DC MOTORS

INTRODUCTION:

The basic principles of electromagnetic induction were discovered in the early 1800's by Oersted Gauss, and Faraday. By 1820, Hans Christian Oersted and Andre Marie Ampere had discovered that an electric current produces a magnetic field. The next 15 years saw a flurry of cross-Atlantic experimentation and innovation, leading finally to a simple DC rotary motor. A number of men were involved in the work, so proper credit for the first DC motor is really a function of just how broadly you choose to define the word "motor."

Construction:

DC motors consist of one set of coils, called armature winding, inside another set of coils or a set of permanent magnets, called the stator. Applying a voltage to the coils produces a torque in the armature, resulting in motion.

Stator

• The stator is the stationary outside part of a motor.
• The stator of a permanent magnet dc motor is composed of two or more permanent magnet pole pieces.
• The magnetic field can alternatively be created by an electromagnet. In this case, a DC coil (field winding) is wound around a magnetic material that forms part of the stator.

Rotor

• The rotor is the inner part which rotates.
• The rotor is composed of windings (called armature windings) which are connected to the external circuit through a mechanical commutator.
• Both stator and rotor are made of ferromagnetic materials. The two are separated by air-gap.

Winding

A winding is made up of series or parallel connection of coils.

• Armature winding - The winding through which the voltage is applied or induced.
• Field winding - The winding through which a current is passed to produce flux (for the electromagnet)
• Windings are usually made of copper.

DC Motor Basic Principles

Energy Conversion

If electrical energy is supplied to a conductor lying perpendicular to a magnetic field, the interaction of current flowing in the conductor and the magnetic field will produce mechanical force (and therefore,mechanical energy).

Value of Mechanical Force

There are two conditions which are necessary to produce a force on the conductor. The conductor must be carrying current, and must be within a magnetic field. When these two conditions exist, a force will be applied to the conductor, which will attempt to move the conductor in a direction perpendicular to the magnetic field. This is the basic theory by which all DC motors operate.The force exerted upon the conductor can be expressed as follows.

F = B i l Newton

Where B is the density of the magnetic field, l is the length of conductor, and i the value of current flowing in the conductor. The direction of motion can be found using Fleming’s Left Hand Rule.

Figure 1: Fleming’s Left Hand Rule

The first finger points in the direction of the magnetic field (first - field), which goes from the North Pole to the South Pole. The second finger points in the direction of the current in the wire (second - current). The thumb then points in the direction the wire is thrust or pushed while in the magnetic field (thumb - torque or thrust).

WORKING PRINCIPLE:

Consider a coil in a magnetic field of flux density B (figure 2). When the two ends of the coil are connected across a DC voltage source, current I flows through it. A force is exerted on the coil as a result of the interaction of magnetic field and electric current. The force on the two sides of the coil is such that the coil starts to move in the direction of force.

Figure 2: Torque production in a DC motor

In an actual DC motor, several such coils are wound on the rotor, all of which experience force,resulting in rotation. The greater the current in the wire, or the greater the magnetic field, the faster the wire moves because of the greater force created.

At the same time this torque is being produced, the conductors are moving in a magnetic field. At different positions, the flux linked with it changes, which causes an emf to be induced

(e = d/dt) as shown in figure 3. This voltage is in opposition to the voltage that causes current flow through the conductor and is referred to as a counter-voltage or back emf.

Figure 3: Induced voltage in the armature winding of DC motor

The value of current flowing through the armature is dependent upon the difference between the applied voltage and this counter-voltage. The current due to this counter-voltage tends to oppose the very cause for its production according to Lenz’s law. It results in the rotor slowing down. Eventually, the rotor slows just enough so that the force created by the magnetic field (F = Bil) equals the load force applied on the shaft. Then the system moves at constant velocity.