Variable speed drives: back to the features!
13 November 2015
Nipun Sibal reminds us that, while we probably concentrated on subjects like speed and torque control in our supplement of twenty years ago, there's a lot more on offer from today's drives.
A variable speed drive (VSD), by dint of its title, is primarily a motor speed control device, but it has evolved over time to do a great deal more. It can be considered as a ‘smart actuator’ fully capable of being connected to, and integrated within, a variety of systems, whether they are related to automation, safety or power conversion.
A modern drive is able to act hierarchically at all levels of an application, be it electrical, mechanical or process control related, by protecting, controlling, and monitoring a huge range of equipment, including pumps, machines of every kind, conveyors, elevators, cranes - the list is endless.
The modern variable speed drive is not just a drive - it is a power conversion product. Essentially, a drive is a frequency converter, if we look at it from an input/output perspective. It is able to deliver full current from 0 to 599Hz, but also up to 2,000Hz as a fundamental frequency. As a power conversion product realised with multiple converters, it is basically a rectifier bridge (ac/dc power converter), with or without regenerative capabilities. It is also a dc/dc power converter and inverter bridge (dc/ac power converter).
It is also possible to electrically connect drives together - for instance, through dc bus sharing architecture, but also to use the power converter as bricks to optimise the power conversion system, connecting them in series or in parallel. And there are many types of filters available to adapt power conversion architectures.
A drive is also an automation safe product; it’s a connectable low level controller because of its embedded capability for fieldbus based or point-to point wired communication systems. Insulated from the mains power supply, it’s a safe and certified device. Its firmware can be updated, configured and programmed locally with external devices such as a keypad, PC, laptop or smartphone. It can store data from its environment and exchange it with connected devices. It can also be embedded with a dedicated communication link to exchange data between drives in an optimal and transparent way.
The modern drive is also a local HMI product. You can quickly start a drive configuration for an application, which can be stored and applied to other drives. You can start, stop, and locally reset a drive, monitor the application and alert the user.
Electrical motor control
Electrical machines universally convert electrical energy into mechanical motion. The fundamental physics remain the same whatever the motor brand. And thanks to a physical model approach, a drive is able to control all types of electrical machines, including induction motors, surfaced/internal permanent magnet motors, and synchronous reluctant motors.
The drive addresses mainly three-phase sinusoidal motors but it is not a physical limitation; with or without a sensor. According to the flux vector control principle, a drive optimises the electrical machine energy consumption and simultaneously maximises the mechanical power capability. The settings functions (manually through nameplate or automatic through auto-tuning) complete the electrical motor control approach.
As discussed above, a motor converts energy from the electrical to the mechanical domain. The mechanical domain is managed by the fundamental principle that links position, speed and torque through dynamic equations. The Drive embeds all types of mechanical control and sequence principles to address torque, speed and position needs for a rigid or flexible load as well as hybrid loads that include a gearbox, with or without a mechanical brake. A drive can also share the torque realisation on mechanical coupling, in multiple drive equipment architecture (master/slave application, for example).
A drive is application oriented, as it is embedded with main application functions (for pump management, for compressor, for fan, and so on) while also providing a flexible control environment. Thanks to a model-based approach, application control functions pertaining to application/process variables (flow, pressure, diameter, distance and so on) can be managed on an individual basis to address specific needs, such as the creation of a new application program and corresponding embedded drive functions.
The modern VSD has evolved over the years to become more of an optimised energy drive, acting efficiently to command, protect, connect and monitor in every field of use. This versatility of application illustrates how far the technology has come in a relatively short period of time, and how many and varied the applications are for VSDs across a broad range of engineering platforms and environments.
The breadth of applications for VSDs brings into question whether the same still fits the solution. VSDs are long overdue for a rebrand that recognises their incredible versatility.
Nipun Sibal is product marketing manager, Schneider Electric
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