Case Study: Direct Current (DC) Motors

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Direct Current (DC) motors
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University 1.0 Introduction
A Direct Current (DC) motor is one of the broader classification of electric motors which operate from a direct current. Nearly all mechanical devices applied in daily life activities are powered by electric motors. Motors transform electrical energy to mechanical energy, hence they are essential for energy conversions in electrical machines.
All types of electrical motors are made up of a stationary field referred to a stator and a rotating field referred to a rotor. According to Krishnan, (2010), DC motors operate through the interaction of an electric current with magnetic flux to generate a torque and rotational speed. D.C motors are of different types but they all operate
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When a DC voltage is supplied to the coil it induces a current flow through the coil. The electric current and the magnetic field will therefore interact resulting to a force, which pushes the coil to move in its direction.

Figure 2: Torque production in a DC motor (Hambley et al., 2008) In a real DC motor, there are many such coils wounded on the rotor, and they will all be pushed in the direction of the force causing their rotation. The current flowing in the coil is directly proportional to the magnetic field produced and sequentially the rate of the motion. In addition, torque is produced concurrently with the movement of the conductors in the magnetic field (Hambley et al., 2008). This causes an induced electromotive force (emf), which counteracts the applied voltage.

Figure 3: Induced voltage in the armature winding of DC motor (Hambley et al.,
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The mode of the electrical connection will also determine the motor operating characteristics. Self-excited and separately excited is where the winding terminals are connected across the input voltage and separate voltage source respectively.
1.5 Efficiency of a DC motor Some power loses takes place as power flows from the input to the output terminals of the motor. Therefore, the motor efficiency can be calculated as the ration of the output power to the input power.
Efficiency = power output / power input
1.6 Rating of a DC motor
DC Motors are rated in terms of the rated voltage, rated power, rated speed and rated load. Rated voltage is the input operating voltage, rated power is the power that the motor is designed to supply to the load. Rated speed is the speed that the motor is intended to operate and the rated load is the load the motor is intended to handle.
1.7 Applications of DC motors
Motors are manufactured in different sizes depending on their applications (Krishnan, 2010). For instance, large motors that operate with a high load up to thousands of Horsepower are applied in industries to run elevators, heavy metal rolling mills etc. In contrast, small motors are applied in low power machines e.g. in robots, automobiles and hand power

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