# Electoral machine lab report

Lab Report

Introduction

The objective of this study was to study the characteristics of a separately-excited DC motor. The characteristics were studied included speed vs. voltage, torque vs. current, speed vs. current, and speed vs. torque.

Motor Speed vs. Armature Voltage

The figure below summarizes the effect of armature voltage variation on the motor speed. Figure 1: Motor Speed vs. Armature Speed

K1 = (n2 – n1)/ (E2 – E1)

= (1584.54 – 172.37)/ (283.55 – 35.05)

485.34/86.03

= 5.974 r/minV

Motor Torque vs. Armature Voltage

The figure below summarizes the effect of armature current variation on the motor torque 0

Figure 2: Motor Torque vs. Armature Current

K2 = (T2 – T1)/ (I2 – I1)

= (0.72 — -0.01)/ (0.73 – 0.26)

= 0.73/0.47

= 1.553 N.m/A

Speed Decrease vs. Armature Current

Note: You did not provide the value of RA and EA

Based on the calculations, the ECEMF and dc motor speed n should decrease as the armature current is increased.

The figure below summarizes the effect of armature current variation on the motor speed Figure 3: Motor Speed vs. Armature Current

DC Motor Speed vs. DC Motor Torque

The figure below summarizes the effect of DC motor speed variation on the DC motor torque Figure 4: DC Motor Speed vs. Dc Motor Torque

No data for the reversed connections!!!!

Discussion/Analysis

In figure 1, the armature voltage EA is directly proportional to the dc motor speed n.The graph confirms that the separately excited dc motor is similar to a linear voltage-to-speed converter, in which higher voltage has produced higher speed. As long as the armature current does not exceed the nominal value, the armature current IA is directly proportional to the dc motor torque as shown in figure 2. This graph confirms that the separately-excited dc motor is similar to a linear current-to-torque converter, provided the armature current does not go beyond the nominal value. In this case, a higher armature current produces a greater torque.

Figure 3 clearly confirms that the dc motor speed n decreases as the armature current IA is increased. This is in line with the prediction made in the previous step. What causes the dc motor speed to reduce when the armature voltage is fixed and the armature current is increased is the back EMF. When the armature rotates, it produces a voltage that counters the power supply voltage. In other words, the voltage generated is of opposite polarity to the voltage of the power supply, commonly referred to as back EMF[CITATION Kel13 p 31 l 1033 ]. This voltage causes the motor windings to receive smaller voltage potential (increases voltage drop), thus reducing the speed (see the relationship between speed and armature voltage in figure one).

Conclusion

This lab exercise allows the student to understand the characteristics of a separately-excited DC motor. As observed in the results, the speed of a separately excited dc motor is directly proportional to the armature current fed to the motor. The dc motor also produces a torque that is directly proportional to the armature current applied to it. One supervising thing to note is that the speed decreases when armature current is increased at a fixed voltage because of the back EMF.

Summarize the results of the reversed connections in the next paragraph.

Work Cited

Keljik, Jeffrey J. Electricity 4: AC/DC motors, controls, and maintenance. Boston: Cengage Learning, 2013. Print.