DC Generator, Overview:
In this article, we will learn about DC machines their construction and working principle and the types of DC generators. In our day to day life we come across many common terms such as generator motor etc. These devices are called as DC machines they either convert the mechanical energy into electrical or electrical energy into mechanical energy. The device which converts mechanical energy into direct type electrical energy is called as a DC generator. The device which converts electrical energy into mechanical energy is called as a DC motor but now we will only focus on the DC generator. A DC machine works on the principle that whenever the conductor cuts the magnetic flux lines and EMF is induced in it which generates the sinusoidal current.
Construction of DC Machine:
Let’s see the construction of a DC machine it is made up of stationary parts such as yoke, poles and brushes and rotating parts such as armature, commutator and bearings.
The yoke is the outer frame of the DC machine made up of a magnetic material such as cast iron the pole every machine has even number of poles divided into three parts as pole core, Pole shoe, and the field winding.
The armature core is cylindrical in shape and is mounted on a circular shaft. Armature is a part of a machine which rotates in a circular direction.
A commutator is a cylindrical body mounted on a shaft along with the armature thus forming a single body. Hence the commutator also rotates along with the armature. The brushes are made up of carbon and mounted on the commutator. They are stationary and do not rotate. The external load circuitry is connected across these brushes as shown. The main function of the bearings is to support the rotating part and allow its smooth motion with minimum friction.
Working Principle of the DC Generator:
This generator works on Faraday’s laws of electromagnetic induction which states that when the conductor cuts the magnetic flux lines an EMF is induced in it the direction of this induced EMF is stated from Fleming’s right-hand rule for DC generators we use Fleming’s right-hand rule as we need to find the current and for DC motors we use Fleming’s left hand rule as we need to find the motion. Now Fleming’s right-hand rule states that if three fingers of a right hand namely thumb, index finger, and middle finger are out sourced so that they are mutually perpendicular to each other and if the index finger is made to point in the direction of the magnetic field then the thumb indicates the direction of the motion of a conductor and the middle finger gives the direction of the EMF induced in the conductor. The induced EMF is given by the equation:
E = BIV sinɵ
Where e equals induced EMF, V equals flux density in Weber per meter square, l equals length of the conductor in meters and theta is the angle between the direction of the motion of the conductor and the magnetic field.
Construction of the DC Generator:
The DC generator is constructed by keeping the two conductors A B and C D between two magnets both these conductors are connected to the commutator and the brushes are mounted on the surface of these commutators to which the external circuit is connected. Whenever the conductors cut the magnetic flux lines the EMF is induced that causes the current to flow through the load circuit. when the conductor A B reaches the vertical position rotating in clockwise direction. We get the maximum current as it continues its motion and reaches to 180 degrees position the current becomes zero momentary. Thus from zero degrees to 180 degrees rotation we get the positive half of the AC current generated. when the conductor A B attains the vertical value at 270 degrees we get the maximum current again and the process continues generating a continuous AC current. Now during the positive half conductor A B touches the brush b1 thus B one attains the positive charge giving the positive cycle at the output. When A B touches the brush B 2 V 2 attains the positive charge providing the positive cycle again the commutators are so designed that for every cycle. The current will flow out towards the circuit given positive output only. The EMF equation for the DC generator is given by:
Eg= PɸN/60 × Z/A
Where P equals number of poles of generator, theta equals flux produced by each pole in Weber, N equals speed of armature in rpm Z equals total number of armature conductors and A equals number of parallel paths in which the total number of conductors are divided. The symbolic representation of DC generator is as shown.
The DC generator is basically divided into two categories:
- Separately excited generator
- Self-excited generator
Self-excited are further divided as:
- Shunt Generator
- Series Generator
- Compound Generator
Separately Excited generator:
In the separately excited DC generator the field winding is supplied from the external separate DC supply the EMF induced is given by the equation:
Eg=V+ Ia Ra+ Vbrush+Armature reaction drop
In self-excited shunt DC generator the field winding is connected in parallel with the armature as the load current increases the armature current increases thus voltage drop laRa also increases as a result of which load voltage P decreases.
Applications of the shunt generator:
Thus we get the following load characteristics the application of the shunt generator is:
- Battery Charging
- Ordinary Lightning
- Power Supply Purposes Etc
When field winding is connected in series with the armature winding while supplying the load the generator is called as the series generator for the series generator we also take drop across the series field winding while finding the EMF equation unlike the shunt characteristics for the series generator the load voltage V increases as the load current increases due to the residual flux retained by the field winding.
Eg=V+ Ia Ra+ Vbrush+Armature reaction drop
The graph starts from the non-zero voltage value.
Applications of the Series Generator:
The applications of the series generator are used as boosters on DC feeders and as constant current generators for welding generators and lamps.
The last type is the compound generator in compound generator the poles of the windings are excited by two independent windings, shunt winding, and series winding; thus the named compound generator if the series winding Shields the shunt field the generator is cumulative but if the series winding opposes the shunt field the generator is called as differential generator. There are two types of the compound generator; short shunt generator and long shunt generator is winding is absent the load characteristics are the same as the shunt generator if the series winding aids the shunt field it gives a positive boost to the voltage and the output voltage increases if the series winding opposes the shunt field it supplies a negative boost due to which the voltage decreases.
