Three Phase Induction Motors

Experiment 2: Induction Motors Byron Ramon Banzon,Lucky Niwre Gerona ,James Ayento Electrical and Electronics Institute University of the Philippines Diliman Quezon City, Philippines

Abstract⎯⎯ An Induction Machine is an electro-mechanical energy conversion device but different from the normal synchronous machine.An Induction Machine uses coils on the outside connected to an AC power source. The changing Alternating Current induces a current in the rotating armature, and that current interacts with the current in the coils to produce a force which makes the armature rotate. The Induction Motor is the widely used AC machine in the industry because of its many applications, simple operations, and low maintenance compared to its synchronous counterpart.A type of Induction Machine is the Wound Rotor Induction Motor,which uses slip rings that are mounted to the motor shaft.These slip rings are used for adding resistance into the rotor windings so that it will obtain a large starting value of torque while having low amount of current flowing in the rotor and stator. In this experiment, we were tasked to explore the properties of the Induction machine and how the different parameters and values affect the functionality of the machine and its components. These are all explored to have a better grasp of its operation.

I. INTRODUCTION An Induction Machines also known as Asynchronous Machines are AC machines which converts electrical energy to mechanical energy using rotating magnetic fields caused by AC supply oscillations but unlike synchronous machines, rotor operates at a different speed relative to the stator magnetic field oscillations. With the changing magnetic field affecting the rotor, a current is induced in the rotor windings, hence the name induction motor, very similar to induced currents in

secondary transformer windings. Two types of these are Slip-ring Induction Machine and Wound rotor Induction Machine Induction Machines, more specifically Induction Motors, are used in many industrial applications like . One common use is the electric fan, which uses a single-phase Induction Motor.

II.EXPERIMENT AND RESULTS A. Starting a Wound Rotor Induction Machine (WRIM) The experiment starts by observing the properties of a WRIM as it is turned on. As three-phase AC voltage is applied, and the properties is shown in table 1.1. Quantity Measured

Value

Starting line current

1.4

Steady state line current

1.1

Motor speed

1191

Rated speed

1440

Table 1.1: Startup properties of WRIM

At startup, a high inrush current is experienced by the stator windings which magnetizes the air gap and is transferred to the rotor windings. This induces an emf across the rotor windings creating current and this starts to

turn the motor also creating torque. The rotation of the rotor also induces an emf which lowers the line current. This explains the values observed, 1.4 A immediately at startup then decreases to 1.1 as the motor spins. The speed of the WRIM is also observed. The rated speed of the motor of 1440 is very far from the actual speed of the motor. This is easily explained by understanding the properties of an Induction motor. An induction motor runs at the speed slower than the rated speed because of the lagging magnetic flux experienced by the rotor relative to the rotating magnetic field in the stator. This lag is important in creating an induced current through the rotor which creates torque. Without the lag in speed, the motor would have 0 torque and will not rotate.

B. Speed Control of a Wound Rotor Induction Machine The next part of the experiment observes the changes in speed as the series rotor resistance and the input voltage across the stator are both varied. Series Rotor Resistance

Line Current

Speed

55

0.3

1191

65

0.3

1171

75

0.3

1120

85

0.3

1050

Input Voltage

Line Current

100

0.3

1684

125

0.4

1721

150

0.4

1735

175

0.45

1756

200

0.5

1768

Table 1.3. Speed Control by varying Input Voltage

As shown in Table 1.3, as input voltage across the stator increases, the speed increases. This is because increasing stator voltage increases the magnitude of the rotating magnetic field of the stator, increasing the induced emf across the rotor, increasing the speed. C. Direction Control of a Wound Rotor Induction Machine The next part of the experiment deals with the direction control of the motor. Similar to synchronous machines, switching any two phases of the supply voltages across the stator reverses the direction of rotation of the rotor. This is because swapping phases changes the polarity of the induced emf across the rotor, changing the current direction which changes the direction of motor rotation.

