The power stage – the tiny substation powering the motor

In an electric drive’s power stage, the mains voltage is converted into the voltage required by the drive. With a digital drive control system, it is not only possible to create a fixed operating voltage, but to control the waveform of the supply voltage such that the rotation speed, torque or even position can be regulated.

Power unit plus regulation

A control device is integrated between the actual motor and the supply grid in order to achieve an adjustable rotation speed, shaft position or desired output torque. It converts the input voltage such that the motor outputs the respective mechanical output and speed demanded by the application.

This kind of control device or actuator consist of two basic units: the power electronics and the control system. The power electronics transform the electrical energy in terms of voltage, current and frequency. The control system is responsible for all regulation, control, communication and protective functions. As such, they issue the appropriate commands to the power electronics. Nowadays, they are generally implemented in the form of digital components, in other words microcontrollers or microprocessors.

 

Different actuators to suit each motor

The actuators differ depending on the motor type:

• In the case of asynchronous, synchronous and brushless DC motors, the rotation speed and torque depend on the frequency and amplitude of the supply voltage. Frequency converters convert the frequency and amplitude of the voltage supplied by the supply grid in such a way that the motor’s rotation speed and direction of rotation can be altered.
   
• In the case of direct current motors, the rotation speed depends, amongst other things, on the level of voltage applied. Power converters are used here. These transform the alternating voltage from the supply grid into a variable direct voltage which, in turn, can be used to continuously adjust the motor’s rotation speed.

 

Semiconductor devices in the power stage

The power electronics generally consist of a rectifier that converts the supply grid’s alternating voltage into direct voltage. This is then converted into an alternating voltage with variable frequency and voltage via a downstream inverter, in order to therefore alter the rotation speed of a three-phase motor electronically. In addition, a “chopper” is also integrated – it is responsible for dissipating braking energy (since current can only flow through the rectifier in one direction).

In order to increase the efficiency of the overall system, the semiconductor elements used in the individual components play a significant role: switching devices such as IGBTs (insulated-gate bipolar transistors) are used with rectifiers; these have the benefit of minimising harmonic distortion. An IGBT is generally used for chopper modules. This switching device ensures that the current produced in regenerative mode is either converted into heat via a brake resistor or is conducted into an accumulator as direct voltage (which is, of course, much more energy-efficient). IGBTs are normally also used in the last phase, the inverter.

In recent years, significant investments have been made in the development of wide bandgap materials. Commercially available SiC and GaN switches are used in applications where small footprint and low power stage dissipation are critical, such as servo drives or automotive drive inverters.

 

Energy efficiency classes

Each of these three components has its own efficiency, and these together determine the overall efficiency of the system. In order to classify the energy efficiency of drive controllers and frequency converters in a power range of 0.12 to 1,000 kW, IEC 61800-9-2 defines three IE classes: IE0 (low) to IE2 (high). A device from efficiency class IE2 has 25% lower losses than a device from class IE1 in this case – and a device from class IE0 has 25% higher losses.

 

Higher efficiency with SiC and GaN

In order to reduce switching losses and conduction losses in the components and increase efficiency, various semiconductor technologies can be used for the design of the power switch:

• IGBT: An "insulated-gate bipolar transistor" is one of the most commonly used power semiconductors. It can switch large loads with very low control powers. IGBT power modules are therefore particularly used for power-hungry applications, as they can improve temperature, weight as well as cost.
   
• Silicon-based MOSFET: Metal-oxide-semiconductor field-effect transistors that are based on Si can be used for various voltages and currents, are a mature technology and are available at a good price/performance ratio.
   
• GaN-based FET: Gallium nitride is a wide bandgap material whose properties enable high current ratings and high switching frequencies with low losses and high breakdown fields. This results in significantly lower losses on system level, meaning power converters can have a more compact construction and improved levels of efficiency can be achieved.
   
• SiC switches: Silicon carbide (SiC) is also a WBG material. It has particularly low losses and works rapidly with high switching voltages and high material temperatures.
   
  • Gallium nitride (GaN) and silicon carbide (SiC) offer a higher level of efficiency compared to the semiconductor technologies that are currently prevalent, such as silicon MOSFETS and IGBTs: the losses are lower, the switching frequencies higher, higher operating temperatures are possible, the level of robustness in harsh environments is further improved and breakdown voltages are higher.
     
  • Therefore, WBG devices offer excellent performance in terms of total losses compared to their silicon counterparts IGBT and MOSFET in the same power range. A drive solution with SiC MOSFETs can therefore reduce losses by up to 50%. SiC MOSFETs also deliver more power with a smaller size; this results in a more robust inverter with lower overall system costs.

 

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