Sally travels to New Zealand and carries a transformer to convert New Zealand’s

Question

Sally travels to New Zealand and carries a transformer to convert New Zealand’s standard 240 V electricity to 120 V so that she can use some small appliances from America on her trip. The transformer connects to a wall socket in New Zealand and provides socket that is equivalent to a wall socket in America.

a. If the tranformer contains a total of 1650 turns, how many turns do the primary and secondary coils have?

b. If Sally plugs in an appliance that uses 100 mA of current, how much current will the transformer draw from the wall?

c. Sally plugs a coffee grinder into the transformer, and the transformer draws 416 mA of current from the wall. What is the resistance of the coffee grinder?

Explanation / Answer

a)

Ep = primary voltage = 240

Es = secondry voltage = 120

Np = number of primary turns

Ns = num,ber of secondry turns

given that : Np + Ns = 1650                 eq-1

Ep / Es = Np/Ns

240/120 = Np/Ns

Np= 2Ns                       eq-2

using eq-1 and eq-2

2Ns  + Ns = 1650

Ns = 550

Np= 2Ns = 2 x 550 = 1100

b)

is = 100 mA

ip = ?

using the formula

ip/is = Ns/Np

ip/100 = 1/2

ip = 50 mA

c)

ip = 416 mA

is = ?

using the formula

ip/is = Ns/Np

416/is = 1/2

is = 832 mA

Es = 120

R = Es /is = 120/0.832 = 144.23 ohm

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