Electric motors host almost all the mechanical movements that surround us. Conversion of energy is the prime use of electric machines. Electrical energy is converted into mechanical energy by motors. Motors are used for powering up devices that are commonly seen in our daily life. Sizes of motors vary from motors taking load of 1000’s of horsepower to smaller application motors. Applications include electric trains, elevators, robots, automobiles etc. Electric motors are subdivided into two segments: Alternating current (AC) and Direct current (DC). In almost all cases, generally, motors include a rotor (armature or rotating field) and a stationery field (stator) that is being operated by the interaction of electric current and magnetic flux to generate rotational torque and speed.
DC motor namely consists of two major parts, that is,
Other subsidiary parts are
All these parts together make up in construction of the DC motor.
Yoke consists of a magnetic frame that is made up of steel or cast iron and is an important component of stator. This forms the protective coating/covering for the inner sensitive parts of the DC motor and supports armature. Yoke houses the field winding and magnetic poles thereby supporting the field system.
Inner walls of the yoke have magnetic poles fitted to it with screws. Magnetic poles construction basically uses two parts, pole core and pole shoe, which uses hydraulic pressure to stack together and followed by attaching to the yoke. The purpose of pole core, with a smaller cross sectional area, is to hold back the pole shoe onto the yoke and the pole shoe is meant to spread flux that is produced over the air gap in between the rotor and the stator so as to reduce any losses due to reluctance. Pole shoe with a cross-sectional area relatively larger than pole core comprises of slots for carrying field windings that is meant to produce field flux.
Field windings are made by wounding field coils over the slots belonging to the pole shoes. As the current flows through the coils, the adjacent poles produce opposite polarities. Electromagnets are formed by the field winding which produces flus around the field within which armature of DC motor rotates and which results in the cutting of flux.
The rotor is attached with the armature winding and due to the rotation of rotor, the windings are subjected to magnetic fields that are altering during its path of rotation and thereby results directly in magnetic losses. Therefore, rotor is created from armature core, which in fact is made of several silicon steel laminations having low hysteresis so as to reduce magnetic losses such as eddy current loss and hysteresis loss. Armature core cylindrical structure is formed by stacking together the laminated steel sheets. Copper is usually used for making windings.
Slots are found in the armature core which is made of the same material as that of core on to which the windings of copper wire with several turns are distributed over the whole periphery of the armature core uniformly. Fibrous wedges are used to shut the slot openings so as to avoid the conductor from moving out caused due to the high amount of centrifugal force that is produced during the armature rotation in the presence of magnetic field and supply current.
Graphite or carbon structures are used to make the brushes for the DC motor that is held in contact with the rotating commutator. The electric current is relayed from the external circuit to the commutator by the brushes which then flow to the armature winding. So, the brush and commutator setup is meant for the transmission of power to the rotor from the static electrical circuit.
Commutator is a cylindrical form of structure that is made from stacking together copper segments and mica used for the insulation between each other. In electric motors, electrical switch, that is rotary, have a moving part called the commutator which is meant to reverse the direction of current flowing between the external circuit and rotor. The purpose of commutator is to bring the supply current to the armature winding, which is placed in the rotor, from the mains via the brushes in the DC motor. Commutator has long life when compared to number of breaks and makes in a circuit in case of a normal operation.
DC motors are different from AC motors due to its operation based on the direct current. That is, DC motors are used for the conversion of DC electrical energy to mechanical energy. The principle behind the working of DC motor is that a conductor carrying current when placed in the magnetic field experience torque and starts moving. This is said to be the motoring action. When the electric current direction is changed, rotation also changes its direction. Here, mechanical force is produced on the interaction between electric field and magnetic field.
Fleming’s left hand rule gives the rule behind the motor’s direction of rotation. According to this rule, left hand’s index finger, middle finger and thumb are held like mutually perpendicular to one other, where magnetic field direction is represented by the index finger, electric current direction is represented by middle finger and thumb shows the direction of the force that is experienced by the DC motor’s shaft.
The magnitude of the force is given by
Force, F = B I L (Newton)
Where, B stands for magnetic field (Wb/m2),
I stand for current (A)
L stands for length of the coil (m).
A coil is placed in a magnetic field with flux density B. A DC voltage source is connected to the two ends of the coil which lets current I to flow through it. Due to the interaction between the electric current and magnetic field, coil experiences a force on the both the sides. Coil starts moving in the direction of force. In DC motor, rotor is wound with several numbers of coils which rotates due to the force. With the current or the magnetic field increasing, the force also increases thereby the coil moves faster. Torque is also produced at the same time while the coils are moving.
Each time when the coil rotates flux linked with it changes thereby an emf is induced. This voltage, induced emf, opposes the voltage which causes current to flow in the conductor and is termed as back-emf or counter-voltage. The current that flows through the armature depends on the difference between the counter-voltage and applied voltage. According to Lenz’s law, the current by the counter-voltage opposes the cause of its origin which ends up with the slowdown of rotor. Gradually, rotor slowdown to the value just enough for the force value produced by the magnetic field (F= BIL) to become equal to the load force that is being applied to the shaft. From now on, the system carries out with a constant velocity.
The significance of back-e.m.f lies in making DC motor a self-regulating machine, which enables the armature current to be drawn by the motor as much as it is required to create the torque that is required by the load. The armature current flow in a DC motor is regulated by the back e.m.f which automatically keeps on changing the current in armature so as to meet the load requirements.
DC motor comes cross a number of losses like
Copper Loss: These losses occur due to the current flowing through the windings, mainly armature and field windings. Armature Copper loss is the loss found in the armature circuit. The armature copper loss is a function of time as the armature current value is determined by load. Field Copper loss is loss found in the field circuit. Field copper loss depends on field circuit resistance and thus remains constant with no variation in circuit resistance. Brush contact loss is due to the resistance offered by brush contacts.
Mechanical Loss: Since there is moving parts and so as the machine runs there is lot of frictional forces to overcome with large expenditure of valuable energy and results in the heating up of rubbed parts. Mechanical losses are independent of the load and depend on the speed which makes it difficult to estimate by calculations directly whereas this can be measured.
Iron Loss: The armature core made of iron is continuously rotating in the magnetic field which creates losses in the core. Therefore, these losses are also known as core losses. Armature core which undergoes reversal of magnetization causes hysteresis loss. This type of loss depends on the iron used in the manufacture of core, frequency at which magnetic reversals occur and the flux density amount.
The back-e.m.f induced in the core is small allowing more current, called the eddy current, to flow through it due to lower resistance offered by core. There is power loss due to this current known as eddy current loss.
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