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Electrical Transformer: 5 Basic Questions You Should Know

Electrical transformers are one of the most important equipment in the power system, Some transformer basics are so important, And every electrician and electrical engineer should know it.

While I was searching for transformer important questions, I found 5 basic questions about transformer core, power rating and power factor. I decided to answer these question to help you all.Let’s get started.

We all know that, iron core and windings are the active parts of the transformer. So, Why do we use the core? Do all transformers have iron core? Why is core laminated? Why is transformer power rated in KVA not KW? and, Do transformers have power factor?

Let’s start to answer these questions one by one.

Why is iron core used in transformers?

Number of winding turns is not the only factor that affect the secondary voltage. Magnetic coupling between the primary and the secondary is also important.

This coupling can be achieved either by decreasing the distance between the windings or by using iron core.

The less the distance between the two windings the higher the secondary voltage. Using the iron core is more effective in this case.

Should the core be iron? I mean, why not to use Aluminum core? Well let’s find out why is iron or steel better than Aluminum.

For a good transformer, it is desirable to have certain properties such as low core losses and infinite or very high permeability.

Therefore selection of core material is very critical as the amount of flux passing from primary to secondary coil and number of turns depends on this factor.

The flux that passes is actually in the form of magnetic fields, which can escape out of the core surface if the material used has less permeability compared to the outside environment.

Air has a very low permeability, and therefore air is not a good option at all to be used in transformer core. Apart from Air, soft and hard magnetic materials can be used for core of transformer.

In Aluminum, a very strong energy is required to energize the primary coil, which results in a lot of energy loss and very poor magnetic properties overall.

In comparison to that, Iron require very less energy for magnetizing, which makes its magnetic properties suitable for a transformer core.

Therefore, in transformers, Iron is preferred and used most of the times.

Is electrical Transformer core laminated?

Yes, Iron core is laminated to overcome the issue of eddy currents losses. Laminated is the opposite of solid core i.e the core is in the shape of very thin layers (0.3mm) isolated with silicon.

When AC supply is connected to the primary coil of transformer, the variation of magnetic field induce a flux.

If the core doesn’t have any lamination, loops of currents (eddy currents) are produced by varying magnetic field produces power loss as a shape of heat, increasing the temperature of core to extreme levels.

Therefore, Transformer core is laminated to mitigate such issues.

Do all transformers have an Iron Core?

Selection of core material for transformers depends on the operating environment and frequency levels.

Besides Iron (solid, carbonyl), some other materials used for transformer core are amorphous steel, silicon steel, amorphous metals, Ferrite ceramics and laminated magnetic cores.

  • Amorphous steel cores has multiple layers of thin metallic tapes, helping in reducing eddy losses. Due to low amount of losses, these core are very much capable of handling high temperatures, and mostly used in transformers operating at medium frequencies and high efficiencies.
  • Amorphous Metals designed for high efficiency and high performance. This material has considerably low level of conductivity, which reduce eddy losses and high response to magnetic fields.
  • Silicon Steel is preferred in low frequency transformers because of increased resistivity and high density of flux. Furthermore, high level of permeability and low amount of losses are some other factors making it a suitable option.
  • Ferrite Ceramicsis used in high frequency transformers with self-insulation properties to prevent eddy losses.

Now, let’s go in to a confusing question about transformer power rating.

Why is transformer rated in KVA and not in KW?

For a transformer’s rating, voltage and current plays the basic role in determining how much power it can supply in overall. But in reality, this information is not enough because of some other factors.

A transformers supply electrical power not mechanical power to different types of loads, and it’s hard to tell which type of load the transformer will supply. So, the power factor of the load may be less than or equal to unity.

Therefore, Power factor comes into the equation. In inductive loads (e.g. motors), some extra power is required which is taken and then released by the load.

This reactive power is not constant, and could vary according to the load power factor. Therefore, it is not correct at all to express the transformer power in Kilo Watt (KW),

Why does transformer nameplate has no power factor?

Transformer power factor is not a constant value like electrical motors, that’s why power factor is not on transformer name plate. The transformer load power factor varies according to the load type.

Power factor is the ratio between real and apparent power. For inductive loads like  three phase high efficiency motor the power factor is about 0.9, while a poor power factor can be 0.75 for single phase motors.

On the other hand, a unity power factor is the best case when the transformer is supplying pure resistive loads like electrical heaters.

So, the transformer power factor depends on the load type, that’s why transformer nameplates don’t mention its power factor.

Can You Leave Transformers Plugged In?

Yes transformers can be left plugged in. It is safe, no harm in this.

There is no safety issue even if the transformer is turned on all the times. But there is always the some sort of power consumed even without a load, so for saving the little amount of energy, turn it off when it is not in the use.

Due to the magnetic core losses in the transformer current draws continuously. But this drawing current is pretty much small and not a notable problem.

If you wired your transformer accurately, connections of the transformer are not exposed, and the ventilation in the transformer is OK, then there should be no problem in turning on the transformer as far as safety issue is concerned.

A power indicator lighten up idea is pretty much cool as external person can understand that there is a live power.

Because it is plugged in, so some wattage will be used by it constantly.

For more Electrical And Workplace Safety I recommend visiting my SAFETYFRENZY Site. It has a lot of useful electrical safety articles.

Do Transformers Draw Current Without Load?

Yes a transformer draws small amount of current without the load.

A transformer has no load means there is no burden i.e load on the secondary winding, the transformer should draw zero current but this is not the actual case.

Transformer on no load draws small amount of current from the primary side to set up the needed magnetic flux in magnetic core.

The name of this current is “No Load Current”. This no load current is also called the magnetizing current. The reason for calling no load current magnetizing current is because it is responsible for magnetization of the Ferro magnetic core.

The no load current of a transformer consists of the two components namely magnetizing component and active or the power component:

Magnetizing Current Im:

Let’s say applied voltage is V1. Magnetizing current is in quadrature with V1. This component is responsible for the flux and not consumes any sort of power.

Active or the Power Component Iw:

It can also be called the working component. Active component Iw is in phase with the V1. It is responsible for the iron losses and also responsible for the minor amount of copper loss.

Working or active component = Iw = I0Cos ϕ0

Magnetizing Component = Im = I0 Sin ϕ0

Power Factor Cos ϕ0= Iw / Im

No load power input formula will be:

P0 = V1 * I0 Cos ϕ0

This no load current is almost 3 to 5% of the full load current. This is for the losses in the transformer. No load current is usually 2-10% of rated current.

No load losses comprises core(iron) losses which includes hysteresis and the Eddy current losses.

Copper losses (I2 * R) are also due to the no load current. No load current is usually represented by the i0.

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