A course in Electrical Machines I
An Electrical Generator converts mechanical energy (or power) into electrical energy (or power).And motor converts electrical energy to mechanical energy.
It is based on the principle of production of dynamically (or motionally) induced e.m.f (Electromotive Force). Whenever a conductor cuts magnetic flux, dynamically induced e.m.f. is produced in it according to Faraday's Laws of Electromagnetic Induction. This e.m.f. causes a current to flow if the conductor circuit is closed.
Hence, the basic essential parts of an electric generator are :
- A magnetic field,
- A conductor or conductors which can so move as to cut the flux.
Simple Loop Generator
A single turn rectangular copper coil 'ABCD' rotates about it's own axis in a magnetic field. In practice, this field can be due to permanent magnets or electromagnets.
The two ends of the coil are connected to slip rings 'a' and 'b' which are insulated from each other and from the central shaft.
Two collecting brushed made up of carbon or copper press against these slip rings in order to collect the current induced in the coil. This current is conveyed to the external resistance 'R'
The rotating coil is referred to as the 'Armature' and the magnets are called 'Field Magnets'
If the coil rotates in clockwise direction, the flux lined with it changes as it assumes successive positions in the field.Hence, an e.m.f is induced in it and this e.m.f is proportional to the rate of change of flux linkages, i.e e=NdΦ/dt
When the plane of the coil is at right angles to the lines of flux, i.e. position '1' in the figure, the flux,Φ, is maximum but the rate of change of flux,dΦ/dt is minimum. This is because the segments 'AB' and 'CD' don't cut the flux but move along parallel to it. Thus, no e.m.f. is induced in the coil.
This position is assumed as the starting position and hence, the angle of rotation is measured from here. As the coil rotates further, dΦ/dt increases till it reaches a maximum value at position 3 where the angle of rotation,θ, reaches a value of 90 degrees. Since the coil plane is now horizontal, the flux linked is minimum, but the rate of change of flux is maximum.
In a similar manner,as the angle varies from 90 degrees to 180 degrees, the flux increases and rate of change of flux decreases till position 5. According to Fleming's Right Hand Rule, the current is found to be flowing from A to B and C to D.
The same process is repeated as the coil continues to rotate through angles 180 degrees to 360 degrees and positions 5 to 8, back to 1. However, by Fleming's Right Hand Rule, the current is found to be flowing from D to C and then B to A
Thus, the direction of current is reversed every half cycle and this is called ALTERNATING CURRENT. This is illustrated in the figure above. A.C current, unlike D.C current does not maintain a constant direction, or even a constant magnitude in one direction.
For making the flow of current unidirectional, slip rings are replaced by split rings. These are made up of conducting cylinders, cut into two halves or segments, insulated from each other by a thin mica sheet or other insulating material.
Slit rings result in unidirectional current, but not this current is not continous like pure DC current.
The basic principle of a generator was explained in the above section. In practice, a generator has the following parts :
1.Frame or Yoke
The yoke serves two purposes. The first one is to provide an outer covering, i.e. it should provide mechanical suppot for the motor. The second use is to provide a path for the magnetic field. Since the field is stationary( caused by DC) there are is no need to laminate the yoke. It is made from cast iron in cheap machines or sometimes using fabricated steel to obtain high values of permeability.
The poles house the field coils around them. They also need not be laminated as they carry stationary field. Poles are usually made from cast iron for small machines and fabricated steel for high power machines. The pole shoes present at the end of poles act as support for the field coils and spread out the magnetic flux generated. The pole shoes thereby, reduce the reluctance of the air gap. This reduces the requirement of field currrent.
The objective of the field coils is to provide the necessary mmf resulting in flux. Now, mmf is the product of number of turns and current. To reduce the current drawn, more number of turns are normally wound on the pole. The windings are usually made with copper. By reducing the currrent, the diameter of wire is reduced, leading to critical copper saving in the field coils.
It is the rotating part of the machine, and is cylindrical in shape. The purpose of the armature is to rotate the conductors in a uniform magnetic field. The armature is slotted and these slots house the armature conductors. The armature conductors are made of copper and are insulated. The field produced from these conductors is not stationary. Therfore, the armature is made from silicon steel stampings to reduce the hysteresis and eddy current losses. There are small key holes present throughout the armature to aid in proper cooling. The armature conductors are placed in the slots in different patterns to control the performance of the machine.
5.Armature windings or conductors
The armature windings as previously specified are made of copper and are insulated from each other. There are placed in the slots present in the armature. The armature windings have diffferent arrangements leading to a different performance of the machine. Some popular armature winding arrangements are Wave, Lap, Drum, etc.
The commutator is one of the most important parts of the DC machine. It is like a rotating switch placed between the armature and the external circuit. It is arranged in such a way that it will reverse the connections to the external circuit at the instant of each reversal of current in the armature coil. The commutator essentially converts alternating current generated in the armature conductors to a unidirectional current. Coomutator is made using high conductivity copper segments seperated by thin layers of mica.
7.Brushes and Bearings
The function of the brushes is to collect current from the commutator segments and supply it to the external load circuit. The brushes are rectangular in shape and rest on the commutator. Brushes are made from a variety of materials like carbon, graphite and copper. Copper brushes are costlier and are used only in machines with very high current ratings. Carbon is the preferred material for making brushes for the smaller rating machines.
Testing of DC Machines
A synchronous electric motor is an AC motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Synchronous motors contain electromagnets on the stator of the motor that create a magnetic field which rotates in time with the oscillations of the line current. The rotor turns in step with this field, at the same rate. The motor does not rely on "slip" under usual operating conditions, and as a result produces torque at synchronous speed. Synchronous motors can be contrasted with induction motors, which must slip in order to produce the rotor magnetic field and to provide torque. The speed of the synchronous motor is determined by the number of magnetic poles and the line frequency. Synchronous motors are available in sub-fractional self-excited sizes to high-horsepower industrial sizes. In the fractional horsepower range, most synchronous motors are used where precise constant speed is required. In high-horsepower industrial sizes, the synchronous motor provides two important functions. First, it is a highly efficient means of converting AC energy to work. Second, it can operate at leading or unity power factor and thereby provide power-factor correction.
Laxmi Nair - B.Tech,EEE,Amrita Vishwa Vidyapeetham
Amit Kumar - B.Tech,EEE, Amrita Vishwa Vidyapeetham