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And there are many famous brand of frequency inverters (ac drives) in the world. Famous brands of this item include ABB, Siemens, Danfoss and some brands from Japan. These brands are used widely in the world. But do you know? Today, many famous brands of inverters have decided to manufacture their frequency inverters (ac drives) in China. Why? Why do they decide to manufacture frequency inverters in China? The reason is the same as that many famous companies manufacture their shoes, their comsume electronics in China. Because the cost of the labour in China is so cheap!!

So these famous brands of frequency inverters (ac drives) closed their factory in Europe, USA or other areas. Or they reduced the manufacturing of frequency inverters (ac drives) in Europe, USA or other areas just to save the cost. Then they can competite with the very cheap price.

Motors which are connected directly to the mains absorb reactive power by induction.

Anything more than a few motors can (and does) increase the cost of electricity provided by the local supply authority.

A way round this is to install power factor correction (PFC) equipment to the satisfaction of the supply authority which involves considerable capital investment.

Since frequency inverters absorb almost no reactive power, installations controlled by these units are efficient to the degree that the need for power factor correction (PFC) equipment is obviated.

Moreover, frequency inverter (AC Drives, frequency converters) controlled motors do not require starters because the frequency inverter eliminates power surges much better than any soft starter or star delta starter.

The removal of power surges (or momentary peaks in demand) can result in cabling of a smaller cross sectional area than would otherwise be the case.

These are all factors which can result in a lower electricity supply tariff.

But what are the other advantages? For air conditioning applications, lower running speeds means lower air movement levels and thus lower noise and draughts.

By running the system constantly to match demand, cut-in and cut-out frequencies are minimised so both the system and the building occupants benefit considerably.

By using frequency inverters (VFDs, ASDs), it is possible to switch and control the system manually or automatically via a PC.

When running a system via automated PC commands, it is possible to select from a variety of fixed speeds, or use control functions such as an integrated PID process controller to monitor and maintain a variety of operating criteria, ie: flow level; temperature; humidity or pressure.

Feedback signal scales can be adjusted to suit whatever measuring device is deployed and can be set to 0-10V, 4-20mA, 0-20mA, 0-5V.

A problem with any ducted system is that if the fan vibration frequency just happened to match the resonance frequency of the ductwork, then one would set up a vibration in harmonic sympathy with the other.

The result is discomfort for building occupiers and accelerated plant wear.

Frequency inverters allow this unwanted noise and vibration to be removed by setting 'skip' frequencies with adjustable 'bandwidths' which can be omitted from the inverter output.

A flying 'restart' circuit is built in which synchronises the inverter to the fan speed in the event that the fan is already cycling when the inverter is switched on and obviates the energy wastage normally experienced when having to halt the rotation and restart from rest.

Moreover, the restart circuit prevents the fan from running backwards in the event that it is freewheeling in the wrong direction when started.

Using an AC drive (frequency converter, vfd) for conveyor control permits an adaptation of the speed to changing needs. A partly loaded conveyor, with a higher speed than necessary, wastes energy and causes unnecessary wear. In controlling conveyors, AC drives (frequency inverters, adjustable speed drives) also improve process control by enabling collection of measurement and supervision information. The soft start of the conveyor using AC drives (VVVF Drives) reduces the stress on the gearboxes when the conveyor is started.

Reliability and serviceability are key issues for a mine operating 24 h/7 d week. The controlled and balanced torque means less stress, wear, and therefore, less maintenance on the motor, gearbox and belts. The inverter units are partly configured in master/follower systems when controlling belts with several driving drums to ensure proper load sharing. "S" curve speed control during acceleration and deceleration also minimize belt dynamic interaction.
 

Variable-speed motor/drive combinations use a synchronous ac motor with an encoder built in. The encoder signals the motor's speed and phase angle to the drive, which must then match its output frequency to the motor speed and its output phase to produce the required torque.

To get the drive's power efficiently out to the load as mechanical power requires having enough voltage to drive enough current through the motor. The output power is, after all, the product of voltage times current. The ratio of the voltage to the current, on the other hand, defines an impedance for the drive/motor combination.

Copper resistance in the motor coils and power cables, as well as contact resistance in all the connections rob the system of power, however. The higher the circuit impedance compared to these parasitic impedances, the more efficient the overall system will be.

The graph below compares motor/drive impedance to a typical parasitic impedance level of one Ohm. Assuming we want the ratio to be one or two orders of magnitude, the chart shows how the supply voltage needed correlates with output power. Small, fractional horsepower motors (red line) work well from a few tens to a few hundreds of Volts. Larger motors require hundreds of Volts to run efficiently. As horsepower requirements climb, so do the drive voltage requirements.

What really drives people crazy is the term "medium voltage drive." It is a purely marketing term. From zero to 600 V is called "low voltage (LV)." "Medium voltage (MV)" is 600 V and above. There is no "high-voltage (HV)" designation.

Comparing this 600 V cutoff to the chart shows that low-voltage drives provide good efficiency up to several horsepower (10,000 W). Above 10 kW, however, medium voltage drives are needed. Voltages above a few thousand Volts, however, are needed only for the relatively few electric motor applications requiring hundreds of horsepower. Most applications requring that much mechanical output are currently served by internal combustion engines.

