Variable Frequency Drives (VfD)

Variable Frequency Drives (VfD)

Introduction

 Variable Frequency Drive (VfD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Other names for a VfD are variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive, micro-drive, and inverter. Variable-frequency drive  also termed adjustable-frequency drive, “variable-voltage/variable-frequency (VVVF) drive”, variable speed drive ) is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage. VfDs are used in applications ranging from small appliances to large compressors. About 25% of the world's electrical energy is consumed by electric motors in industrial applications, which can be more efficient when using VfDs in centrifugal load service; however, VfDs' global market penetration for all applications is relatively small. Over the last four decades, power electronics technology has reduced VfD cost and size and has improved performance through advances in semiconductor switching devices, drive topologies, simulation and control techniques, and control hardware and software. VfDs are made in a number of different low- and medium-voltage AC-AC and DC-AC topologies.

Variable-frequency types

There are three common types of VfDs. Current source inversion (CSI) has been successfully used in signal processing and industrial power applications. CSI VfDs are the only type that has regenerative power capability. In other words, they can absorb power flow back from the motor into the power supply. CSI VfDs give a very clean current waveform but require large, expensive inductors in their construction and cause cogging (pulsating movement during rotation) below 6 Hz.

Voltage source inversion (VSI) drives have poor power factor, can cause motor cogging below 6 Hz, and are non-regenerative. Consequently, CSI and VSI drives have not been widely used.

Pulse-width modulation (PWM) VFDs are most commonly used in industry because of excellent input power factor due to fixed DC bus voltage, no motor cogging, higher efficiencies, and lower cost. A PWM VFD uses a series of voltage pulses of different lengths to simulate a sinusoidal wave  . Ideally, the pulses are timed so that the time average integral of the drive yields a perfect sinusoid. The current method of choice to produce this waveform runs a triangle wave and sine wave through a comparator, and outputs a voltage pulse whenever the sine wave's value is greater than the triangle wave. The current electric component of choice to generate the voltage pulse is the insulated gate bipolar transistor (IGBT), although silicon-controlled rectifiers (SCRs) can work as well. In the near future, injection-enhanced gate transistors (IEGTs) will be used to perform this task. Much more long term, memristors will probably become the component of choice for this task. Memristors are the fourth passive circuit element, linking electric charge and magnetic flux  Regardless of the component used to form the sine wave, the switching action causes problems.

System Description  

Frequency (or hertz) is directly related to the motor’s speed (RPMs). In other words, the faster the frequency, the faster the RPMs go. If an application does not require an electric motor to run at full speed, the VfD can be used to ramp down the frequency and voltage to meet the requirements of the electric motor’s load. As the application’s motor speed requirements change, the VfD can simply turn up or down the motor speed to meet the speed requirement.

A variable-frequency drive is a device used in a drive system consisting of the following three main sub-systems: AC motor, main drive, controller assembly, and drive/operator interface

The first stage of a Variable Frequency AC Drive, or VfD, is the Converter. The converter is comprised of six diodes, which are similar to check valves used in plumbing systems. They allow current to flow in only one direction; the direction shown by the arrow in the diode symbol. For example, whenever A-phase voltage (voltage is similar to pressure in plumbing systems) is more positive than B or C phase voltages, then that diode will open and allow current to flow. When B-phase becomes more positive than A-phase, then the B-phase diode will open and the A-phase diode will close. The same is true for the 3 diodes on the negative side of the bus. Thus, we get six current “pulses” as each diode opens and closes. This is called a “six-pulse VFD”, which is the standard configuration for current Variable Frequency Drives.

AC motor

The AC electric motor used in a VFD system is usually three-phase induction motor. Some types of single-phase motors or synchronous motors advantageous in some situations can be used, but three-phase induction motors are generally preferred as the most economical motor choice. Motors that are designed for fixed-speed operation are often used. Elevated-voltage stresses imposed on induction motors that are supplied by VfDs require that such motors be designed for definite-purpose inverter-fed duty in accordance with such requirements as Part 31 of NEMA Standard MG-1.

