The Right Calculations Add Up To The Ideal Power Current Transformer

This means that if fewer turns are put on the secondary side, we can obtain higher current in the secondary circuit. That is, if we have to replace an electric bulb in the secondary side with one having a higher current rating, we should take off a few turns from the secondary winding.

Or, if we use this transformer to supply some electronic schematic arrangement, we should know its consumption current I_L and wind the appropriate number of turns on the secondary side, as stated by Equation 7. A reasonable zener diode should protect the schematic from overvoltage if this occurs or if the secondary-side induced voltage is higher than allowed.

Therefore, at a given N_P, we can easily obtain the core size using Equation 5 or determine how many primary turns N_P should be wound on the given core having median length l_M. For example, if a current source produces I_P = 1.2 A for the load and we have to feed a 1.7-A incandescent lamp, it would be necessary to design a new transformer for this arrangement.

First, we should get a core suitable for this unit. Let it be a 36- by 23- by 8mm toroidal core with a D_MED of 29.5 mm. Assume that the relative permeability for this core is 3000 and B_SAT = 0.28 T. From Equation 5, we can easily determine the admissible number of turns on the primary side N_P, knowing that l_M = π × D_MED. Therefore:

N_P = (B_SAT / μ_R × μ₀) × l_M/I_P^MAX = 0.28/(4π10^–7 × 3000) × 29.5 × 10^–3/1.2 = 5.736 turns

Assume N_P = 6 turns, which is usually safe because the flux density saturation value was chosen far below the maximum, i.e., there's a safety margin.

Using Equation 7, it's easy to determine the number of turns of the secondary winding as 4.235 turns, which makes the secondary winding 4 turns. Assuming possible losses at the energy transfer, we can verify that this number fits the design pretty well.

It's obvious that in the case when the secondary current should be lower than the primary current, the number of turns on the secondary side would be higher than those on the primary one. That's why current transformers intended for high current measurement have many turns on the secondary side.

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