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Knowing the truth about VFD operation can simplify the selection process for choose a variable frequency drive.

No matter how commonplace variable frequency drives (VFDs) have become to you, somewhere someone is using one or considering using one for the first time (see Figure 1). Think back to when you first thought about applying one of today's pulse-width modulation (PWM)-based VFDs to an ac motor. Chances are you probably had a few misconceptions about their abilities and designs. This article addresses five common VFD myths and corrects misconceptions about their proper usage.

Myth No. 1: The output of a VFD is sinusoidal

People tend to be more familiar with running their ac induction motors using motor starters. With a starter, starting the motor involves connecting the 3-phase leads of the motor to 3-phase power. Each phase is a sine wave with a frequency of 60 Hz and usually has a voltage amplitude of 230 V, 460 V, or 575 V in the U.S. This applied voltage creates a sine wave current waveform with the same frequency if checked at the motor leads. So far, running a motor is quite simple.

What happens at the output of a VFD is an entirely different story. A VFD typically rectifies the 3-phase input to a fixed dc voltage, which is filtered and stored using large dc bus capacitors. The dc bus voltage is then inverted to yield a variable voltage, variable frequency output. The inversion process is carried out using three insulated gate bipolar transistor (IGBT) pairs-one pair per output phase (see Figure 2a). Because the dc voltage is inverted into ac, the output stage is also called "the inverter." The duration for which each IGBT switch in a given pair is turned ON or held OFF can be controlled, which determines the RMS value of the output voltage. The ratio of the output RMS voltage to output frequency determines the flux developed in the ac motor. In general, there is a fixed relationship between the two. When the output frequency increases, the output voltage should also increase at the same rate to keep the ratio constant and thus the motor flux constant. Normally, the relationship between voltage and frequency is kept linear so that a constant torque can be produced. The resulting voltage waveform applied to the motor winding is not sinusoidal (see Figure 2b). Note that sometimes the voltage by frequency (V/f) ratio can be quite nonlinear, which is typical for fans, pumps, or centrifugal loads that do not require constant torque but instead favor energy savings.

What makes this work is that, as the name implies, an induction motor is a big inductor of sorts. A characteristic of induction is its resistance to current changes. Whether a current is increasing or decreasing, an inductor will oppose the change. What does this have to do with the PWM voltage waveform in Figure 2b? Instead of letting the current pulse rise on the same order as the applied voltage pulse, the current will start to rise slowly. When the voltage pulse has ended, the current doesn't disappear immediately, it slowly starts to ebb. In general terms, before the current has fallen back to zero, the next voltage pulse comes along, and the current starts to slowly rise again-even higher than before because the pulses are getting wider. Eventually, the current waveform becomes sinusoidal, albeit with some jagged up-and-down transitions as the voltage pulses start and end (see Figure 3).

Myth No. 2: All VFDs are the same

The common ac VFD of today is a fairly mature product. Most commercially available drives contain the same basic components: a bridge rectifier, a soft-charging circuit, a dc bus capacitor bank, and an output inverter section. Granted, there are differences in how the inverter section does its switching, the reliability of the components, and the efficiency of the thermal dissipation scheme. But the basic components remain the same.

There are exceptions to this "all-the-same" thinking. For example, some VFDs offer a three-level-output section. This output section allows the output pulses to vary from half-bus, voltage-level pulses and full-bus level pulses (see Figure 4).

To achieve the three-level output, the output section must have twice the number of output switches, plus clamping diodes (see Figure 5). The benefit gained by using a three-level output is reduction in voltage amplification at the motor due to reflected wave, lower common-mode voltage, shaft voltage, and bearing current.

The matrix-style inverter is an even more atypical type of VFD. VFDs with matrix-style inverters do not have a dc bus or a bridge rectifier. Instead, they use bidirectional switches that can connect any of the incoming phase voltages to any of the three output phases (see Figure 6). The benefit of this arrangement is that power is allowed to flow freely from line-to-motor or motor-to-line for fully regenerative four-quadrant operation. The drawback is that filtering is required on the input to the drive because extra inductance is necessary to filter the PWM waveform so that it does not affect the input ac lines.

