Microcontroller Based Dc Motor Conditioning and Monitoring System

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Microcontroller based DC motor Speed Conditioning and Monitoring System

Abstract:

The proposed system is based on concept of monitoring and control of the various speed of a DC motor. The speed of DC is monitor by optical decoder sensor and which is being collcted on a microcontroller which will subsequently control the DC motor through a motor driver circuit. The values of the speed also displayed on an LCD. The DC motor speed can be set controlled manually using a keypad.(3)

The proposed project is based on the concept of measuring and displaying the speed of motor. The system uses a hall sensor for sensing and optical encoder unit for speed (RPM) measurement. A magnet attached on the shaft of the motor structure is actuating the hall sensor. The optical encoder is attached with a circular disc attached with the mechanical shaft of the unit. The measured data is being acquired on the microcontroller unit through a sensor assembly. The data is being displayed on a LCD.(3)

Introduction:

A simple DC electric motor. When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation. The armature continues to rotate. When the armature becomes horizontally aligned, the commutator reverses the direction of current through the coil, reversing the magnetic field. The process then repeats.(6)

When a current passes through the coil wound around a soft iron core, the side of the positive pole is acted upon by an upwards force, while the other side is acted upon by a downward force. According to Fleming's left hand rule, the forces cause a turning effect on the coil, making it rotate. To make the motor rotate in a constant direction, "direct current" commutators make the current reverse in direction every half a cycle thus causing the motor to rotate in the same direction.(6)

The problem facing the motor shown above, is when the plane of the coil is parallel to the magnetic field; i.e. the torque is ZERO-when the rotor poles or displaced 90 degree from the stator poles. The motor would not be able to start in this position, but the coil can continue to rotate by inertia.

There is a secondary problem with this simple two-pole design; at the zero-torque position, both commutator brushes are touching across both commutator plates, resulting in a short-circuit that uselessly consumes power without producing any motion. In a low-current battery-powered demonstration this short-circuiting is generally not considered harmful, but if a two-pole motor were designed to do actual work with several hundred watts of power output, this shorting could result in severe commutator overheating, brush damage, and potential welding of the metallic brushes to the commutator.(6)

Unlike the demonstration motor, above, DC motors are commonly designed with more than two poles, are able to start at any position, and do not have any position where current can flow without producing electromotive power.

If the shaft of a DC motor is turned by an external force, the motor will act like a generator and produce an Electromotive force (EMF). During normal operation, the spinning of the motor produces a voltage, known as the counter-EMF (CEMF) or back EMF, because it opposes the applied voltage on the motor. This is the same EMF that is produced when the motor is used as a generator (for example when an electrical load (resistance) is placed across the terminals of the motor and the motor shaft is driven with an external torque). Therefore, the voltage drop across a motor consists of the voltage drop, due to this CEMF, and the parasitic voltage drop resulting from the internal resistance of the armature's windings. The current through a motor is given by the following equation:

I = (Vapplied − Vcemf) / Rarmature...
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