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History of Variable Frequency Drive

• August 13, 2015
When Nikola Tesla developed and patented the AC induction motor in 1924 (as well as the syncronous and split phasemotors) it had the endearing characteristic that it can be run by direct connection to a three phase power source. The motor speed is directly proportional to the applied frequency and is determined by the formula n = 120f/p where n is the synchronous speed of the motor in rpm, f is the frequency of power applied and p is the number of poles on the rotor. Therefore a 2 pole induction motor running at 60 Hz will run at 3600 rpm synchronous speed less the slip required to produce the induction effect at full load. This slip is variable depending on the motor design but for the “standard” NEMA design B motor it is 3 to 5 % making the typical 2 pole motor run at 3500 rpm at full load at 60 Hz. Soon after the AC motor was developed, the idea of varying the speed was considered and the only practical way of doing this at the time was to provide the motor with a variable frequency obtained by using a DC motor turning an AC alternator which allowed a variable frequency.

This was done on a wide range of applications in the 50s, 60s, and 70s. Since the much simpler Ward-Leonard system existed for DC motors, however the major use for such lines was in precision controlled multimotor lines where synchronous AC motors were used for each section and when the master alternator frequency was varied, all the motors would follow together with synchronous accuracy. Such systems were still being installed on new machines as late as the mid 80s when static variable frequency controls became widely used. Static AC variable speed drives that were readily available were of the six step, variable voltage design. Later, when Phillips/Signetics came out with a sine coded PWM chip set, sine coded PWM drives became the norm and six step variable frequency faded into non-use except for unusual applications where the slightly lower loss at full speed, full load was an advantage.

An induction or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is induced by electromagnetic induction from the magnetic field of the stator winding. An induction motor therefore does not require mechanical commutation, separate-excitation or self-excitation for all or part of the energy transferred from stator to rotor, as in universal, DC and large synchronous motors. An induction motor’s rotor can be either wound type or squirrel-cage type.

Three-phase squirrel-cage induction motors are widely used in industrial drives because they are rugged, reliable and economical. Single-phase induction motors are used extensively for smaller loads, such as household appliances like fans. Although traditionally used in fixed-speed service, induction motors are increasingly being used with variable-frequency drives (VFDs) in variable-speed service. VFDs offer especially important energy savings opportunities for existing and prospective induction motors in variable-torque centrifugal fan, pump and compressor load applications. Squirrel cage induction motors are very widely used in both fixed-speed and VFD applications.

Nikola Tesla
A Serbian American inventor, electrical engineer, mechanical engineer, physicist, and futurist best known for his contributions to the design of the modern alternating current (AC) electricity supply system.

Tesla gained experience in telephony and electrical engineering before emigrating to the United States in 1884 to work for Thomas Edison. He soon struck out on his own with financial backers, setting up laboratories and companies to develop a range of electrical devices. His patented AC induction motor and transformer were licensed by George Westinghouse, who also hired Tesla as a consultant to help develop a power system using alternating current. Tesla is also known for his high-voltage, high-frequency power experiments in New York and Colorado Springs which included patented devices and theoretical work used in the invention of radio communication, for his X-ray experiments, and for his ill-fated attempt at intercontinental wireless transmission in his unfinished Wardenclyffe Tower project.

Tesla’s achievements and his abilities as a showman demonstrating his seemingly miraculous inventions made him world-famous. Although he made a considerable amount of money from his patents, he spent a lot on numerous experiments. He lived for most of his life in a series of New York hotels although the end of his patent income and eventual bankruptcy led him to live in diminished circumstances. Tesla continued to invite the press to parties he held on his birthday to announce new inventions he was working on and make (sometimes unusual) public statements. Because of his pronouncements and the nature of his work over the years, Tesla gained a reputation in popular culture as the archetypal “mad scientist”. He died on 7 January 1943.

Tesla’s work fell into relative obscurity after his death, but since the 1990s, his reputation has experienced a comeback in popular culture. His work and reputed inventions are also at the center of many conspiracy theories and have also been used to support various pseudosciences, UFO theories and New Age occultism. In 1960, in honor of Tesla, the General Conference on Weights and Measures for the International System of Units dedicated the term “tesla” to the SI unit measure for magnetic field strength.

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