What are the basic requirements for motor selection (Two)?

11/08/2019

Second, the 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 characteristic parameter of the motor. It is often asked how big the motor is, generally not the size of the motor, but the rated power. It is the most important indicator to quantify the ability of the motor to drag load, and it is also the parameter requirement that must be provided when the motor is selected.

The principle of correctly selecting the motor capacity should determine the power of the motor most economically and reasonably under the premise that the motor can meet the requirements of producing mechanical load. If the power is selected too large, equipment investment increases, resulting in waste, and the motor often runs under load, the efficiency and the power factor of the AC motor are low; conversely, if the power is selected too small, the motor will run overload, causing the motor to prematurely damage.

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

1) The heating and temperature rise of the motor, which is the most important factor determining the power of the motor;

2) Allow short-term overload capability;

3) The starting ability should also be considered for the asynchronous squirrel cage motor.

First, the specific production machinery calculates and selects the load power according to its heat generation, temperature rise and load requirements. The motor then pre-selects the rated power according to the load power, working system and overload requirements. After the rated power of the motor is pre-selected, it is also necessary to perform the heating, overloading capability and the starting capability verification when necessary. If one of the items fails, the motor must be re-selected and checked until all items are qualified. Therefore, the work system is also one of the requirements that must be provided. If there is no requirement, it will be processed according to the most conventional S1 working system; the motor with overload requirement also needs to provide the overload multiple and corresponding running time; the asynchronous squirrel cage motor drives the fan and so on. For inertia loads, it is also necessary to provide a graph of the moment of inertia of the load and the starting resistance torque to check the starting capability.

The above selection of rated power is carried out under the premise of a standard ambient temperature of 40 °C. If the ambient temperature of the motor is changed, the rated power of the motor must be corrected. According to theoretical calculations and practice, when the ambient temperature is different, the power of the motor can be roughly increased or decreased according to the following table.

Therefore, ambient temperatures are also required in harsh climates. For example, in India, the ambient temperature needs to be checked at 50 °C. In addition, high altitude has an impact on motor power. The higher the altitude, the greater the temperature rise of the motor and the lower the output power. And the motor used at high altitudes also needs to consider the effects of corona.

For the current power range of the motor on the market, I would like to cite the data of my company's performance table for reference.

DC motor: ZD9350 (mill) 9350kW

Asynchronous motor: squirrel cage type YGF1120-4 (blast furnace fan) 28000kW

Winding type YRKK1000-6 (raw mill) 7400kW

Synchronous motor: TWS36000-4 (blast furnace fan) 36000kW (test unit reaches 40,000kW)


Third, the rated voltage

The rated voltage of the motor is the line voltage at rated operating mode.

The choice of the rated voltage of the motor depends on the power system voltage and motor capacity of the power system.

The selection of the AC motor voltage level is mainly based on the power supply voltage level of the place of use. Generally, the low-voltage network is 380V, so the rated voltage is 380V (Y or △ connection method), 220/380V (△/Y connection method), and 380/660V (△/Y connection method). The power of the low-voltage motor is increased to a certain extent (such as 300KW/380V), and the current is limited by the wire bearing capacity, or the cost is too high. A high power output needs to be achieved by increasing the voltage. The voltage supply voltage of the high-voltage power grid is generally 6000V or 10000V, and the voltage levels of 3300V, 6600V and 11000V are also available abroad. The advantage of the high-voltage motor is that it has high power and strong impact resistance; the disadvantage is that the inertia is large, and it is difficult to start and brake.

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


Fourth, the rated speed

The rated speed of the motor refers to the speed in the rated operating mode.

Both the motor and the work machine that is being driven by it have their own rated speed. When selecting the speed of the motor, it should be noted that the speed should not be selected too low, because the lower the rated speed of the motor, the more the number of stages, the larger the volume, the higher the price; at the same time, the speed of the motor should not be selected. High, because this 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 rotational speed. T=9550P/n

Therefore, those with low starting and braking requirements can make a comprehensive comparison with several different rated speeds from the initial investment, floor space and maintenance cost of the equipment, and finally determine the rated speed; and often start, brake and reverse, However, the duration of the transition process has little effect on productivity. In addition to considering the initial investment, the speed ratio and the rated motor speed are selected based on the minimum loss of the transition process. For example, the hoist motor needs frequent positive and negative reversal and the torque is very high, the rotation speed is very low, the motor is bulky and expensive.

When the motor speed is high, the critical speed of the motor must also be considered. The rotor of the motor vibrates during operation, and the amplitude of the rotor increases with the increase of the rotational speed. When the amplitude reaches a maximum value at a certain rotational speed (that is, the resonance is usually called), the amplitude increases with the rotational speed after exceeding the rotational speed. Gradually decreasing, and stable within a certain range, the maximum rotational speed of this rotor is called the critical speed of the rotor. This speed is equal to the natural frequency of the rotor. When the rotational speed continues to increase, the amplitude will increase when it is close to 2 times the natural frequency. When the rotational speed is equal to 2 times the natural frequency, it is called the second-order critical speed, and so on, there are third-order and fourth-order critical speeds. If the rotor is operated at a critical speed, severe vibration will occur and the bending of the shaft will increase significantly. Long-term operation will cause severe bending deformation or even breakage of the shaft. The first-order critical speed of the motor is generally above 1500 rpm, so conventional low-speed motors generally do not consider the effect of critical speed. Conversely, for a 2-pole high-speed motor with a rated speed approaching 3000 rpm, this effect needs to be considered and the long-term use of the motor in the critical speed range should be avoided.

In general, the motor is provided with the type of load that is driven, the rated power of the motor, the rated voltage, and the rated speed to roughly determine the motor. But if you want to optimally meet the load requirements, these basic parameters are far from enough. The parameters that need to be provided include: frequency, working system, overload requirement, insulation level, protection level, moment of inertia, load resistance torque curve, installation method, ambient temperature, altitude, outdoor requirements, etc., according to specific conditions.