The different types of adjustable speed industrial motors
■ Permanent magnet synchronous motors (PMSM) are highly efficient AC motors and can enable precise control of torque and speed in high- performance applications that demand high energy efficiency. ■ BLDCs are also used in applications that require high efficiency and precise control and typically have long operating lives. ■ Servo motors can be AC or DC and support rapid, high- precision responses. VFDs with specialized control algorithms can be used with servo motors in robots, computer numerically controlled (CNC) machines, and similar applications. ■ Synchronous AC motors (SMs) are suited for applications that require constant speed and
precise synchronization. While VFDs can control the speed of SMs, other (lower cost) drive options can support constant speed operation. There’s a variety of control algorithms used with VFDs that increase their versatility. For example, there are four primary types of VFD control algorithms just for induction motors: volts-per- Hertz (V/f), V/f with encoder, open- loop vector, and closed-loop vector. All use pulse-width modulation and provide different levels of control over speed and torque. The importance of VFDs in a wide range of industrial applications is evidenced by the development of IEC 61800-9, which is focused on the efficiency and ecodesign of VFDs and related motor drive systems.
BDM, CDM, and PDS There are two sections of IEC 61800-9 related to VFDs. Part 1 delineates the methodology for determining an application's energy efficiency index or reference. Part 2 details methods for evaluating efficiency based on a series of classifications. While the efficiency of VFDs, called basic drive modules (BDMs) in IEC 61800-9, is important, it’s not the primary focus of the standard. The standard is more broadly based and considers complete drive modules (CDMs) that consist of a frequency inverter (the VFD), a feeding section, and input and output auxiliaries (like filters and chokes) and on the power drive system (PDS) that consists of the CDM plus the motor (Figure 1).
Figure 1: IEC 61800-9 efficiency classes apply to the CDM (black section) and PDS (red section) in VFD systems. (Image source: Schneider Electric)
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