AC generators use Faraday's law to create alternating current. Transformers are devices that use electromagnetic induction to increase or decrease the voltage of an AC supply.
Equations
description
equation
Induced e.m.f.
ε=−ΔtΔ(BANcosθ)
Turn ratio equation
npns=VpVs
Variable definitions
quantity name
symbol
derived units
si base units
magneticflux
ϕ
Wb
kgm2s−2A−1
magneticfluxdensity
B
T
kgs−2A−1
area
A
m2
m2
numberofcoilturns
N/n
e.m.f.
ε
V
kgm2s−3A−1
time
t
s
s
voltage
V
V
kgm2s−3A−1
Alternating current generators
Alternating current (AC) is a type of electric current which periodically reverses direction.
Electromagnetic induction is used to create alternating currents in AC generators such as the one shown below:
1.
Slip rings
2.
Brushes
3.
Rotation of the coil
4.
Coil
5.
Circuit
It works on the principle of Faraday's law, which can be represented as:
ε=−ΔtΔ(BANcosθ)
As the coil moves in the magnetic field it changes the magnetic flux and generates a current. This then gets reversed as the coil rotates and so on. The values of the induced e.m.f. over time are shown in the following graph:
1.
Flux linkage
2.
Induced e.m.f
3.
Maximum rate of flux change
4.
No flux changing
5.
Max e.m.f
6.
No induced e.m.f
7.
e.m.f = - gradient of magnetic flux linkage graph
Step-up and step-down transformers
Transformers are devices that use electromagnetic induction to increase or decrease the voltage of an AC supply. A simple transformer is shown below:
When an alternating current is supplied to the primary coil it produces a varying magnetic flux in the iron core which then induces a current in the secondary coil. Depending on the number of turns in the coils, the output voltage can be increased or decreased according to the turn-ratio equation:
npns=VpVs
Where n represents the number of turns in a coil and V the voltage in it. The p and s indicate the primary (input) and secondary (output) coils.
A transformer can be set up in two ways:
Step-up transformer; Increases the output voltage. The number of turns in the secondary coil is higher than in the primary coil so the secondary voltage is higher than the primary voltage.
Step-down transformer; decreases the output voltage. The number of turns in the secondary coil is lower than in the primary coil so the secondary voltage is lower than the primary voltage.
Efficiency in transformers
A 100% efficient transformer would transfer all the power from the primary coil into the secondary coil, Pp=Ps. Since P=IV you can write:
IpVp=IsVs
IsIp=VpVs
This means that when increasing the output voltage, the current decreases and vice versa.
To increase the efficiency of a transformer, you can use low resistance wires or use a core made by layers of iron separated by an insulator, to minimise currents induced by the core itself.
Example
A transformer is used to step down a voltage of 230V to 10V. How many turns must the secondary coil have if the primary coil has 500 turns?
Firstly, write down what you know:
Vp=230Vs=10Vnp=500
Next, write down the turn-ratio equation:
npns=VpVs
Rearrange it to find the number of turns in the secondary coil:
ns=VpVsnp
Substitute the numbers and calculate the value:
ns=23010×500=22(2s.f.)
The number of turns in the secondary coil must be 22.
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FAQs - Frequently Asked Questions
How do AC generators create current?
AC generators consist of a coil rotating in a magnetic field. Due to Faraday's law this induces a current in the coil.
What is a transformer?
A transformer is a device that uses electromagnetic induction to increase or decrease the voltage of an AC supply.
How can you increase the efficiency of a transformer?
To increase the efficiency of a transformer you can use low resistance wires or use a core made by layers of iron separated by an insulator to minimise currents induced by the core itself.