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back EMF (1 Viewer)
- Thread starter dead
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archangel leila
Member
YES!! i absolutely second that desperate plea 
oohhhh how mr swinfield (aka swindog) has tried but he reassures us, coz of his failing attempts, that the syllabus says u don't actually need to go into detail. But pfft who here likes to understand what they learn? *raises hand*
oohhhh how mr swinfield (aka swindog) has tried but he reassures us, coz of his failing attempts, that the syllabus says u don't actually need to go into detail. But pfft who here likes to understand what they learn? *raises hand*
sunjet
Hip-Hop Saved My Life
back emf is the emf induced in the coils as they spin and due to lenz's law it opposes the emf causing the motion.dead said:can someone please explain what back emf is to me, because i dont understand it one bit when the teacher tries to explain it.
velox
Retired
If it were not to exist, it would violate the Law of conservation of energy. It results in heat, because it causes resistance. It is present whenever there is a current carrying conductor present in a magnetic field.
Jaydels
Member
yes...back emf opposes the supply emf in order to limit the amount of energy produced when the coil rotates in the magnetic field. If the emf produced by the rotating coil were to aid the supply emf, the coil would continually increase its speed, which means that energy is being generated "for free" as my teacher says, which as velox said, violates the law of conservation of energydead said:
hey guys, i know this thread is like heaps old but i've got a really good dot point summary of what back emf is. If you don't need it anymore, just ignore this...
Ok, so for the dot point,
* Explain that, in electric motors, back emf opposes the supply emf
An electric motor has an applied voltage, causing a coil to spin in a magnetic field (motor effect). But as soon as it starts to spin, the coil is in fact also acting like a generator and will induce its own efm (known as 'back emf'). From Lenz's Law, this 'back emf' will oppose the motion inducing it, so the induced voltage will tend to negate or work against the applied voltage of the power supply.
When the motor is spinning quickly, (unloaded) the 'back emf' will be large, cancelling out much of the applied emf, leaving a small effective emf running the motor. This leads to a small current flowing. When the motor is under load, it spins slowly so there is only a small 'back emf'. This doesn't cancel out as much of the applied emf from the power supply, so the effective emf running the motor is higher and therefore, so is the current.
And for this dot point;
*Account for Lenz's Law in terms of conservation of energy and relate it to the production of a "back emf" in motor
Lenz's Law states, the direction of the induced voltage will be such as to produce a magnetic affect that opposes the change in flux (or the motion inducing it)
Hence, if the induced emf was in the same direction as the supplied emf, the resulting induced current could sustain the torque, enabling perpetual motion. ie. for a small energy input to start the motor, it would continue to turn (do work), thus breaking the Law of Conservation of Energy, that energy output can never exceed energy input. So, via the Law of Conservation of Energy, the emf produced MUST oppose the supplied emf and this is known as back emf
Ok, so for the dot point,
* Explain that, in electric motors, back emf opposes the supply emf
An electric motor has an applied voltage, causing a coil to spin in a magnetic field (motor effect). But as soon as it starts to spin, the coil is in fact also acting like a generator and will induce its own efm (known as 'back emf'). From Lenz's Law, this 'back emf' will oppose the motion inducing it, so the induced voltage will tend to negate or work against the applied voltage of the power supply.
When the motor is spinning quickly, (unloaded) the 'back emf' will be large, cancelling out much of the applied emf, leaving a small effective emf running the motor. This leads to a small current flowing. When the motor is under load, it spins slowly so there is only a small 'back emf'. This doesn't cancel out as much of the applied emf from the power supply, so the effective emf running the motor is higher and therefore, so is the current.
And for this dot point;
*Account for Lenz's Law in terms of conservation of energy and relate it to the production of a "back emf" in motor
Lenz's Law states, the direction of the induced voltage will be such as to produce a magnetic affect that opposes the change in flux (or the motion inducing it)
Hence, if the induced emf was in the same direction as the supplied emf, the resulting induced current could sustain the torque, enabling perpetual motion. ie. for a small energy input to start the motor, it would continue to turn (do work), thus breaking the Law of Conservation of Energy, that energy output can never exceed energy input. So, via the Law of Conservation of Energy, the emf produced MUST oppose the supplied emf and this is known as back emf
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