Induction Motors - Lab Report Example

NAME: xxx INSTITUTION: xxx UNIT CODE: xxx UNIT NAME: xxx TITLE: SQUIRREL CAGE INDUCTION MOTOR LECTURE: xxx ©2016 Table of Contents Introduction 3 Results 4 1.Motor with Load 4 a.Torque vs Current 4 a.Voltage vs Speed 5 b.Torque vs Speed 6 2.Motor with NO load 9 a.Speed vs Current 9 b.Speed vs Mechanical Power 10 c.Torque vs Speed without Load 11 Discussion/ Analysis 12 References 12 Questions/ Answers 13 Review Questions 15 Introduction An induction motor is a mechanical device which converts electrical energy fed into it into kinetic energy (in the form of torque). When the electrical energy in an induction motor is obtained by the electromagnetic induction from the magnetic field generated by the winding of a stator, the motor is referred to as an asynchronous motor. The electrical current developed by this rotary method is alternating current (AC). A common example of asynchronous motors commonly used is the Squirrel Cage Induction Motor. The application of the motor determines the type, with one- phase motors being used for small loads such as in household appliances, or three- phase motors for heavier industrial drives [Tho95]. Displacement of a magnet rapidly over short- circuited conductors causes a current to flow in the loop. The current flowing in the loop develops a magnetic field that interacts with that of the magnet being displaced, causing the conductors to follow the movement of the magnet. The Squirrel Cage Induction Motor derives its name from the asynchronous induction motor formed when the ends of the bars connecting the conductors are connected in a loop, creating a replica of a squirrel cage. Figure 1: Simple Squirrel Cage Induction Motor This induction motor has traditionally been used in fixed speed service, but in the recent past, they have been deployed widely in variable- frequency drive (VFD) applications. VFD applications with variable torque offer crucial opportunities for energy saving, such as pumps and fans. This report focusses on the operation of AC induction motors using the Squirrel Cage Induction Motor. Results The various parameters pertaining to the operation of the induction motor were measured and recorded, as shown below;- 1. Motor with Load The results collected for the operation of the motor when a load is placed on the system are as shown below;- Table 1: Table showing the Parameters of Induction Motor with Load a. Torque vs Current The torque produced by the squirrel cage induction motor increases as the current flowing in the system also increases. This is because an increase in the current is evidence of an increase in the difference between the rotating magnetic field and the rotor speed. This increase occurs rapidly at the beginning of operation, but gradually evens out as the speeds of the magnetic field and the rotor become almost similar. When there is no further significant increase in the current, the torque generated by the rotor remains constant. It is important to note that the speeds of the magnetic field and the rotor should not be the same, as this would result in no development of torque. This is shown by the graph below;- Figure 2: A graph showing the relationship between the Torque (y-axis) and the Current (x-axis) a. Voltage vs Speed The graph for the rotor speed against the voltage developed in the system is shown below;- Figure 3: A graph showing the relationship between the voltage (y-axis) and rotor speed (x-axis) When the magnetic flux linking a wire loop varies as a function of time, a voltage is induced between the ends of the wire. If the rate of change of the magnetic flux is increased, the voltage drop generated also increases. This change in the rate of increase in the magnetic flux is achieved by increasing the rotational speed of the motor. Therefore, an increase in the rotor speed results in an increase in the voltage recorded. However, the rate of change of the magnetic flux cannot be increased exponentially, and it gets to a point where further increase in the rotational speed causes little to no increase in the voltage, as shown in the graph. This is the maximum voltage that a specific induction motor can achieve. b. Torque vs Speed The table below shows the torque created by the adjacent speed of the rotor for a motor with load. Table 2: Table showing the Torque and Speed of a motor with Load An increase in the difference between the speed of the rotating magnetic field and the rotational speed in a squirrel- cage induction motor causes an increase in the torque generated. The plot of speed versus torque gives a negative gradually sloping line, as shown in the graph below. It is important that the rotational speed of the motor to always be less than that of the magnetic field (synchronous speed) so that torque is always generated in the motor. The synchronous speed given is 1500 rpm while the rotational speed was determined to be 1467 rpm. Figure 4: A graph showing the relationship between speed (y-axis) and torque (x-axis) 2. Motor with NO load The following results show the general operation of the induction motor when no load is applied to the system;- Table 3: Table showing parameters