VFD on guard for energy savings

Recommendation points

• Principles of liquid flow control without VFEP
• The principle of fluid flow control using VFEP
• Application of VREP for borehole pumps
• Advantages of VREP in addition to energy savings
• Manufacturing firms and payback period

The article will talk about how to reduce the power consumption of an already installed or just planned for installation of a borehole pump, explain what a frequency-controlled electric drive is, name its main characteristics, advantages and applications.

The more benefits appear in our house, the greater the burden falls on the shoulders of the energy system. As a result, payments for electricity are growing, and many owners are trying in any way to reduce this cost item, resorting to various energy-saving technologies. Household appliances with an energy class of at least “A”, economical lighting devices and much more can be seen in almost every home. However, for suburban housing with autonomous water supply, in addition to the above methods, one more rather profitable method of energy saving can be proposed.

A large part of electricity is used to move water and gases in devices such as pumps, fans, compressors. Such equipment is used for the operation of the heat supply system, the supply of drinking water (and not only centralized, but also individual), the functioning of the sewerage system, the industrial movement of liquids and gases. In most cases, energy is wasted here. This is due to the fact that consumption in these systems is uneven. It has peaks with maximum load, which take up a very small part of the operating time and rarely exceed a couple of hours per day. It is for these peaks that equipment such as a pump or a blower is designed. Basically, not 100% liquid or gas supply is required, but in the region of 30-40% of the maximum possible load. It is easy to see this on the example of the peaks of water intake in central drinking water supply systems: the morning and evening maximums are opposed to the night minimum. However, the pump continues to run at full capacity all this time and consume 100% of the energy..

To date, there is already a solution to this problem – a frequency-controlled electric drive (VREP), it will be discussed below.

Principles of liquid flow control without VFEP

In order to assess the benefits of the introduction of frequency regulation, recall the commonly used methods of traditional reduction of the supply of liquid or gas. For simplicity, examples will be given on a regular water supply system with a standard pump, leaving aside the movement of air, oil, gases and all kinds of industrial fluids. By the way, the principles will be very similar.

One of the first to consider is regulation with a bypass. This is a bypass line, which is a branch from the main pipeline, which returns part of the fluid already pumped by the pump back to the supply of the same pump. Despite the possibility of a fairly accurate adjustment of the system to the specified water flow parameters, its efficiency is incredibly low.

The next on this list can be considered regulation using valves and other devices installed downstream of the pump and limiting the useful section of the pipeline. This option can also be considered as wasting a considerable part of the electricity in vain, since the created large head is cut off by these devices to the required level.

Another control option is the periodic operation of the pumps. It involves turning on the equipment only to fill the storage tanks with water, after which an automatic stop occurs. Of the ones described above, it has, perhaps, the best efficiency, but it is not without its drawbacks:

• constant starts / stops reduce equipment life;
• there is a risk of water hammer during the next launch, which can damage the pipeline;
• uneven network pressure.

A more reliable option is the simultaneous operation of a group of pumps. This method involves the inclusion of a backup unit with an increase in water intake. However, it also has a bunch of disadvantages. For example, when using pumps of different power and parameters, the operation of the entire system will be unstable. And the cost of such a regulation method is quite high, since it involves the purchase of not one, but several pieces of equipment at once..

The principle of fluid flow control using VFEP

Fluid flow control using a variable frequency drive is designed to reduce the percentage of wasted energy in all areas where electric motors are used, and also there is a variable load.

The composition of such equipment includes not only the operating mechanism of the pump and the electric motor. Here the main role is played by the so-called “frequency converter”, it is also a frequency converter. By means of sensors installed in the network, it reacts to all changes and controls the flow: a given voltage with a certain amplitude is generated at its output, which, in turn, makes the motor and, accordingly, the working mechanism of the pump rotate at a certain (slower) speed. So, when the flow rate increases to the peak, the pump will work with full efficiency, but immediately when the water intake decreases, it will react by reducing the rotation speed of the working mechanism. And, accordingly, reduced energy consumption.

Consequently, the same volume of liquid delivered to the desired tap by a pump with a VFEP will require less funds than a similar scheme with a constant rotation speed of the equipment operating mechanism will consume. This eliminates the need for ineffective control methods such as throttling or the use of bypasses..

