Motors and generators are electrical machines, which are used to do some useful work. These machines use electrical energy to operate and produce power. There are generally two types of machines namely DC (Direct Current) machines and AC (Alternating Current) machines. Here the DC machines (DC motor and DC generator) use, a DC Supply to power it up, whereas an AC machine (AC motor and AC generator) uses an AC supply to power it up. A motor uses electrical energy to produce mechanical energy, whereas a generator uses mechanical energy to produce electrical energy. Even though the relation between both the motor and the generator are opposite to each other, they are much more similar in construction and operation. In this article let’s look at the construction and operation of these machines.
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| Credit (DC Motor): By Dcaldero8983 - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=17240288 |
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Credit (DC Generator): By Biswarup Ganguly, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=31581003 |
DC MOTOR
A motor is generally a rotating machine through which some useful work is done. A DC motor generally contains three main parts, namely,
1. Field Core and Field winding
2. Armature Core and Armature winding
3. Split rings and Brushes
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Construction of DC Motor |
Let’s look at their construction and working of the above parts briefly
FIELD CORE AND FIELD WINDING
A field core in a DC motor is a stationary part that is placed around the body of the motor. It is generally made up of several steel plates that are joined together. A fine thin copper wire, also called as field winding is made to wound on the core of several turns. The combined combination of the field core and field winding makes up the field part. The field part is also called as field poles. When current is passed through the field coils the core gets energized and becomes an electromagnet. An electromagnet is nothing but a magnet made through passing electricity. In such a way, many poles are made around the body of the motor. The number of poles made depends upon the speed of the motor. Due to the large number of poles, a large number of magnetic field lines will be made to flow across the poles.
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Construction of Field Core and Field Winding |
ARMATURE CORE AND ARMATURE WINDING
An armature part is also similar to a field part. But unlike the field, the armature is a rotating part. The construction of the armature is also much more similar to the field. It is also made up of several thick laminations that are joined together, forming the armature core. Similarly, the winding (called as armature winding) is made to wound over the armature. When passing supply to the armature winding, the armature part also acts as an electromagnet and creates its own magnetic field lines.
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Credit (Armature Core and Armature Winding): By Sebastian Stabinger Paethon - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3823475 |
SPLIT RINGS AND BRUSHES
Split rings and brushes are one of the main parts of a DC motor which are generally used to supply power to the armature. The necessity of using split rings and brushes is only for the armature and not for the field, because the armature is in a rotating motion, whereas the field is in a stationary position. So to supply power to the armature even when it is rotating, rings and brushes are used. The split rings also called as commutator, rotate too with the armature. So to supply power to the armature, the brushes are made to lie on the commutator surface.
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Split rings resting on the commutator |
OPERATION
During starting, the field poles are energized
by giving supply, and north and south poles are made respectively. With the formation of the north and south poles, a huge amount of magnetic field is established. At the same time, the armature is also given power, which establishes its own magnetic field. As both parts exert their own magnetic fields, there will be an accumulation of magnetic fields at a certain point which experiences a force on a moving part. According to Lorentz force, whenever a current-carrying conductor is placed in a magnetic field it experiences a force. So thereby, when the armature (current-carrying conductor) is placed in the magnetic field, it starts to rotate. So the motor rotates. This is how the DC motor runs. The more number of coils, the faster the motor rotates.
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Operation of DC Motor |
DC GENERATOR
A DC generator is also the same as the construction of the DC motor. The DC generator also houses the field and armature part as off in the DC motor. The only difference is the principle. A motor operates in the principle of Lorentz force whereas the generator operates in the principle of Faraday’s Law of Electromagnetic Induction.
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Operation of DC Generator |
As a generator converts mechanical energy to electrical energy, a mechanical movement is given from an external rotating machine (called a Prime mover) to the armature. Here only the field part is given the supply and not the armature part. So there will be only one magnetic field present in the machine, established by the field part. When the armature starts to rotate by the mechanical movement given externally, it starts to cut the field lines produced by the field part. So according to Faraday’s law of Electromagnetic Induction whenever a conductor is placed in a varying magnetic field an EMF (Electro Motive Force) is induced. This EMF is nothing but a voltage. So in this situation, the armature is varying and the magnetic field lines are constant. So the EMF will get induced on the armature. If the armature is connected to a load (light bulb, fan, etc.,), a closed path is provided and a current is made to flow, thus doing some useful work. Thus mechanical energy is converted into electrical energy. But as it is a DC generator, the generated power should be a DC supply. But the generated power usually will be AC (Alternating Current) which means the supply gets varied with time. To convert the AC power into DC power, a device called a commutator is placed, which converts AC supply into DC supply. Due to its property of converting one type of power into another, it is also called a mechanical rectifier. To collect the power produced from a rotating commutator and supply it to a load, carbon brushes are used. It always rests on the commutator and collects the DC power produced. In a DC motor, carbon brushes are used to supply power to the armature, whereas in DC Generator it is used to collect power from the armature. Thus by all these arrangements, the given mechanical energy is converted into a DC supply or AC Supply.
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