## Electric Drives Models

The Electric Drives models are designed for engineers from many disciplines who want to incorporate easily and accurately electric drives in the simulation of their systems. The interface presents the parameters of the selected drive in a system-look topology, thereby simplifying the adjustments users may want to bring to the default values. Then they can seamlessly use any other toolboxes or blocksets to analyze the time or frequency behavior of the electric drive interacting with its system. The models are most helpful when a powerful drive has to be carefully maneuvered without ignoring the operating limits of the load on one side and of the power source on the other side. A good example is the electric drive system of a hybrid car that can switch in milliseconds from driving the wheels to recharging the batteries when the brakes are engaged.

Engineers and scientists can work readily with the seven typical direct current (DC) drives used in industries and transportation systems, eight alternating current (AC) drives providing more efficient and versatile motors from traction to positioning devices, and shaft and speed reducer models useful for connecting to the motor a model of load made of Simulink® blocks. An added value of the models are parameters that assure the validity of the motor, the power converters, and the control system. Particular attention was devoted to the motor models by comparing the models' behavior to the published data of the major manufacturers. Numerous examples or case studies of typical drives are supplied with the models. Hopefully, typical user systems are similar to these analyzed systems, thereby saving time in building the practical system and supplying a known reference point in the analysis.

To access the Electric Drives models, at the MATLAB® command prompt, enter:

electricdrivelib

### What Is 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 always contains a current (or torque) regulation 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 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 are examples of moderate performance applications where variable-speed operation means energy savings.

### Electric Drive Components

An electric drive consists of these main components:

• Electric motor

• Power electronic converter

• Drive controller

This diagram shows the basic topology of an electric drive.

Electric Drive Basic Topology

The motor used in an electric drive is either a direct current (DC) 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 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 to convert the desired drive torque/speed profile into triggering pulses for the electronic power converter, taking into account various drive variables (currents, speed, and so on) fed back by the sensors. To accomplish this conversion, the controller is based first on a current (or torque) regulator. The current regulator is mandatory because 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 models, the speed regulator is in series with the current regulator and is based on a PI controller that has three important features:

• The SP rate of change is limited so that the desired speed ramps gradually to the SP, in order to avoid sudden step changes.

• The speed regulator output that is the SP for the current regulator is limited by maximum and minimum ceilings.

• 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.

PI Controller-Based Speed Regulator