Basics Of Motor And Generator

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.

 ALTERNATOR

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.

Basics Of Transformer - 1

• It is a static device.It has constant power and power factor.
• Its efficiency is more than 90%.
• It has two windings, lv, hv, tap winding. It may have tertiary winding to remove harmonics.
• AC is flown through primary winding . This current causes change in magnetic field. This magnetic flux passes through core which is made up of granular material. This change in magnetic field causes induce e.m.f. due to self induction. The change in e.m.f. causes current to flow in secondary due to mutual induction.This induced current opposes the  magnetic field in core due to lenz law.To decrease the effect of secondary current, lv increases its current value.
• Large current is in lv and large voltage is in hv.
• LV is placed near to core in order to reduce losses.
• Flux coupling is maximum in a three leg cored transformer.

CONSTRUCTION

v Over the legs windings are placed in such a manner as core than lv than hv .

v Windings are prepared in the form of dummy. They are then placed over core.

v The core is build up firm interleaved low loss cold rolled, gain oriented silicon steel laminations.

v On the basis of core design it can be shell type or core type.

v All windings are strengthen so as to reduce the effect of inward and outward force in the current carrying wires. Extra strength is provided so as to reduce the risk of breaking of wires during fault.

v Colour coding is provided during transposition  in winding as per their rating(e.g. red, blue, orange etc.).

v In order to prevent deformation when subject to shot circuit force, solid blocks & insulation, backed by substantial supporting frames, are utilized.

v Depending upon rating the winding can be horizontal or vertical, and can be disc, spiral, helical etc.                                                                            

                                                     

v Connections can be of star-delta,star-star, delta –star. These connections can have neutral or not depending upon ratings as higher the rating , higher the losses due to harmonics and this loss can be avoided by neutral terminal in case of star.

v  Inter-layer cooling ducts are provided to ensure that the temperature gradient between windings & oil  is minimized & high life expectancy is achieved. Insulation between layers & Tums is based upon the impulse test level of the voltage.

v It has got oil conservator tank which contains oil to circulate heat.

v All tanks are made from mild steel, which is electrically welded. The design of the tank is such that the base & cover thickness is related to the size & weight of the finished product. The core & coil are fixed to the tank in such a way that the no damage will cause during transportation, The interior of the tank is painted with oil & heat resistant paints. Zinc chromate red oxide primer is applied to the exterior, before applying two coats of weather proof out door type synthetic enamel paint.

v There is a indicator on conservator which tells us the level of oil.

v Depending upon rating of transformer , oil conservator tank can be of many types. It can be single tank filled with both oil and air, or, single conservator can have a partition, one for oil and one for air, or , it can have balloon and oil in same conservator. Air is provided to allow oil to expand when heated.

v A tap changer is provided to change taps in order to change voltage.Depending upon need it can be OFF circuit tap changer or ON load tap changer. Depending upon the rating of transformer it can be manually controlled,  remote controlled(three phase induction motor),or parallel controlled(three tap changers are connected  to single tap changer).

v  Depending upon rating there may or may not be oil conservator for tap changer  box.

v  Breather is provided for ventilation . It has got crystals of silica to absorb moisture during breathing as moisture can destroy transformer.Its colour changes  on absorbing moisture.

Designs:

v  Clear View acrylic.

v  Metallic designs including aluminium wire mesh protection and metallic armors. ( with view window).


v  Normal glass body clear view.


v Air relay is provided which is passed through windings  chamber. Its valve are made up of mercury  compound which on absorbing smoke goes down. Depending on the model, the relay has multiple methods to detect a failing transformer. On a slow accumulation of gas, due perhaps to slight overload, gas produced by decomposition of insulating oil accumulates in the top of the relay and forces the oil level down. A float switch in the relay is used to initiate an alarm signal. Depending on design, a second float may also serve to detect slow oil leaks. If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator. This flow of oil operates a switch attached to a vane located in the path of the moving oil. This switch normally will operate a circuit breaker to isolate the apparatus before the fault causes additional damage.                          


v Ventilators are provided with or without fan for cooling depending upon rating.

                                                                  

v Water circulation tube is provided in case of water cooling.


v Insulators , which are made up of porcelain, are mounted across both lv and hv so that windings do not touch the body of transformer and also if fault occurs , it can be restricted to insulators only.


v Grounding chamber is provided for earthing so that if fault occurs , current goes to the ground rather to go to the body of transformer.


v Current transformer is provided to get  temperature of core. CT takes current from transformer , with the help of transducers, it is converted into temperature. The difference between normal transformer and instrument transformer(e.g. CT), is that in normal transformer flux is constant while in instrument transformer flux is variable i.e. on changing secondary current , primary  current need not be changed.


v Tertiary resistors are assembled across tap changer to avoid losses due to back e.m.f.. Back e.m.f. is produced when magnetic field is suddenly collapsed. When tap position is changed, at that instant magnetic field is reduced to zero from some value suddenly, this creates e.m.f. in opposite direction which is known as back e.m.f.


v A buzzing sound is produced from the core of transformer due to the phenomenon of magnetostriction. It states that when some particular type of materials posses changing e.m.f., then elongation and contraction of that material takes place.

LOSSES

v PROXIMITY   EFFECT   :: When two wires have current in them then both wires will have different magnetic fields associated with them. These magnetic fields will disturb the flow of electron in other conductor and hence opposes the flow due to mutual induction. Hence the overall distribution of current is not uniform in windings and hence flux is not uniform.

v EDDY CURRENT LOSS   :: When any conductor is placed in a changing magnetic field  , then, current is induced in it. This current is known as eddy current. To reduce this current lamination is provided at each layer of core which acts as a insulation layer. Magnetic field is continuously changed due to AC.

v HYSTERISIS LOSS  ::  Hysterisis curve gives the relationship between magnetic field and magnetic field strength. This curve shows that magnetic field is not zero when magnetic field strength is zero.  Thus this reduces the strength of magnetic field. 

v LOSS  DUE TO HARMONICS  :: The hysteresis curve shows that there is residual magnetism present in core. This disturbs the current and hence harmonics are generated.

v LEAKAGE FLUX LOSS   :: As each leg is surrounded by windings and there  is a space between each leg, therefore, flux follows two paths , one through core and the other through air. This path through air causes leakage flux and reduce the magnitude of flux.It is also directly proportional to eddy current loss.

v COPPER LOSS  :: I²R losses are proportional to copper loss. This loss is due to the copper which is use for circulating current in windings. This loss is directly proportional to the heating of transformer.

v LOSSES DUE TO SEQUENCE COMPONENT  :: Sequence component can be classified  as symmetric or asymmetric. Components can further be classified as positive, negative or zero. When the magnetic flux of sequence component is in opposite direction  then main field flux then negative sequence current is induced. Since it increases the rate of cutting flux , therefore eddy current increases and over heating of rotating part increases. This component is basically due to non uniform distribution of resistance in windings. In case of positive sequence , the magnetic flux of this component is in same direction as main field flux,hence it does not affect eddy current.Since transformer has only zero sequence component as it has all non rotator parts , therefore its loss is basically due to zero sequence component. This sequence component is mainly due to fault with ground .

v MAGNETIC SATURATION LOSSES :: Magnetic saturation is the property of conductor in which on increasing magnetic field , magnetic field strength shows  negligible  change. Due to this , large amount of energy is wasted.

This was just a basic introduction to transformers. Hope you liked it, for more reading... just refer my other posts.