If we move the wire through the magnetic field, then a potential difference is
induced across the ends of the wire. But, when the wire stops moving, then the potential difference is lost, if we move the wire back up through the magnetic field then we get the potential difference again. We can notice that the potential difference is now reverse direction this potential difference effect is called induced potential. If we have complete circuit, then, in this circuit we induce a current. This effect is called generator effect.
Like in wire example, direction of the current changes when the direction of movement changes and if movement of circuit stops then the current also stops. Also we can get induced potential or induced current if our circuit is stationary but we move our magnetic field, like shown. One thing we must remeber is that generator effect will occur only if wire or circuit passes through the magnetic field. If wire or circuit moves along magnetic field then we do not get an induced potential difference or current. Size of the induced potential difference or induced current depends in three factors. Induced potential difference or induced current is larger if:
- We use stronger magnetic field
If we move wire or circuit more frequently
If we shape our wire or circuit in the form of the coil
(more turns of the coil, greater induced potential and current in the wire or circuit)
In this example, you can see magnet how move in and out from the coil of a wire, as you can see this also produces an induced current. As we have seen in past examples, direction of the current changes when the direction of movement of the magnet changes. Remember that we can also change direction of the induced current if we switch poles of the magnet. We have to go through one more thing and this thing is quite tricky. As we have seen, when we move a magnet into a coil of wire, a current is induced in the wire. As we already know, induced current create own magnetic field and this magnetic field opposes the movement of the magnet. Let we explain this, when we insert the North Pole into the coil that end of the coil also becomes a North Pole, this repels the magnet (as we know from magnetism, the same poles are repelled), making it harder to push magnet in the coil. When we pull out magnet from the coil, that end of the coil becomes south pole, this attracts the magnet (as we know from magnetism, opposite poles attract each other), making it harder to pull it out. Because the induced current makes it harder to move the magnet, this actually means that we are making work. We are transferring energy from the movement of the magnet into the movement of the current If we go back to our diagram, we can see that if we move the wire through the magnetic field, then a potential difference is
induced across the ends of the wire. How we can determine direction of induced current? To determine direction of induced current we can use Fleming’s right hand rule How can we use Fleming’s right hand rule, first place your thumb, first finger and 2nd finger, in such a way that they are mutually perpendicular to each other, We have to apply right hand in such a way that your thumb points in the direction of motion of the wire Your forefinger points in the direction of magnetic field, then the second finger gives you the direction of induced current. So, with Fleming’s right hand rule we can determine direction of induced current. Shown in this example, 2nd finger points direction of the current and thumb points direction of movement of the wire. Also you can see how direction of induced current changes when we change movement of the wire If you like, you can pause a video and try to use Fleming’s right hand rule.
Faraday’s law of induction is a basic law of electromagnetism that predicts how a magnetic field will interact with an electric circuit to produce an electromotive force (EMF). The equation for the EMF induced by a change in magnetic flux is: Where EMF is electromotive force in volts, N is number of turns of coil, [FI] is magnetic flux in WEBER and T is time in seconds. So, right now we can conclude that three parameters affect size of electromotive force. EMF is proportional to magnetic flux density and number of turns of the coil and EMF is inversely proportional to time (EMF is greatest when the change in time Δt is smallest).
Similarities between motor and generator:
Motor effect is the term used when a current-carrying wire in the presence of a magnetic field experiences a force. Generator effect is the effect whereby a wire or coil moving in a magnetic field generates an induced potential difference or induced current. As we see from definitions these two effects serve different functions, but there is similarity in the principle, in fact, there is similarity from the construction point of view:
- Both motor effect and generator effect require magnetic field to happen.
Armature is used to achieve the effect in both cases so it is always a conductor or a conductive coil. The armature’s role is twofold.
The first is to carry current across the field and second role is to generate an electromotive force (EMF). As we say this two effects serve different functions and they have a lot of differences.
Difference between Motor and Generator:
In case of motor effect electric energy is the input and mechanical energy is the output. Let’s clarify when we put a current-carrying wire in the magnetic field this current-carrying wire experiences a force. In case of generator, mechanical energy is the input and electric energy is the output. Let’s clarify if we move the wire through the magnetic field, or if we move magnet into coil of wire, like shown, then a potential difference is induced across the ends of the wire or If we have complete circuit, then, in this circuit we induce current. Motor effect follow Fleming’s left hand rule in other words, Generator effect follow Fleming’s right hand rule.
Example of Motor and Generator:
If we show examples of motor effect and generator effect. An example of Motor effect is an electric car or bike where electric current is supplied to the machine or device, and it gets converted into mechanical motion and, as a result, the car or bike moves.
The example of Generator is that in power stations the turbine is used as a device which converts mechanical energy of force of water falling from the dam to generate electric energy.