Design of DC Motor Drive (50V, 10A)- Speed Control Position Control - Power Electronics
The DC Motor Drive circuit is a vital circuit in motor control. Precision control is very important in robotics where small movements have to be made (position control) or the motor is to be rotated at a particular speed (speed control).
The design of such a motor driver is an essential part of any bachelor's degree of electrical engineering and is one of the main projects offered for the Power Electronics course.
The primary idea ruling the design is that the speed/position of the motor would be controlled indirectly by controlling the power delivered to the motor. Since we are essentially talking about analog electrical circuits, this means that we shall be controlling the magnitude of voltage and/or the current supplied to the motor.
By using effective controlled power delivery to the motor, we can control motor’s speed. The more the power delivered to the motor (in whatever direction it is), the higher will be the motor’s speed. Direction control shall be achieved by controlling the direction of net current flow (or the net power flow) into the motor.
For any objective to be pursued, it must be measurable. We need some desired specification laid in stone upon which we shall be making appropriate choice for electronic components (MOSFETS or BJTs, which diodes to use, resistors, etc.) Our objective is that the driver must have the following attributes:
- maximum source voltage 50 V.
- maximum load current 10 A.
- switching frequency 5-15 kHz.
- microcontroller based single PWM generator.
- speed control in both directions (CW and CCW).
The methodology employed in this specific circuit shall pertain solely to the control of a simple dc motor. The brushless dc motor may be controlled in a very similar way (as we shall discuss later).
As the first task we shall look at the overview of the project at hand and consider the flowchart shown below depicting the essential elements (or modules) we shall be implementing.
At the top we see an obvious input by the user (or a user defined program) for the desired speed or the position of the motor. This signal may be input in the form of binary digits using a simple ADC (Analog to Digital Converter) implementation. This input is then fed to the micro-controller which judges the signal and shall generate an appropriate signal which shall dictate the amount of power to be delivered to the motor.
Next down in the hierarchy comes the isolation module. Isolation is a must between circuits of different powers. For example, consider the microcontroller part of the circuit. It operates on around 5Vs and few milliamperes of current at max. However, the motor part of the circuit would require heavy current (10 A in our case) and high voltages (50 V in our case). Isolation separates these two portions of the circuit not physically but electrically! I shall be posting another hub soon to describe and explain thoroughly the importance and implementation of isolation in electronic circuits.
Next in line are the Gate Drive Circuits. These circuits are mainly responsible for providing control signals to the gates of the transistors used.
The most important and crucial feature is the H Bridge. It is the most commonly studied and most essential circuit in Power Electronics. The purpose of the H Bridge is simply to deliver controlled power to its load. The load for our concern is the motor for which we shall implement speed and position control.
Finally, data from the motor (its speed, direction, position etc.) shall need to be fed back to the controller to complete the basic feedback control loop. Since again we are transgressing over to low voltage and low current area, an isolation is due.
The circuit can be broken into particularly two modules: the control module (shaded grey in the diagram) and the power module (patterned with dots in the diagram). I shall be describing each part of the circuit individually as to impart the maximum understanding of the such a design and implementation of electronic circuit. The simulations may be performed in LTSpice or Multisim, whichever you are comfortable with.
Follow the links to each stage of the design.
- Stage 1: H Bridge I - Basic Concepts and Ideal Simulation
- Stage 2: H Bridge II - Component Selection and Real Simulation
- Stage 3: Generation of Dead-Time / Dead-Band in Electronic Circuits
- Stage 4: Optical Isolation and Gate Drive Circuits
- Stage 5: Reading Motor Speed Using Optical Encoder
- Stage 6: The Final Circuit(Simulation and Implementation), Evaluation, Improvements and Instructions
If you have any queries or want help on your project / design, fire away and I shall get back to you as soon as possible with as much help as I can provide.
Your comments are most appreciated and would be an enlightening beacon for my hubs to come.