In addition to VFDs with three-level outputs and matrix-style inverters, there are more examples that prove not all ac VFDs are the same.

Myth No. 3: VFDs cure power factor (PF) issues

It is not uncommon to see VFD manufacturers quote PF statistics like "0.98 displacement PF" or "near-unity PF." And it is true that input displacement PF improves after a VFD is installed ahead of an induction motor. The VFD uses its internal capacitor bus to supply any reactive current the motor requires, thereby protecting the ac line from being the source of the reactive current and lowering the displacement PF. However, displacement PF is not the full story.

The full story of the PF calculation is that it must include the reactive power demanded by harmonics that are created when ac voltage is rectified to dc. The diode bridge conducts the current from the ac line to the dc bus in a discontinuous way. It is important to remember that a diode conducts only when the voltage on the anode side is higher than the voltage on the cathode side (forward biased). This means that the diodes are only ON at the peak of each phase during both the positive and negative portions of the sine wave. This leads to a ripple-like voltage waveform. It also causes the input current to be distorted and discontinuous (see Figure 7).

Much can be said about how to calculate harmonics and how to mitigate them. Regardless, to calculate true PF, the effects of harmonics must be included. The following equation indicates how harmonics influence true PF:

Where THD = total harmonic distortion

For the discontinuous input current in the equation, THD would be in the neighborhood of 100% or more. Substituting that into the equation yields a true PF closer to 0.71, compared to a displacement PF of 0.98, which disregards harmonics.

Not to panic though, there are currently many ways to reduce THD. These techniques make use of passive and active methods of making the input current waveform much less distorted and the THD much lower. The aforementioned matrix-style inverter VFD is an example of an active method of THD reduction.

Myth No. 4: You can run a motor at any speed with a VFD

The beauty of using VFDs is that they can vary both their output voltage and output frequency. Their ability to run the motor slower and faster than its nameplate-rated speed is part of why they are specified so often. Take the motor out of the equation, and this myth is actually true. Without the limitations of the motor, the VFD can easily run any frequency within it speed range without limitation. But in reality, the motor is necessary to do real work, and its cooling and power needs start to limit the actual speed range of the motor/drive combination.

Limit No. 1: From a motor cooling perspective, turning some motors too slowly is not a good idea. In particular, totally enclosed fan cooled (TEFC) motors have limitations because the fan that blows air over the motor shell is attached to the motor shaft. The slower the motor is operated, the less cooling air that goes to the motor. Most motor manufacturers specify speed ranges for their motor designs that reflect how slow the motor can be run—especially while loaded. TEFC motors typically are not recommended for operation at full load below 15 Hz (a 4:1 speed range).

Limit No. 2: It is not always stated on the motor nameplate, but mechanically, motors have a speed range limitation. Commonly referred to as the maximum safe operating speed, this speed is tied to mechanical limitations, such as bearings and balance. Some motor data sheets specify the maximum speed.

Limit No. 3: Before the motor reaches its maximum operating speed, it could run out of torque. This speed limitation is not due to cooling or mechanics, but is due to power limitation, which is a product of speed and torque. To be exact, the VFD runs out of voltage. Note that the rotation of the motor also generates a voltage of its own, referred to as back-electromotive force (EMF), which increases with speed. The back-EMF is produced by the motor to oppose the supplied voltage from the VFD. At higher speeds, the VFD must supply more voltage to overcome the back-EMF so that current can still flow into the motor as current is instrumental in producing torque. After a certain point, the VFD cannot push any more current into the motor because the output voltage has reached maximum, and thus the motor torque reduces, which, in turn, reduces speed. This reduction in speed results in lower back-EMF, which, in turn, allows more current to flow into the motor again. There is an equilibrium point where the motor reaches the maximum speed for a given torque condition so that the product of torque and speed equals its power capability.

Let's take a step back. VFDs can produce constant torque from a motor by keeping the V/f constant (see Figure 8).

