Voltage source inversion (VSI) drives have poor power factor, can cause motor cogging below 6 Hz, and are non-regenerative.

CSI VFDs give a very clean current waveform but require large, expensive inductors in their construction and cause cogging (pulsating movement during rotation) below 6 Hz. In other words, they can absorb power flow back from the motor into the power supply. CSI VFDs are the only type that has regenerative power capability. Current source inversion (CSI) has been successfully used in signal processing and industrial power applications. The process also yields overvoltage spikes and harmonic current distortions. However, this frequency conversion process causes 2% to 3% loss as heat in the VFD - caloric energy that must be dissipated.

VFDs manipulate the frequency of their output by rectifying an incoming AC current into DC, and then using voltage pulse-width modulation to recreate an AC current and voltage output waveform. With VFDs, speed control with full torque is achieved from 0 rpm through the maximum rated speed and, if required, above the rated speed at reduced torque. These variable-frequency drives (VFDs) accurately control the speed of standard AC induction or synchronous motors. An ideal sinuosoid and PWM waveform figure.īy the 1980s, AC motor drive technology became reliable and inexpensive enough to compete with traditional DC motor control. These simple motor controls varied the speed and torque, and were the most economical way to do so for a number of decades.įig. In contrast to AC motor speed control requirements, DC motor speed control was achieved by inserting a rheostat into the low-power DC field circuit, which was feasible with available technology. Even decades after the induction motor gained widespread use, changing the frequency for speed control remained an extremely difficult task - and the physical construction of the motor prevented manufacturers from creating motors with more than two speeds.Īs a result, DC motors were necessary where accurate speed control and significant power output were required. However, AC motor speed control requires either varying the magnetic flux or changing the number of poles on the motor. When Tesla first introduced the 3-phase alternating current (AC) induction motor in 1888, he knew that his invention was more efficient and reliable than Edison's direct current (DC) motor.

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