Controller

The VFD controller is a solid-state power electronics conversion system consisting of three distinct sub-systems: a rectifier bridge converter, a direct current (DC) link, and an inverter. Voltage-source inverter (VSI) drives Most drives are AC-AC drives in that they convert AC line input to AC inverter output. However, in some applications such as common DC bus or solar applications, drives are configured as DC-AC drives. The most basic rectifier converter for the VSI drive is configured as a three-phase, six-pulse, full-wave diode bridge. In a VSI drive, the DC link consists of a capacitor which smooth’s out the converter's DC output ripple and provides a stiff input to the inverter. This filtered DC voltage is converted to quasi-sinusoidal AC voltage output using the inverter's active switching elements. VSI drives provide higher power factor and lower harmonic distortion than phase-controlled current-source inverter (CSI) and load-commutated inverter (LCI) drives   The drive controller can also be configured as a phase converter having single-phase converter input and three-phase inverter outputController advances have exploited dramatic increases in the voltage and current ratings and switching frequency of solid-state power devices over the past six decades. Introduced in 1983, the insulated-gate bipolar transistor (IGBT) has in the past two decades come to dominate VFDs as an inverter switching device.

In variable-torque applications suited for Volts-per-Hertz (V/Hz) drive control, AC motor characteristics require that the voltage magnitude of the inverter's output to the motor be adjusted to match the required load torque in a linear V/Hz relationship. For example, for 460 V, 60 \ Hz motors, this linear V/Hz relationship is 460/60 = 7.67 V/Hz. While suitable in wide-ranging applications, V/Hz control is sub-optimal in high-performance applications involving low speed or demanding, dynamic speed regulation, positioning, and reversing load requirements. Some V/Hz control drives can also operate in quadratic V/Hz mode or can even be programmed to suit special multi-point V/Hz paths.

The two other drive control platforms, vector control and direct torque control (DTC), adjust the motor voltage magnitude, angle from reference, and frequency^[14] so as to precisely control the motor's magnetic flux and mechanical torque.

Although space vector pulse-width modulation (SVPWM) is becoming increasingly popular, sinusoidal PWM (SPWM) is the most straightforward method used to vary drives' motor voltage (or current) and frequency. With SPWM control   quasi-sinusoidal, variable-pulse-width output is constructed from intersections of a saw-toothed carrier signal with a modulating sinusoidal signal which is variable in operating frequency as well as in voltage (or current).

Operation of the motors above rated nameplate speed (base speed) is possible, but is limited to conditions that do not require more power than the nameplate rating of the motor. This is sometimes called "field weakening" and, for AC motors, means operating at less than rated V/Hz and above rated nameplate speed. Permanent magnet synchronous motors have quite limited field-weakening speed range due to the constant magnet flux linkage. Wound-rotor synchronous motors and induction motors have much wider speed range. For example, a 100 HP, 460 V, 60 Hz, 1775 RPM (4-pole) induction motor supplied with 460 V, 75 Hz (6.134 V/Hz), would be limited to 60/75 = 80% torque at 125% speed (2218.75 RPM) = 100% power. At higher speeds, the induction motor torque has to be limited further due to the lowering of the breakaway torque of the motor. Thus, rated power can be typically produced only up to 130-150% of the rated nameplate speed. Wound-rotor synchronous motors can be run at even higher speeds. In rolling mill drives, often 200-300% of the base speed is used. The mechanical strength of the rotor limits the maximum speed of the motor.

An embedded microprocessor governs the overall operation of the VFD controller. Basic programming of the microprocessor is provided as user-inaccessible firmware. User programming of display, variable, and function block parameters is provided to control, protect, and monitor the VFD, motor, and driven equipment.

The basic drive controller can be configured to selectively include such optional power components and accessories as follows:

·         Connected upstream of converter -- circuit breaker or fuses, isolation contactor, EMC filter, line reactor, passive filter

·         Connected to DC link -- braking chopper, braking resistor

·         Connected downstream of inverter—output reactor, sine wave filter, dV/dt filter.

Operator interface

The operator interface provides a means for an operator to start and stop the motor and adjust the operating speed. Additional operator control functions might include reversing, and switching between manual speed adjustment and automatic control from an external process control signal. The operator interface often includes an alphanumeric display or indication lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit is often provided on the front of the VfD controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the VFD controller. Most are also provided with input and output (I/O) terminals for connecting push buttons, switches, and other operator interface devices or control signals. A serial communications port is also often available to allow the VFD to be configured, adjusted, monitored, and controlled using a computer

 

Sizing

VFD sizing should always be based on motor current and voltage, not hp.  Motors can be rated for multiple input voltages, but it is extremely important to know the input voltage for the application because VfDs are not rated for multiple voltages and only operate on the voltage for which they are designed, such as 100 to 120 V, 200 to 240 V, 380 to 48 0V or 525 to 600/690 V. A VfD should never be sized solely on the hp rating of the motor. It is critical that other points be taken into account to ensure proper sizing of the VFD. Let us start with FLAs.

