These two types of transformers- step up and step down transformer are used to step-up voltage from low value to high value according to the turns ratio of primary and secondary windings.
Step-up transformer
This transformer has less primary winding than secondary winding so, we find that the current that flows through primary winding is greater than secondary winding. According to this equation, a step-up transformer is;
P= V * I (for an ideal transformer that means no losses in power )
Such that :
P: power in watt
V: Voltage in volt
I: Current in Ampere
So, Power in primary = Power in secondary (for an ideal transformer)
Vp * Ip = Vs* Is
Vp: primary voltage of the transformer
Vs : secondary Voltage of Transformer
Ip: Primary current of the transformer
Is: secondary current of the transformer
As example :
A single-phase ideal transformer of 100 watts with a primary winding of 5 turns and a secondary winding of 20 turns and a primary voltage of 100 volts. How do you calculate the currents at primary and secondary sides and voltage at secondary side? Here is the answer.
Solution:
P = V * I
100 = 100 * Ip
Then …. Ip = 1 ampere
For
So , Vs = 400 volt
For an ideal transformer :
Power in primary = Power in secondary (for an ideal transformer)
Vp * Ip = Vs* Is
100 * 1 = 400 * Is
So, Is = 0.25 Ampere
According to what we explain; you must deduce that step-up transformers are present in power plants that operate megawatts of power. Also in machines that require high voltage to work like X-ray and micro-wave.
Step down transformer
This transformer has greater primary winding than secondary winding. So, we find that the current that flows through the primary winding is less than secondary winding. Also, the primary voltage is higher than the secondary voltage which makes step-up transformer the best in domestic consumption.
Now you may be asking, can we use the same transformer of step-up or step-down transformer at the same time? In other words, if I have a step-up transformer can I use it as a step-down transformer and vice versa?
As we mentioned earlier, the main task of the transformer is stepping up or stepping down voltage from the primary side to the secondary side assuming that losses are eligible.
Any transformer designed as a step-up or step-down transformer has a primary and secondary winding designed to carry specified current. So, if I have step-up transformer of 1000 watt with this parameter:
Vp = 100 volt , Np = 100 Turns , Ns = 1000 turns
So,
P = V * I
1000 = 100 * Ip
Then …. Ip = 10 amperes ( so we use wires for these winding with cross-section area withstand this current )
For
Vp/Vs = Np/Ns
100/Vs = 100/1000
So , Vs = 1000 volt
For an ideal transformer :
Power in primary = Power in secondary (for an ideal transformer)
Vp * Ip = Vs* Is
100 * 10 = 1000 * Is
So, Is = 1 Ampere (so we use wires for these winding with cross-section area withstand this current )
From the previous example, we conclude that if I use a step-up transformer as a step-down transformer, the secondary winding (now it primary) will burn because these winding are designed to withstand less current.
So, we can’t use a step-up transformer as a step-down transformer and vice versa.
Applications of a step-up and step-down transformer
Step-up transformers are used at the power plant to decrease currents before the transmission process along transmission lines
Step=down transformers are used at substations to decrease voltage to distribute to customers.