Step Up vs Step Down Transformer: Real Cases Reveal the Truth

Transformers are a kind of electric device used to change the voltage levels. They are an important part of our lives and they help make our lives more convenient. Generally, transformers can be divided into two types – step up transformers and step down transformers.

This article will give you a clear understanding of step up vs step down transformer, and help you make a wise choice when faced with the options.

What Is Step Up and Step Down Transformer?

Let’s first understand what a transformer is, and then it will be much easier for you to figure out the differences between the step up and step down transformers.

What Is a Transformer?

A transformer is used to transform the voltages from one level to another level. It can change the voltage from a lower level to a higher level to transmit power efficiently to a long place. It also can change the voltage from a higher level to a lower level for safe and direct use for residents, factories, shopping malls, etc.

Its working principle is electromagnetic induction, which was first discovered by Faraday in 1831. The transformer consists of a closed core and two coils insulated from each other. When the coil on one end (primary side) connects to an alternating current (AC) power supply, a changing magnetic flux will be generated inside the core and transferred to the coil on the other end (secondary side). Voltages will be induced in the coils at both ends.

At this time, when the secondary side connects to loads such as bulbs, then the secondary circuit is closed. The induced voltage on the secondary side drives the current to flow, and the bulb will light up.

So, what determines the voltage level across both ends? The answer is the number of turns of the coil.

What Is a Step-Up Transformer?

If we wind more coil turns on the secondary side than that of the primary side, guess what will happen? In this case, a higher voltage will be generated on the secondary winding. That’s how the step-up transformer comes.

Let’s take a simple example. If you bought a machine from abroad, and this machine requires a 220V high voltage to operate, while the standard voltage in your country is 110V. In this case, you need to buy a step up transformer to increase the voltage from 110V to 220V, otherwise your machine can’t be drived.

What Is a Step-Down Transformer?

Contrary to the step up transformer, a step down transformer’s coil turns on the primary side are more than that on the secondary side. It converts a higher input voltage to a lower output voltage for end use.

Let’s keep it simple. If you buy a home appliance from abroad that requires a 120V rated voltage, while the standard voltage in your country is 220V, then you need a step down transformer to decrease the voltage from 220V to 120V for normal use.

Step Up and Step Down Transformer True Cases

Case Background

OMEK team participated in a subway project in 2021. There were two stations far apart, and two 7.5kW pumps needed to be supplied with stable power. The long distance transmission faced a serious problem of voltage loss.

Ordinary Plan

Use 240mm² large cross-section cables to meet the deviation requirement of ±5% for the voltage at the motor terminals of the pumps. However, the cost of the 240mm² large cross-section cables is not a small number. This solution is very costly.

OMEK’s Solution

OMEK’s engineer team adopts an optimized power distribution scheme – let the step-up and step-up transformers work in coordination, and select 70mm² cables to meet the requirements.

Step-up transmission scheme: Use a step-up cast resin dry type transformer to increase the 380V low voltage from the substation to a 660V high voltage level for transmission. This plan reduces energy loss during long-distance transmission and avoids the a big voltage drop.

Step-down transmission scheme: Use OMEK’s 100 kVA 3 phase step down transformer to decrease the 660V high voltage where transmitted to the pump end to 380V of the pump rated voltage, ensuring a stable voltage of the motor terminal voltage between 365.92V and 396.09V.

This step up and step down power distribution solution provided by OMEK engineers enhances the voltage transmission efficiently, reduces the cable costs, brings true economic benefits to this metro project!

Click here to contact our engineer for professional advices.

Step Up and Step Down Transformer Working Principle & Design

The core principle of both step up and down transformers are all based on electromagnetic induction.

1) Primary coil leads to a variable magnetic field set up
The primary coil connects to the AC current input, and the magnitude and direction of the current are constantly changing constantly. Alternating flux will be prodcued in the closed iron core.

2) Magnetic flux delivery to the secondary coil
The core of the transformer generally adopts silicon steel, or amorphous alloy materials, which can have high magnetic permeability. Then the alternating magnetic flux generated by the primary coil is almost entirely transferred to the secondary coil through the core.

In OMEK, our design team worked out the stepped core and 3D core amorphous alloy transformers for more efficient magnetic flux delivery. These designs ensure 99% of the alternating magnetic flux generated by the primary coil is transmitted to the secondary coil. If you are interested in the transformers of these designs, feel free to contact us here.

3) Induced voltage generation on the secondary coil
When alternating magnetic flux passes through the secondary coil, an induced voltage and alternating current will be induced at the secondary coil.

The biggest difference of their designs is in their coil turns ratio. The voltage magnitude is directly proportional to the number of turns in the coil. The side with more turns has a higher voltage, while the side with fewer turns has a lower voltage. This is expressed by a specific formula:

V_primary / V_secondary = N_primary / N_secondary

Applications: Where Are They Used?

