P18: Motors, Generators and Transformers Flashcards
Describe the magnetic field patterns with like poles facing each other.
Bend away from the midpoint.
Describe the magnetic field patterns with unlike poles facing each other.
The field between the poles is uniform.
What is special about a uniform field?
The magnetic field lines are parallel to each other.
What metal is used for the core of electromagnets?
Iron (Easily magnetised and demagnetised)
Name for metals that can be magnetised/ demagnetised easily
ferrous metals
4 examples of electromagnets
> Scrapyard cranes > Circuit breakers > Electric bells > Relays
Increasing _____ makes the magnetic field of an electromagnet stronger.
Current
What surrounds an electromagnets core?
A coil of insulated wire
How can the size of the electromagnetic force be increased
Increasing the current Using a stronger magnet
In the motor effect, when is the force greatest?
When the wire is perpendicular to the magnetic field.
What are the magnetic field lines around a wire?
Circles centred on the wire in a plane perpendicular to the wire.
Reversing the direction of the current…
…reverses the magnetic field lines
The Motor Effect
When a CURRENT is passed along a wire in a magnetic field, A FORCE IS EXERTED on the wire by the magnetic field.
In the motor effect, when is the force zero?
When the wire is parallel to the magnetic field.
Fleming’s Left Hand Rule
Movement
Field (N to S)
Current
Commutator for DC
Split Ring- Reverse the current on every half turn to keep the motor spinning constantly.
Commutator for AC
Slip-ring - Doesnt alternate the current because it alternates already.
Electromagnetic Induction
The process of inducing a potential difference/voltage in a wire by moving the wire so it cuts across the lines of force of a magnetic field.
The Generator Effect
The production of voltage using a changing magnetic field.
What never changes in a DC generator?
The Voltage’s Polarity
What is a simple AC generator made up of?
A coil that spins in a uniform magnetic field.
When is the size of an induced p.d/voltage on an AC generator zero?
When the coil is perpendicular to the magnetic field lines.
When is the size of the induced p.d/voltage on an AC generator greatest?
When the coild is parallel to the direction of the magnetic field.
What types of current does a generator work with?
AC
The Solenoid Rule
Direction of coil tells us which pole it corresponds to.
North- iN
sOUTh- OUT
4 facts about electromagnetic induction.
- No current when the wire is stationary
- A current is generated when the magnet moves but not the wire.
- Faster movement = More current
- Current reverses when the direction of motion reverses.
What must there be for electromagnetic induction to occur?
A changing magnetic field.
3 Acronyms for Transformers
Vicky needs 5 A’s
Voltage Can Make Vicky Cry
PC Ian Can’t Solve Crime
A A A A A
V C M V C
PC IC SC
3 steps of a transformer based on the acronyms
- An alternating voltage in the primary coil creates an alternating current there.
- This creates an alternating magnetic field in the iron core
- This induces an alternating voltage in the secondary coil, which induces an alternating current.
A step up transformer…
Has more turns on the secondary coil, so increases voltage.
A step down transformer…
Has less turns on the secondary coil, so decreases voltage.
The higher the grid voltage…
… the better the efficiency of the transformer.
Equasion for transformer efficiency:
Vp / Ip = Vs / Is
V = Voltage
I = Current
The transformer equation:
Vp / Vs = (No. of turns) Np / Ns
The faster the coil rotates…
…The greater the frequency of the AC current
+
The larger the peak value of the AC current.
Switch mode transformer.
A transformer that works between 50 000 Hz and 200 000 Hz
What is the core of a switch mode transformer made of?
Ferrite
Advantages of a switch mode transformer
> Lighter and Smaller
> Higher frequency
Voltage of the National Grid
132 000 V
Why does the voltage of the national grid being higher make it more efficient.
If the voltage was lower more curren would be needed for the same amount of power to be transferred. The cables would overheat and result in more wasted energy.
What direction are the magnetic field lines in?
North to South
