Frequently Asked Questions in Electrician Supervisor Interviews

Frequently Asked Questions for Electrician Supervisor Interviews

Do you know what questions to ask when interviewing electrician supervisors? Let’s take a look at the common electrician supervisor interview questions below!

Frequently Asked Questions for Electrician Supervisor Interviews

1. What are the advantages and disadvantages of belt drive?

Answer: Advantages: smooth transmission, no noise; overload protection; The transmission distance is large; the structure is simple, the maintenance is convenient and the cost is low.

Disadvantages: The transmission ratio cannot be guaranteed; the structure is not compact enough; the service life is short; the transmission efficiency is low; not suitable for high temperature, flammable, and explosive situations.

2. How does a three-phase asynchronous motor rotate?

Answer: When the three-phase alternating current flows into the three-phase stator winding, a first rotation is generated in the stator cavity. magnetic field. Under the action of the rotating magnetic field, the rotor conductor that is stationary before rotation is equivalent to the magnetic field lines that the rotor conductor relatively cuts the magnetic field, thereby generating an induced current in the rotor conductor (the principle of electromagnetic induction). Since the conductors in the rotor are always arranged symmetrically, the electromagnetic force generated on the conductors is in exactly the opposite direction, thus forming an electromagnetic torque and making the rotor move. Since the current in the rotor conductor is generated by the rotating magnetic field of the stator, it is also an induction motor. And because the speed of the rotor is always lower than the speed of the rotating magnetic field of the stator, it is also called an asynchronous motor.

3. What are the transmission methods between motors and machinery?

Answer: ① Copy wheel direct transmission; ② Belt transmission; ③ Gear transmission; ④ Worm transmission; ⑤ Chain transmission ; ⑥ Friction wheel transmission.

4. In a single-phase bridge rectifier current, what will happen if a diode is connected incorrectly in reverse? What will happen if a diode is internally broken?

Answer : In single-phase bridge rectifier current, if a diode is connected incorrectly, it will cause a short circuit in the power supply. If one diode is internally open, single-phase half-wave rectification results.

5. What are the advantages and disadvantages of hydraulic transmission?

Answer: Advantages: It can carry out infinite speed changes; the movement is relatively stable; it is fast, has small impact, and can start, brake and change at high speed. direction; can automatically prevent overload; is easy to operate; has a long service life; is small in size, light in weight and compact in structure.

Disadvantages: easy to leak, no components require high manufacturing precision; low transmission efficiency.

6. What issues should be paid attention to when using a low-voltage test pen?

Answer: The following points should be noted: (1) Before testing, test on a charged object that is confirmed to be charged. To prove whether the electric test pen is good. (2) Insulated shoes should be worn when using. (3) When testing under bright light, pay attention to avoid light and observe. (4) When some equipment is working, the shell will be charged due to induction. Testing with an electric test pen will not give correct results. You must use a multimeter to measure whether it is actually charged or charged by induction. (5) For charged objects with voltages below 36V, electroscope pens are often ineffective.

7. What are the main causes of friction between the stator and the rotor of the motor? What are the consequences of the friction between the stator and the rotor?

Answer: The main reasons for the friction between the stator and the rotor of the motor are: : The bearing is damaged; the rotor sinks due to bearing wear; the rotating shaft is bent and deformed; the base and end cover are cracked; the end cover seam is not tightly closed; the inside of the motor is too dirty, etc. The friction between the stator and the rotor will cause the motor to vibrate and make strong noises, causing the friction surface to generate high temperatures and even smoke and fire, causing the insulation to burn and become brittle and even burn the windings.

8. Briefly describe several electric braking principles of three-phase asynchronous motors?

Answer: (1) Reverse braking: The characteristic is that the motor runs in the electric state, but the electromagnetic rotation The torque is opposite to the direction of motor rotation.

(2) Regenerative braking (feedback braking) When the motor speed is ngt; n1, the direction in which the rotor bar is cut by the rotating magnetic field is exactly opposite to that in the electric state (nlt; N1), so The direction of the induced electromotive force in the rotor conductor changes, and the direction of the rotor current also becomes opposite to that in the electric state. At this time, the direction of the torque generated by the motor is opposite to that in the electric state.

9. What are the causes of nonlinear distortion in AC voltage amplification circuits? What impact does static value have on distortion?

Answer ① Improper acquisition of static value will cause Nonlinear distortion. In addition, temperature changes and other disturbing signals (such as power supply voltage fluctuations, load changes, external electromagnetic phenomena) will cause nonlinear distortion; ② If the static value is not obtained appropriately, after the AC signal is added, the transistor may Entering the saturation state or cut-off state means entering the nonlinear region of the transistor, which will cause nonlinear distortion.

10. The rated voltage of a three-phase asynchronous motor is 380V. When the three-phase power line voltage is 380V, what kind of connection should be used for the stator winding? When the three-phase power line voltage is 660V, what kind of connection should be used? Which connection method should be used?