Step-Up and Step-Down Transformers: Different Positions in the Power Grid

The step-up transformers and step-down transformers work together in the power grid. Not a single one can be omitted.

The voltage generated by power plants is usually between 10-20 kV. The step-up transformers then increase the voltage to 110 kV, 220 kV, or even above 500 kV. Why do this? Because the higher the voltage, the smaller the current, and the less power is wasted due to wire heating. The purpose of doing this is to reduce energy loss in long-distance power transmission.

The elevated higher voltage electricity passes through high-voltage transmission lines and is transported to cities or industrial areas. Once it reaches the destination, the step-down transformers will gradually reduce the high voltage to a safe level.

The electricity after being stepped down is ultimately sent to households, factories, stores, etc., for use by various appliances and equipment.

In summary, in the entire power grid system, the step-up transformers are for efficient transmission, and the step-down transformers are for safe use. They work together to complete the tasks of power transmission and distribution.

Uses of Step-Up Transformer

In power grid systems, step-up transformers are generally located at the starting point of power transmission. Its two main uses: one is for long-distance power transmission, and the other is to minimize energy loss. Their most common application scenarios are power stations and high-voltage transmission towers, etc.

The GSU transformer (generator step up transformer) is a typical large step up transformer. It connects between the generator outlet and the high voltage transmission line. This kind of step up transformer is widely used in the following places:
– Large-scale thermal power plant
– Nuclear power plant
– Hydropower station

Uses of Step-Down Transformer

Step-down transformers are generally located at the end of the entire power transmission. Its main function is to reduce the voltage to a safe and usable level. For example:

– Provide suitable operating voltages (440V, 600V, etc.) for large motors and electric furnaces
– Power the machine control panel, relays and PLC (24V, 110V)
– Provide a high current and low voltage output for the electric welding machine to generate the high heat required for welding.

How to Choose the Right Transformer: Key Factors

1. Define the Key Specs

Firstly, you should define the specifications you require. Such as below items:
– Power (kVA), Input/Output Voltage, Frequency, Phase (single or three phase transformer).
– Insulation Class, Cooling Type. (Read more to understand the Different Transformer Cooling Types)
– A step up photovoltaic transformer 12v to 240v for off-grid solar? Or a 12v step down transformer for LED lighting? Define your exact need.

2. Consider the Cost & Performance:

For example, a step up transformer’s price depends on:
– Power rating capacity and efficiency class (IE3, IE4).
– Material (copper vs. aluminum windings). Copper transformer usually costs more.
– Customization and protection rating (IP Code).

3. Specialized Solutions:

– CT Transformer: Used for metering and protection, often paired with power transformers.
– Stepped Core Transformer: The best choice for high-efficiency, low-heat applications.

Can We Reverse Feed a Transformer?

Since we can change the voltage levels on both the primary winding and secondary winding via adjusting the coil turns, can we reverse feed it? Simply put, can we regard the secondary winding as the primary winding and regard the primary winding as the secondary winding?

What exactly reverse feed a transformer?

Common practice (normal feed)
A step down transformer is to decrease higher voltage (such as 660V) to lower voltage (such as 400V), and a step up transformer is to rise a lower voltage (such as 380V) to higher voltage (such as 660V). The input and output terminals are fixed. They are the original design of the transformer manufacturer.

Reverse feed
Take this step down transformer (660V→400V) for example, if we connect the power source to the original secondary winding (400V), and connects the load to the original primary winding (660V), it will become a 400V→660V step-up transformer.

Not All Transformers Can Reverse Feed

Reverse feed is to change the wiring direction. The same transformer can be switched over between step up and step down. Although it is reversible in principle, there are strict limitations to reverse feed. If you reverse feed a transformer, you need to consider the following requirements and results.

1. The convert ratio matching must be met.
For example, the convert ratio of a common step down transformer is 660:400 (appoximately 1.65). When used in reverse, it is 400:660 (also ≈1.65). The output voltage can only be adjusted per this fixed ratio and can’t be adjusted casually.

2. The power will be discounted.
The heat dissipation and insulation of the transformer is designed for the normal feed of the tranformer, when it is used in reverse, the actual output power may only be 70%-90% of the rated power.

3. The protection device may fail.
The short circuit protection and other protection designs were set based on the primary current. After reversing, the current direction changes, and the protection logic may be disordered and ineffective.

4. Special transformers are prohibited.
Rectifier transformers, special isolation transformers and transformers with tap changer can’t adopt reverse feed. Conventional two-winding transformers can feed power in reverse due to their symmetrical design. However, the design of the special transformer is asymmetric. Reverse wiring will disrupt its original design logic.

OMEK suggests not reverse feed casually unless it is in urgent need or emergency repair. It is just a temporary replacement plan. For a long power transmission and distribution needs, a combination scheme of step-up and step-up transformers is recommended.

Add our WhatsApp +86 18553105918 to have a direct communication with our engineer. Let us know your application and you will get a targeted solution!