¿CÓMO CALCULAR LOS KVA NECESARIOS?

DIMENSIONING THE POWER OF A GENERATOR SET

One of the most important factors when deciding on a particular generator is to correctly choose the kVA of the equipment. We must correctly select the power of the generator set (electrical power) according to the needs of our installation. Based on the required power we can calculate through a series of calculations or tables the power that our generator must have.

Would you like to know how to handle these much-needed calculations? Take note.

HOW TO CALCULATE THE KVA NECESSARY?

Properly dimensioning the power of the generator set is one of the most important factors when buying one of these equipment. Taking into account that a correct choice of generator is important to ensure the life of the machine and minimize fuel consumption, to avoid unnecessary maintenance costs.

Incorrect calculation in the dimensioning of the group can cause serious problems after installation. What are these errors?

  • If we oversize the power of the generator set, we will be raising the price of the equipment unnecessarily, in addition we will be wasting the capacity of use of the equipment and raising in excess the consumption of fuel with the expense that entails.
  • On the other hand, if we underestimate the power of the group, we run the risk of overloading the generator, shortening the service life of the group, or in the worst case we can cause a fault that will be very expensive to repair, or even cause all the connected equipment to stop To operate.

The safest way to measure the power of a generator is to let yourself be advised by a professional in the industry. Although it will require clear and precise information on the part of the end user, as for example, activity that realizes, equipment that uses, place of installation, conditions of use, periods of activity, etc. All these questions are aimed at selecting the generator that best suits your needs. Therefore it is advisable to be suspicious of those who recommend a particular generator set without doing this type of question, since they are important factors when choosing a particular model and / or power.

Dimensioning the power of a generator

What data helps us to select the right power for our generators.

  1. The first data to be taken into account when selecting a generator is with which current type of current work the equipment to be maintained by the generator, single-phase or three-phase. It is important to choose a single-phase or three-phase model depending on the operation of your equipment. It must be taken into account that the three-phase models share the power load in three shots so that the individual load of one equipment can not be more than 1/3 of the total power of the generator set.
  2. To make the approximate calculation of the kVA necessary to correctly dimension the group that we wish to acquire, it is advisable to base it on another data that is usually known in advance, the motor power of the object of study, or the sum of powers if the object of the study counts with more than one, which would normally be expressed in CV or kW. This information is usually easy to locate since most manufacturers etch it onto plates or stickers in a visible place on the machine.
  3. Another factor to note is that the motors absorb more kVA during startup than during their nominal operation (ie during their operation), this data is reflected in the attached tables. In a first column (Start) we observe the value necessary for starting and in a second column (Operation) the value in operation.

With these data in mind it is possible to perform the calculation by following one of the following methods: By conversion tables or by simple mathematical calculation. Both the tables and the basic calculations that we will show below, allow to know in a simple way the power necessary for each case. Obviously this result is approximate although it offers a more than sufficient approximation.

Thanks to this system we avoid falling into the error of other calculation systems that do not take into account something as important as engine starts. These peaks of power produced during the starting of the motors can cause great problems if they are not taken into account in advance in the calculation.

Tables of calculation of the power of a generating set.

This method allows to convert the power, in kW or CV, known of the machines that we are going to support with the generating set, in a fast and simple way to kVA. Depending on the type of start of the machine, we will use one table or another. For this type of calculations it is always necessary to have as a reference the value kVA needed at start-up time.