When the output frequency is increased, the voltage increases linearly. The problem arises when the frequency is raised beyond the base frequency of the motor, most commonly 60 Hz in the U.S. Beyond the base frequency, the output voltage cannot increase, which causes the V/f ratio to reduce. The V/f ratio is a measure of the magnetic field strength in the motor and reducing it reduces the torque capability of the motor. Hence, the ability to have the motor produce rated torque at higher-than-base speed must decline at a rate of 1/frequency, so that the product of torque and speed, which equals power, is constant. The region of operation above base speed is called the constant power range, while operation at speeds below the base speed is called the constant torque range (see Figure 9).

Myth No. 5: A VFD's input current should be higher than its output current

Perhaps this is not a myth but a misunderstanding. Some VFD users check their output and input currents with a current clamp meter or by using VFD display monitors and find that the input current is much lower than the output current. It doesn't seem to align with the idea that the VFD should have some losses due to its own thermal component losses, so input should always be slightly higher that output. The concept is correct, but it is power, not current that should be considered:

The voltage portion of the preceding power equation is straightforward. The input voltage is always at the ac line voltage. The output voltage varies with the speed per the V/f pattern. The current components of the equation are a bit more complex. The key to understanding current components is knowing that a typical induction motor has two current components: One is responsible for producing the magnetic field in the motor, which is necessary to rotate the motor; while the second component is the torque-producing current, which, as the name suggests, is responsible for producing torque.

The drive consumes input current proportional to the motor's active torque demand, or load. The current needed for producing the magnetic field typically does not vary with speed and is provided by the drive's main dc bus capacitors, which are charged during power up of the VFD. Under low torque conditions, the output current may seem to be much higher than the input current because the input current mirrors only the torque-producing current plus some harmonics but does not include the magnetizing current. The magnetizing current circulates between the dc bus capacitors and the motor. Even at full load conditions, the input current will typically be lower than the motor current because the input still does not have any magnetizing current component in it.

Remember, we are balancing input and output power. For example, consider a fully loaded motor at low speed. The input voltage is at the rated line while the output voltage will be low due to the low speed. Because of the full load on the motor, output current will be high. To balance the power equation, the input current must be lower than the output current.

Paul Avery is a senior product training engineer at Yaskawa America Inc. He has been a VFD technical trainer with the company since 2000 and has a degree in electrical engineering from the University of Michigan.

This article appears in the Applied Automation supplement for Control Engineering and Plant Engineering.

Paul Avery, Yaskawa America Inc.

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When someone ask Why using AC Driives/Inverter/VFD/VSD ???

"AC Drive Efficient"

Process variables, including pressure and flow of gases and liquids, have long been regulated using mechanical clutches, throttles, and adjustable inlet guide vanes. These schemes waste energy, require frequent maintenance, and provide inaccurate control. Adjustable frequency drive control provides more efficient, maintenance-free performance, with more accurate control. They have become the preferred method of control for variable speed applications. These drives provide many benefits over traditional control methods. The benefits are both cost and performance related and include:
• use of the rugged, squirrel cage induction motors for reduced cost and easy application,
• advanced performance from digital microprocessor control and serial communications,
• competitive first costs by using standard off-the-shelf components.

Most adjustable frequency drives consist of four basic sections (see figure1):
• The converter section rectifies the AC line input power into the DC bus/filter.
• The DC bus/filter section smooths the DC ripple.
• The driver-regulator section consists of the control, measurement, logic, and command circuits necessary to integrate the drive elements into a system.
• The inverter section converts the filtered DC bus into an AC output.

The voltage level, power level and type of inverter technology determine the size and type of power semiconductors in the converter and inverter sections.

Most inverter sections consist of one of the following solid state switching devices:
• Bipolar transistors have a higher switching rate than silicon control rectifiers or gate turn-off thyristors. However, current ratings limit their application to low- power applications where high switching rates are required.
• Insulated gate bipolar transistors represent state- of-the-art technology in power semiconductor devices. They have very fast turn-off times, allowing them to switch at rates of up to 15-20 Khz. The current waveform is nearly sinusoidal, reducing peak currents by as much as 42% compared with bipolar transistors. The result is higher available torque throughout the speed range. This eliminates motor noise and reduces motor losses and heating.