​D. Effect of Mechanical Loading Unloaded

Low Load

Mediu m Load

High Load

Load Resistance

+∞

#6

#4

#2

Line Current

1.2

3.5

2.7

1.85

Speed

1712

1504

1579

1648

Table 1.2. Speed Control by varying Series Resistance

In Table 1.2, the effect of varying resistance to speed is shown. The speed of the motor decreases as the resistance is increased. This is because increasing rotor resistance decreases the voltage experience by the load.

Speed

Table 1.4. Effects of Mechanical Loading

By coupling another machine to the output of the WRIM, mechanical loading is simulated. Load resistance is also varied. Shown in Table 1.4 is unloaded properties of the motor as a baseline. When mechanical load is connected, the speed of the motor decreases caused by losses in the load side, and it decreases even further when load resistance is decreased. This is because when mechanical loading is present, motor speed will immediately decrease and this decrease in speed decreases the induced emf by the rotor to the stator, therefore increasing the line current. The decrease in the resistance of the load increases the current through the load, and this decreases the speed even more, which is reflected in the data recorded in Table 1.4.

E. Other Operating Modes The remaining part of the experiment observes other operating modes of WRIM. This part of the experiment has warnings to consider to ensure safety. The first part disconnects rotor windings while the motor is running and the next part is disconnecting stator windings. Based on the recorded data in Table 1.5, disconnecting one phase of the rotor, the stator line current is the same while rotor speed decreases significantly, this is because the effective induced emf is decreased by ⅓ because one phase is disconnected which decreases the total speed is also decreased by ⅓. When all rotor phases are disconnected, the motor stops because there is no induced emf in disconnected rotor windings. All Rotor Phase connected

One rotor phase disconne cted

All rotor phase disconnec ted

Line Current

1.4

1.4

1.4

Speed

1190

790

0

Table 1.5. Effects of disconnecting rotor windings

In Table 1.6, stator windings are disconnected and properties are recorded. When one phase is disconnected, the line current increases. This is because when one phase is disconnected, the induced emf across the rotor is decreased and the rotor speed is decreased which is also seen in the table. With the rotor speed decreased, the induced emf by the rotor is decreased, increasing the line current. When all phases are disconnected, There is no rotating magnetic field and thus, no induction of emf occurs, and no rotation occurs and the motor does not run. All stator phase connected

One stator phase disconnected

All stator phase disconnected

Line Current

1.0

1.4

0

Speed

1691

1638

0

Table 1.6. Effects of disconnecting stator windings

III. CONCLUSION The Induction Motor is the most used AC to DC motor because of its relatively cost, adequate efficiency, and low maintenance. Because of its ubiquity, understanding its capabilities and knowing its properties is an important piece of information to have. The experiment showed the important factors to remember from startup, operation to turning it off. Furthermore, each kind of Induction Machine can be converted to each other using wye-delta conversions . IV. PERSONAL REFLECTION Induction Motors are used in many applications and gaining knowledge of its operation is important when dealing with the industrial machines and appliances which utilize a WRIM. This is to ensure safety and to be able to utilize the machine to its true capabilities. - Gerona, Lucky Niwre M.

Controlling the Wound Rotor Induction Motor was a fun experience since I learned that it can be modified to suit any problem. -Byron

As discussed earlier, the Rotor resistance controls the motor speed because when the rotor resistance increases, the induced emf is essentially shared by the rotor resistance and the rotor windings. The higher the rotor resistance, the lower the effective emf experienced by the rotor winding effectively reducing the motor speed.

V. REQUIRED DISCUSSION 1.Discuss the different starting techniques that you can use to start a WRIM. What motor starting technique(s) did you use in the exercise? According to Liang, X., there are 4 different starting techniques in starting an Induction Motors [5]. Direct On Line Starter, Star-Delta Starter, Auto-transformer Starter, and Rotor Resistance starter. The technique utilized is Direct On Line Starter technique, because it does not need any connected resistance and it has a high inrush or startup current. 2.Discuss the different speed control techniques that you can use in a WRIM.What speed control technique(s) did you use in the exercise? The two speed control techniques used in the experiment is Rotor Resistance and Input Voltage varying.