of induction motor subjected to NO load a. Speed vs Current As the rotational speed of the motor increases, the current developed in the system reduces. At the start of operation, the difference between the speed of the magnetic flux and rotor speed is high, generating a voltage drop between the ends of the wire loop, which causes an electric current to drop. However, as the motor continues running, the rotor speed gradually catches up to the speed of the magnetic flux, reducing the voltage drop that develops. At the nominal rotational speed, a relatively stable voltage drop is achieved, which means that the current flowing in the system becomes the minimum. This relationship is shown in the graph below;- Figure 5: A graph showing the relationship between the current (y-axis) and speed (x-axis) b. Speed vs Mechanical Power The graph of the rotational speed of the motor against the mechanical power consumed is shown below;- Figure 6: A graph of the mechanical power (y-axis) against the rotor speed (x-axis) At the start of operation of the motor, the current in the system is very high, as shown by the current – rotor speed graph shown above. This causes the mechanical power consumed by the motor to be relatively high when the motor is started, as it is a function of the current. However, as the motor picks up its rotational speed, the current in the system gradually drops, and likewise, the mechanical power consumed also goes down. When the minimum value of the current in the system is achieved, this corresponds to the minimum value of mechanical power consumed by the motor. This is shown by the graph above. c. Torque vs Speed without Load An increase in the difference between the speed of the rotating magnetic field and the rotational speed in a squirrel- cage induction motor causes an increase in the torque generated. The plot of speed versus torque gives a negative gradually sloping line, as shown in the graph below. It is important that the rotational speed of the motor to always be less than that of the magnetic field (synchronous speed) so that torque is always generated in the motor. The synchronous speed given is 1500 rpm while the rotational speed was determined to be 1467 rpm. Figure 7: A graph showing the relationship between the torque (y-axis) and rotor speed (x-axis) Discussion/ Analysis When a stator is winded up in an induction motor, a rotating magnetic field is set up through the rotor. The motion of this magnetic field interacts with the conductors to create a voltage drop in the loop, which causes an electric current to flow. The flow of this current leads to the development of a secondary magnetic field which is attracted to, and follows the primary magnetic field. The interaction of these two magnetic fluxes causes a force acting tangent orthogonal to the rotor, which results in torque which turns the shaft. The conductors (rotor) follow the magnetic field at a slower rate since the secondary magnetic flux in the conductors only occurs when the primary magnetic field passes over the conductors. The difference in the rotational speed of the magnetic flux and that of the rotor is referred to as slip. An increase in the speed of rotation of the magnetic flux will result in an increase the voltage drop developed in the conductors, which increases the current flowing in the system. This also has the effect of increasing the rotor speed which leads to an increase in the torque transferred to the shaft. However, further increase in the rotor speed and torque developed will lead to a point of instability where the motor cannot maintain its operation at such levels, and as such motor speed and torque output are reduced. This point is known as the breakdown torque. Nonetheless, this is usually more than the locked- rotor torque, which is the torque output at zero motor speed. At start of operation of the motor, the slip is very large, which causes a large current to flow within the conductors. This translates to a very high mechanical power consumption, as this is a function of the current. However, as the motor continues operation, the slip is reduced and the current flowing through the conductors is likewise reduced. This results in the mechanical power consumption during normal operation to be less than during start – up. References Tho95: , (Thompson, 1895), Questions/ Answers 7. Clockwise; ; No – 8. 10. No 11. An increase in the motor torque results in an increase in the rotor speed recorded 12. The nominal torque for the motor is 0.32 N.m. Increasing the torque on the system reaches a point of instability called the breakdown torque of 0.59 N.m. The speed and torque of the system reduce until the locked rotor torque of 0.57 N.m at zero speed 13. Yes Yes The reactive power is very crucial to create the magnetic field in the system. 14. As the motor speed decreases, the line current flowing through the conductors increases 15. Starting current – 1.19 A 16. Counter-clockwise Yes Review Questions 1. b. synchronous speed 2. a. known as slip 3. c. it requires reactive power to create the rotating magnetic field 4. a. It increases 5. c. The motor reverses its direction of rotation Read More

