Motors and Generators Flashcards
what is a transformer?
a device which changes the potential of AC electricity by utilising the principle of electromagnetic induction
what does a transformer consist of?
a transformer usually consists of:
- two coils, the primary and secondary coil
- a soft iron core in which the coils are wrapped around
- fans to keep the transformer cool
why is AC power passed through a step up transformer before it is transmitted to the consumer?
-increasing the potential of the electricity helps to minimise power losses that occur as a result of high current, as P=I^2*R. By increasing this potential, current is reduced
this in turn saves costs and power
why are transformers used in electricial networks?
- transformers help to minimise losses in power during AC transmission
- transformers also are used to decrease the potential of electricity in small household appliances, increasing safety
- transformers may also increase the potential for use in some appliances such as Televisions.
what benefits has AC generators and transformers had on society and the environment.
- AC generators create electricity which has its ability to have its potential changed by a transformer. this means that the potential of electricity during transmission can be increased to minimise power losses
- because this potential can be changed, AC generators can be made close to the resources which are needed to fuel them, savings costs and making the urban areas less polluted
- AC power can be mass produced and distributed extensively, creating an economy of scale and providing this power to the average consumer. this also means that people dont need to burn coal and fuel in their home, meaning less pollution in urban areas.
what were the negative impacts of AC generators and transformers on society and the environment?
- AC generators made many unskilled jobs redundant, increasing unemployment levels
- increased risk of electrocution
- caused a large scale production of pollution in the form of sulfur, nitrogen, oxides, carbon dioxide : major contributor to global warming
- increased dependency on electricity
- polluted the rural areas
what is my assessment of the impact of AC generators on society and the environment?
AC generators created electricity which could be transmitted economically to the average consumer. it had many labour saving benefits but had a dramatic impact on the environment, being one of the major contributors to global warming. all in all, it had a large impact on society and the environment.
where do energy losses occur during electrical transmission?
energy losses may occur in the:
- generator
- transformer
- power lines and power poles
power is mainly lossed through eddy currents and heat.
why is their energy losses during transmission of electricity?
- because of resistance, a high resistance in a wire correlates to it becoming very hot. to counter this, transformers make sure the current in these transmission wires are very small
- because of eddy currents due to changing flux. these can occur in power poles, generators and in transformers. according to lenz law, a current will be induced in a conductive material in order to oppose a change in flux. as AC is constantly changing direction, it is constanstly producing a change in flux, leading to formation of eddy currents.
how is power loss in transformers minimised?
- fans are often used to keep the transformer cool
- the soft iron/ferrite core is often laminated in order to reduce the size of eddy currents. these laminations are insulated by lacquer.
- the core can me made out of non conductive material but still ferromagnetic including ferrites; complex iron oxides
- immersing the transformer in non-conductive oil. this oil is circulated around the transformer helping to dissipate heat.
- the transformer can be placed above the ground out of the sun to reduce exposure to the sun and better the circulation and ventilation of air
- often transformer cases have heat sinks which increase surface area for heat dissipation
what is a rectifier?
a rectifier is a device which can convert between AC and DC electricity.
why are transformers and rectifiers found in small househole devices like mobile phones or electric toothbrushes?
because those often contain batteries which can only operate on low, DC potentials (voltages).
how do induction cooktops work?
induction cooktops usilise an electrical circuit below the cooking surface consisting of a coil connected to an AC supply.
as AC is fed into the coil, this creates a varyiing magnetic field and hence a change in flux in the pot or pan being heated. eddy currents then form in these pots or pans, heating it up and its contents. this is called resistive heating
what are some advantages of induction cooktops?
- they are very efficient, all the heat/power goes in to heating the pan
- it is safer than other cooktops, only the pan or pot becomes hot, not the ceramic surface.
how have eddy currents been utilised in electromagnetic braking?
whats so great about electromagnetic braking?
eddy currents are used in trains to brake them. they are also used in some theme park rides.
the relative motion between the wheel of a vehicle and a magnetic filed creates a change in flux and thus eddy currents in the wheel. this effect then creates a braking force.
The great thing about electromagnetic braking is that it works best at very high speeds, because the more change in flux the more emf and hence greater braking force, providing a smooth and controlled stop. also there is no friction involved, and hence it provides less wear and tear to the braking system and thus it is more efficient then conventional braking systems used in cars etc.
what are the advantages of AC generators?
