Figure 8, Drive Output Waveform Components
The speed at which power devices switch on and off is the carrier frequency, also known as the switch frequency. The higher the switch frequency, the more resolution each PWM pulse contains. Typical switch frequencies are 3,000 to 4,000 times per second (3KHz to 4KHz). (With an older, SCR-based drive, switch frequencies are 250 to 500 times per second). As you can imagine, the higher the switch frequency, the smoother the output waveform and the higher the resolution. However, higher switch frequencies decrease the efficiency of the drive because of increased heat in the power devices.
Shrinking cost and size
Drives vary in the complexity of their designs, but the designs continue to improve. Drives come in smaller packages with each generation. The trend is similar to that of the personal computer. More features, better performance, and lower cost with successive generations. Unlike computers, however, drives have dramatically improved in their reliability and ease of use. And also unlike computers, the typical drive of today doesn’t spew gratuitous harmonics into your distribution system-nor does it affect your power factor. Drives are increasingly becoming “plug and play.” As electronic power components improve in reliability and decrease in size, the cost and size of VFDs will continue to decrease. While all that is going on, their performance and ease of use will only get better.
Sidebar: What if you have SCRs?
With the large installed base of SCRs, you might want to know how these operate. An SCR (originally referred to as a thyristor) contains a control element called a gate. The gate acts as the “turn-on” switch that allows the device to fully conduct voltage. The device conducts voltage until the polarity of the device reverses-and then it automatically “turns off.” Special circuitry, usually requiring another circuit board and associated wiring, controls this switching.
The SCR’s output depends on how soon in the control cycle that gate turns on. The IGBT output also depends the length of time the gate is on. However, it can turn off anytime in the control cycle, providing a more precise output waveform. IGBTs also require a control circuit connected to the gate, but this circuitry is less complex and doesn’t require a reversal of polarity. Thus, you would approach troubleshooting differently if you have an SCR-based drive.
This information has been provided by: ABB Inc. – Drives and Power Electronics