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

Fig: Step-up and Step-down Transformer Construction

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

We all are familiar with the Transformer and its different types and uses; we may have someone very expert and working with a transformer in our limited field or someone newer to the introduction 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".

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 the 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 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 a step-up or vice-versa. There is some safety measure for machine and man or property. So, let's find the actual fact.

Case-1:
Simply there are more turns on the secondary coil for the step-up transformer than on the primary coil, wherein a step-down transformer has fewer turns on the secondary coil than the primary coil;


Case-2:
If you look outside of a transformer the terminal notations if the primary side U V W. On the secondary side, u v w is there capital alphabet notation is primary if the notation smaller alphabet is secondary;


Case-3:
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 probing for voltage at the secondary winding. Easily we can determine transformer is step-up or step-down;


Case-4:
We can look at the rating plate to determine the turn 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;


Case-5:
By 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;


Case-6:
We can check the type of primary and secondary side bushing, if available. The size of cables on both sides and the tap changer position which is always on the HV side;


Case-7:
If possible find the nameplate so we can easily find the transformer type, such as if there are 2 voltages written in the following form 230/115 or any such two figures, the ratio- numerator/denominator proves the transformer is step-up or step-down;

Case-8:
We can check the thickness of the insulation and the thickness of the conductor material if possible. Thicker insulation is used on the high-voltage side and a thicker conductor is used on the low-voltage side.

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

Difference between Step-up and Step-down Transformer:

Step-up Transformer	Step-down Transformer
The output voltage of the Step-up transformer is more than the source voltage.	The output voltage of the 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 the Step-up Transformer is greater than its primary voltage.	The secondary voltage of the Step-down Transformer is less than its primary voltage.
The number of turns in the primary winding is less than in the secondary winding.	The number of turns in the primary winding is more than in the secondary winding.
The primary current of the transformer is more than the secondary current.	The secondary current is more than the primary current.
A Step-up transformer is generally used for power transmission. The generator Transformer in powers the plant is one example of a 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 consumption voltage level or to the transmission voltage level; two- to provide "galvanic isolation" between the source power and the driven load. 

Come across these 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 at the same level that is applied in the input. But, if the number of coils in each winding is different, then the output voltage level will be different from 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.

You may know the details about the electrical transformer from the following articles:

 

1. Working Principle of Transformer;
2. Transformer Construction;
3. Core-type Transformers;
4. Shell-type Transformers;
5. Elementary Theory of an Ideal Transformer;
6. E.M.F. Equation of Transformer;
7. Voltage Transformation Ratio;
8. Transformer with losses but no Magnetic Leakage;
9. Transformer on No-load;
10. Transformer on Load;
11. Transformer with Winding Resistance but no Magnetic Leakage;
12. Equivalent Resistance;
13. Magnetic Leakage;
14. Transformer with Resistance and Leakage Reactance;
15. Simplified Diagram;
16. Total Approximate Voltage Drop in Transformer;
17. Exact Voltage Drop;
18. Equivalent Circuit Transformer Tests;
19. Open-circuit or No-load Test;
20. Separation of Core Losses;
21. Short-Circuit or Impedance Test;
22. Why Transformer Rating in KVA?;
23. Regulation of a Transformer;
24. Percentage Resistance, Reactance, and Impedance;
25. Kapp Regulation Diagram;
26. Sumpner or Back-to-back-Test;
    
27. The efficiency of a Transformer;
28. Condition for Maximum Efficiency;
29. Variation of Efficiency with Power Factor;
30. All-day Efficiency;
31. Auto-transformer;
32. Conversion of 2-Winding Transformer into Auto-transformer;
33. Parallel Operation of Single-phase Transformers;
34. Questions and Answers on Transformers;
35. Three-phase Transformers;
36. Three-phase Transformer Connections;
37. Star/Star or Y/Y Connection;
38. Delta-Delta or ∆/∆ Connection;
39. Wye/Delta or Y/ Connection;
40. Delta/Wye or ∆/Y Connection;
41. Open-Delta or V-V Connection;
42. Power Supplied by V-V Bank;
43. Scott Connection or T-T Connection;
44. Three-phase to Two-Phase Conversion and vice-versa;
45. Parallel Operation of 3-phase Transformers;
46. Instrument Transformers;
47. Current Transformers;
48. Potential or Voltage Transformers.