Inverter air conditioners

Inverter air conditioners

Introduction

To understand DC Inverter ACs, we first need to understand how they work. Air conditioners use a refrigerant (a gas which exists in low pressure but turns liquid as the pressure increases), which is compressed and liquefied by the compressor. It is then cooled down in the condenser and later allowed to expand (become a gas) in the evaporator, albeit in a controlled manner. Those who have studied physics, would know that this expansion causes a cooling effect which, in turn, causing the condenser to become warm.

The same principle is used in refrigerators; they move the heat from the inside to the outside. When an AC is in cooling mode, the heat from the room is removed and pushed towards the outdoor unit of the split AC. If the AC has the ability to heat, it can reverse the process, the energy is absorbed from the outside while heating the room. In most cases, the reverse process, i.e. heating mode, consumes less power than the cooling mode because the compression energy is added to the overall heating cycle.

The remaining two important components of an AC are its motored fans. The outdoor fan chucks the air out of the condenser to the outside. However, the indoor unit makes use of the blower and louvers/fins to dissipate the cool air (or hot air if the AC is in heating mode) to the room.

Until a few years ago every single air conditioner unit was those of stable speed when we switched on our unit with a remote or wired remote our unit began to run and it worked at its full power, at its 100% of capacity, when it reached to the adjustable temperature or when we stopped it manually, the unit “stopped” completely and it didn’t keep throwing cold or heat (heat pump),the air conditioner stable speed units only have 2 speeds (0% and 100%) and when you start them they pass from being completely stopped to be at their full capacity because they give their maximum capacity right after switching on them. That’s the reason we could feel in the intensity of current of our house or office creating in many occasions a lowering in the light intensity of our room, dining hall, etc. It was noticed like the bulb didn’t give off much light just when you switched on the air conditioner (voltage descent) due to the big energy consumption the unit needs to pass from 0% to 100%, later the situation was settled and there weren’t more intensity descents (until the next starting), that’s why after starting the unit directly to 100% of its nominal capacity and maintain itself through all time the unit was running, we noticed that feeling of an intense cold so many people miss, the inverter air conditioner units don’t work this way, because these begin running slowly and then they accelerate to its full capacity reaching a 140% of its nominal capacity, but after reaching the desired temperature the unit scales down the power used and operates at much lower power  levels .

Non-Inverter Air Conditioners

Non inverter or fixed speed air conditioners operate on a fixed amount of power at a fixed speed. This means the compressor has to stop and start again to maintain the desired room temperature..In this type of arrangement the compressor is either off or on. When it is on, it works at full capacity consuming full electricity. When the thermostat reaches the required temperature setting, the compressor stops and the fan continues to function. When the thermostat senses that the temperature in the room has fluctuated, the compressor starts again.

Inverter Air Conditioners

This technology was developed in Japan and is now being used globally for air conditioners and refrigerators alike. In inverter air conditioning systems, the speed of the compressor varies to ensure energy efficient operations and precise cooling or heating as required.

A DC inverter converts AC current to DC current. It then uses a modulation technique (called Pulse Width Modulation) in another inverter to produce AC current based on the desired frequency and voltage. The variable frequency AC drives the brushless motor or induction motor. The speed of the motor is proportional to frequency of the new AC voltage. Thanks to this, the compressor can now run at multiple speeds.

The inverter technology acts like an accelerator in a car. When the compressor requires more power, it gives it more power. When it needs less power, it provides less power. In this kind of setting, the compressor is always turned on, but draws less power or more depending on the temperature of the incoming air and the setting adjusted in the thermostat. The speed and power of the compressor is also adjusted accordingly.

Major difference between the two

An air conditioner with DC inverter technology has the ability to control its cooling (or heat) transfer rate by changing the output from its compressor. The rate can be modified depending on the requirements of the room. To put it more simply, the AC uses electricity depending on the environment’s requirements.

The compressor in a common AC runs at full 100 percent all the time. When it is not needed it shuts off. So a common AC uses start/stop cycles to control the temperature.

When using a DC Inverter in an air conditioner, it uses a processor to check the surrounding temperature and adjusts the speed (power input) of the compressor based on it. It can provide different levels of cooling or heating. So the DC Inverter AC regulates its output capacity to uniformly control the temperature of the room.

Both the inverter and non-inverter systems offer similar functions but differ in terms of compressor motor type running in the system. The compressor is responsible for compressing the refrigerant into liquid after which it shuts off and allows it to expand. As this process takes place, the refrigerant begins to cool thus producing the desired effect of cooling. Inverter air conditioner units are designed in such a way that they save up to 30-50% of electricity units consumed as compared to a regular air conditioner.

The magnitude of cooling or heating required by an air conditioning unit varies depending upon the outdoor temperature and the amount of heat in the room. When the cooling or heating effect needs to be amplified, the compressor will operate at a higher speed and will upsurge the amount of refrigerant flow.

