back emf. (1 Viewer)

[atBondi]

Ok, a little confused cause i have contradicting notes? :S

One of my advantages for DC generators is :
• No back emf once current is flowing.

but, when i was doing this question,
" a back emf is: "
A. found in AC motors only.
B. found in DC motors only
C. adds to supply voltage
D. subtracts from supply voltage.

and, i understand why the answer would be D, and there would be back emf in DC motors due to cutting of flux and inducing another current.... but then why do my notes say " no back emf once current is flowing"??
Something to do with one is genrator and one is motor?

 

[Aquawhite]

Your notes seem to be flawed and jumbled up. I suggest that you return to your textbook and take the correct notes.

 

[atBondi]

would you mind explaining to me if there is back emf in DC motors and generators?
because i got those notes from my teacher.. so either im interpreting it wrong..or...

 

[adomad]

would you mind explaining to me if there is back emf in DC motors and generators?
because i got those notes from my teacher.. so either im interpreting it wrong..or...

back EMF only occurs in motors...

when u have a motor, you have a supply emf causing the coil to rotate as the current interacts with the magnetic field and the coil/ armature rotates (the motor effect). since you are cutting flux lines i.e. a change in flux, an emf will be induced in such a way to to oppose the original magnetic field that induced it. hence the back emf will flow against the supply emf and hence it is called back emf.

with a generator, you don't have a supply emf so thus there is no back emf. it is just an emf. Lenz's law will still apply as the magenetic field produced by the current in the wire will oppose the magentic field hence if you stop spinning the handle, the coil stops spinning.

 

[atBondi]

Ahhh that makes sense, however when u say no back emf is in generators..is that only in d.c?? Or does that apply to a.c as well cause i am sure my teacher said negative for a.c generators was back emf, so is she teaching it very wrong...

 

[atBondi]

can anyone answer my question?

 

[alcalder]

Firstly, consider a generator.

Movement is provided to the coil that turns in a magnetic field.
This movement, produces a current in the coil.

Now we must remember Lenz's Law that says anything produced will be in such a direction as to oppose the change that caused it. (In accordance with Law of Conservation of Energy.)

Therefore, the current produced in the coil by its turning will now generate its own magnetic field which will actually be opposite to the one in which the coil is turning.

If the coil is not forced to turn, it will thus come to a halt.
Net magnetic field = Applied magnetic field - opposing mag field


Now, if we consider the motor.

We provide current to a coil, to make it turn in a magnetic field. This turning now produces it's own current (like in the above example of a generator). Lenz's Law again and we need to remember that the current produced by the motor will be in opposition to the current we feed into the coil to make it move.

It is this opposing current that provides the BACK EMF. V = IR (current produced and resistance of the coil)

So,
net voltage in coil = supply voltage - back emf

Now, if there is no load on the motor no current will flow and motor will increase in speed until back emf = supply voltage so that there is eventually no net voltage in the coil and hence no force on the coil and the coil will rotate at a constant rate.
F = BIAsinX

Remember, F=ma (if F=0, then a=0, v is constant)

If there is a load on the motor, current flows (there is back emf) and the coil cannot rotate as fast. So, back emf<supply voltage, and there is a FORCE that will do the WORK. But remember

I = V/R

So the net voltage AND the resistance of the coil determine the current that can flow.

So, if a motor is OVERLOADED, the coil rotates too slowly and the back emf is SMALL resulting in a large net voltage and hence large current that could burn out the motor.

Whether a motor is AC or DC does not change this because essentially the current flowing in the coil emulates AC no matter what because of the commutator ring in the DC motor assembly.

I hope that helps clear it up a bit.

 

[adomad]

there is no back EMF in any generator. if there is no supply EMF then what is the induced EMF going to go against. back EMF exist in a motor because you have a coil with a EMF in it causing it to spin. as it spins, the wire is cutting flux ( or there is a change in flux), so an EMF is induced. using Lenz's law, we know that the EMF flows in the opposite direction to the supply or applied EMF.

as explained in the post above by alcalder, the result of lenz's law is that the EMF will be produced in such a way to oppose the original magnetic field that induced it. so the equivlent to "back EMF" in this case is really "back magnetic-field".