back emf (1 Viewer)

[stinger]

is it right that eventually in a motor, the back emf will equal the supply emf (input voltage)?

i also read, that the motor will keep on turning even though there is no current in the coil when: back emf = supply emf

how does this happen? inertia? wouldn't friction with the air or something cause it to slow down

 

[gnrlies]

Well i suppose theoretically the back EMF could equal the supply EMF, but this could only occur in a perfectly efficient motor. No real motor is perfectly efficient so this would not be the case...

As for the rotor still turning, well I dont quite know what you meant, but Inertia would be the only force making it move as the supply emf = back emf there would be no resultant force therefore the inertia of the object should keep it moving.

But i dont think you really need to know that for the course. I think you probably only need to know why a rotor would come to a stop at 90 degrees to the field without a split ring commutator, and that interia is what keeps it going when it is at that point of zero torque force.

 

[howareya]

can someone explain back EMF i dont get it

 

[wogboy]

can someone explain back EMF i dont get it

As a motor spins when you apply power (e.g. from a battery) to it, while the motor is spinning it also acts like a generator (i.e. generates an emf as it spins just like a generator). This emf generated by it is called the "back emf", and this back emf always opposes the supplied emf in direction. The size of the back emf depends on how fast your motor is spinning. Spinning fast ==> larger back emf, spinning slow ==> smaller back emf.

is it right that eventually in a motor, the back emf will equal the supply emf (input voltage)?

Nope, impossible. Back emf is always smaller than the supply emf. As you increase the total conductivity (i.e. reduce the resistance) of the circuit (including the resistance of the power supply, connecting wires and motor coils), the back emf approaches the supplied emf however it never actually reaches it, unless your entire circuit has 0 ohms of resistance (i.e. everything conducts PERFECTLY). You can only "0 ohms" with superconducting wires and coils, but there still remains a problem which is the resistance of the power supply. It is impossible to have a superconductive power supply. If it was possible you could generate infinite power (how cool would that be? ).

Consider this, if there was a superconducting power supply and you connected a superconductor across its positive & negative terminals, there would be infinite current and infinite power flowing in the circuit! Can this happen?

 

[stag_j]

try thinking about the forces involved... while a force acts there will be acceleration, but we know that the motor will not continue to speed up infinitely - it will reach a maximum at some point.
as the speed increases, the back emf will increase accordingly, so eventually the back emf will be close to the supply emf, thus there will be negligable force acting.
in reality they won't ever be quite equal, since there will be friction, resistance etc as other people have explained.