Innovations in electric motor technology: the first 100 years
The concept of mechanical motion from an electrical current source is not new, but the discoveries of the past provide the foundation for today’s advanced motor developments. The earliest such example can be traced back to Hans Christian Oersted in 1820 when current from a battery was observed to deflect a compass needle. That same year, Andre-Marie Ampere invented what is now known as the solenoid coil. It was Michael Faraday, however, who set the stage for what we now know as the induction motor. Faraday’s 1821 experiments demonstrated rotation of a wire in a circular orbit around an electromagnet.
Over the next 70 years, numerous electrical giants built on Faraday’s experiments, progressing toward a true induction motor and pushing inventors to fund and create systems capable of powering these new devices. Names include Joseph Henry, Werner Siemens, Nikola Tesla, Charles Bradley, and George Westinghouse.
The Russian inventor Michael Dolivo-Dobrowolsky made two critical advances: improvements to the Tesla three-phase induction motor and the revelation that Delta and Wye three-phase connected motors only require three current carrying conductors (unlike two-phase motors of the time, which required four conductors). Many of the design characteristics of the Dolivo-Dobrowolsky three-phase induction motor (circa 1890) are still used today.
In the 20th century, advancement continued during the industrial revolution. More applications are converted from human, beast, and steam power to electromechanical means, which drives innovation in materials, construction methods, and electrical power transmission systems. One concept of vital importance is speed control. The ac induction motor’s speed control is relegated to one of two methods: gear reducers and the number of poles in the motor; both are set in stone without some fairly significant engineering changes.
Pursuant to the equation RPM = 120 x Applied Frequency / # of Poles, the speed of a motor can be affected by the number of poles in the electromagnetic winding. Fewer poles results in a higher output revolutions per minute (RPM), more poles equals a lower RPM. Since the U.S. has standardized on 60 Hz, this is a constant in the RPM equation.
Speed control with a VFD
Fast forward to the ac variable frequency drive (VFD) and the number of poles is no longer a driving factor nor is applied frequency a constant. Previously, motor starting methods for ac induction motors were either across the line or soft starting (reduced voltage starting or split winding delta/wye). Either way, it is a pure sine wave. With the VFD, instead of a pure sine wave there are pulses of voltage in a method called pulse width modulation (PWM).
The ac voltage is converted to dc then inverted back to ac and the voltage is pulsed though a solid-state switch, which is called an insulated-gate bipolar transistor (IGBT). An IGBT allows the user to actively modulate the motor’s speed by adjusting the Volts/Hertz ratio. Therefore, 460/60 is full speed and 230/30 equals half speed, etc. This method of control, however, causes different electrical stresses on a motor, which necessitated further motor innovation and led to the inverter duty rated motor. This new type of motor combats the electrical phenomenon that results from a VFD’s pulsing output.
Avoiding motor damage
In an across-the-line motor starter frequency is constant, and voltage has an upper limit-no more than what is supplied from the utility. With a VFD, frequency is no longer constant and voltage is switched (pulsed) thousands of times per second from zero to peak to mimic a sinusoidal wave form familiar to the motor. Peaks may reach 1,500 V or higher. It is these peaks in voltage that can damage the windings in a motor that is not suited for the application.
Inverter duty rated motors combat this problem by using winding insulation that eliminates the occurrence of corona inception voltage (CIV). In short, CIV is the point at which a conductor’s insulation becomes conductive, which is not good.
This article is not meant to be a comprehensive look at all motor technologies, but a brief look at the history of the ac induction motor and the notable names and innovations in the early part of its history. Current innovations, such as the permanent magnet ac motor (PMAC), are being refined to bring higher efficiency, cooler running temperatures, and incredible speed and torque accuracy. As a result of semiconductor technology and microprocessor advances, the VFD has become smarter, more efficient, and more accurate. It makes a person wonder what will happen in the next 100 years.
Dan Messina, CEP, is power technical consultant, Rensenhouse Electric Supply in Kansas City, Mo., part of CED’s Industrial Solution Network. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, email@example.com.
Keywords: Motors, variable frequency drives, VFDs
- Michael Faraday’s experiments in 1821 demonstrated rotation of a wire in a circular orbit around an electromagnet, the beginnings of the induction motor.
- Michael Dolivo-Dobrowolsky improved the Tesla three-phase induction motor and discovered that Delta and Wye three-phase connected motors only require three current carrying conductors.
- Motor systems made a huge advance with the variable frequency drive (VFD).
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