The components of the driver-regulator section may use either analog or digital techniques. However, most adjustable frequency drive manufactures are turning to the flexibility of the digital microprocessor- driven regulators.

Microprocessors allow manufacturers to include optional control schemes via software modifications. They allow additional functions to be supplied at minimal incremental costs to the user. Furthermore, microprocessors provide enhanced fault diagnostics since most fault data can be stored and viewed at a later time. These features add to the performance and the ability of a single drive design to meet a wide variety of application requirements. As a result of microprocessor-based digital control, adjustable frequency drives are easy to start-up and operate, are resistant to damage and have simple troubleshooting procedures.

Another benefit of microprocessors that contributes to advanced system integration is serial communications protocol for control and monitoring. Many adjustable frequency drives use programmable logic controllers to manage data collection from peripheral controls. This allows the operator to customize the drives with software-executed features and programmable parameters.

A pulse width-modulation inverter controls both the width of, and the spacing between, DC pulses precisely. This allows the inverter to simulate a sinusoidal shaped output pattern. In turn, the output voltage has lower harmonic content. Pulse width- modulations have a wide speed range, smooth low speed operation, multi-motor operation ability, and a high-input power factor. Microprocessors provide improved modulation techniques. Higher-speed switching devices, such as insulated gate bipolar transistors, are making the pulse width-modulation inverter the standard for the 1-500 HP range.

The characteristics of the drive and motor must be considered as a system when applying adjustable frequency drives.

The duty cycle of the inverter-motor combination must be checked at all load conditions to ensure the combination is suitable for the application. It is also important to understand the load requirements. Depending on the application, the load can be classified as one, or a combination of three basic load profiles. (See figure 2).

Constant torque

Constant torque implies that any speed in the operating range requires the same amount of driving torque. Conveyors are an example of a constant torque application.

Adjustable frequency drives with many control features are ideal for conveyor applications. With the advanced factory automation, energy-savings during transportation has become essential. Inverters start up from low frequency and low voltage then increase both frequency and voltage. Current and torque are much smaller during acceleration compared to the cross-line start with commercial power supplies. Therefore, an inverter drive can eliminate the reduced voltage start unit.

Since current during acceleration is less, motor heating is reduced, allowing frequent run/stop operations. For example, feed conveyors running at constant speed consume more energy than a conveyor that runs only as fast as necessary. This happens even when there is no material to be fed. If a conveyor changes its speed rapidly, work pieces may be damaged. Such troubles can be avoided and product quality can be stabilized since inverters can change speed slowly by soft start/stop time adjustment. Additional benefits of adjustable frequency drives for conveyor applications include:

• Motors do not require space for speed change as do mechanical converters so that smaller size and lighter weight drives can be used.
• Since totally enclosed fan cooled motors can be used, they are suitable for conveyors under adverse conditions such as constant feeders with excessive dust, sive dust, paint lines with adhesive compositions, or bottling lines.
• Brush or commutators are not part of the induction motor design so maintenance is eliminated.
• Inverters can be used for existing motors under some conditions. By applying an inverter to existing circuits, conveyors which operated at a constant speed can be run at stepless variable speeds.
• A wide range of speed control, up to 40:1, can be controlled remotely by changing frequency reference for optimum speed.
• Since phase-rotation switching of inverter transistors performs the forward/ reverse operation, conventional main circuit contactors, are not required.

Variable torque

Often, variable torque loads require the driving torque to vary proportionally with the speed of the load. HVAC systems are an example of a variable torque application.

Centrifugal fans and pumps are sized to meet the maximum flow rate required by the system. In most applications however, maximum demand volume is required for only a small percentage of the total operating time. Most of the operating time is spent providing 40% to 70% capacity. For centrifugal devices, the torque varies by the square of speed and the horsepower varies by the cube of speed. Reduced fan speed achieves reduced air volume and reduces motor power consumption.

