DC Motors

A DC motor is an electro-mechanical device in which a DC(direct current) input results the functioning of motor. The direction of rotation depends on the polaratiy of connected supply across the terminal of motor.
The first DC electrical motor was demonstrated by Michael Faraday in England in 1821.

Classification of DC motor

Shunt Wound DC Motors
Series Wound DC Motors
Compound Wound DC Motors
Permanent Magnet DC Motors
Brushless DC Motors
Universal DC Motors

Features of DC motors:

Encoder/potentiometer determines the accuracy and resolution of the servo motor

A servo motor has 5-10 times rated torque for short periods

Stays cool because the current draw is proportional to load

Maintains usable high speed torque of 90% of NL RPM

performs quietly at high speeds silently

Has a resonance-free and vibration-free operation

High Torque to Inertia Ratio can rapidly accelerate loads

The servo motor can approach 90% efficiency at light loads

Construction of DC motor:

A simple motor has six parts:

Armature or rotor:It contain a rectangular coil made of insulated copper wire wound on a soft iron core. This coil wound on the soft iron core forms the armature. The coil is mounted on an axle and is placed between the cylindrical concave poles of a magnet.

Commutator:A commutator is used to reverse the direction of flow of current. Commutator is a copper ring split into two parts. The split rings are insulated from each other and mounted on the axle of the motor. The two ends of the coil are soldered to these rings. It rotates along with the coil. Commutator rings are connected to a battery. The wires from the battery are not connected to the rings but to the brushes which are in contact with the rings.

Brushes:Two small strips of carbon, known as brushes press slightly against the two split rings, and the split rings rotate between the brushes. The carbon brushes are connected to a D.C. source.

Shaft: It is a circular rod connected to armature of motor to connect load.

Field magnet: There are two kinds of magnets used as field magnet for motors. They are permanent magnet and electromagnet. These magnets are used to generate magnetic field/magnetic force for the functioning of motor.

DC power supply

Functioning of DC motor:

Followed by the Fleming’s left hand rule, when an electric current passes through coil in magnetic field, the magnetic force produces a torque which turns the shaft of DC motor.

The direction of the flux is dependent on the direction of the current flow.

If we look at the air gap between two magnets that have their opposite poles facing each other, we would see magnetic field from the N to S poles.

Now, if we place a current carrying conductor in the air gap of two magnets, the lines of flux in the air gap will be affected.

On the side of the conductor where the lines of flux oppose each other, the magnetic field will be made weaker. On the side of the conductor where the lines of flux are not opposing each other, the magnetic field will be made stronger.

Because of the strong field on one side of the conductor and a weak field or, the other side, the conductor will be pushed into the weaker field.

In simple words to understand the concept, we know that energy flow from high level to low level. Similarly the rotor will move along with the direction of magnetic field ie. from strong magnetic field to a weak magnetic field.

Now, lets apply this principle to the operation of the DC motor. The armature of the motor is a loop of wire (current carrying conductor) which is free to rotate.

The field magnets are permanent or electro magnets with their N and S poles facing each other to set up the lines of flux in the air gap.

The armature is connected to the commutator which rides along the brushes which are connected to a DC power source.

The current from the DC power source flows from the positive lead, through the brush through one commutator section, through the armature coil, through the other commutator section, through the brush and back to the negative lead.

This current will generate lines of flux around the armature and affect the lines of flux in the air gap.

On the side of the coil where the lines of flux oppose each other, the magnetic field will be made weaker. On the side of the coil where the lines of flux are riot opposing each other, the magnetic field is made stronger.

The torque(turing effort) available at the motor shaft is determined by the magnetic force(flux) acting on the armature coil and the distance from the center of rotation of that force.

The flux is determined by the current flowing through the armature coil and strength of the field magnets

Important concepts:

Back EMF:

There is a corollary of motor ie. Every motor is a generator.
The current flowing through the armature coils in the motor sets up a magnetic field that is not aligned with the field generated by the permanent magnets. This creates a torque that turns the motor, Conversely, the turning of the armature through the magnetic field induces an electromotive force (EMF) in the armature.Since this EMF is in the opposite direction(opposite in polarity) to the voltage applied to the motor by the battery, it is called a “back EMF.”

It is the effect of Lenz's law of electromagnetism.

In a motor using a rotating armature and, in the presence of a magnetic flux, the conductors cut the magnetic field lines as they rotate. The changing field strength produces a voltage in the coil(armature); the motor is acting like a generator. (Faraday's law of induction.) This voltage opposes the original applied voltage; therefore, it is called "counter-electromotive force", With a lower overall voltage across the armature, the current flowing into the motor coils is reduced.

In motor control, the term "Back-EMF" refers to using the voltage generated by a spinning motor to infer the speed of the motor's rotation.

As the motor speed increases the eddy currents and the resulting back EMF also increase. When the motor reaches its maximum operating speed back emf will be generated at a constant rate. When a load is applied, the speed of the motor is reduced, which reduces the back emf and hence increases current in the motor.

If the load stops the motor from moving then the current may be high enough to burn out(* To avoid burnouts due to high initial electric currents at the start of motor we use starters in DC motors) the motor coil windings.

Generally, the load slows the armature down and so the current increases as the back emf is decreased. This produces an increase in current and torque to cope with the increased load.

Characteristics:

Speed-Torque characteristic:It is the curve between speed N and armature torque Ta of a d.c. motor. It is also known as mechanical characteristic

torque-Armature current:It is the curve between armature torque Ta and armature current Ia of a DC motor. It is also known as electrical characteristic of the motor.

speed-Armature current:It is the curve between speed N and armature current Ia of a d.c. motor. It is very important characteristic as it is often the deciding factor in the selection of the motor for a particular application.

Types of DC Motors:

Brushed DC motor:
1. Permanent magnet DC motors :
1. Shunt wound DC motor:
2. Series wound DC motor:
3. Compound wound DC motor:
3. Universal DC motor:

DC Motor Drivers:

The Drivers are used by motors for their safe operation and avoid loading effect.

There are three general types of D.C. motor control: manual, semi-automatic and automatic.

Manual control directly connects a D.C. motor to the input power line or mains. Operator intervention is required.

Semi-automatic control uses switches or sensors (.e.g., limit, pressure, temperature, float level, flow, proximity, timing and photo-sensitive switches) to control a magnetic contactor or starter which, when enabled or closed, will connect the motor to the input power line.

In semi-automatic operation, an operator is needed to start or stop the motor but the rest of the operation is controlled by the sensors or switches.

Automatic control is similar to semi-automatic control with one important difference: no operator intervention is required. For example, a thermostat in an air conditioning system or a refrigerator will turn a compressor motor on or off to maintain the set point temperature automatically.

Disadvantages and problems of brushed DC motor:

The brushes eventually wear out.

Because the brushes are making/breaking connections, you get sparking and electrical noise.

The brushes limit the maximum speed of the motor.

Having the electromagnet in the center of the motor makes it harder to cool.

The use of brushes puts a limit on how many poles the armature can have.

Applications of brused DC motor:

In wheeled robots

Toys

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