DC MOTORS
A simple DC motor has a coil of wire that can rotate in a magnetic field. The current in the coil is supplied via two brushes that make moving contact with a split ring. The coil lies in a steady magnetic field. The forces exerted on the current-carrying wires create a torque on the coil.
SPLIT RING
It is a type of ring which rectifies the emf. As we know that
emf = − dφ/dt
This means that when coil is moved towards magnet , its rate of change of flux is negative
emf = -(10-1)=negative value.
And when it moves away from magnet then emf will be positive as
emf= -(1-10)=positive value
Therefore naturally we obtain sinusoidal emf. But we have to obtain same torque and due to this reason split ring is used because it will disconnect the ring when emf is changing sign and moves due to inertia and hence we get constant torque throughout.
The force F on a wire of length L carrying a current i in a magnetic field B is iLB times the sine of the angle between B and i, which would be 90° if the field were uniformly vertical. The direction of F comes from the right hand rule.The two forces shown here are equal and opposite, but they are displaced vertically, so they exert a torque. The coil can also be considered as a magnetic dipole, or a little electromagnet.
DC GENERATORS
Now a DC motor is also a DC generator. The coil, split ring, brushes and magnet are exactly the same setup as the motor above, but the coil is being turned, which generates an emf.
The ends of the coil connect to a split ring, whose two halves are contacted by the brushes. Note that the brushes and split ring 'rectify' the emf produced i.e. when sign of emf changes than there will be no more contact between armature and stator and hence torque is due to inertia only.
If we want AC, we don't need rectification, so we don't need split rings, but we will use slip ring. In slip ring there is no dead zone i.e. it will circulate current
Here the two brushes contact two continuous rings, so the two external terminals are always connected to the same ends of the coil. The result is the unrectified, sinusoidal emf given by NBAω sin ωt.
Back emf
Every motor is a generator. This is true, in a sense, even when it functions as a motor. The emf that a motor generates is called the back emf. The back emf increases with the speed, because of Faraday's law. So, if the motor has no load, it turns very quickly and speeds up until the back emf, plus the voltage drop due to losses, equal the supply voltage. The back emf can be thought of as a 'regulator': it stops the motor turning infinitely quickly. When the motor is loaded, then the phase of the voltage becomes closer to that of the current (it starts to look resistive) and this apparent resistance gives a voltage. So the back emf required is smaller, and the motor turns more slowly.
AC MOTORS
With AC currents, we can reverse field directions without having to use brushes. This is good news, because we can avoid the arcing, the ozone production and the ohmic loss of energy that brushes can entail. Further, because brushes make contact between moving surfaces, they wear out.
The first thing to do in an AC motor is to create a rotating field. With single phase AC, one can produce a rotating field by generating two currents that are out of phase using for example a capacitor. By using capacitor , the two currents are 90° out of phase, so the vertical component of the magnetic field is sinusoidal, while the horizontal is cosusoidal . This gives a field rotating counterclockwise.
In a capacitor, the voltage is a maximum when the charge has finished flowing onto the capacitor, and is about to start flowing off. Thus the voltage is behind the current. In a purely inductive coil, the voltage drop is greatest when the current is changing most rapidly, which is also when the current is zero. The voltage (drop) is ahead of the current.
If we put a permanent magnet in this area of rotating field, or if we put in a coil whose current always runs in the same direction, then this becomes a synchronous motor. Under a wide range of conditions, the motor will turn at the speed of the magnetic field.
Single phase Induction motors
Now, since we have a time varying magnetic field, we can use the induced emf in a coil – or even just the eddy currents in a conductor – to make the rotor a magnet. That's right, once you have a rotating magnetic field, you can just put in a conductor and it turns. This gives several of the advantages of induction motors: no brushes or commutator means easier manufacture, no wear, no sparks, no ozone production and none of the energy loss associated with them.
Three phase induction motors
Single phase is used in domestic applications for low power applications but it has some drawbacks. One is that it turns off 100 times per second (you don't notice that the fluorescent lights flicker at this speed because your eyes are too slow: even 25 pictures per second on the TV is fast enough to give the illusion of continuous motion.) The second is that it makes it awkward to produce rotating magnetic fields. For this reason, some high power (several kW) domestic devices may require three phase installation. Industrial applications use three phase extensively, and the three phase induction motor is a standard workhorse for high power applications. The three wires (not counting earth) carry three possible potential differences which are out of phase with each other by 120°.In the figure below arrow shows the direction of magnetic field and it will be changing as it has moving magnetic field.
Thus three stators give a smoothly rotating field.
If one puts a permanent magnet in such a set of stators, it becomes a synchronous three phase motor.
LINEAR MOTORS
A set of coils can be used to create a magnetic field that translates, rather than rotates. The pair of are pulsed on, from left to right, so the region of magnetic field moves from left to right. A permanent or electromagnet will tend to follow the field
.
So would a simple slab of conducting material, because the eddy currents induced in it comprise an electromagnet. Alternatively, we could say that, from Faraday's law, an emf in the metal slab is always induced so as to oppose any change in magnetic flux, and the forces on the currents driven by this emf keep the flux in the slab nearly constant.