Lecture 3: Two-Phase Servomotors PDF

PEME421: Special Electrical Machines Dr. Sayed M. Said Lecture 3 Two-Phase Servomotors Villamos Energetika Tanszék Villamos Művek és Környezet Csoport Two-Phase Servomotors An ac servomotor is an induction motor with two primary windings mutually displaced in...

PEME421: Special Electrical Machines Dr. Sayed M. Said Lecture 3 Two-Phase Servomotors Villamos Energetika Tanszék Villamos Művek és Környezet Csoport Two-Phase Servomotors An ac servomotor is an induction motor with two primary windings mutually displaced in magnetic position from one another by 90° electrical degrees. It has low inertia and high-resistance rotor, thus, giving a speed- torque curve that is linear in shape. 10/18/2022 Special Electrical Machines 2 Construction of Two-Phase Servomotors 1. Stator 2. Rotor 10/18/2022 Special Electrical Machines 3 Construction of Two-Phase Servomotors Stator: The stator is similar to that of the split-phase induction motor. It has two windings called control winding and reference winding displaced by 90° electrical angle with each other. The two windings may be identical or not depending on the applications. Rotor: The rotor is generally of two types; one is squirrel cage type while the other is drag cup type. The squirrel-cage rotor is usually small in diameter to keep the mechanical inertia as low as possible, and the rotor has high resistance in order to obtain linear speed-torque characteristics. To reduce the inertia further, a drag cup type of rotor construction is used. There are two air gaps in this construction. The drag-cup rotor consists of a cup of a nonmagnetic conducting material such as copper. This rotor is used for low output motors (few watts) in order to minimize the moment of inertia. The drag- cup rotor can be described as a special from of squirrel cage rotor in which the rotor teeth are removed, the rotor core is held stationary, and the squirrel cage bars, and the end rings are replaced by a cylindrical cup. 10/18/2022 Special Electrical Machines 4 Construction of Two-Phase Servomotors Rotor: 10/18/2022 Special Electrical Machines 5 Construction of Two-Phase Servomotors Rotor: 10/18/2022 Special Electrical Machines 6 10/18/2022 Special Electrical Machines 7 Principles of operation: Two phase balanced operation: The two windings have the equal number of turns of the same cross section, i.e., symmetrical two-phase windings. Balanced two-phase supply is applied. 10/18/2022 Special Electrical Machines 8 Principles of operation: Two phase balanced operation: 10/18/2022 Special Electrical Machines 9 Principles of operation: Two phase balanced operation: 10/18/2022 Special Electrical Machines 10 Principles of operation: Unbalanced operation of 2-phase servomotor: The 2-phase windings are assumed to be balanced. i.e., the two windings have the same number of turns with the same conductor's cross-section area, and they are displaced by 90° electrical angle in space. Unbalanced voltages are applied: 10/18/2022 Special Electrical Machines 11 Principles of operation: Unbalanced operation of 2-phase servomotor: This unbalanced supply can be analysed into positive and negative balanced voltages: 10/18/2022 Special Electrical Machines 12 Principles of operation: Unbalanced operation of 2-phase servomotor: 𝒂 = −𝒋, 𝒂𝟐 = −𝟏, 𝒂𝟑 = 𝒋 10/18/2022 Special Electrical Machines 13 Principles of operation: Determination of machine performance using the sequence components of voltages: 10/18/2022 Special Electrical Machines 14 Principles of operation: Determination of machine performance using the sequence components of voltages: 10/18/2022 Special Electrical Machines 15 Principles of operation: Determination of machine performance using the sequence components of voltages: Equivalent circuit 10/18/2022 Special Electrical Machines 16 Principles of operation: Determination of machine performance using the sequence components of voltages: + ve sequence response: 10/18/2022 Special Electrical Machines 17 Principles of operation: Determination of machine performance using the sequence components of voltages: - ve sequence response: 10/18/2022 Special Electrical Machines 18 Example 10/18/2022 Special Electrical Machines 19 Example cont. 10/18/2022 Special Electrical Machines 20 Example cont. 10/18/2022 Special Electrical Machines 21 Example cont. 10/18/2022 Special Electrical Machines 22 Example cont. 10/18/2022 Special Electrical Machines 23 Example cont. 10/18/2022 Special Electrical Machines 24 10/18/2022 Special Electrical Machines 25 10/18/2022 Special Electrical Machines 26 10/18/2022 Special Electrical Machines 27 Servomotor torque-speed Curves: 10/18/2022 Special Electrical Machines 28  Servomotor torque-speed Curves: Assume 90° phase shift between Va and Vb, we can define the quality K such that: 10/18/2022 Special Electrical Machines 29 Servomotor torque-speed Curves: Torque-speed characteristic for any value of K can be easily determined without using the equivalent circuit by the following equation: 10/18/2022 Special Electrical Machines 30 Servomotor torque-speed Curves: Thus, to calculate the speed-torque characteristic for K=1/2: 10/18/2022 Special Electrical Machines 31 Features of Servomotors: 1. Light in weight for quick response. 2. Robust in construction. 3. It is reliable, and its operation is stable in nature. 4. Smooth and noise free operation. 5. Large torque to weight ratio. 6. Large resistance to reactance ratio. 7. No brushes or slip rings are required. Hence maintenance free. 8. Driving circuits are simple to design. 9. The negative slope of the torque-slip characteristics adds more friction improving the damping. This improves the stability of the motor. This features is called internal electric damping of two-phase AC servomotor. 10/18/2022 Special Electrical Machines 32 Applications of Servomotors: 1. This motor is used to activate movements in robotics for giving the arm to its exact angle. 2. This motor is used to start, move and stop the conveyor belts carrying the product along many stages. For example, bottling, packaging, and product labelling. 3. This motor is built into the camera to correct a lens of the camera to improve out of focus images. 4. This motor can be used to control the robotic vehicle by controlling robot wheels and speed generating plenty torque to move and also start and stop the robot. 5. It can be used in solar tracking systems to set the angle of the solar panel so that panel stays to face the sun. 6. This motor is used in milling machines for metal cutting and forming to provide specific motion. 7. This is used in Textile industries to control and knitting machines like spinning, and weaving. 8. This motor is used in automatic door opening and closing in public places like hospitals, theatres, and supermarkets. 10/18/2022 Special Electrical Machines 33  Q&A 10/18/2022 Special Electrical Machines 34  Thank you for your attention! 10/18/2022 Special Electrical Machines 35