11.2.4 Transformer Calculations

• The transformer equation is:

• Where:
  • N_s = number of turns in the secondary coil
  • N_p = number of turns in the primary coil
  • ε_s = output voltage from the secondary coil (V)
  • ε_p = input voltage in the primary coil (V)

There are two types of transformers:

• Step-up transformer (increases the voltage of the power source) where Ns > Np
• Step-down transformer (decreases the voltage of the power source) where Np > Ns
• For an ideal transformer, there is no electrical energy lost and it is 100% efficient
• This means the power in the primary coil equals the power in the second coil;

• Where:
• • I_p = current in the primary coil (A)
  • I_s = output current from the secondary coil (A)

Worked Example

Step 1: List the known quantities

• • Primary voltage, ε_p = 0.5 kV = 0.5 × 10³ V
  • Secondary voltage, ε_s = 100 V
  • Number of turns in the primary coil, N_p = 3000 turns

Step 2: Write down the transformer equation

Step 3: Rearrange for number of turns in the secondary coil

Step 4: Substitute in the values

= 600 turns