let's start Selection of motor

The basic content required for motor selection includes: the type of load to be driven, rated power, rated voltage, rated speed and other conditions.

01. The type of load driven

The principle of selecting a motor is to choose a motor with a simple structure, reliable work, and convenient maintenance on the premise that the performance of the motor meets the mechanical requirements.

The load is stable, and the continuous running production Machinery with no special requirements for starting and braking should be preferred to ordinary squirrel cage asynchronous motors, which are widely used in machinery, water pumps, fans, etc.

For production machinery that is frequently started and braked and requires larger starting and braking torques, such as bridge cranes, mine hoists, air compressors, and irreversible steel rolling machines, winding asynchronous motors should be used.

Where there is no requirement for speed regulation, a constant speed or improvement of power factor is required, synchronous motors should be used, such as medium and large capacity water pumps, air compressors, hoists, mills, etc.

Production machinery that requires a speed regulation range of 1:3 or more, and requires continuous, stable and smooth speed regulation, should use separately-excited DC motors or synchronous motors, such as large precision machine tools, planers, rolling mills, hoists, etc.

The production machinery that requires large starting torque and soft mechanical characteristics uses series-excited or compound-excited DC motors, such as trams, electric locomotives, and heavy-duty cranes.

02. Rated power

The rated power of the motor: refers to the output power, that is, the shaft power, also called the capacity, which is the iconic parameter of the motor. People often ask how big the motor is. Generally speaking, it does not refer to the size of the motor, but the rated power. It is the most important index to quantify the motor's drag load capacity, and it is also a parameter requirement that must be provided when the motor is selected.

The principle of correct selection of motor capacity should be the most economical and reasonable decision on the power of the motor on the premise that the motor can meet the requirements of the production mechanical load. If the power is selected too large, the equipment investment will increase, resulting in waste, and the motor often runs under load, and the efficiency and power factor of the AC motor are low; on the contrary, if the power is selected too small, the motor will run with overload, causing the motor to run prematurely damage.



There are three factors that determine the main power of the motor:

1) The heating and temperature rise of the motor are the most important factors that determine the power of the motor;

2) Allow short-term overload capacity;

3) For asynchronous squirrel-cage motors, the starting ability should also be considered.

03. Rated voltage

The rated voltage of the motor: refers to the line voltage in the rated working mode.

The choice of the rated voltage of the motor depends on the power supply voltage of the power system to the enterprise and the size of the motor capacity.



The choice of AC motor voltage level is mainly determined by the power supply voltage level of the place where it is used. Generally, the low voltage network is 380V, so the rated voltage is 380V (Y or △ connection), 220/380V (△/Y connection), 380/660V (△/Y connection). The power of the low-voltage motor is increased to a certain extent (such as 300KW/380V), and the current is limited by the carrying capacity of the wire, and it is difficult to increase, or the cost is too high. Need to increase the voltage to achieve high-power output. The power supply voltage of the high-voltage power grid is generally 6000V or 10000V, and there are also 3300V, 6600V and 11000V voltage levels abroad. The advantages of high-voltage motors are large power and strong impact resistance; the disadvantages are large inertia, which makes it difficult to start and brake.

The rated voltage of the DC motor should also match the power supply voltage. Generally 110V, 220V and 440V. Among them, 220V is a common voltage level, and high-power motors can be increased to 600~1000V. When the AC power supply is 380V and the three-phase bridge silicon controlled rectifier circuit is used for power supply, the rated voltage of the DC motor should be 440V. When the three-phase half-wave silicon controlled rectifier power supply is used for power supply, the rated voltage of the DC motor should be 220V.

04. Rated speed

Rated speed of the motor: refers to the speed in the rated working mode.

The motor and the working machinery driven by it have their own rated speeds. When choosing the speed of the motor, it should be noted that the speed should not be too low, because the lower the rated speed of the motor, the more the number of stages, the larger the volume and the higher the price; at the same time, the speed of the motor should not be selected too low. High, because it will make the transmission mechanism too complicated and difficult to maintain.

In addition, when the power is constant, the motor torque is inversely proportional to the speed.

Therefore, those who do not have high requirements for starting and braking can make a comprehensive comparison of several different rated speeds from the aspects of equipment initial investment, floor space and maintenance costs, and finally determine the rated speed; and often start, brake and reverse, However, if the duration of the transition process has little effect on productivity, in addition to considering the initial investment, the speed ratio and the rated speed of the motor are mainly selected based on the minimum loss of the transition process. For example, the hoist motor needs frequent forward and reverse rotation and large torque, the speed is very low, the motor is bulky and expensive.

When the motor speed is high, the critical speed of the motor needs to be considered. The rotor of the motor will vibrate during operation. The amplitude of the rotor increases with the increase of the speed. At a certain speed, the amplitude reaches the maximum value (that is, the usual resonance). After this speed, the amplitude increases with the speed. Gradually decrease and stabilize within a certain range, the speed with the largest amplitude of the rotor is called the critical speed of the rotor. This speed is equal to the natural frequency of the rotor. When the speed continues to increase, the amplitude will increase when it approaches twice the natural frequency. When the speed is equal to twice the natural frequency, it is called the second-order critical speed, and analogously, there are third-order, fourth-order and other critical speeds. If the rotor runs at a critical speed, severe vibration will occur, and the curvature of the shaft will increase significantly. Long-term operation will also cause severe bending and deformation of the shaft, or even breakage. The first-order critical speed of the motor is generally above 1500 rpm, so the conventional low-speed motor generally does not consider the influence of the critical speed. Conversely, for a 2-pole high-speed motor, where the rated speed is close to 3000 rpm, this effect needs to be considered, and it is necessary to avoid long-term use of the motor in the critical speed range.

Generally speaking, the motor can be roughly determined by providing the type of load driven, the rated power of the motor, the rated voltage, and the rated speed. But if the load requirements are to be optimally met, these basic parameters are far from enough. The parameters that need to be provided include: frequency, working system, overload requirements, insulation level, protection level, moment of inertia, load resistance torque curve, installation method, ambient temperature, altitude, outdoor requirements, etc., which are provided according to specific conditions.