Full load amps FLAs are the amps on which the motor will draw when operating at full load and full speed. The actual measured FLA may be lower than the nameplate based on the actual loading of the motor (this may be because of a reduced load or a slower speed). It is critical when sizing the VfD to use the motor FLA plus any additional overload that the application may require. This additional overload rating may actually push the VfD selection to the next size, so it is important to use care in sizing the VFD for this additional overload.

Service factor amps. In some pumping applications, the pump manufacturer may size the motor to operate into its service factor to get the pump to maintain flow or head conditions as required. When operating in this condition, it is important to verify the continuous amps required to operate the motor in its new normal conditions. These overload motor amps are used to size the VfD. Sometimes the motor nameplate details the service factor amps of the motor, otherwise it is critical to multiply the running amps by the service factor to ensure proper VFD sizing. The overload rating of the VfD will typically have the VfD running into its intermittent output current rating (which the VfD can run into for one minute out of every 10 minutes, depending on manufacturer), so the VFD should be sized so it is possible to see this overload operation as its continuous amp rating.

For high overload usage of the VFD, it is important to understand how the motor will be used so it can be sized to accommodate the motor overload protection required for more difficult applications such as conveyers, cranes, positive displacement pumps and blowers. These constant torque applications may require 150 to 160 percent overload of motor current when in an overload condition. This overload condition may be required to provide the necessary torque to overcome the starting inertia or during heavy operating conditions in some loads like rock crushers, positive displacement pumps, belt presses, cranes or other applications.

Again, it is critical when sizing the VFD to know as much about the operating conditions as possible. Different manufacturers use different terms when describing the overload operation like high overload, constant torque or automation/industrial VFD to alert the user to the VFD’s overload sizing. For applications such as centrifugal pumps, fans, blowers or other variable torque loads, these applications may only require 110 to 120 percent overload current for one minute. This overload may be required to provide the necessary torque to overcome the starting inertia or operation during heavy conditions in some loads like pumping solids or other viscous materials. Most pumping and fan VFDs today are sized with variable torque as their primary sizing.

 Other Factors

It is also important to know the type of motor in the installation. For example, be careful when operating a submersible motor. Submersible motors are not National Electrical Manufacturers Association (NEMA) B-type motors and typically draw more current per hp than conventional NEMA B-type motors. Low-rpm motors sometimes require more current to operate. Equipment alignment will always be better if the correct nameplate motor current is determined and the VfD is sized based on those facts.

Single-phase input is another useful application of a VfD for phase conversion. When only single-phase power is available, some VfDs can use single-phase input power to provide three-phase output power and operate a three-phase motor. Some manufacturers provide specific VfD units designed for this while others simply oversize the VFD and derate for this feature. De-rating the VfD may not be fully UL listed. Users should be cautious in their search for VFDs and purchase fully tested UL VfDs certified on single-phase inputs with three-phase outputs.

High-altitude installations also require special attention. As the air thins with higher altitude, the ability of the air to transfer heat out of the VFD is reduced. The higher the altitude, the less heat the VfD can exhaust, requiring a de-rating or reduction in its ability to produce output current. This is different by manufacturer and size, so for the best information, visit the VFD manufacturer’s website or call the tech support hotline.

Higher ambient temperature can also affect the VfD size since heat dissipation is the big design issue with VfDs. The higher the ambient air temperature, the hotter it is when running the VfD. The issue is that the VfD is unable to cool itself as it generates heat while producing current to run the motor. With a high ambient temperature, the VfD is unable to dissipate the generated heat it needs to dissipate and keep itself cool. The sizing may require the need to de-rate for higher ambient temperature or cool the VFD with cooler air. An air conditioner is not always the solution. Sometimes an exhaust fan may change enough air to allow the VfD to operate satisfactorily. The de-rated value varies by manufacturer and by size, so no good rule applies other than to visit the manufacturer’s website or call the tech support hotline.

Low ambient temperature can also be a potential pitfall. VfDs need a certain amount of heat to keep all components operating at their optimal designs. The low thermal rating needs to be taken into account when mounting VfDs in unconditioned or outside installations. Capacitors, insulated-gate bipolar transistors and LCD displays are some of the components that do not operate well in below-freezing temperatures or when frozen. If a VfD is unpowered, has been in a below-freezing temperature and is frozen, it is crucial that the drive be warmed up before powering because this extreme condition could cause the capacitors to explode.

