Modern low-voltage drives are almost all the same these days. They utilize a diode or SCR front end to rectify AC to DC which sits on dc bus capacitors, and the inverter section is made up of transistors (IGBTs specifically) to create the output to the motor. Troubleshooting low voltage drives is by no means easy, but a basic understanding of how they are laid out is extremely helpful to narrow down where the problem lies. For a troubleshooter, knowing their “guts” are all essentially the same certainly simplifies things.
In the medium voltage drive world however, the topology varies greatly from manufacturer to manufacturer. The “guts” of one drive are not the same as another. The cascaded H-bridge topology created by Robicon allowed them to dominate the market for many years. Many of you work at a plant that has or had a Robicon Harmony or Perfect Harmony series drive, and many of them are still running today! Because Robicon had the patent for the cascaded H-bridge design (this patent has since expired, which has lead to many “robiclone” drives entering the market), other topologies emerged as manufacturers joined the race to sell medium voltage drives.
Current Source/Voltage Source:
Current Source drives have been around since the 1970s and is a very mature technology that has long since been phased out in low voltage drives. As the name suggests, current source drives are fed with constant current while voltage source drives are fed with constant voltage. Allen Bradley’s Powerflex 7000 is the most widely known current source Medium Voltage Drive. While a rock-solid design, the drive is subject to line notching issues that can interfere with a utility line without proper upstream filtration. The DC link utilizes a large inductor instead of capacitors, which is why you often see current source drives used in very large horsepower applications because the size is less compared to a voltage source alternative that uses capacitors. Nidec is one of the manufacturers known for large, current source drives.
Voltage Source drives utilize a much more modern design that is almost universally used in modern low voltage drives. Voltage source drives utilize capacitors in the DC link that stores voltage, and that voltage is switched on and off to the motor to create the output.
Three Sections of A Modern VFD:
Converter:
This section converts the AC to DC by using diodes or SCRs. It is also referred to as the “rectifier” section. In certain Medium Voltage Variable Frequency Drives, this is done at the cell level, where each power cell has its own diode bridge that converts AC to DC; this is typical of the Siemens Perfect Harmony Series Drives and the Allen Bradley Powerflex 6000 drives.
Other models, like the Toshiba T300MV2, have a the converter section upstream of the cells that feeds DC to each individual cell.
DC Link:
Often called the “DC Bus”, this section stores and smooths the DC voltage and is the bridge between the converter and inverter sections. In low voltage drives, the DC link is composed of electrolytic capacitors, but because the design varies so much in medium voltage, you might find electrolytic capacitors, film type capacitors, oil-filled capacitors, or even large inductors in current-source drives.
Inverter:
The inverter section is the section responsible for the output to the motor. In voltage-source drives, the inverter section is typically made of IGBTs which act as switches to deliver voltage and current to the motor. You may have heard VFDs refereed to as “inverters”, which comes from this DC back to AC inversion that takes place. Current source drives, like the Allen Bradley Powerflex 7000 use SGCTs to create the output.
Making Drives Motor Friendly:
When medium voltage variable frequency drives were first introduced, the peak-to-peak spikes that resulted from transistors turning on and off, was extremely damaging to motors who’s insulation was not rated for it. This problem has been well known for years which is why most low voltage drives are running “inverter duty” motors, which have beefier insulation to withstand the voltage spikes. Since medium voltage motors are not typically inverter duty rated, manufacturers had to come up with a different output topology to limit the voltage spikes. By adding additional IGBTs and driver circuits, the output “steps” can be increased, reducing the amount of voltage being switched at a time, which in turn, reduces the peak-to-peak spikes that can damage a motor. The resultant waveform is much more sinusoidal and “motor friendly”. The higher-level topology also allows for a much longer motor lead length as well without the use of an output filter. The Toshiba T300MV and Yaskawa MV1000 are two examples of modern drives that use multi-level topology.
Confused yet? The differences that exist within the Medium Voltage VFD landscape makes it tough to troubleshoot. Never fear! At EMA, we’re well versed in all types of medium voltage drives and have experience working on almost every make and model of medium voltage variable frequency drives. In over 34 years, there’s little we haven’t seen! Do you need help in applying, troubleshooting, or starting up a Medium Voltage VFD? Give us a call, contact us via email, or click the chat on the site to talk to us today. Let us prove to you that No One, ANYWHERE, Is Better At Drives Than We Are!