AC motor & back emf (1 Viewer)

[MyLuv]

In the success one book,they said:"a.c motor continously produces a back_emf limits itself and so doesnt need a series resistor at start"
I dont get it since DC motor also has back_emf and both of them(AC+DC)spin slowly at the beginning thus small back_emf and need a resistor to prevent damage to motor,isnt it???
Can some1 explain plz???

 

[+:: $i[Q]u3 ::+]

with the dc motor - the current goes thru the coil. at the beginning the coil's turning real slow. So the magnetic flux thru the coil is changing at a slow rate. So there's small back emf and u need a resisor.

But with an ac induction motor.. (i'm only considering this one cuz it's the only one i know =P) the current produces that shifting b-field. Which induces currents in the rotor, which causes it to turn. In AC motors, it IS the back emf that causes it to turn.
with dc - it's the supplied emf. the back emf opposes it.

hmm.. have i gotten myself confused?

 

[jims]

perhaps its refering to an AC induction motor.

 

[jims]

nuts
i didnt even bother to write all that out but still got beaten

 

[ND]

Originally posted by +:: $i[Q]u3 ::+
But with an ac induction motor.. (i'm only considering this one cuz it's the only one i know =P) the current produces that shifting b-field. Which induces currents in the rotor, which causes it to turn. In AC motors, it IS the back emf that causes it to turn.
with dc - it's the supplied emf. the back emf opposes it.

hmm.. have i gotten myself confused?

Yep that's right (except we call it induced emf if it's not opposing the applied emf), but i think the question is refering to synchronous ac motors.

I thought that ac motors did use resistors for that reason... I don't understand how it would limit itself, now i'm interested.

 

[MyLuv]

So if they ask something like that,we should assume its the AC induction motor right???
Btw, I've read somewhere that most AC motor are induction motor not synchronous.

 

[ND]

Originally posted by MyLuv
So if they ask something like that,we should assume its the AC induction motor right???
Btw, I've read somewhere that most AC motor are induction motor not synchronous.

Nah if you get asked that, i'd try and make something up on the spot. That statement is definately not refering to induction motors (and if asked in an exam, it would specifically say synchronous, so there is no need to assume). While the induced current in induction motors is definately limited, that's not the reason it doesn't need a resistor. The reason for a resistor is to stop a large current flowing which could result in damage to the wire or something. In an induction motor this is not an issue at all. If you find out how they limit themselves, please let me know.

 

[+:: $i[Q]u3 ::+]

dont' ac motors limit themselves cuz the degree of induced emf is dependent on the rotating b-field (50hz) and not on currents thru the coil?
and what are synchronous ac motors? (must we know about them? ^^)

 

[ND]

Originally posted by +:: $i[Q]u3 ::+
dont' ac motors limit themselves cuz the degree of induced emf is dependent on the rotating b-field (50hz) and not on currents thru the coil?

Yeh that's right, but this question was not related to induction motors. No one would ever consider putting a resistor in an induction motor, there is just no need.

and what are synchronous ac motors? (must we know about them? ^^)

Synchronous ac motors are just like dc motors, you know, with the slip rings.

 

[Constip8edSkunk]

hmmm i thought AC sybchronous motors as it only runs on 50Hz, therefor 3000 rpm needed a small startup DC current to reach that initial speed before changing to AC current?

Perhaps thats what the original statement is refering to induction motors to purely to state theres no need? strange yes.....

id b interested 2 know the answer too

edit: also maybe coz AC synchronous motors can only run at 50Hz, there'll always be a large back EMF so the current will always be small? as it is induced by the alternating current rather than the rotation speed, the load wouldnt affect the back EMF?

 

[ND]

Originally posted by Constip8edSkunk

edit: also maybe coz AC synchronous motors can only run at 50Hz, there'll always be a large back EMF so the current will always be small? as it is induced by the alternating current rather than the rotation speed, the load wouldnt affect the back EMF?

What do you mean when you say because they run at 50Hz there'll always be a large back emf? Why would there be a large back emf even when the coil is rotating slowly?

But yeh, i was thinking that maybe it lies in the alternating nature of the current, but i can't think how...

 

[Constip8edSkunk]

ok maybe not that large but the back emf shouldnt vary.

i think i have a rough idea now

in DC motors, the back emf is caused by the rate of relative motion of the coil with the magnetic field, hence the rate of rotation(dependent on the load) can change the back emf, there4 the current in teh coil

but in AC synchronous motors, the back emf is caused by the alternating current in the wire rather than the coils motion, therefor the rate at which it turns shouldnt change the magnitude of teh back emf and as the AC current has a set frequency, the back emf is constant, therefore current is constant at a managable level so no resistance is required

 

[MyLuv]

Originally posted by Constip8edSkunk

but in AC synchronous motors, the back emf is caused by the alternating current in the wire rather than the coils motion,

Not really,back_emf in AC Motor is still due to its motion in magnetic field just as DC motor.(Consider the graph of emf produced by DC generator ,DC current varies too).Therefor,an AC sychronous motor still needs a resistor to prevent damage to it.
Btw,I think only AC Induction(most AC motors are) doesnt need a series resistor.

 

[+:: $i[Q]u3 ::+]

lol... i've been sitting here for 20 minutes trying to figure out how the hell AC synchronous dont' need a resistor...
and i refresh it and i see someone's answered =)
*sigh of relief* is myluv right?? *someone plz tell me
cuz an ac motor is the same as a dc job - except for the rings yeah?

 

[Constip8edSkunk]

Originally posted by MyLuv
Not really,back_emf in AC Motor is still due to its motion in magnetic field just as DC motor.(Consider the graph of emf produced by DC generator ,DC current varies too).Therefor,an AC sychronous motor still needs a resistor to prevent damage to it.
Btw,I think only AC Induction(most AC motors are) doesnt need a series resistor.

hmmm.....

edit:
perhaps its because the DC current does not reverse. the transformers is based on the same principle.... yet only AC current will function with it... even the pulsating DC still require a switch to make it work eg in the induction coil.

therefore i think the pulsating DC current will not produce a back emf on its own, but the ac motor still will.

 

[+:: $i[Q]u3 ::+]

i'm askin my teacher tomorrow =P
*narrows eyes at the ppl who started this thread and confused an innocent bystander...* lol...^^

 

[ND]

Originally posted by Constip8edSkunk
hmmm.....

edit:
perhaps its because the DC current does not reverse. the transformers is based on the same principle.... yet only AC current will function with it... even the pulsating DC still require a switch to make it work eg in the induction coil.

therefore i think the pulsating DC current will not produce a back emf on its own, but the ac motor still will.

I'm still not sure on what you mean it will produce it on its own. The only way back emf is produced is by lenz law, so back emf is proportional to the angular velocity of the coil.

Anyway i had a look at a textbook, and as you said skunk, they use a DC current to start up, therefore a resistor is used.

I guess the statement was talking about induction motors then?/ Pretty stupid, i can't imagine why anyone would even think an induction motor would need a resistor at start-up.

 

[+:: $i[Q]u3 ::+]

went to school today.
couldn't find my phys teacher... *um.. that may have because i couldn't be bothered looking very hard... =P
but i asked our grade's no.1 physics man and he said ac synchrous should need a resistor as well. So if he's gettin it wrong, i'm gettin it wrong with him! ^^

 

[Constip8edSkunk]

well i was thinking along lenz's law being an application of induction and vice versa, so... but ... *chucks argument in the bin*

i'll just follow the argument that it needs resistor

hmm ill see if i can find my phys teacher tomorrow

 

[ND]

I think this has been resolved - synchronous ac motors use dc current to startup, and therefore use a resistor.