- they create electricity which can be transmitted long distances with little power loss, all thanks to the transformer
- slip rings provide less wear and tear than split ring commutators. they also provide more electrical contact and thus can draw higher currents
- AC generators have less moving parts and thus require less maintance than DC generators. they produce less heat, less noise, less friction and are more compact than DC generators.
what are the disadvantages of AC generators?
- AC power is a hell of a lot dangerous than DC power
- The output of AC generators need to be constantly shielded so that energy is not lost to the environment by induction, as AC is always creating a change in flux
- High potential AC is constantly subject to arcing and so are more dangerous to their surroundings than DC. stronger insulation is required as compared to DC generated power.
what factors affect the magnitiude of the force on a straight conductor in a magnetic field?
- the strength of the external magnetic field
- the magnitude of the current in the conductor
- the length of the conductor in the field
- the angle between the conductor and the external magnetic field
how does magnetic field affect the force on a current carrying conductor?
force is proportional to the strength of the magnetic field, B
therefore the greater the external magnetic field, the greater the force experienced by the wire
how does current affect the force on a current carrying conductor?
the greater the current, the more electrons are in motion, hence the greater force on the conductor
the force on the conductor is the sum of all the forces on the individual electrons
what affect does length have on the force experienced by a current carrying conductor?
greater the length, the more electrons experience a force at the same time and hence greater total force
how does the angle between a conductor and the external B field affect the force on that conductor?
the magnitude of the force is proportional to the component of the field that is at right angles to the conductor
the force is a max when the conductor is at right angles to the field
the force is a min when the conductor is parallel to the field
what is the equation of the force on a current carrying conductor in a magnetic field?
F=BILsinx
F is force
B is magnetic field strength
L is the length of the conductor
x is the angle between the conductor and magnetic field
what is torque?
torque is the turning effect of a force acting on an object
how do i achieve the greatest torque possible on an object that can be rotated around a pivot axis?
i must apply a force as far from the pivot point as possible and the force must be at 90 degrees to a line joining the point of application of the force and the pivot axis
since torque is greatest when it is applied at right angles, what is torque in terms of components?
torque is the product of the tangential component of the force and the distance the force is applied from the axis of rotation
what are the two formulas for torque?
T=fd and T=fdsinx
where f is the force, d is the distance from the pixot axis the force is applied and x is the angle between the force and the line joining the point of application to the pivot axis
what is the motor effect?
the motor effect is the conversion of moving electric charge in a magnetic field into kinetic energy.
what causes the motor effect?
the motor effect is caused by the interaction of the magnetic field produced by a wire and the external magnetic field in which the wire is placed. this interaction creates a force.
how can i determine the direction of a force caused by the motor effect?
by using the right hand palm rule where
- the thumb is current
- the fingers is B field
- palm is force
what are the main features of a DC motor?
- magnets
- iron core
- armature
- power source
- rotor coils
- split ring commutator
- brushes
- axle
distinguish between a stator and rotor
a stator is the stuff that is stationary in a generator or motor
the rotor is the stuff that is moving in a generator or motor
what is the stator and what is the rotor in a DC motor?
stator
- magnets
- brushes
- power source
rotor
- split ring commutator
- rotor coils
- amarature
- iron core
what are the two ways in which the required B fields in DC motors can be produced?
the external B field can be made by either
permanent magnets –> they ara shaped and fited to fit around the armature
current carrying coils/electromagnets—> made by winding coils of wire around iron cores attatched to the case of the motor and passing current through these coils. these coils are wound in such a way in which pairs of coils face the rotor coils creating a north and south pole, just like permanent magnets
who was Michael Faraday?
Michael Faraday was an english scientist who is credited for discovering the phenomena of electromagnetic induction
what is electromagnetic induction?
electromagnetic induction is the generaion of an EMF(electromotive force) or current due to the relative motion between a ocnductor and a magnetic field
what was Michael Faradays first experiment?
two coils were wound around an iron core, one connected to a power source while the other connected to a galvanometer
when the coil connected to the source was powered, a sudden a slight effect was seen on the galvonometer as a reading, meaing current was induced in the wire
the same effect had been observed when the current was switched off, but the deflection of the galvonometer was in the opposite direction
what was Michael Faraday’s second experiment?
he wound an insulated copper wire around a soft iron ring connected to a galvonometer
when he moved a magnet through the ring, movement was observed of the needle in the galvonometer
what did Faraday conclude about the results of his two experiments?
he concluded that a changing magnetic field can induce a current or electromotive force (EMF) in a circuit
