If your company is considering purchasing Medium Voltage Variable Frequency Drives, there are multiple factors to consider besides upfront cost. We cannot cover all of these in this short article, but these are a few to think about during the decision-making process.
Unlike low voltage drives, where there are two very distinct markets for two very distinct load types: Constant and Variable Torque, most medium voltage drives are built for variable torque applications such as pumps and fans since the majority of motors that size utilizing VFDs are running variable torque applications. There are, however, many constant torque applications such as extruders, grinders, and conveyors that utilize medium voltage variable frequency drives. Variable torque drives are designed to provide full load torque at high speeds, which is in line with how a variable torque application works. If your application will require full load torque throughout the speed range, you’ll need to purchase a medium voltage VFD that is rated for constant torque. EMA can help size this appropriately.
There is no universal capacitor specification for medium voltage drives that calls for what type of capacitor needs to be used on the DC Bus. The three most common are electrolytic, film-type, and oil-filled. The overall cost of ownership varies greatly depending on what type of capacitor your medium voltage variable frequency drive utilizes because the lifespan of each type is different. The overall lifespan of the three most common capacitor types are:
- Electrolytic: 7-10 years
- Film-Type: 15 years
- Oil-Filled: 20-30 years
For medium voltage drives, different manufacturers utilize different methods of dealing with pre-charging or “softly” charging the capacitors in the DC Bus bank. For more on the purpose of pre-charge circuitry, see here. Toshiba and TMEIC are two manufacturers (there are others) who utilize pre-charge circuity to reduce the effects of inrush current on the drive’s components and transformers. Siemens and Rockwell utilize the impedance of the phase-shifting transformer in lieu of an actual pre-charge circuitry, which can be problematic because the initial inrush can cause significant stress on the transformer and cause eventual damage
Maintenance Time and Cost
Medium voltage variable frequency drives are often on critical applications where downtime is extremely costly. We recently talked to a cement plant that told us that downtime costs them $300,000 per day! When every second costs money, having a drive that is easily repairable can save thousands of dollars. Many MV drives use power cells aka power arms on the output which are modular and can be swapped in 30 minutes or less. It is highly suggested to purchase at least 1 spare power cell during the initial drive purchase and to rotate the spare into the drive during scheduled preventative maintenance. Other spare parts such as power supplies, control cards, and fuses should also be shelved because you simply cannot count on the manufacturer to have these parts when you need them; it is not uncommon for there to be very long lead times on very common medium voltage VFD parts. EMA stocks many spare parts for medium voltage variable frequency drives, so if you’re in that bind, please contact us.
All variable frequency drives create an output to the motor by switching the DC bus into a variable voltage output. “Topology” refers to how the VFD goes about creating that output and this varies from manufacturer to manufacturer. Some medium voltage variable frequency drives are more “motor friendly” than others because their output is closer to a pure sine wave. The tradeoff to that is it utilizes more components which adds more points of failure. The amount of distance between the motor and the drive and the construction of the motor plays a major factor in this as well.
Are you confused enough now? Purchasing an expensive piece of equipment such as a medium voltage variable frequency drive can be overwhelming, but WE CAN HELP. Our unbiased, 3rd party approach can help you make the right decision. Contact us today and let us prove that No One ANYWHERE Is Better At Drives Than We Are!