In China, ac drives (frequency changers, VFDs) are classified with different voltages:
1. Low voltage (LV) ac drives (frequency inverters, frequency converters), the voltage range: 110V, 220V, 380V

2. Medium voltage (MV) ac drives (frequency inverters, frequency converters), the voltage range: 660V, 690V, 1100V, 2300V

3. High voltage (HV) ac drives (frequency inverters, frequency converters), the voltage range: 3kv, 3.3kv, 6kv, 6.6kv, 10kv, and above.

But in many other countries, ac drives (frequency changers, VFDs) are classified with different voltages:

1. Low voltage (LV) ac drives (frequency inverters, frequency converters), the voltage range: From zero to 600 V

2. Medium voltage (MV) ac drives (frequency inverters, frequency converters), the voltage range: 600 V and above

3. High voltage (HV) variable frequency drives: no high voltage designation

AC drives  (Frequency Converters, Frequency Changers) are used in a wide variety of industrial applications. For example, AC drives are often used with fans to provide adjustable airflow in large heating and air conditioning systems. The flow of water and chemicals in industrial processes is often controlled by adjusting the speed of pumps.

What's more, variable speed AC drives (adjustable speed drives) are commonly used in more complex and difficult environments such as water and wastewater processing, paper mills, tunnel boring, oil drilling platforms or mining.

The main components of an AC drive includes rectifier, DC circuit and inverter

1. Rectifier unit

The AC drive is supplied by the electrical network via a rectifier. The rectifier unit can be uni- or bidirectional. When unidirectional, the AC drive can accelerate and run the motor by taking energy from the network. If bidirectional, the AC drive can also take the mechanical rotation energy from the motor and process and feed it back to the electrical network.

2. DC circuit

40% of all energy in Europe and North America is consumed in buildings. The biggest share of this energy is consumed in heating, ventilation and air conditioning (HVAC) applications.

With the rising energy cost and concerns about the CO2 levels and global warming, it is crucial to use all means available to reduce the energy consumption in heating, ventilation and air conditioning (HVAC) applications. The savings potential in HVAC applictions is big.

The key thing is to start looking more at lifetime costs of HVAC system, where energy cost plays a big role, rather than the initial investment in HVAC system. To give an example, 90% of the lifetime costs of the pump or fan is energy.

1. Fans and pumps

Using an AC drive to control the fan or pump output rather than using dampers, vanes, valves or on/off control brings substantial energy savings, if the required output is less than nominal most of the time.

The AC drive (VFD) controls the speed of the pump and fan by changing the electrical energy supplied rather than damping the air- or water flow. It is like reducing the speed of a car by pressing less on the accelerator instead of using the brake to slow down the speed. The payback time of an AC drive is typically one year or less.

An inverter converts the DC electricity from sources such as batteries, solar panels, or fuel cells to AC electricity. The electricity can be at any required voltage; and in particular it can operate AC equipment designed for mains operation, or rectified to produce DC at any desired voltage.

The applications of an inverter include:

1. DC power source utilization

2. Uninterruptible power supplies (UPS)

3. Induction heating

4. HVDC power transmission

5. Variable-frequency drives (VFDs, frequency inverters)

6. Electric vehicle drives

7. The general case

AC variable frequency drives on Pumps

In centrifugal pumping applications, a control valve is usually employed for flow control. By contrast, the traditional way to control flow for positive displacement pumps was to return part of the fluid back into the pump by means of a bypass valve.

If a variable speed drive (VSD) is employed with a positive displacement pump, the energy consumed is directly proportional to speed. Thus the energy consumed by a centrifugal pump (using a throttle valve) and positive displacement pump (using a bypass valve) as the net flow varies from 40% to 100%.

Variable speed drives (VSDs) have been successfully used to large boiler feedwater pumps in power plants, hot water circulation pumps in commercial buildings, and for waste water treatment plants.

AC variable frequency drives on Fans and Blowers

Fans and blowers, used in heating, ventilation and air conditioning (HVAC) and boiler applications, are generally considered oversized to account for contingencies. Outlet dampers and inlet vanes modulate the airflow by essentially presenting a resistance to it. In turn, the energy consumed by this method is only slightly lower than that consumed while operating with dampers fully open, whereas inlet vanes are somewhat more efficient.

The percentage of energy consumed by a fan system controlled using traditional outlet dampers, inlet vanes, and compares these with a VSD replacement when the net flow varies from 20 - 100%.

The advantages, gained in both productivity improvements and reduced energy consumption by using AC variable speed drives (VSD) on pumps, fans, compressors and other equipment, has been widely documented in the past few years.

Variable speed drives (VSDs) can reduce energy costs and prolong the life of equipment by adjusting motor speed to meet load requirements. For example, by lowering fan or pump speed by 15-20%, shaft power can be reduced by as much as 30%.

There are many advantages to variable speed drives (VSDs) on pumps, fans, compressors and other equipment over other forms of control.

1. Energy is saved by replacing mechanical fluid flow controllers with integrated motor speed control. Generally, these energy savings translate into cost savings and a reduction in greenhouse gas emissions for a given level of production. The soft start characteristic of VSDs eliminates voltage dips and reduces starting shock on motor, couplings, gears and driven equipment, in turn reducing maintenance.