Heat, power losses, and harmonics. The first problem a VFD manufacturer needs to address is heat. Although VFDs are highly efficient devices, manufacturers are unable to produce an ideal set of components. The heat lost in the drive is governed by the following equation:

H_loss= P_t (1-η)

Where H_loss is the power lost (W), P_t is the power through the drive (W), and η is the efficiency of the drive. Usually, VfDs have an efficiency rating between 95% and 98%. This means the amount of air that must be moved through the drive is governed by the equation:

m = H_loss÷(C_pΔT) = P_t(1-η)÷(C_pΔT)

Where m is the mass flow rate (kg/s), C_p is the specific heat of air [kJ÷(kg×K)], and ΔT is the difference in temperature between the incoming air and the outgoing air (K). This heat can cause significant cooling costs to be added into the design, especially if the drive is unable to be placed in an unclassified location (area free of flammable gases or particles). If the drive must be placed in a classified location, then the airflow going to the drive will need to be purged and pressurized.

Heating is only one of the problems with VFDs. The other major problem lies with system harmonics. A picture of the PWM and the harmonics they cause   The irregularities in the sine wave are called harmonics. In an ideal power circuit world, these harmonics should not exist. They do nothing but cause problems. Fortunately, there are a number of ways to mitigate harmonics.

One of the simplest methods of dealing with harmonics is to place a sine wave filter on either side of the VfD. On the line side, these are typically called line reactors and have reactance values anywhere between 1.5% and 5.0% impedance. Higher impedance not only stops more harmonics, but it also limits the power going to the VfD.

Another tactic that can be used on the line side of the VFD is to place capacitors at a common bus. Because the impedance of a capacitor is inversely proportional to the frequency of a signal, the harmonics see a short through the capacitor and travel through the capacitor to ground, hopefully ignoring the other loads on the bus. VfDs may also use an active front end to limit the harmonics that the line side sees. An active front end has another IGBT switching at an inverse voltage as the main IGBT, but it is placed through a high pass filter so that the fundamental power signal goes to ground. The summation of the two harmonic signals ideally should be zero. If an active front-end drive is not suitable for some reason, a passive front-end VfD might be procured. Passive front-end VFDs use multiple phase-shifting transformers and diode bridges to mitigate harmonics.

The more pulses a passive front-end VFfD has, the fewer problems with harmonics exist. The trade-off is that the line voltages must be well balanced, and with each additional phase shifting transformer there is increased cost and a loss in efficiency. In extreme cases, an isolation transformer might be procured. Although this is one of the most effective ways to prevent harmonics from spreading, it's also one of the most costly.

If harmonics are not sufficiently mitigated on the line side of the VfD, crosstalk and overheating could become issues. Overheating could either cause bus sizes to be de-rated or increase cooling costs. Crosstalk is defined as the signal from one circuit interfering with another circuit. Generally speaking, it is a larger issue than overheating. An example of this is a radio just slightly out of tune. Although it is possible to hear the music through the static, the static is annoying. Crosstalk is an annoying thing in telecommunication circuits. In power circuitry, crosstalk will cause overheating and frequency relay trips.

Just as harmonics left unchecked on the line side can cause problems, they can create issues on the load side as well. This is because of the nature of waves. For example, a small force exerted on a Slinky at either end will cause a high amplitude sine wave. Electromagnetic waves act in the same fashion, meaning a small amount of reactance can cause large voltage spikes. Because this reactance is inductive in nature, most output filters are capacitors connected in a delta configuration. Ideally, this should make the reactance portion of the impedance go to zero. If the impedance is matched properly, then this does not occur.

A note of caution: Capacitors connected on the load side of the VFD can create a large number of problems, up to and including destroying a drive. Therefore, it's wise to check with the drive manufacturer before installing a sine wave filter on the load side of the VfD. On rare occasions, an active filter may be used. Although these tend to work well, they are rather expensive and usually have to be custom designed.

Benefits

Energy savings. Many fixed-speed motor load applications that are supplied direct from AC line power can save energy when they are operated at variable speed by means of VfD. Such energy cost savings are especially pronounced in variable-torque centrifugal fan and pump applications, where the load's torque and power vary with the square and cube, respectively, of the speed. This change gives a large power reduction compared to fixed-speed operation for a relatively small reduction in speed. For example, at 63% speed a motor load consumes only 25% of its full-speed power. This reduction is in accordance with affinity laws that define the relationship between various centrifugal load variables. AN actual audit of a industry utilizing VfDs  is presented as follows:

Savings Textile Mill
	11-Sep	Jan-14	diff %
kWh	1489019	1515574	1.78
bags	6532	5932	-9.19
kWh/bag	227.9576	255.4912	12.08
Rs./month	22335285	22733610	26.75
savings /month		179849.74
savings/year		2.16	Rs. M
Inv		12.88	Rs. M
payback		5.97	Years

 The above suggests that the investment in VfDs has a payback period of less than 6 years , there was an element of subsidy involved and the actual pay back period nay be  less  than 8 years or so,

If you have an application that does not need to be run at full speed, then you can cut down energy costs by controlling the motor with a variable frequency drive, which is one of the benefits of Variable Frequency Drives. VfDs allow you to match the speed of the motor-driven equipment to the load requirement. There is no other method of AC electric motor control that allows you to accomplish this.

