Motors and Generators Flashcards
(14 cards)
The Motor Effect
A current (I) carrying conductor in the presence of a magnetic field (B) experiences a force (F). This force is at right angles to both the current and the field.
The motor effect force
F=BIlsintheta
Force between two parallel conductors
F=kI1I2l/d
Define torque
The turning effect of a force on an object caused when the force does not act through the object’s centre of mass. It is the product of the applied force and the distance form the centre of turn. The unit of torque is the Newton Metre (NN)
Torque formulae
Torque=Fd
If the torque is not acting at right angles to the object then the torque (turning effect) is calculated by torque=Fdsintheta
Torque(coil)=nBIAcostheta
Law of Electromagnetic Induction
A conductor in the presence of a changing magnetic field will have induced within it EMF. If a circuit is provided current will flow.
Define EMF
Is the electromotive force and can be considered as voltage or potential difference across the ends of a conductor. Once a potential exists, the electrons will tend to move from one end of the conductor to the other is a circuit is provided.
Magnetic Flux
The number of field lines passing through a given area in a coil or circuit. It is measured in Webers (Wb)
Magnetic Flux Density (B)
This is the same as magnetic field strength. Measured in Teslas (T). Defined as the number of magnetic field lines per 1m^2
Faraday’s Law of Electromagnetic Induction
The generated potential difference difference is proportional to the rate of change of magnetic flux through a coil.
Formula
EMF=-nd.magnetic flux/dt
Lenz’s Law
Full definition:
The direction of the induced EMF is such as to set up a current whose associated magnetic field would partly cancel out the change in flux that produced it.
Or
An induced EMF tries to set up a current whose magnetic effects oppose the current that induced it.
Most simply
The induced EMF opposes the change that causes it.
Or
The induced current will give rise to a magnetic field that opposes the one that created it.
Back EMF
An electromotive force that opposes (according to Lenz’s Law) the current flow in a circuit/coil when the coil of a motor rotates, a back EMF is induced in the coil due to its motion in the external magnetic field.
Eddy currents
Eddy currents are induced circular currents produced in a large or flat conductor when a changing magnetic flux passes through or around this conductor. All eddy currents oppose the motion between the magnetic field and the conductor in which the current is induced in accordance with Lenz’s Law.
Transformer
A magnetic circuit with two multi-turn coils wound onto a common core. They are devices for transferring electrical energy from one circuit to another while changing the size of alternating (AC) voltage.
