Electric motor, how to dimension?

The electric motor is one of the greatest inventions of mankind, being present in several types of equipment, from a clock to industrial machines having as its main character to transform electrical energy into mechanical energy. In this article we will cover industrial motors, give tips on how to dimension motors, calculations to find a motor’s synchronous speed, torque, power, work, electric motor categories and understand factors that must be taken into account for motor dimensioning.

Electric motor categories

According to NBR 17094, there are three categories that make up the three-phase induction motors, where each one adapts to a type of load.

Category N

Category N engines are most commonly found on the market, being present in pumps, machine tools, fans and other utilities. This category has a working current considered normal, a normal starting torque (torque) and a low slip.

Category H

Motors in this category are more used in jobs where higher starting torques are required, operate with normal working current, low slip, however, these categories are higher than in the N category, the crushers are equipment that has motors in this category.

Category D

Used in elevators and machinery that require a load with higher starts, motors in this category have normal use loads and relatively high slip, with a rate of more than 5%, one of the characteristics is that this engine model is used wherever there are periodic peaks.

Factors that must be taken into account

There are two important pieces of information that must be taken into account when designing the engine to obtain the maximum possible power.

It is necessary to observe the temperature where the engine will be installed and the altitude, every manufacturer has a table showing the correction factor, which should be used to define the available power, maximum power.

Three-phase motor.

All electric motors need protection components, to be installed correctly it is necessary to make some calculations for the correct dimensioning of their protection components, each one with different characteristics, such as circuit breakers, contactors, overload relays, fuses, among others.

Once you have understood the main factors that are necessary when dimensioning an electric motor, to dimension them it is necessary to collect information, such as: the mains voltage, the motor supply voltage, the environment where the motor will be installed if the engine will be exposed to dust, moisture, chemical elements or gases, what the engine torque is, the number of phases and poles of the engine, power in KW, frequency of the mains, whether the engine will run continuously or only for certain periods, the type of load to be connected to the engine and the load application cycle.

Calculations for engine sizing.

We will calculate the engine power with the following data provided below as an example:

Mains voltage: 380 V
Frequency: 60Hz
Motor supply voltage: 380V.
Environment where the motor will be installed, we will use Zone 2 where the degree of protection is IP66.
Direct coupling with a force of 280 Nm (motor torque).
Current: 4 A.
Efficiency: 80%
Four-pole three-phase motor.

Formula to calculate rpm:

Ns = Synchronous speed in RPM
P = Number of poles
F = Frequency in Hz

Calculating the synchronous speed of an engine

Ns = (120 x F) / No. of poles.
Ns = (120 x 60) / 4
Ns = 7200/4
Ns = 1800 rpm

Formula to calculate power:

For the correct dimensioning of an engine, the power must be given in KW.

P = Power.
V = Voltage.
I = Current.
Ef = Efficiency.

Calculating the power of an engine that has 380V, a current of 4A and 80% efficiency:

P = V x I x Ef
P = 380 x 4 x 80%
P = 1216 W

After you have found the engine power, you must consult the manufacturer’s catalog and check which engine is within the constructive characteristics that will be required. The power to be used must be greater than the value obtained in the calculations.

Formula to calculate the work of an engine:

W = Work (N xm).
F = Newton force (N).
D = distance in meters (m).

Which engine should be used to be able to lift a 30 kg load at a distance of 6 m?

Knowing that 1 Kgf is equivalent to 9.8 N approximately, then to know the value of 6.5 Kg in Newton just make a simple rule of three and the result will be the product between Kgf and N.

X = 6.5 x 9.8 = 63.7 N

The next step is to apply the values ​​found to the formula above.

W = F x D
W = 63.7 x 8
W = 509.6 Nm

Formula to calculate the torque of an engine:

It is important to note that, as the engine work, the unit of measurement for torque is also Newton per meter, but they are different quantities, so do not confuse.

T = Torque (N xm).
F = Newton force (N).
D = distance in meters (m).

What is the torque of an engine that produces a force of 58N at a distance of 40cm from the center of its axis?

T = F x D
T = 58 x 0.40
T = 23.2 Nm

Currently, with the available technologies, it is possible to easily identify which engine will be required for each activity performed, as they are software made available by the manufacturers themselves for the dimensioning of the engines.