Basic of Electrical Machine - An electrical machine is an apparatus that converts energy in three categories i.e.Generator which converts mechanical energy to electrical energy, Motor which converts electrical energy to mechanical energy and Transformer which changes the level of voltage of an alternating current.
All three types of the machine do there working by involving a third domain which is called magnetic field. Without involving magnetic field electrical machines cannot work. For an Electrical machine either it is a generator, motor, or transformer all work on the same principle.
All three types of the machine do there working by involving a third domain which is called magnetic field. Without involving magnetic field electrical machines cannot work. For an Electrical machine either it is a generator, motor, or transformer all work on the same principle.
Electrical machines are divided into three categories
- Generator
- Motor
- Transformer
- AC Motor
- DC Motor
The machine which converts electrical energy into mechanical energy is called a motor . The motor which used DC electrical energy and convert into mechanical energy is called DC motor. DC motor is seldom used in ordinary applications because almost all Electric Supply companies provide alternating current.
Principle of working of DC motor
Before talking about the working principle of DC motor, the electromagnetic drag must be known to use because it is only responsible for the rotation of the armature in the particular machine. In DC motor, armature generates a magnetic field due to induction current.
This magnetic field tends to rotate armature in its direction of rotation, this effort is called electromagnetic drag. The drag provides mechanical energy in the form of armature's rotations in DC motor.
The working principle of the DC motor is based on the principle that when a current-carrying conductor is placed in a magnetic field then this conductor experiences a force perpendicular to the direction of the magnetic field.
This magnetic field tends to rotate armature in its direction of rotation, this effort is called electromagnetic drag. The drag provides mechanical energy in the form of armature's rotations in DC motor.
The working principle of the DC motor is based on the principle that when a current-carrying conductor is placed in a magnetic field then this conductor experiences a force perpendicular to the direction of the magnetic field.
Construction of DC motor
DC motor and Generator have similar construction therefore the same machine can be used as either a DC motor or DC generator. A DC motor has different parts and now we will describe each part one by one:-
Stator
The stator is the stationary part of the motor and it holds the magnetic field winding and receives the electrical power from an external source. Mainly it is designed to produce magnetic flux.
Field winding
The stator consists of field winding and these winding are made with field coil wound in the slots of the pole shoe in such a way that when field current flow through it then adjacent poles have opposite polarity. Basically winding forms an electromagnet and produces a magnetic field within the air gap.
The rotor is the rotating part of the motor. it is also called armature where electrical energy converted into mechanical energy. The rotor is placed between the stator. When generated magnetic field in stator is linked with rotor winding then the rotor conductor experiences a torque and start to move.
The armature winding of a DC motor is attached to the rotor or the rotating part of the machine. Armature winding is placed in the slot of the rotor and the rotor is the rotating part of the machine hence armature winding is subjected to the alternating magnetic field.
Due to this alternating magnetic field, hysteresis and eddy current loss occur in the armature winding. To minimize these losses, the armature core is made of high grade still and silicon material. Armature windings are made in two way
Due to this alternating magnetic field, hysteresis and eddy current loss occur in the armature winding. To minimize these losses, the armature core is made of high grade still and silicon material. Armature windings are made in two way
In this winding, the number of parallel paths between the conductor is equal to the number of poles.it means that the two terminal of each pole is connected in parallel on the commutator.
2. Wave winding
In this winding, the number of parallel paths between conductors is always equal to irrespective of the number of poles. That means that the two terminal of each pole of the stator are connected in series with each other and the final two terminals are connected to the stator.
Commutator and Brushgear of DC Motor
The commutator of a DC motor is a cylindrical structure made of up of copper segment attached together but insulated from each other by mica or other insulating material.
In the case of a DC motor, the function of the commutator is to provide the supply current from the external source to the armature winding over a rotating structure through the brushes. The Brushes are made with carbon or graphite, structure making sliding contact over the rotating commutator.
In the case of a DC motor, the function of the commutator is to provide the supply current from the external source to the armature winding over a rotating structure through the brushes. The Brushes are made with carbon or graphite, structure making sliding contact over the rotating commutator.
Yoke
The Yoke or frame of the DC motor is made up of cast iron or steel and it forms an integral part of the stator. The main function of the Yoke is to provide mechanical support over inner complex part of the motor and also provide support to the armature and the field winding system.
Working of DC motor
when a current-carrying conductor in place in a magnetic field then this current-carrying conductor experiences a force perpendicular to the direction of the current and magnetic field. this is the basic principle on which a DC motor work.
In the figure given below the basic construction of a two-pole DC motor is shown in which the current-carrying conductor is kept between the two poles of the magnet. when the switch is on a rotational torque will act on both side of the current-carrying conductor and due to this torque the current-carrying conductor tends to rotate and the direction of rotation is given by Fleming's left-hand rule.
In the figure given below the basic construction of a two-pole DC motor is shown in which the current-carrying conductor is kept between the two poles of the magnet. when the switch is on a rotational torque will act on both side of the current-carrying conductor and due to this torque the current-carrying conductor tends to rotate and the direction of rotation is given by Fleming's left-hand rule.
The torque will act as the conductor until the conductor is in the magnetic field. The motion which is produced by this torque will not be uniform, to get uniform motion the field system is inclined properly.
Some term used in DC Machine
Back EMF
When the rotor starts rotating in the magnetic field then the rotor conductor cut the magnetic field line then an EMF is induced in the rotor conductor. This induced EMF has a tendency to oppose applied external voltage according to Lenz law. this induced EMF is called back EMF. The induced back EMF depends on the field, numbers of conductors, speed of the Rotor, and the number of poles.Torque
The turning moment of a force about an axis is called torque. In a motor, the armature rotates about the shaft. Torque depends on back EMF and armature current and it is given by the product of back EMF and armature current. The produced torque is further divided into two Part:-Armature torque
Since we know armature has a conductor and every conductor has its own torque and the vector sum of all individual conductors torque is called armature torque. this torque depends on magnetic flux, number of the conductor, number of the pole, number of parallel paths on commutator, and armature current.Shaft Torque
The torque at the shaft to doing the useful work is called shaft torque.it is generally given in horsepower. It is the rated torque of the machine.Speed of DC motor
The speed at which the shaft rotates is called the speed of the DC motor. it is generally given in revolution per minute(rpm). The speed of the motor depends on back EMF, magnetic flux density, number of poles number of conductor on the armature.Speed of Motor is measure in Revolution per minute i.e RPM.
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