On the contrary, during moderate outside temperatures for example, when the cooling and heating capacity needs to be reduced, the compressor will run at a low speed and will decrease the amount of refrigerant flow. When the inverter air conditioner is switched on, the compressor operates at a high speed in order to cool or heat the area rapidly. As room temperature approaches the desired temperature, the compressor slows down, maintaining a constant temperature. Any spontaneous fluctuation in the room temperature will be detected and promptly adjusted to bring the room temperature back to the set temperature.

There are many benefits of the inverter ac First of all, it requires less electricity in comparison to traditional air conditioning unit. And the other benefit is that it comes with a compressed circuit board that is able to handle various cooling loads. You will not get this feature in the non-inverter air conditioners. Moreover, the traditional type of air conditioner or non-inverter type of air conditioner offers less cooling than the inverter ac. Inverter ac is not prone to voltage fluctuations like the traditional one. It is durable and requires less maintenance than the non-inverter type of air conditioners.

                         
The inverter type ac is energy efficient. This is one of the key features of this air conditioner. It consumes around thirty percent less electricity in comparison to the traditional and non-inverter type air conditioners. In this developed system, you do not need to keep the system on and off to get the required temperature. It comes with an automatic system that makes necessary adjustments to maintain temperature

Operating principles

The Inverter technology   is the latest evolution of technology concerning the electro motors of the compressors. An Inverter is used to control the speed of the compressor motor, so as to continuously regulate the temperature. The DC Inverter units have a variable-frequency drive that comprises an adjustable electrical inverter to control the speed of the electromotor, which means the compressor and the cooling / heating output. The drive converts the incoming AC current to DC and then through a modulation in an electrical inverter produces current of desired frequency. A microcontroller can sample each ambient air temperature and adjust accordingly the speed of the compressor.. The biggest difference between inverter and non-inverter AC is the fact that the motor of the inverter compressor has a variable speed. The speed of the non-inverter compressor is fixed. This means that it operates either at full or minimum speed. A censor in the inverter adjusts the power according to the temperature in the room, lowering the electrical consumption and saving energy.

Efficiency and Operating cost

The inverter air conditioning units have increased efficiency in comparison to traditional air conditioners. A conventional AC starts multiple times and always runs at peak capacity. This means that every time it starts it needs extra electricity to jump start its stationary motors and compressor, i.e. torque current. Without being too technical, you can think of it this way, it needs more power when it starts and it starts too many times. It also runs at full capacity each time requiring a lot of current.

In comparison, a DC Inverter AC never turns off its compressor or motors. It reduces the electricity it needs and constantly keeps cooling (or heating) the room. When it does start the first time, a DC Inverter AC starts off slowly so it doesn’t require a massive torque current. Inverter ACs can start at low voltages as well thanks to the inversion mechanism. By running in a low power consumption state, DC Inverter ACs save a lot of electricity.

DC Inverter ACs isn’t as effective when running at full capacity, however, when they are running at partial capacity, which is how ACs are normally used, they can save a lot of energy.

The inverter AC has lower operating cost and with less breakdowns .The inverter AC units is more expensive in upfront cost than the constant speed air conditioners, but this is balanced by lower energy bills. The payback time is approximately two years depending on the usage.

 Due to the sophisticated operational method of the inverter, its compressor does not work at its full capacity all the time. When the speed is lower, the needed energy is lower too and you pay less money for electricity.

 The inverter AC is able to cool or heat your room faster than the non-inverter. This is due to the fact that in the beginning of the process, the inverter uses more power than the non-inverter and diminishes the power when it gets close to the desired temperature. The inverter air conditioner units may vary its capacity according to their needs; this makes the consumption to get adapted and lowers the electrical consumption significantly.

The stable speed units were very old in comparison to that, for they don’t have such electronics and they couldn’t modify its way of running and they worked in the same conditions though the weather was a 24ºC normal day or an extreme hot day with 38ºC, some stable speed units had pressure relief valves and condensing controls or different fan speed in the condenser, but this is nothing in comparison with all the things an inverter unit can do, regulating the gas flow with the electronic expansion valve, reducing or increasing the compressor speed, such as the condenser fan motor does. 

Cooling capacity of 1 ton is equal to 3.517 kW of power. For 1 ton AC inverter, power consumption of ac = cooling capacity/EER = 1.5*3.517/2.7=1.954 kW. AC consists of two units, Indoor unit which is called the evaporator and the Outdoor Unit which called the Compressor.

If you have a 1-DC Inverter AC, it will save you around 40-50 percent in electricity bills under normal usage. 3-DC and 4-DC Inverter ACs can achieve over 60 percent in savings. You probably won’t get 75 percent in savings under normal usage and extreme heat in Pakistan. However, if your AC has the heating-mode, it will cause more cost savings compared to the cooling mode.

Normal conditions mean that the user does not set temperatures to extremes. If the temperature is set to extremely low (for cooling) or extremely high (for heating) then you cannot expect such huge savings. However, even at extremes, DC Inverter ACs beat the conventional ACs.