Electric motor systems are responsible for more than 65% of the power consumption in industry today. Optimizing motor control systems by installing or upgrading to VfDs can reduce energy consumption in your facility by as much as 70%. Additionally, the utilization of VFDs improves product quality, and reduces production costs.  , returns on investment for VFD installations can be as little as 6 months. By operating your motors at the most efficient speed for your application, fewer mistakes will occur, and thus, production levels will increase, which earns your company higher revenues.  

Equipment Life- Your equipment will last longer and will have less downtime due to maintenance when it’s controlled by VfDs ensuring optimal motor application speed. Because of the VfDs optimal control of the motor’s frequency and voltage, the VfD will offer better protection for your motor from issues such as electro thermal overloads, phase protection, under voltage, overvoltage, etc.. When you start a load with a VfD you will not subject the motor or driven load to the “instant shock” of across the line starting, but can start smoothly, thereby eliminating belt, gear and bearing wear. It also is an excellent way to reduce and/or eliminate water hammer since we can have smooth acceleration and deceleration. Electrically, VfDs run at a high power factor. Any class of induction motors usually has a low power factor at half and three-quarters load (0.75 to 0.85). This actually decreases the life of the motor, because the unnecessary increase in current overheating the winding insulation. VfDs bypass this problem by running the load at a frequency below the fundamental

Speed Control- The most obvious reason to procure a VfD is speed control. This is usually done for process, operation, and economic benefits. One economic benefit comes from the reduction of maintenance when using a VfD, especially not having to deal with the DC motor carbon brushes or mechanical speed-control gearboxes (transmissions). The most obvious economic benefits of VfDs occur with fans and pumps. The power that a pump or fan consumes is directly proportional to the cube of the velocity. This means if an operator can run a fan at 80% of full speed, it theoretically uses 51% of full load power.

Starting current - VfDs also optimize motor starting characteristics. VfDs bring motors up to full speed quickly and by drawing only 100% to 150% of full load amps (FLAs). This ability to start at normal FLA is very important if the power supply cannot withstand the normally six times FLA across-the-line starting draw, or even the 350% FLA soft-start device current. VFDs do this by managing the magnetic flux of an induction motor. Magnetic flux is directly proportional to the voltage and inversely proportional to the frequency. By keeping the flux constant, the inrush current does not exceed the FLA rating of the motor, and full torque is maintained. This is a significant improvement on a soft-start, which has significant voltage drop problems and cannot start under full load.

Another potentially useful aspect of VFDs  is the maintenance of a   constant torque, and constant horsepower. The constant torque region is fairly self explanatory; the VfD is regulating the flux so that the current is constant. Once the VfD surpasses the rated system frequency, the voltage cannot increase due to the physical constraints of the system. Because the voltage is static — and the frequency is increasing — the flux is forced to decrease. When this occurs, the current and torque are forced to decrease as well. This is called field weakening. Although not necessarily a good thing, it can be useful if there is a need to power a partial torque load above the rated speed. In addition to this capability, VfDs can also take any form of input power whether it's single-phase AC, 3-phase AC, or DC. VfDs fed from a DC source still power an AC load without an internal rectifier.

Power Grid- VfDs also have some applications on the power grid. One classic example of this is a doubly fed induction generator, in which the VfD can force a fixed frequency and voltage signal out of a variable-speed (frequency) input. This is commonly seen in wind turbines and other small hydroelectric generation projects that will be connected to the power grid. Other renewable energy sources, such as photovoltaic cells, can use VfDs to act as an inverter before connecting to the power grid, although inverters with buck-boost technology are more common.  

Conclusion

 Whenever a load has either a variable torque or a variable speed, a VfD should be considered. A VfD might be considered if a large motor has a problem with voltage drop, torque, or inrush current during start-up. Even though VfDs undoubtedly solve their fair amount of problems and provide substantial energy savings, the heat they generate must be dissipated — and the harmonics they produce must be mitigated.