Power Transformers: What You Need to Know

When it comes to power transformers, the first thing you’ll come across is the power rating. This is usually listed in VA (volt-amperes), kVA (kilovolt-amperes, which is 1,000 VA), or MVA (megavolt-amperes, which is 1,000,000 VA).

What is VA?

VA stands for volt-amperes, which just means a unit that measures electrical power.

VA vs. Watts: What’s the Difference?

Both watts (W) and volt-amperes (VA) measure electrical power, but they mean slightly different things:

• Watts (W) measure “real power,” which is how much power is actually being used or consumed.
• Volt-amperes (VA) measure “apparent power,” which is the total power that’s available, but not all of it is being used. This is because voltage and current can get out of sync with each other. In simple terms, VA shows the potential power in the system, while watts show the actual power being used.

What affects the Size of a Transformer?

Transformers can vary in size based on how they’re designed and cooled. For smaller transformers (under 10kVA), natural air cooling is often enough. Larger transformers might use forced air cooling, or be immersed in oil (which can either circulate on its own or be pumped). All cooling methods affect the transformer’s overall size.

The materials in the transformer—like the copper or aluminium used for windings and the type of electrical steel for the core—also affect size and efficiency. Copper windings are more efficient than aluminium windings, and electrical steels vary in their efficiency dependant on the quality of the electrical steel used. The current density designed within the windings and flux density designed within the electrical core are fundamental to the overall efficiency and size of the transformer. The design of the transformer’s windings and core (like current density and flux density) are key to how well it performs. Plus, the insulation’s temperature rating (usually anywhere from 105°C to 180°C) plays a big role in how compact and efficient the transformer can be.

Frequency’s Role in Transformer Size

The supply frequency (measured in Hz) can also impact transformer size. Generally, if you double the frequency of the supply voltage, you can cut the size of the transformer in half. For example, in low-power applications (up to 2kW), transformers in “switched-mode power supplies” can run at frequencies up to 10kHz, which makes them way smaller than your typical 50/60Hz transformer.

Single-Phase vs. Multi-Phase Transformers

• Single-phase transformers usually range from small sizes up to around 75kVA.
• Three-phase transformers start at less than 1kVA and can go all the way up to several MVA. These are commonly used for larger applications.

For three-phase transformers, it’s important to consider the phase relationship (how the phases are arranged), which should be specified when placing an order. There are also other configurations like 3-phase to 1-phase, Scott-connected, Le Blanc, zigzag, and phase-shifting transformers, which are custom-designed based on specific needs.

Protecting Transformers: IP Ratings and Enclosures

Transformers need to be protected, not just from the environment but also from electric shock. Smaller transformers might be housed in control panels or machines, where the casing offers protection. For larger transformers, the housing is critical for heat dissipation.

Protection is typically indicated by an IP rating, which measures how well the transformer is protected from things like dust and water. Common ratings range from IP23 (some protection) to IP65 (full protection). When choosing an enclosure, you’ll want to consider how well it lets heat escape while still protecting against shock and external elements.

How to Specify a Transformer

When you’re specifying a transformer, there are a few key details to consider:

• Power rating (VA, kVA, MVA)
• Phase type (single-phase, three-phase, or other custom configurations)
• Vector grouping (for three-phase systems)
• Input voltage range
• Input frequency (in Hz)
• Output voltage and required power for each output
• Type of protection (to meet specific needs)
• Special heat loss or efficiency requirements
• Any other unique needs for your application

To learn more, reach out to our sales team.