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 SimPowerSystems 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.

 

 

 

 

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