Most pumps are centrifugal and their operation is defined by two independent curves. One is the pump curve—a function of the pump geometry and motor characteristics. The other ¦s the system curve which depends on the geometry of the piping and valves connected to the pump. The intersection of these curves determines the natural operating point. If the system is part of a process that requires adjustable flow rates, then some method is needed to alter either the system characteristics or the pump parameters. These methods include valves or throttling to change the system curve or an adjustable frequency drive on the pump to modify the pump curve.

Constant Horsepower

Constant horsepower loads require high torques at low speeds and low torques at high speeds. Machine tool applications are perfect examples of such loads.

In gear-type speed changers, spindle speed can be selected only in steps so that delicate peripheral speed constant control is not available. Although dc spindles can make stepless speed changes, they are expensive, inefficient, and require regular brush maintenance. Using adjustable frequency drives for spindle drives eliminate these problems. Since a standard motor can achieve stepless spindle drive speed changes, the clutch mechanism can be eliminated. An adjustable frequency drive used in place of a gear-type or dc spindle provides the following benefits:

• higher accuracy in cutting of soft workpieces
• elimination of brush maintenance
• improved efficiency-field winding is not needed.
• increased machine output

Automated grinding is another constant horsepower application. Grinding speed requirements range from 20,000 to 180,000 rpm. At the beginning of a typical grinding cycle the wheel moves toward the work-piece at a fast feed rate. Then, at a safe distance, the feed rate slows to the normal grinding rate as it approaches the surface. However, due to variations in part dimensions, the distance traveled to reach the work piece at the slow rate can be significant.

To overcome this hurdle, some adjustable frequency drives offer a load sensing circuit to produce a signal that is a function of the current drawn by the grinding wheel drive motor. With this feature, the grinding wheel advances at a fast rate toward the work piece until it makes contact. Within 20 to 40 milliseconds after the wheel contacts the work piece, the adjustable frequency drive produces a signal that adjusts the feed rate to the normal lower value.

"Conclusion"

The use of adjustable frequency drives makes an important contribution to the efficient operation of industrial and commercial equipment.

The advances in the area of adjustable frequency drives have led to high-tech cost effective solutions that offer substantial advantages over both mechanical systems and dc drives.

The most effective adjustable frequency drives use state-of-the-art technology. Pulse width-modulation combined with microprocessor control provides optimum induction motor control and superior flexibility. The combination of insulated gate bipolar transistors and surface mounted device technology have allowed a more compact and less complex design with reduced costs compared to other variable speed drive technologies.

https://www.yaskawa.com

An ac variable frequency drive must simultaneously control output frequency and voltage to efficiently control the speed of a three phase induction motor.

Frequency controls the motor's speed Common 60Hz induction motors are typically offered at no load speeds of 3600rpm (2 pole), 1800rpm (4pole), and 1200rpm (6 pole). Applying 60Hz to a 4 pole motor will produce a motor speed of 1800rpm at no load. Actual speed at any applied frequency is influenced by motor load requirements. If frequency is cut in half (30Hz), then motor speed is cut in half.

Voltage is applied in proportion to frequency to achieve rated motor torque. If the motor is running half speed (30Hz), the voltage applied is also cut in half. Failure to reduce applied voltage with reduced speed will result in excessive current draw and motor overheating.

Pulse Width Modulation (PWM) is the present state of the art method used to control frequency and voltage. An AC power source is connected to the drive rectifier, converted to DC, and then "inverted" in a logic controlled output of DC pulses of varying width (voltage) and polarity (frequency). A motor is an inductive device constructed of coils of wire embedded in iron. The motor's inductance resists the rapid voltage changes, averaging (smoothing) the pulses and making them appear to the motor as a 3 phase sine wave.

There are three major elements in the PWM process:
Rectifier - converts AC power source to DC.
DC Bus - pulsating DC is smoothed by large capacitors. A measurement at the output of this section indicates a DC voltage equal to the AC peak value of approx. 1.4 times the AC input.
Inverter - receives instructions from control logic; converts DC to variable frequency variable voltage 3 phase PWM output.

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