When selecting a motor, how do you choose power and torque?

How to select power and torque when choosing an electric motor?

The power of the electric motor should be selected according to the power required by the production machinery, and try to make the motor run under the rated load. The following two points should be noted when selecting:

① If the motor power is selected too small. If the motor power is selected too small, there will be a “small horse-drawn cart” phenomenon, resulting in long-term overloading of the motor. So that the insulation due to heat and damage. Even the motor was burned.

② If the motor power is selected too large. There will be a “big horse-drawn cart” phenomenon. Its output mechanical power can not be fully utilized, power factor and efficiency are not high, not only the user and the grid is not good. And also cause a waste of electricity.

To correctly select the power of the motor, the following calculations or comparisons must be made:

P=F*V/1000(P= calculated power KW, F= required tension N, line speed M/S) For constant load continuous working mode, the required motor power can be calculated according to the formula: P1(kw) : P=P/n1n2 n1 is the efficiency of the production machinery; n2 is the efficiency of the motor, that is, the transmission efficiency. The power P1 calculated by the above formula is not necessarily the same as the product power. Therefore. The rated power of the selected motor should be equal to or slightly greater than the calculated power.

Besides. The most common method is to choose the power of the motor by analogy. It's called analogy. It is compared with the power of electric motors used in similar production machinery.

Specific practices are as follows:

Learn how much power motor is used by similar production machinery in this unit or other units nearby, and then choose a similar power motor for test. The purpose of the test run is to verify whether the selected motor matches the production machinery.

The method of verification is:

Make the motor drive the production machinery to operate, measure the working current of the motor with a clamp ammeter, and compare the measured current with the rated current marked on the nameplate of the motor. If the actual working current of the electric power machine is not much different from the rated current marked on the nameplate. It indicates that the power of the selected motor is appropriate. If the actual working current of the motor is about 70% lower than the rated current marked on the nameplate. It indicates that the power of the motor is too large, and the motor with smaller power should be replaced. If the measured operating current of the motor is more than 40% greater than the rated current marked on the nameplate. It indicates that the power of the motor is too small, and the motor with larger power should be replaced.

It is suitable for the mutual conductance between the rated power, rated speed and rated torque of the servo motor, but the actual rated torque value should be the actual measurement, because of the energy conversion efficiency problem, the basic value is generally the same, and there will be a slight reduction.

Follow-up question:

What if I'm using an infinite speed control? In terms of motor output power and torque, what are the characteristics and differences between AC variable frequency speed regulation and DC speed regulation?

Answer:

On AC variable frequency speed regulation and DC speed regulation 1: The development of inverter DC motor drive and AC motor drive has been born in the 19th century, it has a history of more than 100 years, and has become the main driving device of power machinery. However, due to technical reasons, for a long period of time, the non-variable speed drive system accounting for about 80% of the entire electric drive system is used in the AC motor (including asynchronous motor and synchronous motor), and in the need for speed control of the drive system is basically the use of direct current motor.

Compared with the DC motor, the AC motor has the following advantages:

(1) Strong structure, reliable work, easy maintenance; 

(2) there is no commutation spark, can be applied to the presence of flammable and explosive gases in harsh environments; 

(3) It is easy to manufacture AC motors with large capacity, high speed and high voltage.

Therefore, for a long time, people hope to replace the DC motor with adjustable AC motor in many occasions, and a lot of research and development work has been carried out in the speed control of AC motor. However, until the 1970s, the research and development of AC speed control system has not been able to get really satisfactory results, and therefore limited the promotion and application of AC speed control system. It is precisely for this reason that baffles and valves have to be used to adjust wind speed and flow in electric drive systems that require speed control such as fans and pumps, which are widely used in industrial production. This approach not only increases the complexity of the system, but also causes a waste of energy.