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a few questions on motors (1 Viewer)

mr EaZy

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i'd appreciate any help :)

1) why would AC motors burn out when heavily loaded (or stall?)?

2) is there an equation for magentic force? coz i came across it in a 2001 trial and i think its old syllabus :rolleyes:

3) compare the FUNCTION of a split ring and slip ring commutator.


4) what does the AC induction motor have to do with 50 Hz?

5) What features of an AC generator make it more efficient than a DC generator?



its a handful, but i hope it benefits everyone :)
 

d_elmo

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1) ?

2)?

3) the slip rings in an AC motor are there basically to complete the circuit (as far as i know) connecting the coils of the motor to the power source (with brushes). The split ring commutator is there in order to alternate the direction of the current every 180 degrees, which allows the armature to continue spinning in one direction

4) I think its that AC power supplies alternate the current 50 times per second - it has a frequency of 50Hz

5)?

i think i need to study more, too many question marks for my liking :p hope i helped u with those 2 anyways
 

unmentionable

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1) its due to back emf
when the motor is heavily loaded it rotates slowly therefore inducing a small back emf
since total voltage = emf from source - back emf
as back emf is small, the total voltage is large
and therefore burns out or stalls
 

jumb

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mr EaZy said:
i'd appreciate any help :)
5) What features of an AC generator make it more efficient than a DC generator?
This probably has something to do with how their energy is transported. I think their production is about the same.
 

senso

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2) not reall y i dont think. Theres a couple with magnetic force in them in the data sheets, like F = BIlsin@,
But if you mean flux, then Flux = BA
 

DavidB

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5. AC Vs DC

The relative advantages and disadvantages of AC and DC generators relate to two features of their design: DC generators use a split-ring commutator, while AC generators use slip rings; and in DC generators the output current is induced in the rotor, whereas the roles of the rotor and the stator can be reversed in an AC generator.

The commutator of a DC generator consists of a number of metal bars separated by narrow gaps filled with insulating material. As the brushes remain in contact with the commutator under spring pressure, they are constantly striking the leading edge of each successive bar. This wears the brushes and they need to be replaced regularly. The commutator bars also wear down until the insulating material between them prevents the brushes from making proper contact with the bars, reducing the efficiency of the generator. Pieces of metal worn from the commutator bars can become lodged in the gaps, causing a short between bars and reducing the output of the generator.

In contrast, the slip rings of an AC generator have continuous, smooth surfaces, allowing the brushes to remain continuously in contact with the slip ring surface. Thus the brushes in an AC generator do not wear as fast as in a DC generator. There is no possibility of creating an electrical short circuit between segments in an alternator because the slip rings are already continuous. An AC generator therefore requires less maintenance and is more reliable than a DC generator. Most commercial generators are AC generators.

In a DC generator the current is generated in the rotor and is then drawn from the windings through the commutator and out via the brushes. The larger the current required, the heavier the rotor coils must be, placing high demands on bearings and supporting structures. In addition, drawing large currents through the commutator-brush connection increases the likelihood of electric arcs forming as the brush breaks contact with each bar in turn. This reduces the efficiency of the generator and creates radio “noise”. This limits the usefulness of DC generators to relatively low current applications.

In an AC generator designed for high current applications, such as in a power station, the current is produced in the stator windings rather than in the rotor. The rotor is used to create the field magnetization that induces the AC current in the stator when the rotor is rotated. It is much easier to draw the current through a fixed connection in the stator rather than through a commutator from a moving rotor. Thus AC generators are better suited to high current demands than DC generators.

An advantage of AC generators is that they can easily be designed to produce three-phase electricity by the use of six stator poles and a single electromagnet rotor. The coils are mounted in opposing pairs spaced evenly around the stator, and connected in pairs to the three phases of the power supply. The rotor induces alternating current in successive pole pairs. The sinusoidally varying voltages are then 120 degrees out of phase with each other. AC generators are ideal for generating electricity on a large scale for distribution over a wide area.

NJOY!!
 

ontherun

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for part 4 mr eazy
i read somewhere that an ac motor alternates only 50Hz in australia!
i dunno if its true though
 

DavidB

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Oh yeh i forgot about qn 2.

F(magnetic) = q.V.B.

where q= charge of particle.
v = speed of particle (m/s)
B = Magnetic feild.

However i think they did remove this from our syllabus, but i learnt it anyways because J.j. Thompson used it in his experiment.

Peace out.
 
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DavidB

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ontherun said:
for part 4 mr eazy
i read somewhere that an ac motor alternates only 50Hz in australia!
i dunno if its true though
well actually our home power supplies, (240V)...is an alternating current, and the frequency of oscillation is 50Hz. But nope nuthn u need to know at all.
 

CrashOveride

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quick q,
the torque always acts so as to position the coil such thats its perpendicular to the magnetic field. at this point, its done its job and the torque is then zero, but the force acting on the wires is at its maximum, but its because then that this force is always opposite in direction which means its net effect is zero right? so the only reason the coil keeps moving when perpendicular to the magnetic field is coz of inertia ?
 

CrashOveride

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also with radial magnetic fields, when they curve the ends of the magnets, it says they do this so that the coil is always parallel to the field, and hence maximum toruqe can be maintained. But isnt the field like coming in at all directions..complicating matters ?
 

tempco

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It's curved so the plane of the coils are always parallel to the magnetic field... as for the field coming out in all directions, field lines come out at right angles to the surface of the magnet... by having curved magnets, the curvature of the magnet surface compensates for the varying angles/distances between the coil plane and the magnets.
 

grimreaper

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yeah the only reason it keeps going when it's perpendicular to the field is because there's nothing stopping it
 

tempco

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I use the iron ring experiment... that's from Jacaranda.
 

DavidB

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wait a secont...

do we have to know the experiment that Faraday carried out?? and what dot point are you guys talkn about?!
 

mr EaZy

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al right thanx everyone.

now solomon, if u inisist on writng "peace out" do it in ur signature.
u need to know that he discovered EM induction and how he did it. Use examples like him moving a mganet through a solenoid with an attached galvanometer. many possibilities
u prob know them already :)
 

Xayma

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Well technically the iron ring isnt a moving magnet but a collapsing and strengthing magnetic field which has the same function. Both should be fine as long as you state that both had the same effect as a moving magnet.
 

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