MY QUESTION IS, DOES LENZ"S LAW ONLY APPLY TO MOTORS AND HENCE BACK EMF? (1 Viewer)

[Mellonie]

SO IS LENZ"S LAW USed to describe the BAck Emf in motor? I mean thats the only way i can understand, back emf.

Is my thinking correct, when lenz says THe current opposes the force which createdit. HE is saying the current is in the other direction to the rotation of the armetur, so an opposite force is acting on the armeture(THe current)... AND THIS IS THE BACK EMF< THE SLOW DOWN EFFECT???

 

[DigitalFortress]

hahahha whoa...

 

[Will Hunting]

Ha ha, you're wigging me out with that crazy CAPS on - caps off typing style, Mel!

Whenever there is relative motion between a conductor and a magnetic field, a back EMF will be induced in opposition to the direction of the motion, or the direction of the applied force. This is the case for both DC and AC motors, as the case of relative motion presents in both scenarios.

In DC motors: A current is supplied, in single phase, to the conductor, which, by the motor effect, experiences a force and a torque because it's in a magnetic field. Now, because it is moving, there is relative motion between the conductor and the stationary magnetic field, by Lenz' Law inducing an emf that opposes the force experienced by the armature, or a BACK EMF!

In AC motors: Same deal, but current is supplied in three - sometimes four - phases (using solid state devices) to create changing magnetic fields and to induce a current in the conductor, which, by the motor effect, experiences a force due to its presence in the magnetic field of the electromagnets (the ones that are being supplied with the current, and whose changing flux is causing induction to occur). Now, the conductor is moving in a magnetic field, causing a back emf in opposition to the direction of the motion.

 

[Mellonie]

Back Emf

alryt, so back emf is an induced current (eddie currents) in the armateur. THese eddie currents induced in the atmeteur, are in the opposite direction of the rotation of armateur??? SO the back emf, is the eddie current induced in armetur. THe eddie currents cause this back emf as the current opposes the direction which caused it..., a slow down effect hence takes place in the motor, (the law of conservation applies now)

 

[gordo]

read carefully

The emf induced in an electric circuit always acts in such a direction that the current it drives around a closed circuit produces a magnetic field which opposes the change in magnetic flux which produces the emf.

 

[Mellonie]

read carefully

The emf induced in an electric circuit always acts in such a direction that the current it drives around a closed circuit produces a magnetic field which opposes the change in magnetic flux which produces the emf.

So the circuit which is acting in the oppsite direction to the force which crated it, produces a magnetic field. THAt magnetic field is in the opposite direction to the rotation of armateur (magnetic flux), which produces the back emf, to act on the armeteur?

 

[jsttesting]

Hey guys,

Just to further add on...I've got a series of questions...

1. In AC motors, does the back emf keep changing it's direction, because of the fact of the alternating current reversing it's direction every half revolution? And in DC motors, the back emf is only in one direction throughout, due to the nature of the DC current (i.e. one direction only), right?

2. What about AC and DC MOTORS? Is there some kind of back emf acting on them?? I'm taking a guess and saying that the AC and DC current INDUCED into the coils CAN be technically called the 'back emf', but what about eddy currents?

Thanks....

 

[Will Hunting]

Hey man,

1. No, in AC motors, back emf always moves in the same direction. This is because it is a product of the motion of the armature through the magnetic field, and this rotation is always in the same direction. It doesn't matter that output alternates, because the motor always spins in the same direction anyway.

2. See my first post in this thread. Back emf applies in both cases. Eddy currents are experienced when sheet metal is moved through a magnetic field, and don't occur in wires.

 

[rama_v]

Hey guys,

2. What about AC and DC MOTORS? Is there some kind of back emf acting on them?? I'm taking a guess and saying that the AC and DC current INDUCED into the coils CAN be technically called the 'back emf', but what about eddy currents?

Thanks....

I think you are referring to AC and DC generators...I dont think back emf is termed eddy currents, but they work on the princple of Lenz's Law. So yes both DC and AC generators produce back emf. This emf causes a torque in the opposite direction to which the rotor is being rotated.

 

[jsttesting]

I think you are referring to AC and DC generators...I dont think back emf is termed eddy currents, but they work on the princple of Lenz's Law. So yes both DC and AC generators produce back emf. This emf causes a torque in the opposite direction to which the rotor is being rotated.

Oh yeah.....oops.....yeah i meant AC and DC generators...

thanks...

 

[Mellonie]

Is eddie currents apparent in both Ac motor and Dc motor

BAsically the question is, is eddie currents in dc motors, and ac motors? cuz some people have told me, only in ac induction current their is eddie currents!
LIke thats confusing if eddie currents r what creates the back emf, due to magnetic field... then shouldnt eddie currents be in dc and ac motor???

BY THE Way someone on this post talked about genearators and back emf together! bAck emf is only apparent in motors! that is a really important factor to ge ur head around...

 

[rama_v]

Teh quick answer to this is no, there are no eddy curents in ordinary DC motors. Why? A definition of an eddy current is basically a swirling curernt produced on teh surface of a conductor due to a change in magnetic flux. A back emf does not swirl around like an eddy, and back emf does not occur on the surface of a conductor. They are two different thigns, but the only thing they have in common is that they both work on Lenz's law. Please feel free to corect me if I am wrong, but this is how I perceive it to be.

Ok now there are definitely eddy currents in AC induction motors. That is the way that induction motors work - teh changign magnetic field created by the AC as it is passed through teh stators create eddys in the rotor which cause it to turn, I think.