Application of VREP for borehole pumps

As mentioned above, frequency regulation technology can also be applied in an ordinary country house. It is possible to implement it on a hot water supply system, heating or on a borehole pump. Let us consider the last option in more detail, since it is there that the most pronounced and understandable for an ordinary person is the frequency of the load:

• night is the minimum, in the situation under consideration it is often equal to zero;
• morning – maximum (washing, shower, cooking breakfast, etc.);
• day – medium (washing, cooking, cleaning);
• evening – maximum (shower, bath, cooking, and so on);
• night is another minimum.

The division, of course, is conditional, however, the owners of suburban housing sometimes very clearly feel peaks when, due to a decrease in pressure in the network, water from the tap flows with a weak pressure. What is not observed at other times of the day.

Today, it is possible not only to buy a ready-made pump with a variable drive, but also to retrofit an already installed frequency converter. The latter, when used in conjunction with a submersible pump, must have the following set of functions:

1. Built-in PID (sometimes PI, but these are less common on the market) controller.
2. Energy optimization capability allowing normal voltage derating with low drive load.
3. Possibility of restarting the drive after any failure or error of the automation without human intervention.
4. Motor overload protection.
5. Motor overheating protection.
6. Short circuit protection.
7. Protection of the pump from dry running, i.e. from running without water when the level in the well drops below the suction pipe. The pumped liquid is a cooling and lubricant for the unit, therefore its absence in a running pump leads to overheating and rapid failure.
8. “Sleep” mode when using the converter in conjunction with a pump that is limited to work at very low speeds.
9. Archive of accidents. This option is indispensable when clarifying the features of the device’s operation in specific circumstances with repeated (periodic) failures.
10. Scalar (volt-hertz U / f) or vector control for more precise adjustment of the drive, ensuring normal smooth (no jerking) operation of the mechanism.

As for the choice of equipment, the following points can be highlighted:

1. When choosing a “chastotnik”, more attention should be paid not to power, but to the rated current, and a certain margin should be provided. This is due to the fact that the rated current of submersible electric motors is slightly higher than that of standard models of motors..
2. The overload capacity of the installed “frequency” must be large enough (over 120%), otherwise this drawback must be compensated for by the motor power, which will be slightly increased.
3. In cases where the transmitter is to be placed in an unheated room, it must have a suitable operating temperature range and an appropriate protection class..

In addition to the main equipment, you should pay attention to the cable – it must be of a large cross-section in order to prevent voltage loss along the length. As an additional protection, you can install a motor choke, it additionally protects against large current leaks and overload protection. A line choke can also be installed in front of the inverter (converter), which, in turn, eliminates problems when operating from a distribution transformer.

Advantages of VREP in addition to energy savings

In addition to saving energy, equipping pumps with a variable electric drive has other positive aspects..

Firstly, the resource of the equipment is significantly, almost doubled, as the number of starts and stops is reduced.

Secondly, it is possible to significantly reduce the capacity of the storage tank, since when the flow rate increases, the pump automatically starts working with greater efficiency. In order that any maximum consumption does not lead to a decrease in pressure in the network, a pump with a deliberately higher power can also be provided for the installation – the power consumption will increase slightly.

Another positive factor is smooth starts and stops, which negate the likelihood of a water hammer in the network. As a result, not only the equipment, but the water supply itself will last longer than usual..

Manufacturing firms and payback period

There are various manufacturers of variable frequency drives on the market. You can see a variety of products from renowned world-class companies such as ABB and SIMENS, and samples of domestic production. The cost of eminent brands will be appropriate, but as for the quality, it is quite possible to meet it with Russian companies..

As for the payback period, in each case it will be calculated individually. Usually funds are fully covered by savings for a period of six months to two years, but there may be isolated exceptions.

The following pattern can be distinguished – the more power the pump has, the more expensive it will cost, respectively, and a frequency-controlled electric drive to it will be more expensive than a less powerful analogue. But such a pump also consumes more electricity – therefore, the savings when using the “frequency” will be more significant and it will pay for itself earlier.

Another fact: the installation of a frequency-controlled electric drive will justify itself earlier in the network, in which the uneven operation is more pronounced, and the peaks (maximum load) occur rarely and are short-lived.

In conclusion, I would like to note that it would be good to apply this method of regulation not only at home. For many enterprises, this energy saving measure would help reduce the energy intensity of production. Utilities would spend less on transporting water in heating and water supply systems. Plus, VFD technology isn’t just applicable to pumps. It can be successfully used in any areas where electric motors are used: elevators, hoists, any hydraulic components of mechanisms, and others. By switching to rational consumption of electricity, we reduce the load on CHP and NPP, which ultimately has a positive effect not only on the material condition of the state, but also on the ecology of the region..