How do you know if a transformer is step up or step down?

Step-up and Step-dowon Transformer
Fig: Step-up and Step-down Transformer Construction

What is meant by step-up and step-down transformer?

We all are familiar with the Transformer and its different types and uses; we may someone very much expert and working with a transformer at our limited field or someone newer to introduce level. The fact is that sometimes we face a few critical problems in our working field. Today we will discuss and share our knowledge about how we find the transformer is step-up or step-down when there is no identification marking.

"A transformer that increases voltage from primary to secondary is called a step-up transformer which is designed as more secondary winding turns than primary winding turns. Conversely, a transformer designed to do just the opposite is called a step-down transformer".

In very simply we can say a transformer can be used for both step-up and step-down. All that depends on the number of primary and secondary winding numbers. If we connect the primary side to AC supply mains and secondary side to some load, if the secondary has less number of winding-turns there will be less voltage drop, so then it will function as Step-Down and in the either case as the winding-turns are more there will be more voltage drop so it will function as a Step-Up transformer. 

But, in practice, it’s not so easy to use a step-down transformer as step-up or vise-versa. There is some safety measure for machine and man or property. So, let find the actual fact.

Simply there are more turns on the secondary coil for step-up transformer than the primary coil, wherein a step-down transformer has fewer turns on the secondary coil than the primary coil;

If you look outside of a transformer the terminal notations if primary side U V W. In secondary side u v w is there capital alphabet notation is primary if notation smaller alphabet is secondary;

We can test to find the primary and secondary using a safe low voltage like- hooking it up to an A/C power source around 20-30V at the primary winding and probe for voltage at the secondary winding. Easily we can determine transformer is step-up or step-down;

We can look at the rating plate to determine the turn’s ratio or voltage ratio and check the connections. If the in-feed is on the lower voltage winding, it is a step-up transformer. If the in-feed is on the higher voltage winding, it is a step-down transformer;

Measuring the dc resistance of the transformer wingdings we will get an idea. Say, if the transformer is a 10:1 step down transformer then the dc resistance of the primary winding will be perhaps 10 times greater than that of the secondary;

We can check the type of primary and secondary side bushing, if available. The size of cables on both side and the tap changer position which is always on the HV side;

If possible to find the nameplate then easily we can find the transformer type, such as if there are 2 voltages written in following form 230/115 or any such two figures, the ratio- numerator/denominator proves the transformer is step-up or step-down;

We can check the thickness of insulation and thickness of conductor material if possible. Thicker insulation is used in the high voltage side and a thicker conductor is used in the low voltage side.

See How Step-Up & Step-Down Transformer Work?

Difference between Step-up and Step-down Transformer:

Step-up Transformer
Step-down Transformer
The output voltage of Step-up transformer is more than the source voltage.
The output voltage of Step-down transformer is less than the source voltage.
LV winding of the transformer is the primary and HV winding is secondary.
HV winding of the transformer is the primary and LV winding is secondary.
The secondary voltage of Step-up Transformer is greater than its primary voltage.
The secondary voltage of Step-down Transformer is less than its primary voltage.
The number of turns in the primary winding is less than the secondary winding.
The number of turns in the primary winding is more than the secondary winding.
Primary current of the transformer is more than the secondary current.
Secondary current is more than the primary current.
Step-up transformer is generally used for power transmission. Generator Transformer in power the plant is one example of Step-up Transformer.
Step-down The transformer is used in power distribution. The transformer in a residential colony is one example of a step-down transformer.

Why Transformers are used?


There are two reasons to use a Transformer; one- to change the voltage level in a system either to meet the consume voltage level or to transmission voltage level; two- to provide "galvanic isolation" between the source power and the driven load. 

Come across this two most basic, a transformer is designed as two separate coils of wire on a common iron core.  If the number of turns in each of the two coils is the same, then the output voltage of the transformer will remain the same level that applied in the input. But, if the number of coils in each winding is different, then the output voltage level will be different by the input voltage, depending on the ratio of the number of coils.

Example: If a transformer coil winding ratio is 10:100; means-
If we apply 100V on the side with 100 coils, the voltage on the other side will be 10V. 

If you apply the 100V on the side with the 10 coils, the voltage on the other side will be 1000V.

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