b. Torque vs Speed The table below shows the torque created by the adjacent speed of the rotor for a motor with load. Table 2: Table showing the Torque and Speed of a motor with Load An increase in the difference between the speed of the rotating magnetic field and the rotational speed in a squirrel- cage induction motor causes an increase in the torque generated. The plot of speed versus torque gives a negative gradually sloping line, as shown in the graph below. It is important that the rotational speed of the motor to always be less than that of the magnetic field (synchronous speed) so that torque is always generated in the motor.

The synchronous speed given is 1500 rpm while the rotational speed was determined to be 1467 rpm. Figure 4: A graph showing the relationship between speed (y-axis) and torque (x-axis) 2. Motor with NO load The following results show the general operation of the induction motor when no load is applied to the system;- Table 3: Table showing parameters of induction motor subjected to NO load a. Speed vs Current As the rotational speed of the motor increases, the current developed in the system reduces.

At the start of operation, the difference between the speed of the magnetic flux and rotor speed is high, generating a voltage drop between the ends of the wire loop, which causes an electric current to drop. However, as the motor continues running, the rotor speed gradually catches up to the speed of the magnetic flux, reducing the voltage drop that develops. At the nominal rotational speed, a relatively stable voltage drop is achieved, which means that the current flowing in the system becomes the minimum.

This relationship is shown in the graph below;- Figure 5: A graph showing the relationship between the current (y-axis) and speed (x-axis) b. Speed vs Mechanical Power The graph of the rotational speed of the motor against the mechanical power consumed is shown below;- Figure 6: A graph of the mechanical power (y-axis) against the rotor speed (x-axis) At the start of operation of the motor, the current in the system is very high, as shown by the current – rotor speed graph shown above. This causes the mechanical power consumed by the motor to be relatively high when the motor is started, as it is a function of the current.

However, as the motor picks up its rotational speed, the current in the system gradually drops, and likewise, the mechanical power consumed also goes down. When the minimum value of the current in the system is achieved, this corresponds to the minimum value of mechanical power consumed by the motor. This is shown by the graph above. c. Torque vs Speed without Load An increase in the difference between the speed of the rotating magnetic field and the rotational speed in a squirrel- cage induction motor causes an increase in the torque generated.

The plot of speed versus torque gives a negative gradually sloping line, as shown in the graph below. It is important that the rotational speed of the motor to always be less than that of the magnetic field (synchronous speed) so that torque is always generated in the motor. The synchronous speed given is 1500 rpm while the rotational speed was determined to be 1467 rpm. Figure 7: A graph showing the relationship between the torque (y-axis) and rotor speed (x-axis) Discussion/ Analysis When a stator is winded up in an induction motor, a rotating magnetic field is set up through the rotor.

The motion of this magnetic field interacts with the conductors to create a voltage drop in the loop, which causes an electric current to flow. The flow of this current leads to the development of a secondary magnetic field which is attracted to, and follows the primary magnetic field. The interaction of these two magnetic fluxes causes a force acting tangent orthogonal to the rotor, which results in torque which turns the shaft. The conductors (rotor) follow the magnetic field at a slower rate since the secondary magnetic flux in the conductors only occurs when the primary magnetic field passes over the conductors.

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