DC Electric Motor Electro Magnet Permanent Magnet Magnetic Field Around a Current Carrying Conductor Magnetic Force On A Current – Carrying Conductor The magnetic force (F) a conductor experiences is equal to the product of its length (L) within the field, the current I in the conductor and the external magnetic field B. Magnetic Force is Proportional to: • Length of the conductor • Current • Magnetic Field The force on a current-carrying conductor in a magnetic field When a current-carrying conductor is placed in a magnetic field, there is an interaction between the magnetic field produced by the current and the permanent field, which leads to a force being experienced by the conductor: Fleming’s left-hand rule • The directional relationship of I in the conductor, the external magnetic field and the force the conductor experiences I B F Motion of a current-carrying loop in a magnetic field F Rotation I N brushes L R S F Commutator (rotates with coil) Vertical position of the loop: Rotation N S Electric Motor • An electromagnet is the basis of an electric motor • An electric motor is all about magnets and magnetism: A motor uses magnets to create motion. • Opposites attract and likes repel. Inside an electric motor, these attracting and repelling forces create rotational motion. • A motor is consist of two magnets. Parts of the Motor • Armature or Rotor (electro-magnet) • Commutator • Brushes • Axle • Permanent Magnet • DC power supply of some sort Motor Illustration Armature •The armature is an electromagnet made by coiling thin wire around two or more poles of a metal core. •The armature has an axle, and the commutator is attached to the axle. •When you run electricity into this electromagnet, it creates a magnetic field in the armature that attracts and repels the permanent magnets. So the armature spins through 180 degrees. •To keep it spinning, you have to change the poles of the electromagnet. Commutator and Brushes • Commutator is simply a pair of plates attached to the axle. These plates provide the two connections for the coil of the electromagnet. • Commutator and brushes work together to let current flow to the electromagnet, and also to flip the direction that the electrons are flowing at just the right moment. • The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the magnet. The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator. Spinning Armature Operating principle • When the field coil is energized, it creates a magnetic field (excitation flux) in the air gap, in the direction of the radii of the armature. This magnetic field “enters” the armature from the North pole side of the field coil and “exits” the armature from the South pole side of the field coil. • The conductors located under the other pole are subject to a force of the same intensity in the opposite direction. The two forces create a torque which causes the motor armature to rotate (see Figure 1). Operating principle When the motor armature is powered by a DC or rectified voltage supply U, it produces back emf E whose value is: E = U – RI where RI represents the ohmic voltage drop in the armature. The back emf E is linked to the speed and the excitation by the equation: E=kωΦ Where: k is a constant specific to the motor ω is the angular speed Φ is the flux Operating principle This equation shows that at constant excitation the back emf E (proportional to ω) is an image of the speed. The torque is linked to the field coil flux and the current in the armature by the equation: T=kΦI If the flux is reduced, the torque decreases. Example of Motor Your Simple Motor Types of DC Motor Parallel excitation (separate or shunt) • The coils, armature and field coil are connected in parallel or supplied via two sources with different voltages in order to adapt to the characteristics of the machine (e.g.: armature voltage 400 volts and field coil voltage 180 volts). • The direction of rotation is reversed by inverting one or other of the windings, generally by inverting the armature voltage due to the much lower time constants. Most bidirectional speed drives for DC motors operate in this way. Series wound The design of this motor is similar to that of the separate field excitation motor. The field coil is connected in series to the armature coil, hence its name. The direction of rotation can be reversed by inverting the polarities of the armature or the field coil. This motor is mainly used for traction, in particular on trucks supplied by battery packs. In railway traction the old TGV (French high-speed train) motor coaches used this type of motor. More recent coaches use asynchronous motors. Compound wound (series-parallel excitation) • This technology combines the qualities of the series wound motor and the shunt wound motor. This motor has two windings per field coil pole. One is connected in parallel with the armature. A low current (low in relation to the working current) flows through it. The other is connected in series. • It is an added flux motor if the ampere turns of the two windings add their effects. Otherwise it is a negative flux motor. But this particular mounting method is very rarely used as it leads to unstable operation with high loads.