Answer: When the three-phase power line voltage is 380V, the stator winding should be connected in delta; when the three-phase power line voltage is 660V, the stator winding should be connected in star.

11. What are the common methods for reducing the speed of DC motors?

Answer: Taking shunt motors as an example, there are the following three types.

① Reduce the terminal voltage: This method has the disadvantage of lowering the terminal voltage and causing a decrease in the excitation current, so it is rarely used. ② Increase the excitation current: that is, increase the magnetic field. However, this method is limited by magnetic saturation and has certain limitations. ③ Connect a resistor in parallel in the armature circuit to reduce the armature terminal voltage. This method is easy to implement, so it is generally used.

12. What are the main causes of friction between the stator and rotor of the motor? What are the consequences of the friction between the stator and the rotor?

Answer: The main reasons for the friction between the stator and the rotor of the motor are: : The bearing is damaged; the rotor sinks due to bearing wear; the rotating shaft is bent and deformed; the end cover of the machine base is cracked; the end cover seam is not tightly closed; the inside of the motor is too dirty, etc. The friction between the stator and the rotor will cause the motor to produce strong vibrations and noises, causing the friction surface to generate high temperatures and even smoke and fire, causing the insulation to burn, become brittle, and even burn the windings.

13. Why are closed-circuit shunt circuits often used in multimeter DC current measurement circuits?

Answer: Because in closed shunt circuits, the contact resistance of the transfer switch and the shunt resistance are The resistance value has nothing to do with it. Even if the contact is poor or the range is changed under load, it will not have any adverse effects on the meter head, which can ensure the accuracy of the measurement and the safety of the meter. If an open-circuit shunt circuit is used, the contact resistance will be connected in series with the shunt. The resistance increases the error, and when the contact is not good or the range is changed under load, a large current will directly pass through the meter and burn out the meter.

14. Please explain the difference between magnetic field intensity and magnetic induction intensity?

Answer: The magnetic field intensity is represented by h, and the magnetic induction intensity is represented by B. Both can describe the strength and weakness of the magnetic field. direction, and are all related to the current and distribution of the excitation magnetic field. But H has nothing to do with the magnetic field medium, while B is related to the magnetic field medium. The unit of H is A/m (press/meter), while the unit of B is T (Tesla). H is often used in calculations related to magnetic fields, and B is often used when qualitatively describing magnetic fields.

15. What issues should be paid special attention to during the use and maintenance of DC torque motors?

Answer: The armature current of a DC torque motor must not exceed the peak current during operation to avoid demagnetization of the magnets. , the torque decreases. When taking out the rotor, the stator must use a magnetic short circuit to protect the magnet, otherwise it will also cause the magnet to demagnetize.

16. What are the general regulations for the installation of explosion-proof electrical equipment?

Answer: ① Electrical equipment and circuits, especially electrical equipment that generates sparks during operation, should be installed in explosion-proof areas as much as possible. Outside of hazardous locations, if it needs to be located in a hazardous location, it should be installed in a less dangerous location and effective safety measures should be taken. ② Electrical equipment and circuits within explosion hazardous areas should be protected from mechanical damage and should meet the requirements of environmental conditions such as anti-corrosion, moisture-proof, sun protection, wind and rain protection, and wind and sand protection. ③ Electrical equipment in explosion-risk locations should be equipped with reliable overload and short-circuit protection. ④ In places with explosive hazards, the sockets that must be installed should be installed in places where explosive mixture is not easy to accumulate; local lighting fixtures should be installed in places where gas is not easy to impact in the event of an accident.

17. What are the methods to adjust the average value of the output DC voltage of the chopper?

Answer: The output voltage of the chopper is a rectangular pulse wave voltage. Adjust the average output voltage. There are three methods of value: keep the pulse period T unchanged and change the pulse width τ; keep τ unchanged and change T; change τ and T at the same time

18. Why does the AC contactor overheat when it is operated frequently?

Answer: When the AC contactor is started, due to the large gap between the iron core and the armature and the small reactance, the excitation current that can pass through the coil is very large, often more than ten times greater than the operating current. If it is started frequently, the The excitation coil passes through a large starting current, which causes the coil to overheat, and in severe cases, the coil will be burned.

19. How to correctly dismantle and repair an asynchronous motor?

Answer: Before dismantling and repairing an asynchronous motor, various preparations should be made, such as the tools used and inspection before disassembly. and record work. Steps to disassemble the motor: ① Disassemble the belt or coupling. Mark the belt and coupling before disassembling. Remove rust during installation and clean before resetting; ② Disassemble the end cover: first remove the bearing cover, and then Take the end cover and mark the front and rear covers. Reset according to the marks during installation; ③ Disassemble the rotor: Wear-resistant thick paper should be placed between the stator and the rotor to prevent damage to the fixed value winding. If the winding is heavy, you can use lifting When installing the rotor of the equipment, first check whether there are impurities inside the stator, then install the shaft extension end cover first, and then install the rotor together with the fan and back cover. ;