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Three Main Components of an Electric Drive |
An electric drive is a system that performs the conversion of electric energy into mechanical energy at adjustable speeds. This is the reason why an electric drive is also called adjustable speed drive (ASD). Moreover, the electric drive, as we will see later, always contains a current (or torque) regulation in order to provide safe current control for the motor. Therefore, the electric drive torque/speed is able to match in steady state the torque/speed characteristics of any mechanical load. This motor to mechanical load match means better energy efficiency and leads to lower energy costs. In addition, during the transient period of acceleration and deceleration, the electric drive provides fast dynamics and allows soft starts and stops, for instance.
A growing number of applications require that the torque and speed must vary to match the mechanical load. Electric transportation means, elevators, computer disk drives, machine tools, and robots are examples of high-performance applications where the desired motion versus time profile must be tracked very precisely. Pumps, fans, conveyers, and HVAC (heat, ventilation, air conditioners) are examples of moderate performance applications where variable-speed operation means energy savings.
An electric drive has three main components:
The electric motor
The power electronic converter
The drive controller
The following figure shows the basic topology of an electric drive. Beside the three main components, the figure shows an electric power source, a mechanical load, electric and motion sensors, and a user interface.
Electric Drive Basic Topology
The motor used in an electric drive is either a direct current (DC) motor or an alternating current (AC) motor. The type of motor used defines the electric drive's classification into DC motor drives and AC motor drives. The ease of producing a variable DC voltage source for a wide range of speed control made the DC motor drive the favorite electric drive up to the 1960s. Then the advances of power electronics combined with the remarkable evolution of microprocessor-based controls paved the way to the AC motor drive's expansion. In the 1990s, the AC motor drives took over the high-performance variable-speed applications.
The power electronic converter produces variable AC voltage and frequency from the electric power source. There are many types of converters depending on the type of electric drive. The DC motor drives are based on phase-controlled rectifiers (AC-DC converters) or on choppers (DC-DC converters), while the AC motor drives use inverters (DC-AC converters) or cyclo converters (AC-AC converters). The basic component of all the power electronic converters is the electronic switch, which is either semicontrolled (controllable on-state), as in the case of the thyristor, or fully controllable (controllable on-state and off-state), as in the cases of the IGBT (insulated gate bipolar transistor) and the GTO (gate turn off thyristor) blocks. The controllable feature of the electronic switch is what allows the converter to produce the variable AC voltage and frequency.
The purpose of the drive controller is essentially to convert the desired drive torque/speed profile into triggering pulses for the electronic power converter, taking into account various drive variables (currents, speed, etc.) fed back by the sensors. To accomplish this, the controller is based first on a current (or torque) regulator. The current regulator is mandatory because, as mentioned previously, it protects the motor by precisely controlling the motor currents. The set point (SP) of this regulator can be supplied externally if the drive is in torque regulation mode, or internally by a speed regulator if the drive is in speed regulation mode. In the Electric Drives library, the speed regulator is in series with the current regulator and is based on a PI controller that has three important features. First, the SP rate of change is limited so that the desired speed ramps gradually to the SP, in order to avoid sudden step changes. Second, the speed regulator output that is the SP for the current regulator is limited by maximum and minimum ceilings. Finally, the integral term is also limited in order to avoid wind-up. The following figure shows a block diagram of a PI controller-based speed controller.
Block Diagram of the PI Controller-Based Speed Regulator
For each electric drive application, the mechanical load to be driven has a specific set of requirements. The torque/speed possibilities of the electric drive can be represented as a speed versus torque graph consisting of four quadrants. In the first quadrant, the electric torque and the speed signs are both positive, indicating forward motoring since the electric torque is in the direction of motion. In the second quadrant, the electric torque sign is negative and the speed sign is positive, indicating forward braking since the electric torque is opposite to the direction of motion. In the third quadrant, the electric torque and speed signs are both negative, indicating reverse motoring. In the fourth quadrant, the electric torque sign is positive and the speed is negative, indicating reverse braking. The drive braking is handled either by a braking chopper (dynamic braking) or by bidirectional power flow (regenerative braking).
The following figure illustrates the four-quadrants operating region of an electric drive. Each quadrant has a constant
torque region from 0 to +/- nominal speed
and
a region where the torque decreases inversely with the speed from
to
the maximum speed
. This second region is a constant
power region and is obtained by decreasing the motor magnetic flux.
