EE208 - DC Motor

EE208

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Title: DC Motor

Aim: To determine the characteristics of a dc motor excitation

Apparatus: See manual for apparatus.

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THEORY:  

A separately excited DC motor has a field system having a separate power supply. The rated field current here is 1.6 Amp with 75 Ohms resistance. Accordingly V_f is chosen.

            The armature gets an excitation from the rectifier according to the firing angle chosen. As the firing angle decreases from 90 degrees towards zero, the voltage applied to the armature increases due to which the speed also increase. If the torque demanded is increased, then the armature current (l_a) drawn by the motor also increases. The inductance of the armature decided whether current can sustain itself in a continuous manner or not even though firing angle may be large.

The basic equations of the DC motor are

V_a = E_b + I_aR_a............................................(1)

Where E_b = back emf of the motor = K.I_f.w     (w being the speed in radians per sec, If being the field current and  K being a motor constant)

V_a= Armature voltage
I_a = Armature current and R_a = armature resistance
T = K I_f I_a

Thus,   

Speed w = (V_a/KI_f)  - (TR_a/2KI_f).........................................(2)

From the above equation, it can be said that as the torque demand increases for a given V_a, the speed ‘w’ decreases. The aim of this experiment is to check how the DC motor speed is changing as the firing angle of the converter changes , i.e., as V_a changes.
The output of the converter is given by V_a = (2*Vm *Cosα)/π

Where V_m = peak of the supply voltage and  α is the delay angle of the converter. Here the firing angle is chosen to be 80 degrees.
 

For Procedure and Circuit Diagram, See your Electrical Laboratory Manual on pages 33-34

 Fill Up Tables in Your Manual and Continue To….

PRECAUTIONS:

For Precautions, See General Electrical Lab Precautions

Answers to Questions:

1.     The nature of the speed-torque characteristics of a DC motor with constant excitatation is described by a linear –ve slope graph.

a.     Ideal no-load speed: at the ideal no-load speed, the back emf is equal to the driving source voltage.

b.     Change in V does not affect the speed-torque characteristics, increase in Ra shifts the slope to the right and vice-versa, lowering of the speed of the motor increases the field current If, but this is not usually reliable.


2.     Losses include; Electrical/Copper losses (Armature loss, Field Losses), Magnetic Field Losses and Mechanical Losses.

3.     This makes the motor run below the base speed. Because the high armature current will flow through this resistor, considerable power is consumed, and the overall efficiency of the motor is reduced accordingly.

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About Uniben Engineering

Stephen Djes is a passionate Graduate of Engineering from the University of Benin, and he is geared towards helping fellow engineering students in the great institution of UNIBEN to do better at academics.