Transformer formulas

How much current can be drawn from a 150 VA transformer if it has an output voltage of 40V?

The current (I) can be determined by dividing the VA rating by the voltage.

I = VA / V = 150/40= 3.75A

What is the kVA rating of the transformer if it has a maximum output voltage of 230V and supplies a current of 14A?

kVA = V x I / 1000

230V x 14A / 1000 = 3.22VA

(answer times by 1000 if value needed in VA)

What is the formula to find the line current (iL) by a transformer if a 15 kVA three-phase transformer has an output line voltage of 400V.

15kVA must be converted into VA for this formula so it must be multiplied by 1000. √ might be expressed as 1.732 for some formula.

By transportation: iL = 15000 VA / √3 / 400V = 21.65A

Calculate output voltages for a transformer having a turns ratio of 2:9 is connected to a 200 V supply.

There are 60 turns on the primary winding and 270 on the secondary winding (which still gives a 2:9 ratio). If these numbers are inserted into the ratio formula:

Vs = Vp x Ns / Np

Vs = 200V x 270t / 60t = 900

Determine the volts per turn on a single-phase transformer that has 2,500 turns on its primary winding and 750 turns on its secondary winding whilst being fed from a 230 V AC supply.

Vp/Np = 230/2500t = 0.092V per turn

(each turn has 0.092 turns across it)

What is the output kVA of a single-phase transformer being used to supply a 230V/110V SELV? The transformer’s primary winding is fed from a 230V 50Hz AC supply. The 80 turn secondary winding needs to provide an output current of 200 A at 110V.

kVA out = Vs x Is / 1000

110V x 200A / 1000 = 22kVA

Determine the primary current for a single-phase transformer being used to supply a 230V/110V reduced low voltage supply system. The transformer’s primary winding is fed from a 230V 50Hz AC supply. The 80 turn secondary winding needs to provide an output current of 200A at 110V.

Ip = Vs x Is / Vp

110V x 200A / 230V = 95.65 A

Determine the primary current of a single-phase transformer being used to supply a 230V/110V reduced low voltage supply system. The transformer’s primary winding is fed from a 230V 50Hz AC supply. The 80 turn secondary winding needs to provide an output current of 200A at 110V.

For the primary: Vp/Np = 230V/167t = volts per turn

For the secondary: Vs/Ns = 110V/80t = 1.83 volts per turn.

What is the formula for volts per turn for each windings for a single-phase transformer being used to supply a 230V/110V reduced low voltage supply system. The transformer’s primary winding is fed from a 230V 50Hz AC supply. The 80 turn secondary winding needs to provide an output current of 200A at 110V.

For primary: Vp/Np = 230/110 = 1.38 volts per turn

For secondary: Vs/Ns = 110V/80t = 1.38 volts per turn