The Best Applications for Variable Frequency Drives (VFDs)

Advances

The most commonly used motor in building HVAC applications is the three-phase, induction motor, although some smaller applications may use a single-phase induction motor.
VFDs can be applied to both.
While VFD controllers can be used with a range of applications, the ones that will produce the most significant benefits are those that require variable speed operation.

For example, the flow rate produced by pumps serving building HVAC systems can be matched to the building load by using a VFD to vary the flow rate.

Similarly, in systems that require a constant pressure be maintained regardless of the flow rate, such as in domestic hot and cold water systems, a VFD controlled by a pressure setpoint can maintain the pressure over most demand levels.

The majority of commercial and institutional HVAC systems use variable volume fan systems to distribute conditioned air. Most are controlled by a system of variable inlet vanes in the fan system and variable air volume boxes.

 

As the load on the system decreases, the variable air volume boxes close down, increasing the static pressure in the system. The fan’s controller senses this increase and closes down its inlet vanes. While using this type of control system will reduce system fan energy requirements, it is not as efficient or as accurate as a VFD-based system.

Another candidate for VFD use is a variable refrigerant flow systems. Variable refrigerant flow systems connect one or more compressors to a common refrigerant supply system that feeds multiple evaporators. By piping refrigerant instead of using air ducts, the distribution energy requirements are greatly reduced.
Because the load on the compressor is constantly changing based on the demand from the evaporators, a VFD can be used to control the operating speed of the compressor to match the load, reducing energy requirements under part-load conditions.

Additional VFD Applications

While the primary benefit of both of these VFD applications is energy savings, VFDs are well suited for use in other applications where energy conservation is of secondary importance. For example, VFDs can provide precise speed or torque control in some commercial applications.
Some specialized applications use dual fans or pumps. VFDs, with their precise speed control, can ensure that the two units are operated at the desired speed and do not end up fighting each other or having one unit carry more than its design load level.

Advances in technology have increased the number of loads that can be driven by the units. Today, units are available with voltage and current ratings that can match the majority of three-phase induction motors found in buildings. With 500 horsepower units or higher available, facility executives have installed them on large capacity centrifugal chillers where very large energy savings can be achieved.

One of the most significant changes that has taken place recently is that with the widespread acceptance of the units and the recognition of the energy and maintenance benefits, manufacturers are including VFD controls as part of their system in a number of applications. For example, manufacturers of centrifugal chillers offer VFD controls as an option on a number of their units.
Similarly, manufacturers of domestic water booster pump systems also offer the controls as part of their system, providing users with better control strategies while reducing energy and maintenance costs.

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Cable Spacing as a Means of Noise Mitigation

Separation distances

In situations where there are a large number of cables varying in voltage and current levels, the IEEE 518-1982 standard has developed a useful set of tables indicating separation distances for the various classes of cables.
There are four classification levels of susceptibility for cables.
Susceptibility, in this context, is understood to be an indication of how well the signal circuit can differentiate between the undesirable noise and required signal. It follows that a data communication physical standard such as RS-232E would have a high susceptibility, and a 1000-V, 200-A AC cable has a low susceptibility.

IEEE 518 – 1982 standard

The four susceptibility levels defined by the IEEE 518 – 1982 standard are briefly:

Level 1 (High) – This is defined as analog signals less than 50 V and digital signals less than 15 V. This would include digital logic buses and telephone circuits. Data communication cables fall into this category.
Level 2 (Medium) – This category includes analog signals greater than 50 V and switching circuits.
Level 3 (Low) – This includes switching signals greater than 50 V and analog signals greater than 50 V. Currents less than 20 A are also included in this category.
Level 4 (Power) – This includes voltages in the range 0–1000 V and currents in the range 20–800 A. This applies to both AC and DC circuits.

The IEEE 518 also provides for three different situations when calculating the separation distance required between the various levels of susceptibilities. In considering the specific case where one cable is a high-susceptibility cable and the other cable has a varying susceptibility, the required separation distance would vary as follows:

Both cables contained in a separate tray:

• Level 1 to level 2-30 mm
• Level 1 to level 3-160 mm
• Level 1 to level 4-670 mm

One cable contained in a tray and the other in conduit:

• Level 1 to level 2-30 mm
• Level 1 to level 3-110 mm
• Level 1 to level 4-460 mm

Both cables contained in separate conduit:

• Level 1 to level 2-30 mm
• Level 1 to level 3-80 mm
• Level 1 to level 4-310 mm.

The figures are approximate as the original standard is quoted in inches.

Trays and conduits

A few words need to be said about the construction of the trays and conduits. It is expected that the trays are manufactured from metal and be firmly earthed with complete continuity throughout the length of the tray. The trays should also be fully covered preventing the possibility of any area being without shielding.

Briefly galvanic noise can easily be avoided by refraining from the use of a shared signal reference conductor, in other words, keeping the two signal channels galvanically separate so that no interference takes place.

 

Electromagnetic induction can be minimized in several ways. One way is to put the source of electromagnetic flux within a metallic enclosure, a magnetic screen. Such a screen restricts the flow of magnetic flux from going beyond its periphery so that it cannot interfere with external conductors. A similar screen around the receptor of EMI can mitigate noise by not allowing flux lines inside its enclosure but to take a path along the plane of its surface.

Physical separation between the noise source and the receptor will also reduce magnetic coupling and therefore the interference.

 
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An example how to calculate voltage drop and size of electrical cable

Input information

Electrical details:

Electrical load of 80KW, distance between source and load is 200 meters, system voltage 415V three phase, power factor is 0.8, permissible voltage drop is  5%, demand factor is 1.

Cable laying detail:

Cable is directed buried in ground in trench at the depth of 1 meter. Ground temperature is approximate 35 Deg. Number of cable per trench is 1. Number of run of cable is 1 run.

Soil details:

Thermal resistivity of soil is not known. Nature of soil is damp soil.

Ok, let’s dive into calculations…

• Consumed Load = Total Load · Demand Factor:
  Consumed Load in KW = 80 · 1 = 80 KW
• Consumed Load in KVA = KW/P.F.:
  Consumed Load in KVA = 80/0.8 = 100 KVA
• Full Load Current = (KVA · 1000) / (1.732 · Voltage):
  Full Load Current = (100 · 1000) / (1.732 · 415) = 139 Amp.

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How to wire a HA 3/4 Rectangular Connector

HA Series connectors are used whenever space is limited. They provide the smallest possible footprint in a 10A power connector. Wiring a HA 3 series connector takes less than 10 min. Watch this video to learn how

 

Visit the Lapp Group website to access our comprehensive line of industrial connectors. 

http://lappusa.lappgroup.com