The output of three-phase asynchronous motor should be selected according to the power required by the production machinery, and the three-phase asynchronous motor should be operated under rated load as much as possible. When selecting the power size of the motor, the purpose should be appropriate.
About the selection method of power size of three-phase asynchronous induction ACmotor.
Selecting a three-phase asynchronous motor with appropriate power will neither destroy the electric energy nor overload the motor for a long time. If the motor power is too low. The "small carriage" phenomenon occurs, causing the motor to be overloaded for a long time, causing the insulation material to be damaged by heat. Even if the motor burns out, if the motor power is too large, the "big marathon" phenomenon will occur. Its mechanical output power can be fully utilized, the power factor and power are not high, and it also generates electric energy.
Method 1: Use the formula to calculate the power required by the three-phase asynchronous motor:
- For the continuous operation method, the load power (that is, the power on the mechanical shaft) Pl (kW) is known. The following formula can be used to calculate the power P (kW) of the motor: P=P1/n1 n2 Where n1 is the power of the production machine and n2 is the power of the motor. This is the transmission power.
According to the power obtained by the above formula, because the power and so on do not need to be very accurate, and the mechanical energy loss has certain variables, the rated power of the selected motor should be equal to or slightly greater than the calculated power.
If an electric motor with a power of 0.8 is selected, try to find out how many kilowatts the motor power should be. Solution: P=P1/n1n2=3.95/0.7*0.8=7.1kW Since there is no specification of 7.1kW, a 7.5kW three-phase asynchronous motor is selected.
- Short-term operation rated motor. Compared with the motor with the same rated power during continuous operation. Small components, large maximum torque, and low price. If conditions permit, a three-phase asynchronous motor with a short-term rating of S2 should be selected as much as possible.
- For three-phase asynchronous motors with an intermittent operation rating of S3, select the output size. According to the size of the load freewheeling rate, a three-phase asynchronous motor with an S3 operating system is selected, which is specially used for intermittent operation. The calculation formula for load Fs% continuation is Fs%=tg/(Tg+to)×100%, where Tg is the working time (unit: min), t is the suspension torque (unit: min); Tg ten to is the cycle time (unit: min)
Method 2: Use the simulation method to select the power of the three-phase asynchronous motor. The so-called analogy, which is comparable to the performance of the three-phase asynchronous motor used in similar production machines.
The specific method is: understand the power consumed by similar production machines near this unit or other units, and then select nearby motors for trial operation. The trial operation is intended to check whether the selected motor is compatible with the production machine. The verification method is: make the electric hoist drive the production machine to operate. Use a clamp ammeter to measure the working current of the three-phase asynchronous motor, and compare the measurement result with the rated current specified on the motor nameplate.
Method 3: Evaluation method is suitable:
- If the actual working current of the motor does not deviate significantly from the displayed nominal current. This indicates that the performance of the selected motor is sufficient.
- If the actual working current of the motor is about 70% lower than the rated current specified on the nameplate. Then it indicates that the motor power is too high (i.e. the "big carriage" should be replaced with a motor with lower power).
- If the measured motor working current is more than 40% higher than the rated power flow specified on the nameplate. This indicates that the motor power is too low (i.e. "small carriage"), that is, a larger power motor should be replaced.