Four-Quadrant Operation of an Electric Drive
The AC and DC library allows two levels of simulation, detailed simulations or average-value simulations. The detailed simulations use the Universal Bridge block to represent the detailed behavior of rectifier and inverter controlled drives. This simulation level requires small simulation time steps to achieve correct representation of the high frequency electrical signal components of the drives.
The average-value simulations use average-value models of the power converters. When simulating in average-value mode, the electrical input and output currents and voltages of the power converters driving the electrical motors represent the average values of the real-life currents and voltages. By doing so, the high frequency components are not represented and the simulations can use much bigger time steps. Each power converter average-value model is described in the detailed documentation associated with each DC or AC model type. The time step used in a drive at average-value level can usually be increased up to the smallest controller sampling time used in a model. For example, if a drive uses a 20 μs time step for the current loop and a 100 μs time step for the speed loop, then the simulation time step in average-value mode can be increased up to 20 μs. Simulation time step guidelines are given in the detailed documentation of each model.
Switching between the detailed simulation level and the average-value simulation level can easily be done via the new GUI associated with each model, as explained in Selecting the Detailed or the Average-Value Model Detail Level.
The drive models supplied in the library are relatively complex and involve a large number of parameters. The Electric Drives library provides new GUIs for all models. The new GUIs offer all the functionality you would expect from existing Simulink masks, plus some new features, as outlined below.
The general layout of the GUIs is identical to the layout of Simulink masks. A short description of the model appears at the top, parameters are entered in the middle portion, and buttons are placed at the bottom.
The parameters section is divided in three tabs at the top level, for all drive models supplied in the library. You enter parameters related to the electric machine, converters and DC bus, and controller in the first, second and third tabs, respectively. The following figure illustrates the Self-Controlled Synchronous Motor Drive GUI with the Controller tab active.
The new GUIs offer the same functionality as Simulink masks. You can enter as parameters numerical values, valid MATLAB expressions, and MATLAB variables. An important difference between these GUIs and Simulink masks is that you can only enter a single value in each input field (e.g., vectors and arrays are not allowed).
The new features (with respect to Simulink masks) are outlined below.
The new GUIs are designed to signal invalid parameters as early as possible. Hence, if you enter an invalid constant (for example 1.2.3 or --2) in a drive model's GUI, an error will be flagged as soon as you move away from the invalid parameter (for instance if you try to change another parameter in the GUI). Variables are treated slightly differently. If you enter a variable name that has yet not been defined in the MATLAB workspace, parameter validation is deferred until you start the simulation of the diagram that contains the model.
You can see in the previous figure that the GUIs include the usual buttons found at the bottom of Simulink masks, plus two new ones, Load and Save. The Save button enables you to save in a file the complete set of parameters if the GUI. The format of the file is the standard MATLAB binary (.MAT) format. The Load button enables you to recover a previously saved set of parameters for a given drive type (e.g., AC1, DC2, etc.). When you load a set of parameters, the drive type of the saved parameters is compared to the drive type of the model you are loading the parameters into, to ensure that you are loading parameters compatible with the model.
When you use the Load button, the dialog that appears will point to the directory in your MATLAB installation that contains the standard sets of parameters supplied for all the drives in the library.
However, when you use the Save button, the dialog that appears will point to the current working directory in the MATLAB workspace.
There is a Schematic button in the top right corner of the controller tab in all drive models. When you click this button, the control schematic of the drive model will appear in a new window.
You can select the detailed or the average-value simulation level by using the Model detail level menu located in the lower part of the GUI. Remember to modify the simulation time step with respect to the model detail level used.
You can select either the load torque or the motor speed as mechanical input. Use the Mechanical input menu located in the lower part of the GUI. Note that if you select the motor speed as mechanical input, the internal mechanical system is not used and the inertia and viscous friction parameters are not displayed. You have to include these parameters in the external mechanical system.
It is important to note that if you decide to disable the link between a drive model and its library, the new GUI will no longer be available for that particular instance of the model. Double-clicking on the model in such conditions will simply open the subsystem, as in the case of an unmasked Simulink subsystem. You can then enter parameters in the individual masks of subsystems that the drive model is composed from.
Note that in the standard (e.g., linked) situation, these masks are disabled to ensure that the top-level GUI is the only place where parameters can be changed. This is required to ensure proper synchronization of the two levels of user interfaces (e.g., the new GUI and the underlying subsystems' masks).
 | About the Electric Drives Library | Simulating a DC Motor Drive |  |
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