Noise level and other benefits

The variable speed function makes the inverter AC unit’s quieter, extends life of their parts and the sharp fluctuations in the load are eliminated.  As the compressor motor of the inverter air conditioner does not turn on and off all the time, but keeps working at low power, the operation is more quite. The technology of the inverters not only makes cooling and heating more efficient, but it also makes the AC’s life longer As the compressor motor of the inverter air conditioner does not turn on and off all the time, but keeps working at low power, the operation is more quite.
The inverter air conditioner units get more comfort and faster than the stable speed ones.the inverter units reach to an assigned temperature faster than the stable speed units, apart from that, being capable of regulating their refrigerating capacity these maintain the room in a much more precise way than the stable speed ones because they go reducing their capacity slowly while reaching the desired temperature, maintaining that temperature “never stopping”. And that is put in inverted commas because I have heard that a lot. The inverter units never stop, my machine never stops”.

 Speed in cooling and warming.

The inverter units start slowly, it’s like a long-distance race, starting this way there’s neither a change in the intensity of current nor we feel a change in the intensity of bulbs, but they began to increase its capacity until they reach a 140% capacity in some cases.  In comparison to steady speed air conditioner units which either go or don’t go, the inverter air conditioner units have a wide range of operations, that’s the reason why catalogues used to show three different capacities: the nominal capacity, which is the “real” capacity or “basic” of that air conditioner and it’s the one we have to use to do the numbers more accurately (there are so many people who are wrong when taking as a reference the maximum capacity), the minimum capacity and the maximum capacity.

  The inverter air conditioner units  do stop  but only a few times In comparison to a stable capacity unit which switches itself on when the temperature goes a degree over our assigned temperature (cold mode) and it stops when it exceeds -1ºC, the inverter units work in a different way, these go reducing its capacity slowly when they reach their objective, and that’s the reason why seldom they exceed or reduce the temperature more than what it’s needed because it will keep itself a few degrees over or below our assigned temperature. It doesn’t mean it doesn’t stop, it does, but in general reducing its capacity and therefore its consumption is enough to keep the desired temperature.

Much more capacity

The inverter air conditioners are way much more efficient than those which aren’t inverter and they achieve an energy coefficient wider. Thanks to its electronics and the obvious progressions over these years, an actual inverter unit can reach a 5, 15 energy coefficient, that in comparison with a 2,76 of a non-inverter unit but same model, it is something to be aware of, it is almost the double of capacity and for that it means the half of consumption when doing the same work and air-conditioning the same room. Just for this reason choosing between an inverter and non-inverter one should be an easy choice to anybody.

Wider working range

Another inverter air conditioner characteristic which makes it better than a not inverter one is its working range, for example in heat mode. When the outside temperature reaches between 5 and 0ºC, a not inverter unit starts to fall its capacity heavily, even becoming ineffective and useless if the temperature descends even more, This is due to working in heat mode, the condenser will be cooling and it reaches to a point when it produces frost and it finally freezes itself, making impossible the heat or cold exchange with the outside. This is a solution with the called “defrost”, which almost every inverter unit has. With this defrost mode, what the unit does during a few minutes is to send the heat that should be going to the outside in order to warm our house, it sends it to the external unit to defrost it, loosing with this process the heat that should be impelling in the internal unit. After that, the unit will work normally until the next defrost, which will be more usual and shortened in time if there is more cold and humidity in the outside.

The inverter units don’t have such problems, or, if they have it is much less pronounced due to its electronics regulate the gas flow for this not to happen thanks to measurements, being able to work to -15ºC and loosing much less capacity than not inverter air conditioner units. There also exists special inverter units which works to -25ºC, giving a full capacity when they reach -15ºC

What Are the Different Types of DC Inverter ACs

Some manufacturers claim they have 3-DC Inverter ACs, All DC Inverter ACs, 4-D Inverter ACs, etc, while others simply claim to have Inverter ACs. They also claim to have varying savings percentages from 60 percent to 75 percent. The reason behind that is, some manufacturers employ older technologies and only upgrade their compressors to DC inverters. Others also convert the indoor and outdoor fan motors hence, completely upgrading all motors to DC inverters.

Those, which claim to save about 50 or 60 percent in electricity bills and simply state DC Inverter (or only Inverter) ACs, are using the inverter tech for the compressor only. 3-DC Inverter or 4-D Inverter systems, sometimes referred to as All DC Inverter, upgrade other motors or major electrical components as well. This means more savings as more electrical components get the ability to vary their speeds. In short, a number besides the “D” or “DC” mean the number of components using the inverter technology.

Conclusions

The inverter unit gives more comfort, capacity, saving, and other benefits and it has more working possibilities given the weather. On the contrary, the benefit of a not inverter unit will be that it costs less (upfront cost) and it also will cost less if it need to be repaired, because they are easier and cheaper to fix.  Not only do these ACs help save money, their use is good for the environment. The DC inverter technology is being used in other products as well

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.