3.Describe how the input voltage affected the behavior of the WRIM at no load.Explain your observations. Also discussed earlier, input voltage directly affects speed by varying the rotating stator magnetic field, thus varying the induced emf across the rotor. As input voltage increases, the rotor speed also increases

5.Explain how connecting the WRIM to an electrically loaded shunt DC Generator can simulate mechanical loading. When the DC Generator and WRIM is coupled, the DC Generator acts as a mechanical load because it resists the rotation of the WRIM. This simulates loading because it uses up the motor’s rotation. 6.Describe how changes in the mechanical load affected the behavior of the WRIM.Explain your observations. As earlier discussed, the mechanical load changes properties of the WRIM, recorded in Table 1.4. The

7.Describe how interchanging two supply terminals affect the direction of rotation of the WRIM.Explain your observations. Interchanging the two supply terminals in the stator would change the direction of the magnetic field to the opposite direction thus reversing the direction of rotation of the WRIM.

8.Describe how disconnecting one phase of the rotor circuit of the WRIM affected the behavior of the machine.Explain your observations. Disconnecting one phase of the rotor circuit reduced the speed of the motor .

4.Describe how the series rotor resistance affected the behavior of the WRIM at no-load.Explain your observations.

9.Describe how disconnecting one phase of the stator circuit of the WRIM affected the behavior of the machine.Explain your observations. Disconnecting one phase of the stator circuit reduced the speed of the motor .

10.What happens to the WRIM when all phases of the rotor circuit are opened?Explain your observations. As stated in Table 1.5, when all phases of a rotor circuit are disconnected, the line current remains the same but the motor stops completely since the rotor windings are open, and thus no current runs through the windings even when there is still a rotating magnetic field in the stator. 11.What happens to the WRIM when all phases of the stator circuit are opened?Explain your observations. Table 1.6 records the properties of the motor when all windings are disconnected. The line current becomes zero and the motor stops completely because the rotating magnetic field in the stator disappears because the stator windings are already open. 12.Is it safe to operate the WRIM with at least one phase out of service?Why or why not? No,since having at least one phase out of service means the two other phases are doing additional work of one phase which leads to the increase in current flowing in the two phases and will cause the windings to overheat.

13.In addition to the WRIM,induction machines can also be constructed as a squirrel cage induction machine.What are the similarities and differences of SCIM and WRIM in terms of construction? Both Squirrel Cage and Wound Rotor Induction machines have windings in the stator which carry 3 phase AC supply. SCIM and WRIM differ in the windings of the rotor. In a squirrel

cage induction motor, the rotor windings have bars in slots that run around and across the rotor and are connected or shorted to each other, while WRIM is constructed similar to a synchronous machine which has 3 phase windings but the ends of the windings are connected to slip-rings

14.What are the similarities and differences of SCIM and WRIM in terms of operations? Comparing a SCIM and a WRIM in terms of operation, the WRIM is equipped with a secondary resistance which protects the machine from high currents which aids in producing a high starting torque immediately, from 0 to full speed. The downside of a WRIM is that it is more expensive and needs more maintenance compared to SCIM. The SCIM has no secondary resistance high enough to protect it from inrush currents and thus, it needs to be started more slowly compared to the WRIM. Furthermore, the squirrel cage induction motor cannot be configured to control the output torque of the motor.

15.Why is it not advisable to go above rated voltage,assuming everything else is held fixed? When the voltage applied is higher than the rated voltage, the motor windings can be damaged because it would generate more power and produce more heat, lowering the motor lifetime. Furthermore, higher voltage produces higher currents which affect the magnetic field to saturate, which wastes a significant amount of power..

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4. S. C. Bhargava (2013). Electrical measuring instruments and measurements. Hyderabad: BS Publications/CRC. 5. Liang, Xiaodong; Ilochonwu, Obinna (Jan 2011). "Induction Motor Starting in Practical Industrial Applications". IEEE Transactions on Industry Applications. 47 (1): 271–280. doi:10.1109/TIA.2010.2090848. Retrieved 15 November 2018