DIRECT START
Power
Engine
Start
Light
Start
Medium
Start
Heavy
kW CV Start
(kVA)
Operation
(kVA)
Start
(kVA)
Operation
(kVA)
Start
(kVa)
Operation
(kVa)
0.18 0.25 0.7 0.3 0.9 0.3 1.2 0.3
0.37 0.5 1.4 0.5 1.9 0.5 2.3 0.5
0.55 0.5 2.1 0.7 2.8 0.7 3.5 0.7
0.75 1 2.8 0.9 3.7 0.9 4.6 0.9
1.1 1.5 4.2 1.4 5.6 1.4 6.9 1.4
1.5 2 5.6 1.9 7.4 1.9 9.2 1.9
2.2 3 8.3 2.8 11 2.8 13.8 2.8
 3 4 11 3.7 14.7 3.7 18.4 3.7
 4 5.5 15.2 5 20.3 5 25.3 5
 5.5 7.5 21 7 27.6 7 34.5 7
 7.5 10 27.5 9.2 36.8 9.2 46 9.2
11 15 41.5 13.8 55.2 13.8 69 13.8
Single Phase 230/220 50 Hz (Nominal)
Three Phase 400/380 Volts 50 Hz (Nominal). Star alternator connection.
Three Phase 230/220 Volts 50 Hz (Nominal). Alternator connection in triangle.
START STAR-TRIANGLE
Power
Engine
Start
Light
Start
Medium
Start
Heavy
kW CV Start
(kVA)
Operation
(kVA)
Start
(kVA)
Operation
(kVA)
Start
(kVa)
Operation
(kVa)
3 4 3.7 3.7 4.9 3.7 6.1 3.7
4 5.5 5 5 7 5 8.4 5
5.5 7.5 7 7 9.2 7 11.5 7
7.5 10 9.2 9.2 12.2 9.2 15.3 9.2
11 15 13.8 13.8 18.4 13.8 23 13.8
15 20 18.4 18.4 24.5 18.4 38.3 23
18.5 25 23 23 30.6 23 38.3 23
22 30 27.6 27.6 36.8 27.6 46 27.6
25 34 31.3 31.3 41.7 31.3 52.1 31.3
30 40 36.8 36.8 49 36.8 61.3 36.8
37 50 46 46 61.3 46 76.7 46
45 60 55.2 55.2 73.6 55.2 92 55.2
55 75 69 69 82 69 115 69
75 100 92 92 122.7 92 153.3 92
90 125 115 115 153.4 115 191.6 115
110 150 138 138 184 138 230 138
Starting at 4 kW, the starter is usually star-triangle.

Principles of calculation for the power of a generating set.

The above data is based on the following calculations. So we can get any other value that is not reflected in the previous conversion tables. To calculate, we will use the following formula:

kW / cos PHI = kVA

If we only have the CV of the engine we can carry out the conversion easily with the following formula.

kW= CV * 0.736

The cos PHi is a constant fact that we know in advance.

cos PHI = 0.81

As indicated above, it is necessary to take into account the peak power at the time of starting the engine, so we add the correction factor (X) according to the requirement of starting each machine.

According to your requirement at start-up the correction factor is as follows:

  • Light start : (X) = 3

(Turbines, Fans, Surface pumps, Machine tools, Vacuum starters, etc.)

  • Medium start : (X) = 4

(Reducers, Conveyor belts, Submersible pumps, Compressors, Starter machines under load, etc.)

  • Heavy start : (X) = 5

(Cranes, Lifting equipment, Cutting discs, Floor polishers, Loaders with large loads, etc.)

  • In operation : (X) = 1

Taking into account the correction factor, the direct start calculation formula is expressed as follows:

(kW * (X)) / 0.81 = kVA

O well,

(CV Nominales * 0.736 * (X) ) / 0.81 = kVA

Taking into account that the requested energy point is 1/3 at star-triangle start, the formula will be expressed as follows.

(1/3) * (kW * (X)) / 0.81 = kVA

O well,

(1/3) * (CV nominales * (X) * 0.736 / 0.81) = kVA

Taking into account that in operation the correction factor is 1 and the same formula is used as the direct start, the formula will be expressed as follows:

kW  / 0.81 = kVA

O well,

(CV Nominales *  0.736) / 0.81 = kVA

In the same way as in the conversion tables it is always advisable to take the start value as a reference.

Considerations to take into account

It is recommended to use any of the above methods that if we are not sure of the type of boot of the machine we use the most unfavorable values of heavy starting, to avoid being short in the calculations.

It is also appropriate to add a 20% power to the power calculation obtained to avoid that the generator set works at 100% of its capacity, which extends the useful life of the generator set and avoid potential problems.

Because a generator power with the star-triangle starter system is much smaller, in some cases it is convenient to convert single-starter to star-triangle to save power. It is advisable to consult the manufacturer if the engine of the group allows it and the installer if this change is possible.

Keep in mind that a change of this type can increase the cost of installation. Although obviously acquiring a lower power group can lead to a large reduction in the cost of acquiring the group, so that the increase in the installation can be easily assumed.

As a last remark, it is advisable to program the startup of the motors so that the startup occurs in a staggered way, thus avoiding excessive power peaks when starting all unison.

Are you looking for an experienced generator supplier?

In Tecnics Grupos Electrógenos, S.L. We have more than 50 years of experience in the sector, designing, manufacturing and selling generators.

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