How to Choose a Transformer: Transformer Calculator And Sizing

Figure 1: Transformer

Read more

A transformer is a passive electrical device that uses electromagnetic induction to transfer electrical energy from one circuit to another. Selecting the proper transformer for an application requires careful consideration of several factors, such as voltage, current, type of load, efficiency, environment, and cost. This article gives a step-by-step guide on choosing a transformer and analyzing various powers in a transformer.

Table of contents

• Selecting a transformer
• Transformer's rated load calculator
• Transformer load capacity
• Transformer efficiency calculation
• Transformer wattage calculator
• Transformer losses
• How to reduce transformer losses
• FAQs

Selecting a transformer

Read our overview article on transformers for more information on how they work, their construction, and their types. Transformers are available in multiple designs, specifications, and sizes. It is important to consider the following:

• Step up or down: Choose a step-up or step-down transformer depending on the specific application requirements. For instance, while working on a rectifier, to reduce the supply voltage from 230 to 12 volts, choose a 230-12 volt step-down transformer.

Note: The voltage and frequency of the main power supply vary from one country to the other. Countries like the USA use the main power supply of 120 volts operating at 60 Hz, whereas those in Europe like the UK, Netherlands, and Germany operate at 230 volts and 50 Hz. Choose the transformer accordingly. See the complete list of countries with their corresponding mains power supply voltage and operating frequencies for more details.

Figure 2: Step-down (left) and step-up (right)

• Voltage and current specifications: The next step in transformer selection involves determining the load's current and voltage requirements.
  • If a building has an AC power supply of 240V and requires 120V for electrical equipment, choose a 240-120V transformer.

Note: Generally, the primary and secondary voltages are listed in the transformer specifications. But in exceptional cases like a current transformer, the primary and secondary currents are listed instead.

• VA: Determine the power required by the load (VA) when selecting a transformer, as transformers are usually specified by their primary and secondary voltages and VA values. Determining these values in advance is necessary to select the proper transformer for the specific application.

Transformer's rated load calculator

The rated load (apparent power) of a transformer is measured by VA (Volt-Ampere). The rated power represents the total amount of power the transformer can deliver to the load. To calculate the rated load needed:

1. Determine the current and voltage requirements of the load.
2. Multiply the load current in amps (I) and voltage (V) values to get the VA (or rated load) of the transformer.

1. For a single-phase transformer,

2. For a three-phase transformer,

Note: Small transformers rate their deliverable power in volt-amperes, but much larger power transformers are rated in units of Kilo volt-amperes (kVA), and Mega volt-amperes (MVA). Therefore, VA, kVA, and MVA are similar units. For example, 6 VA is equal to 0.006 kVA.

Read our articles on single-phase and three-phase transformers for more information on their working and design.

Transformer load capacity

Transformer load capacity is the maximum amount of electrical power that a transformer can safely and efficiently handle without overheating or causing damage. For example, a transformer with a 200 VA rating can handle 200 volts at one ampere of current or 100 volts at two amperes of current.

Knowing the VA value of the transformer, it is possible to determine the current across the load by rearranging the equation for VA.

1. Obtain the VA rating of the transformer.
2. Determine the load’s voltage required.
3. Rearrange the equation for VA to solve for current. For example, for a single-phase transformer.

Note: Loading a transformer above 80% of its VA rating is not recommended. After calculating the VA, divide the value by 0.8 to get the minimum VA rating needed. An electrician or electrical engineer should review and confirm all values for safety reasons.

Example

Calculate the rated power of a single-phase transformer with a load voltage of 24 volts and a maximum load phase current of 5 amperes.

The VA rating of the transformer = (V × I)= (24 × 5) = 120 VA

Dividing the VA by 0.8, the revised VA = 150

Choose a transformer with a VA rating of approximately 150 for the load.

Goto Tianya to know more.

Transformer efficiency calculation

The efficiency of a transformer is given by:

When dealing with transformers, the primary power is generally referred to as volt-amps (VA) to distinguish it from the secondary power. Therefore, the efficiency is given by:

Transformers have high efficiencies due to the absence of moving parts. Transformers are also called static AC machines. The efficiency of a well-designed power transformer is very high, averaging over 98 percent. The only losses are due to core losses, maintaining the alternating magnetic field, resistance losses in the coils, and power used for cooling.

Transformer wattage calculator

Power factor

In an AC circuit, the power factor is the cosine of the phase difference between voltage and current. The power factor represents the fraction of the rated power (apparent power) that is consumed (true power) by a circuit.

• PF: Power Factor