7.1 - Alternating Currents & Transformers Flashcards
What is the definition of an alternating current?
A current which periodically varies between a positive to a negative value with time.
What does A.C look like plotted on a graph?
Sinusoidal as the current varies every half cycle.
What is peak current/ peak voltage defined as?
The maximum value of the alternating current or voltage.
What is the rms value of an alternating current defined as?
The square root of the mean of the squares of all the values of the current in one cycle.
In an A.C circuit, how do electrons move in the current carrying wire?
They move back and forth with simple harmonic motion.
What is the rms of A.C equivalent to?
The direct current that produces the same power.
What is the equation to calculate the rms value of A.C?
Iᵣₘₛ = I₀/√ 2
I₀ = peak current
What is the definition of the rms value of an alternating voltage?
The square root of the mean of the squares of all the values of the voltage in one cycle.
What is the rms value of alternating voltage equivalent to?
The d.c voltage that produces the same power.
What is the equation to calculate the rms value of an alternating voltage?
Vᵣₘₛ = V₀/√ 2
V₀ = peak voltage
What is the rms value defined as?
The steady direct current, or voltage, that delivers the same average power in a resistor as the alternating current, or voltage.
How do you calculate the average power of a A.C circuit?
Average power = Iᵣₘₛ x Vᵣₘₛ
What are the applications of alternating current/voltage?
- Mains electricity is supplied as alternating current by the National Grid
What is the voltage and frequency produced by the mains electricity that goes into households?
- 230V (rms value)
- 50Hz
What does CRO stand for?
Cathode-Ray Oscilloscope.
What is a CRO used for?
It is a laboratory instrument used to display, measure and analyse waveforms of electrical circuits (can be used as an a.c and d.c voltmeter).
What is A.C voltage on an oscilloscope represented as? (what does it look like on the screen)
Transverse wave.
Since A.C voltage is displayed as a transverse wave on an oscilloscope, what can properties be measured?
- frequency, time period and peak voltage.
What is D.C voltage represented as on an oscilloscope? (what does it look like on the screen)
As a horizontal line at the relevant voltage.
Since D.C voltage is displayed as a horizontal line on an oscilloscope, what can properties be measured?
-x-axis is time, y-axis is voltage
- therefore voltage level is shown
What is the time-base on an CRO?
how many seconds each division represents.
(s/div, s/cm)
What happens when the time-base is turned off on a CRO for A.C?
Only a vertical line on the voltage-gain axis is seen with its relevant amplitude.
What does voltage-gain mean on an CRO?
The amount of voltage per division (per square).
What is a transformer
A device that changes high alternating voltage at low current to low alternating voltage at high current, (vice versa).
Why were transformers designed, what do they help with?
They reduce heat energy lost whilst electricity is transmitted down electrical power lines from power stations to the national grid.
What are transformers made up of?
- A primary coil
- A secondary coil
- An iron core.
What is the structure of a transformer?
The primary and secondary coils are wound around the soft iron core.
There is a magnetic field in the iron core.
Why is the soft iron core of a transformer necessary?
- It focuses and directs the magnetic field from the primary and secondary coil.
- It can easily be magnetised and demagnetised
What happens in the primary coil of a transformer?
- An alternating current producing an alternative voltage is applied.
- this creates an alternating magnetic field inside the iron core and therefore a changing magnetic flux linkage
What happens in the secondary coil of a transformer?
- A changing magnetic field passes through the coil through the iron core
- This results in a changing magnetic flux linkage in the coil and therefore an emf is induced
- This emf produces an alternating output voltage from the secondary coil (same frequency as input voltage)
What is the transformer equation?
Nₛ/Nₚ = Vₛ/Vₚ
Nₛ = number of turns in the secondary coil
Nₚ = number of turns in the primary coil
Vₛ = output voltage from the secondary coil
Vₚ = input voltage in the primary coil
What is a step up transformer?
A transformer that increases the voltage of the power source.
Nₛ>Nₚ
What is a step down transformer?
A transformer that decreases the voltage of the power source.
Nₚ>Nₛ
When is a step up transformer used?
Between power stations and transmission wires.
When is a step down transformer used?
Between the transmission wire and buildings.
What is the transformer efficiency equation?
efficiency = Iₛvₛ/IₚVₚ
Iₛ = output current from the secondary coil
vₛ = output voltage from the secondary coil
Iₚ = current in the primary coil
Vₚ = voltage in the primary coil
What can the ideal transformer equation also be written as? (ratios)
Iₛ/Iₚ = Vₚ/vₛ = Nₚ/Nₛ
What creates eddy currents?
- A changing magnetic field (and flux) from the alternating current/voltage
- This creates a changing magnetic field in the core that acts against the field that induced them
- An emf is therefore induced
- A current flows, as the core is made from a conducting material
How are eddy currents reduced?
- Laminating the iron core with layers of insulation
- Having a core made from a high resistivity material
How does laminating the core reduce eddy currents?
The core is built of thin layers of metal instead of solid metal, reducing the eddy currents.
The laminations are insulated from each other, so the current doesn’t flow between them.
How do the wires in a transformer contribute to inefficiencies?
- Coils of wire have resistance
- This causes heat energy to be lost from the current flowing through the coils
- The larger the current, the greater the amount of heat energy lost
What are 3 things that create inefficiencies in transformers?
- Induced eddy currents
- The reversal of magnetism
- Poor insulation between the primary and secondary coil
What are 4 ways to reduce energy loses in transformers?
- Making the core from soft iron
- Laminate the core
- Use thick wires - reduces resistance
- Using a core that allows all the flux due to the primary coil to be linked to the secondary coil
Why are step up transformers used to increase the voltage generated in power stations?
- voltage increases and current decreases (through transmission lines)
- decreased current decreases heat energy loses
