of core materials.
This magnetic field then induces a voltage in the secondary winding, but at a lower voltage level than the primary winding.
A transformer works on the mutual induction principle, which we call Faraday’s Law of Electromagnetic Induction, which states that the magnitude of voltage is directly proportional to the rate of change of magnetic flux.
What is the Core
The magnetic core is made of iron or other magnetic materials with high magnetic permeability added to different metals.
These materials can be stamped and formed into required shapes according to different requirements. When the copper wire is wound on the iron core, the current passing through the coil will generate magnetic flux.
High permeability iron core has higher inductance, but it is also easy to magnetic saturation. It will cause eddy current loss in the coil and iron core, resulting in temperature rise.
For solving this problem, the core material with low magnetic permeability or the air gap given by the core can be used to reduce the eddy current loss.
When designing a transformer, engineers need to understand the characteristics table of the iron core and the physical property table of the material in detail.
In order to solve magnetic saturation and reduce eddy current loss, we will design and use thin metal iron sheets to be wound into rings or stamped into laminations of the shapes like EI, EE, UU… etc.; this is able to let the core has a small air gap to reduce eddy current loss, if only a single sold iron core, it is easy to generate eddy current.
In order to solve the severe eddy current loss, sometimes two or several pieces of different thin metal sheets are needed to increase the air gap but also reduce the magnetic flux. Therefore, high magnetic permeability nickel steel sheets are added to make up for the low magnetic permeability caused by the air gap during design.
In addition, the selection of iron cores of various shapes and sizes should consider saving material cost and efficiency.
What is the winding
Two insulated copper wires are separately wound on the frame core to generate mutual inductance.
The condition for us to design a transformer is to use an insulated copper wire with a small resistivity and good conductivity, that the transformer can have good efficiency.
Generally used in transformer windings is copper, which has good electrical conductivity and very low resistance compared to other materials.Although gold, silver, and platinum have very low electrical resistance and conductivity, but they are quite expensive. Only when the components are used in space communications, advanced instruments, or military communications, otherwise these kind of materials are seldom to use.
Transformer Working how to work
As we know, transformers work on the principle of mutual induction. That is, when the current in one coil changes, a current is also induced in the other coil.
Every transformer consists of two coils or several windings: a primary and a secondary. The primary winding is connected to the AC source and the secondary winding is connected to the load. When the AC current is connected to the primary winding, magnetic flux will be generated, and when the secondary winding senses the magnetic flux of the primary winding, an electromotive force (call: EMF) will be induced. The strength of the generated EMF depends on the number of turns of the secondary coil winding.
N1>N2
where N1 = Number of turns in the Primary coil
N2 = Number of turns in the Secondary coil
The relation between voltage and the number of turns in a coil is: Vp/Vs = Np/Ns
where Vp = Voltage in the primary coil
Vs = Voltage in the secondary coil
Np = Number of turns in the primary coil
Ns = Number of turns in the secondary coil
What is a step-down transformer?
A transformer which winding turns of primary is more than secondary, it is called a step-down transformer.
We can understand the formula of the relative relationship between the number of winding turns and the voltage mentioned above.
If the number of turns on the primary is greater than the number of turns on the secondary, the voltage produced by the secondary will be less than the primary input voltage.
Hence, we get a lower voltage in the secondary coil of the step-down transformer. Simply put, a step-down transformer converts a higher-voltage power supply to a lower voltage power supply.
Step-Down Transformer Formula
Here is an example to understand the above process:
Suppose the input voltage is AC 240 V.
If Np (Number of turns of the primary coil) = 20000
Ns (secondary turns) = 100,
It can be obtained from the following formula - Vs (voltage at the secondary coil): Vs = (Vp * Ns)/Np
Step-Down Transformer Types
Single Phase Step Down Transformers
Three Phase Step Down Transformers
Multi-output Step-Down Transformers
Application
Step-down transformers are used in power adaptors and connect rectifiers to change the voltage from AC to DC.
Some new designs are thinking of the small size and reduced weight which will use on electronic switching power supplies.
Other applications include:
● Power transmission lines
● Welding machines
● Voltage stabilizers and inverters
Some Common Questions about Step-Down Transformers
Q1. Can step-down transformers be used to increase the voltage?
No, step-down transformers are designed to reduce the voltage of an AC power supply. To increase the voltage of an AC power supply, you would need to use a step-up transformer.
Q2. How do you calculate the voltage output of a step-down transformer?
The voltage output of a step-down transformer can be calculated using the following formula:
V_output = V_input * (N_secondary / N_primary)
● V_output: the output voltage
● V_input: the input voltage
● N_secondary: number of turns in the secondary winding
● N_primary: the number of turns in the primary winding.
Q3. How do you select the appropriate size of the step-down transformer?
The appropriate size of a step-down transformer for a particular application depends on the voltage and current requirements of the load (i.e. the device or devices that the transformer will be powering).
You will need to know the voltage and current ratings of the load and select a transformer with a primary voltage rating that is higher than the supply voltage and a secondary voltage rating that is equal to or higher than the load voltage.
The current rating of the transformer should be equal to or greater than the load current. It is important to select a transformer that is appropriately sized to avoid overloading or damaging the transformer or the load.
Q4. How do you connect a step-down transformer?
Step-down transformers are typically wired in a Delta or Wye configuration. In a Delta configuration, the primary and secondary windings are connected in a triangular pattern. In a Wye configuration, the primary and secondary windings are connected in a Y-shaped pattern. The specific wiring method will depend on the transformer and the application. It is important to follow the manufacturer’s instructions when wiring a step-down transformer.
Q5. How do you test a step-down transformer?
There are several ways to test a step-down transformer:
● Open circuit test: This test measures the voltage of the primary winding with the secondary winding open.
● Short circuit test: This test measures the current of the primary winding with the secondary winding shorted.
● Load test: This test measures the voltage and current of the primary and secondary windings with a load connected to the secondary winding.
● Temperature rise test: This test measures the temperature increase of the transformer under load conditions to ensure that it is operating within safe limits.
Q6. What are some factors that can affect the performance of a step-down transformer?
Some factors that can affect the performance of a step-down transformer include:
● The quality of the materials used in the transformer
● The design of the transformer
● The load placed on the transformer
● The frequency and voltage of the AC power supply
● The ambient temperature and humidity
● The presence of external electromagnetic interference (EMI)
Q7. What are some common problems with step-down transformers and how can they be resolved?
Some common problems with step-down transformers include:
● Overheating: This can be caused by overloading the transformer, using it at higher-than-rated frequencies, or operating it in high ambient temperatures. To resolve this issue, reduce the load on the transformer, use it at the rated frequency, and ensure proper ventilation.
● Humming or buzzing: This can be caused by magnetic fields interacting with other nearby electrical devices or by mechanical issues with the transformer. To resolve this issue, move the transformer away from other electrical devices or have a professional inspect and repair the transformer.
● Poor voltage regulation: This can be caused by an incorrect transformer size or load, or by a faulty transformer. To resolve this issue, ensure that the transformer is correctly sized and rated for the load and have a professional inspect and repair the transformer if necessary.
References: ElectronicFORU.com
By Ashwini Kumar Sinha and Vinay Minj
Date: January 5, 2022
