a very quick q (1 Viewer)

[Sirius Black]

An electric motor is used to lift load when its load decreases, are the rotational speed and back emf decreasing, and current is increasing(due to less eddy current)?

 

[Budz]

hey i dont understand what ur asking

 

[Sirius Black]

okay the question is- when the load on an electric current with constant voltage supply decreases, wat happens to its rotational speed, back emf, and induced current?

 

[Budz]

What do u mean by load?

 

[Sirius Black]

okay, i can't make the q any simpler-you just consider load as the mass of the object which needs to be lifted up

 

[Budz]

Ahhh i get it
Well if the load decreases the rotational speed increases and hence back emf increases. Although back emf will oppose the rotational speed it is not as significant as reducing the load so the motor will increase in speed. However total current remains the same...
Ummmmm..... Think of a power drill
If you push on the trigger the drill bit will rotate, the harder u push the less resistance hence the faster rotational speed. According to Faraday the quicker it turns the greater the induced back emf, If you drill into something, say wood due to the extra 'load' the drill slows, back emf reduces, however the trigger is still impressed, the resistance on the curicuit has not changed, the total current remains the same....

Now the question is, if the back emf changes shouldn't over all current change aswell, No because when dealing with motors will only reference initial supplied current...

Hope that helps.

 

[Sirius Black]

Ahhh i get it
Well if the load decreases the rotational speed increases and hence back emf increases. Although back emf will oppose the rotational speed it is not as significant as reducing the load so the motor will increase in speed. However total current remains the same...

er in this case, if back emf increases then the induced current must increases as well hen the total current (=constant supplied current-high eddy current)should decrease

 

[Iron_Scarecrow]

I got to start listening in class.

 

[Budz]

back emf increases then the induced current must increases

Um there is no induced current in a motor apart from the back emf, iI think u mean the supplied current, which again does not change. the supplied current remains constant, the total current within the loop fluctuates in accordance to the load, because back emf changes.

 

[Sirius Black]

The back emf is actually induced by the induced current i.e. change in magnetic flux-->induced current-->back emf-->opposes flux

 

[Budz]

Yeah so whats your question.

 

[Budz]

you say the induced back emf is due to the induced current. which is true because where there is an electromoive force there is a current. But its still does not affect the initial supplied current

 

[richz]

ok, if the load's mass (thats wat i'm assuming ur trying to say) decreases the motor will rotate faster so the back emf increases. The smaller the back emf is the greater the current flowing through the coil. So in this case the current increases as well.

hope that helps

 

[Sirius Black]

ok, if the load's mass (thats wat i'm assuming ur trying to say) decreases the motor will rotate faster so the back emf increases. The smaller the back emf is the greater the current flowing through the coil. So in this case the current increases as well.

hope that helps

Sorry.-ur statement seems to be contradictory as you said back emf increasesbut then you said "The smaller the back emf..."
I still think the total current is decreasing coz the supplied current is constant but there is greater induced current due to faster rotation hence result in greater back emf(emf=-delta flux/delta t=-BLv)

 

[richz]

yeah thats just a different sentence, i was just stating the relationship between back emf and the current flowing in the coil.

 

[Budz]

Yer but that means if the back emf increases the total current dcreases that means the speed will decrease hence the back emf will decrease hence the total current will increase meaing the loop will spin quicker hence the back emf will increase and then the current will decrease then the speed will decrease and back emf will decrease and ...... so on... and so on...
it will just keep changing speed bak emf and total current in loop in a contiuously