Modern Variable Frequency Drives are Already Good – But They are Getting Even Better

Variable frequency drives have emerged as a surefire way to reduce energy costs in induction motor systems. From pumps and fans to material handling and industrial processes, VFDs help save many millions of kilowatt-hours around the world each and every year.


And energy savings are only part of the VFD value proposition. VFDs can help extend the working life of induction motors-by allowing them to operate at lower speeds for significant portions of their lifecycle. VFDs can also improve process control capabilities. In fact, the most advanced vector controlled drives, when paired with appropriate feedback devices in a closed-loop control system, can offer positioning performance close to that of servo systems.

One thing speeding the adoption of VFD technology is the fact that it continues to grow more efficient and reliable due to continuous improvements in the underlying power electronics, such as the insulated gate bipolar transistor (IGBT) technologies developed and employed by Fuji Electric. IGBTs have also seen dramatic improvements in power densities, allowing VFDs to become more compact.


A related technology trend that’s helping VFDs get better all the time involves the ready availability of low-cost, high-performance processors. More computing muscle allows VFD to run more complex control algorithms at higher speeds, which further enhances the control capabilities of VFDs.

Taken together, technological advances in power electronics and computing power will take VFDs to new levels of performance and cost effectiveness in the coming years. Here’s a look at how these technology trends have transformed some of Fuji Electric’s newly developed VFDs.

General Purpose Performance Boost


In some ways, today’s VFD technology has already progressed to the point that it meets the vast majority of general purpose application needs. Fluid control applications, such as pumps and fans, are already well served by existing drives. So are many material handling and process control applications.

VFDs have also become much more reliable and efficient over the years. Today’s low voltage drives routinely offer efficiencies in excess of 95 percent up from efficiencies as low as 80 percent just a decade ago.


As for reliability, modern VFDs typically outlast other components of a motor-driven system. At Fuji Electric, for example, our general purpose VFDs exhibit a failure rate below 0.1 percent even after more than a hundred thousand hours at 40°C.

To say that most applications are well-served by existing VFD technology, however, is not to say that there is no room for improvement. There are growing number of applications that can benefit from improved control performance, in terms of response times or the ability to control speed and torque accurately. Applications that push the envelope for general purpose drives tend to be those that have fast process speeds, braking requirements or impact loads (see Figures 1-5).


New multi-functional drives have emerged to fill the performance void between lower performing general purpose drives of years past and much more costly servo systems that would be engineering overkill. Think of these drives as high-perfonnance general purpose drives. One recently introduced example of this new class of drive is Fuji Electric’s FRENIC-MEGA drive.

Compared to earlier general purpose drives, FRENIC-MEGA improves control performance and application flexibility with support for not just traditional v/f control but also for three different types of vector control-PG, sensorless and dynamic torque.

When used with the optional PG vector control, the drive’s performance far exceeds what many engineers would expect from a general purpose drive.

The more advanced general purpose drives also share some other technical characteristics.

Improved Reliability:

Avoiding drive related downtime has become more important than ever as general purpose drives take over more control tasks. One route to enhanced reliability is improved increased durability to overload conditions.

Custom Logic:

The more advanced general purpose drive typically support user customizable, sequential logic functions. While not a replacement for dedicated PLC in applications with high I/O counts, this built in logic capabilities can close high-speed control loops and execute time-critical logic that is closely related to the drive application.


As general-purpose drives move into more difficult process control applications, connectivity via Ethemet TCP/IP, DeviceNet, Profibus and other industry network standards has become essential.

Future Drives Approaching Servo Performance

Moving a notch up the performance spectrum are the true high-performance drives that take positioning and control functionality beyond even the best general-purpose drives. One such drive is the FRENIC-VG, Fuji Electric’s next-generation product.

These FRENIC-VG series drives have a speed response of 600 HZ, or six times better than previous high performance models (See Table 1). Accuracy has improved too. Torque control accuracy is ±3 percent, while speed control accuracy is ±0.005 percent when using a PG card.


The performance gains are due in part to the VG series’ use of dual processors, which doubled the processing power available to crunch control algorithms quickly. The previous high-performance drives, by contrast, had a single processor.

To make the VG series as adaptable as possible, it supports a lineup of interface cards, including the E-SX high-speed synchronized communications card and a PG interface card. A safety card will be added to the lineup soon as will integrated servo functions. The VG series conforms with common safety standards, including ISO 13849-1 safety standards for EN terminals of inverters and IEC 61508 SIL2 for the optional cards.

The FRENIC-VG series is intended for applications that need tighter control than possible with a general-purpose VFD but still less than a full-blown servo system. Among these applications are those, like steel making equipment, that require precise torque control across the entire speed range. Cranes and heavy-duty material handling systems are also a good fit for the VG series, which can accommodate rapidly changing torque requirements. Additional applications involve industrial machines, such as stamping presses or automotive testing equipment, which require responsiveness at high speeds.


Reforming Capacitors on VFDs

Manufacturers highly recommend reforming capacitors on VFDs that have been stored, without power, for more than 1 year.

Material Required

  • Appropriate single phase or three phase power supply.
    220-230VAC Drives = power supply of 220VAC
    380-480VAC Drives = power supply of 220VAC
    500-600VAC Drives = power supply 300-330VAC
    660-690VAC Drives = power supply 300-330VAC
  • Miscellaneous material for connection of power supply.


All Variable Frequency Drives utilize Electrolytic Capacitors for storage of voltage in the inverter section of the drive. These electrolytic capacitors are made up of basically 3 parts. Two conductive plates (usually one being a metal substance similar to aluminum foil and the other being a porous material impregnated with an acidic substance similar to battery acid) that sandwich a layer of insulator material. The insulator material is generally made up of an oxide that is created when voltage is applied to the capacitor during manufacturing of the capacitor. Subsequent to this initial power up, every time the capacitor receives a charge it “rebuilds” this layer of oxide. As time elapses and there is no voltage applied to “reform” this layer the layer begins to degrade. This layer “thickness” is the determining factor for the voltage rating of the capacitor. If the layer degrades to a certain point one of two failures will occur. 1. The two conducting materials will begin to conduct current at a high rate, and this will cause a boiling of the liquid inside the capacitor. Once this boiling begins the pressure will rise internally and the capacitor will rupture. This failure usually results in a complete destruction of the drive as these capacitors are saturated internally with an acidic mixture. 2. The inrush current will arc across the oxide insulator and create a bridge between the plates. This bridge will short the plates and will cause a direct short in the power circuit that can result in other damage to the electronic components. At a minimum this second failure will require the damaged capacitor to be changed.

VFD Capacitor Reforming


If a VFD has not been powered up for 9 months to 1 year it is REQUIRED that the capacitors be “reformed” prior to the drive being put into service. For 220-230V and 380-480V models apply supply voltage of approximately 220Vac, three-phase or
single-phase input, 50 or 60 Hz, without connecting a motor to the output. This voltage should be applied for a period of 1 hour. After this energizing process, a wait period of 24 hours before installing or utilizing the drive for motor control is vital. Part of the oxide insulation is formed during this “resting” period. For 500-600V, 500-690V and 660-690V models use the same procedure applying a voltage between 300 and 330Vac to the inverter input.