Interview skills of property electrician?

Interview questions 1. What's the difference between DC motor and AC motor? Why can both be rotated?

Answer: AC motor is an alternating magnetic field generated by alternating current, which rotates asynchronously. The DC motor moves by the magnetic field generated by the main pole and commutator. The characteristic of DC motor is large torque.

Interview question 2. Explain what measures (electrical and mechanical) need to be taken before using the new motor.

Answer: Before using the new motor, do insulation inspection, check whether there are mechanical cracks in the shell and whether the bearings should be oiled. The new motor depends on what kind of motor, power, speed and even torque, whether it is connected to 220V power supply or 380V power supply, star or triangle. Generally, a new triangular motor with a power of more than 4.5KW is selected to measure the insulation between three phases and the insulation of each relative shell. Mechanically, the rotor rotates flexibly, the bearing has no abnormal noise, the end cover screws are fastened, and the end cover, base and foundation have no cracks.

Interview question 3. How many cable specifications can you name (as listed in; 0.5mm square,1.0mm square,1.5mm square ——)

Answer: The cables commonly used in factories are all below 180 square meters. Check how many kinds there are.

Interview question 4. When the inverter is running with motor, use Fluke digital multimeter to measure the frequency of the inverter outlet terminal. Why do you sometimes measure the frequency of several thousand hertz?

Answer: It is impossible for multimeter to measure the voltage and frequency of inverter. Fluke digital multimeter is special. Never used it? Because the inverter frequency modulation mode V/F is different, and the multimeter is calculated according to a certain frequency, so the general multimeter can not reflect the normal reading. The multimeter can definitely measure the voltage, and the frequency of the inverter is directly proportional to the voltage.

Interview question 5. What is the function of the braking resistor of the frequency converter?

Answer: The braking resistor of the inverter is used to consume the regenerative voltage generated by the motor and will not damage the IGBT module of the inverter. Only when the motor starts frequently or the heavy object falls, the frequency converter can be equipped with a braking resistor.

Interview question 6. The motor needs to be started frequently. Which frequency converter (inductor or capacitor) should I use?

Answer: Choose capacitive inverter.

Interview question 7. A motor runs with a gearbox, and the gearbox is 10: 1. The gearbox is broken. Can you change it to a frequency converter?

Answer: No, because the ratio is the same and the output power is different, there are still many problems.