By Peter Wolff , of Armstrong Fluid Control Technology
OVER the last 150 years, development of the electric motor has gone in fits and starts. Initially, all electric motors used direct current (DC). But with the advent of alternating current (AC) and amid the “battle of the currents” between industrial development giants George Westinghouse and Thomas Edison, Nikola Tesla unveiled his induction motor, a new design that converted the power of alternating electrical current into rotational torque.
The asynchronous AC motor was created. The combination of high starting torque and low maintenance saw it quickly adopted for applications that were previously the realm of steam, water, belts/pulleys and horsepower. The reluctance motor, using magnets embedded in the rotor, was also invented in the 19th century. It was handicapped by low efficiency when driven directly from the AC supply. This would change at the end of the 20th century.
Variable Speed Drives
The invention and mass manufacturing of electronic variable speed drives (VSDs) began in the 1960s and 1970s. Industrial pioneers developed several methods to convert the mains electrical supply at its constant frequency (50 or 60 hertz [Hz],depending on geography) to a variable frequency on the output, resulting in a variable speed induction motor driving the pump or fan. With a suitable control system, plant speed could be adjusted to suit a variable load.
The first generation of VFDs was expensive, large and unreliable. They would regularly trip themselves out to self-protect. Operators responded by often demanding that a bypass system of the VFD to the motor be engineered in case of failure. This made them even more costly, bulky and complicated. Thirty years later, VFDs were smaller, affordable and reliable. The size reduction allowed them to be decentralized and mounted locally.
OEMs of pumps took a step further in the 1990s and integrated them into their products and, in the years since, have added self-adjusting smart speed controllers and web connectivity. This simplifies the design and install of hydronic systems while also delivering more reliable outcomes. The pump controllers’ capabilities often include accurate measurement of flow rate and pump generated head—information that can help to deliver higher process efficiency. A benefit of these improvements has also been the removal of bypasses, rendered redundant just as the starting handle became for cars and automobiles after electric self-starters were introduced.
Permanent magnet motors—surface mount and interior mount
In motor rotors fitted with permanent magnets, the stator winding no longer has to expend power inducing magnetism in it. But how are the magnets best configured in or on the rotor? As their names imply, surface permanent magnet (SPM) motors have magnets on the surface of the rotor and interior permanent magnet (IPM) motors have them embedded inside the rotor, shaped to suit the drive application. SPM motors deliver good starting torque, but the arrangement of the magnets generates back EMF in the stator at speed, reducing efficiency. SPM motors are speed limited due to the mechanical limitation of the attachment of the magnets to the shaft.
An IPM motor, embedded in the rotor, allows the shape of the magnets to be optimally configured so the lines of magnetic flux reduce the back EMF at high speeds. Also, no mechanical speed limits apply as they do to SPM.
The SYN RM IPM motor
Placing PM in the slots in the synchronous reluctance motor produces the IPM SynRM motor. It does not generate back EMF when, at speed, the rotor is aligned to the stator field at one angle and at startup, another angle that generates high torque. One other benefit is that the motor shaft runs cool, improving bearing life. The SYN RM IPM motor uses both reluctance torque and magnetic torque, managing their complementary properties to give high torque at startup with high efficiency and smooth operation at speed. In addition, the power factor is improved. Also, this type requires less magnetic material than a conventional IPM motor.
This article is a shortened version. For the full transcript, visit: https://blog.armstrongfluidtechnology.com/motor-variable-speed-controller-technology
