Applied Mechatronics Fran Marquez and Gabriel Domingues Lund, 29/11/2018 Industrial Electrical Engineering and Automation Lund University, Sweden
1. Crash course on Electrical Machines S N S N
1. Crash course on Electrical Machines S N S N
1. Crash course on Electrical Machines
What is an electric motor? A device that converts electromagnetic energy into mechanical energy. Basic principles: - Equal magnet poles repel each other while opposite magnet poles attract each other. - A moving electric charge (like a current flowing in a wire) creates a magnetic field. Ampère s law - A moving electric charge (like a current flowing in a wire) exposed to a magnetic field experience a force. Lorentz Force 2010 Fran Márquez 5
Why electric motors? - Electric motors have replace hydraulic / pneumatic actuators in most applications due to: Improved controllability Higher efficiency (2x to 4x) Reduced maintenance Clean and tidy (no leaks, no plumbing work, no need for high pressure equipment ) - Still for extremely high torques and power hydraulic systems are used. 2010 Fran Márquez 6
AC/DC? DC Motor Simple principle Easy to control Cheaper electronics Lower efficiency Maintenance of brushes Heavier AC Motor Higher torque & power ratio High efficiency Low maintenance Complex control and electronics More expensive 2010 Fran Márquez 7
The DC Motor (I) armature excitation field commutator armature winding brushes 2010 Fran Márquez www.wikipedia.org 8
The DC Motor (II) - Simple 2 pole motors are not practical: 0 Torque point (neutral point) Short circuit of the power supply - A higher pole number solves those problems 2010 Fran Márquez www.solarbotics.net 9
The Maths... Torque production: Lorentz force Force due to Electric field Force due to Magnetic field A positive charged particle will accelerate in the same linear direction as the electric field E, but will curve perpendicularly to both the instantaneous velocity vector v and the magnetic field B. 2010 Fran Márquez 10
For a single turn For the complete motor: 2010 Fran Márquez 11
More Maths... Induced voltage: Faraday s law For a single turn For the complete motor: 2010 Fran Márquez 12
DC Motor Model 2010 Fran Márquez 13
Summarizing: In a DC Motor: Torque is proportional to the armature current Back emf is proportional to the motor speed 2010 Fran Márquez 14
Speed Control of a DC motor
Electrical Model Voltage Torque
Mechanical Model Torque Speed
DC Motor Model Very small Time constant Bigger time constant Almost a First order system
Controllers: 1. Open Loop 2. Close loop (PI) Tune by choosing damping and bandwidth Tune as a 1st order system Tune by hand
Open loop Speed control of a DC Motor - Simple Proportional controller in open loop (Stationary error) - Adding and integrator in close loop will improve the performance 2010 Fran Márquez 21
Close Loop Speed control of a DC Motor 2010 Fran Márquez 22
Close Loop Speed control of a DC Motor Why a PI!? Why not a PID? Easier to tune More stable and robust Matching performance if well tuned
Tuning the PI controller: Damping and bandwidth Close loop transfer function
Simulink...
Tuning the PI controller: First order system Advantages: First order behaviour No overshoot Quick and stable
Simulink...
How to make your controller more robust! 1. Feedforward the Back EMF 2. Anti-Windup 3. Active damping
Simulink...
How to does that looks like:
Field weakening -Whatis the max. speed achievable for a certain DC link voltage? - How could that limit be extended? - What are the implications of increasing the speed over the nominal speed for: a) The torque delivered by the machine b) The power delivered by the machine 2010 Fran Márquez 31
2010 Fran Márquez 32