AC Drives and Soft Starter (1)

There are usually several choices for starting motors. Two of these, ac variable frequency drives (VFD’s) and soft starters, seem to have similar characteristics. Terms and descriptions used in product literature are nearly the same. Even the list of possible applications is similar. However, the technology and performance are significantly different. When these differences are understood, it becomes clear when and where to properly apply each of them.

The objective of this paper is to provide the basic technical information to understand the differences. First covered are the operating principles of the VFD and soft starter. How motor performance is affected is the other key to selection of the proper starting method. Finally, guidelines will then be presented.

Variable Speed Drives
The VFD works on the principle that the ac line voltage is converted to a dc voltage. This dc voltage is then inverted back to a pulsed dc whose rms value simulates an ac voltage.
The output frequency of this ac voltage normally varies for 0 up to the ac input line frequency. On certain applications the frequency may actually go above the line frequency.
Though high performance current regulated ac drives capable of operating in “torque mode” are available, the more prevalent volts per hertz drive is addressed here. The most common VFD’s manufactured today work using pulse width modulation to create the output sine wave. The conducting components used in drives are diodes, SCR’s, transistors and IGBT’s. These inverters have three distinct and different sections to their power circuits as shown in the typical inverter block diagram figure 1 below.
The first section uses a diode or SCR full-wave bridge to convert the ac line voltage to dc. Filtering of this dc is done in the second section with a capacitor to supply the inverter bridge with a stable dc power source. A dc link choke is normally present on 10 horsepower and larger drives. The final section uses a transistor or IGBT bridge to deliver a pulse width modulated (PWM) dc voltage to the motor. The effective rms voltage delivered to the motor is dependent on the fundamental output frequency that the inverter bridge is commanding. This is what leads to the term “volts per hertz drive.”
The control or logic section of the inverter and user programmed settings determine the frequency output of the inverter. During acceleration, the frequency will vary according to a predetermined algorithm such as linear ramp or s-curve, from minimum or 0 Hz up to commanded speed.
The drive can also be programmed to skip over certain frequencies that may cause a mechanical resonance.


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