 

[Mellonie]

Teh quick answer to this is no, there are no eddy curents in ordinary DC motors. Why? A definition of an eddy current is basically a swirling curernt produced on teh surface of a conductor due to a change in magnetic flux. A back emf does not swirl around like an eddy, and back emf does not occur on the surface of a conductor. They are two different thigns, but the only thing they have in common is that they both work on Lenz's law. Please feel free to corect me if I am wrong, but this is how I perceive it to be.

Ok now there are definitely eddy currents in AC induction motors. That is the way that induction motors work - teh changign magnetic field created by the AC as it is passed through teh stators create eddys in the rotor which cause it to turn, I think.

This is a feature of a DC MOTOR_ from the hsc online syllabus

The armature carries the rotor coils. The iron core greatly concentrates the external magnetic field, increasing the torque on the armature. The laminations reduce eddy currents which might otherwise overheat the armature.

THsi contradicts what u r saying totally, becuz they r saying laminations r used to reduce eddie currents in a dc motor on a armetuer. So i'm correcting u cuz ur wrong

 

[rama_v]

I should have been more clear. There are no eddy currents in the coils...only back emf is produced in the coils. The eddy currents produced are in the iron core of the armature and as u said the laminations reduce the surface area for teh eddy currents to circulate hence reducing their strength.

 

[MuffinMan]

"an induced emf always gives rise to current that creates a magnetic field that opposes the original change in flux through the circuit"

This is a consequence of the Principle of Conservation of Energy. The minus sign in Faradays law is placed there to remind us the direction of the induced emf

USING LEN'Z LAW
----------------------

When determining the direction of the induced emf it is useful to use the field line method for representing magnetic fields (represent magnetic fields as field lines). THE DIRECTION OF THE INDUCED CURRENT CAN BE DUDUCED USING THE RIGHT HAND RULE FOR COILS. The thumb points in the direction of the induced magnetic field within the coil, the curl of fingers holding the coil show the direction of induced current in the coil.

LENZ LAW AND THE PRINCIPLE OF CONSERVATION OF ENERGY
-----------------------------------------------------------------------------------

The principle of conservation of energy states

"Energy cannot be created nor destroyed, but it can be transformed from one form to another"
To create electrical energy in a coil, work must be done. Energy is required to move a magnet towards or away from the coil. Some of this energy is transformed into electrical energy in the coil

LEN'Z LAW AND THE PRODUCTION OF BACK EMF IN MOTORS
-----------------------------------------------------------------------------

It has been shown that emf is induced in a coil that is rotating in an external magnetic field. The emf is produced because THE AMOUNT OF MAGNETIC FLUX THAT IS THREADING THE COIL IS CONSTANTLY CHANGING AS THE COIL ROTATES. The emf induced in the motor's coil, as it rotates in an external magnetic field, is in the OPPOSITE DIRECTION TO THE INPUT VOLTAGE or SUPPLY EMF. If this was not the case, the current will increase and the motor coil will go faster and faster forever. The induced emf produced in the rotation of a motor coil is known as the BACK EMF because it is in the opposite direction to the supply emf.

The speed of the armature coil increases until the back emf is equal to the supply emf. When this occurs there is no voltage across the coil and therefore no current flowing through it. With no current flowing through it there is no net force acting on it and the armature rotates at a CONSTANT RATE

Hope this helps

 

[Mellonie]

I should have been more clear. There are no eddy currents in the coils...only back emf is produced in the coils. The eddy currents produced are in the iron core of the armature and as u said the laminations reduce the surface area for teh eddy currents to circulate hence reducing their strength.

NAr its cool, its just u said that their was only eddie currents in ac induction motor... But eddie currents exist in all motors... THat was my basic question. I understand how back emf works, so for people who keep writing long answers, explaining it... i get that secton. THe fact is eddie currents are the currents which produce a magnetic field, which is in the opposing diferction of the emf, we call this opposing force back emf.

But the fact was eddie currents are apparent in all motors... because if eddie currents aernt induced to create a opposind field, then theirs no back emf. Back emf works on the concept of eddie currents.

Does everyone agree?

 

[MuffinMan]

NAr its cool, its just u said that their was only eddie currents in ac induction motor... But eddie currents exist in all motors... THat was my basic question. I understand how back emf works, so for people who keep writing long answers, explaining it... i get that secton. THe fact is eddie currents are the currents which produce a magnetic field, which is in the opposing diferction of the emf, we call this opposing force back emf.

But the fact was eddie currents are apparent in all motors... because if eddie currents aernt induced to create a opposind field, then theirs no back emf. Back emf works on the concept of eddie currents.

Does everyone agree?

As regards to this...

Induced currents do not occur only in coils and wires. They can also occur

*when there is a magnetic field acting on a part of metal object and there is relative movement between the magnetic field and the object

*when a conductor is moving in an external magnetic field

* when a metal object is subjected to a changing magnetic field

Such currents are known as eddy currents. Eddy currents are an application of Lenz's Law. The magnetic fields set up by eddy currents oppose the change in the magnetic field acting on the regions of the metal objects

 

[Mellonie]

Exactly so eddie currents are apparent in ac motors, dc motors and ac induction motors!

 

[Will Hunting]

read carefully

The emf induced in an electric circuit always acts in such a direction that the current it drives around a closed circuit produces a magnetic field which opposes the change in magnetic flux which produces the emf.

Produces a magnetic field which produces a force which opposes... etc. etc.

If there is a change in magnetic flux, then coil and magnet have been put in relative motion. If they have been put in relative motion, then a force has been applied to one. If a force has been applied to one, then to oppose the change, an equal and opposite force must result.