Nominal chain. What is it and how to calculate?

As we already know, three-phase induction electric motors have several characteristics, factors that make a total difference, both for dimensioning cables, protection devices and for their respective functions. In this article, we will understand what is rated current, starting current and how to calculate the rated current of a three-phase induction motor, come on guys!

Rated current: What is it?

The rated current is nothing more than the current expected from a particular piece of equipment or machine, that is, the rated current is the amount of amps required for the machine to function satisfactorily. It is important to highlight that not all equipment or machines consume exactly the rated current, it is necessary to calculate this rated current, but generally, the rated current is indicated by the manufacturers of the machines and equipment.

The rated current can also be used to express the maximum electrical current capacity that a given device, machine or device is capable of withstanding without being damaged, thereby ensuring good performance.

The rated current, besides serving for the dimensioning of conductors and protection devices, it is also possible to use it as a parameter pass to indicate the electrical power of electrical machines and equipment, being a very common practice.

What is rated current and how to calculate.

Rated current: Electric machines.

The nominal current in electric machines is the electric current value foreseen in the calculation of a given machine, and soon after its construction, it is verified how much the electric machine is capable of withstanding the intensity of electric current so that it has the maximum efficiency possible, without damage and maintaining the machine in good condition. It is worth noting that these tests performed on electrical machines are provided for in the machine manufacturing standards.

Starting current: Electric machines.

We can define the starting current to refer to the electrical current that a given machine (motor) requires in a period of time, so that at the initial moment the electrical energy begins to circulate through the electrical machines, and then its rotor starts to move from zero speed, until the final instant when the full speed of the rotor is reached.

In order to make it easier to understand, the starting current behaves like an energy surge in the electrical network, due to its high-intensity value, so the current varies with the intensity over the time the motor was given the start, and the final value of the current is much less than the initial value.

Rated current in three-phase motors: How to calculate?

To calculate the rated current of three-phase induction motors, we need to have some data at hand, which are found on the motor plates. We will calculate the rated current (In) for a three-phase induction motor that is being powered by a three-phase 220V system, with a power of 3CV, a power factor of 0.85, and a yield of 87.5%. The calculation for nominal current is very simple, just use the formula below:

Formula for calculating rated current.

Before starting the calculations to find the rated motor current, it is necessary to take into account that the motor power is given in watts (W) and not in horsepower (CV), as shown in the example we are analyzing. So we must convert from CV to W, we will use a rule of three, as shown in the image below:

Converting horsepower to watt.

Now that we have converted the power, just replace the values ​​given in the text in the formula, remembering that we cannot insert the direct 87.5% in the formula, it is necessary to divide by 100 before, otherwise the result will be completely different. As we can see in the image below, we first solve the denominator and then perform the division.


Calculating the rated current of a three-phase motor.

The video below shows exactly how to do the calculations to find the rated current of three-phase induction motors, complementing the topic covered.