79 Key Professional Knowledge Areas in Electrical Engineering （51-60）

By clyde March 8th, 2025 16 views

51.Why are the low-voltage windings of a transformer placed inside, while the high-voltage windings are outside?

The arrangement of windings in a transformer is determined by multiple factors, but for most transformers, the low-voltage winding is placed inside the high-voltage winding, primarily for insulation considerations. Theoretically, regardless of whether the high-voltage or low-voltage winding is placed inside or outside, the transformer can still function to change voltage. However, since the transformer core is grounded, placing the low-voltage winding closer to the core makes it easier to meet insulation requirements. If the high-voltage winding were placed near the core, due to its high voltage, achieving the required insulation would require extensive use of insulating materials and a larger insulation distance, increasing the winding volume and wasting insulating material. Additionally, voltage regulation is achieved by adjusting taps on the high-voltage winding, which is easier to implement when the high-voltage winding is placed outside the low-voltage winding.

52.How does a three-phase induction motor work?

When three-phase AC current flows through the three-phase stator windings, a rotating magnetic field is generated inside the stator cavity. Before rotation begins, the stationary rotor conductors experience relative motion with respect to the rotating magnetic field, effectively cutting the magnetic flux lines. This induces a current in the rotor conductors (based on the principle of electromagnetic induction). These induced currents interact with the magnetic field, generating electromagnetic forces that cause the rotor to move. Since the rotor conductors are symmetrically arranged, the forces acting on them are opposite in direction, creating an electromagnetic torque that causes the rotor to rotate. Because the rotor current is induced by the rotating magnetic field of the stator, this type of motor is called an induction motor. Furthermore, since the rotor speed always lags behind the synchronous speed of the stator's rotating magnetic field, it is also referred to as an asynchronous motor.

53.Why can't a transformer step down DC voltage?

A transformer can change voltage only under conditions where alternating current generates alternating flux in the primary winding, which induces a voltage in the secondary winding. The magnitude of the induced voltage is proportional to the rate of change of the flux. When DC current is applied, the current magnitude and direction remain constant, so no alternating flux is produced in the core. With constant flux and zero flux change rate, the entire DC voltage drops across the very low-resistance windings, causing extremely high currents and nearly short-circuiting the transformer. Alternating current, being time-varying, produces alternating flux in the core when applied to the primary winding. If the number of turns in the secondary winding exceeds that of the primary, the voltage increases; if fewer, it decreases. Since DC has a constant magnitude and direction, applying steady DC to the primary winding produces constant flux in the core, which cannot induce a voltage in the secondary winding, making voltage transformation impossible.

54.What are the transmission methods between electric motors and mechanical systems?

① Direct coupling via a flexible coupling;
② Belt drive;
③ Gear drive;
④ Worm gear drive;
⑤ Chain drive;
⑥ Friction wheel drive.

55.What routine inspections should be performed on operating transformers?

① Check if the sound is normal;
② Inspect for oil leakage or seepage, check the color and level of the oil;
③ Ensure current and temperature do not exceed allowable values;
④ Verify that bushings are clean and free from cracks, damage, or discharge marks;
⑤ Confirm proper grounding of the transformer.

Methods for drying a transformer:
① Induction heating method;
② Hot air drying method;
③ Oven drying method.

56.How is a no-load test performed on an electric motor?

Before the test, inspect the motor for any issues. Then, connect it to a three-phase power supply and allow it to run unloaded. During operation, check the sound, bearing performance, and three-phase power supply. Typically, the no-load current of large-capacity, high-speed motors is 20-35% of the rated current, while for small-capacity, low-speed motors, it is 35-50% of the rated current. The no-load current should neither be too high nor too low and must be balanced among the three phases. The no-load test should last at least one hour, during which the motor temperature rise should be measured. The temperature rise should not exceed the limits specified by the insulation class.

57.How is a short-circuit test performed on an electric motor?

In a short-circuit test, braking equipment is used to fix the rotor so it does not rotate. A three-phase autotransformer is used to gradually increase the output voltage from zero. When the current reaches the motor's rated current, stop increasing the voltage. The voltage at this point is called the short-circuit voltage. For a 380V-rated motor, the short-circuit voltage typically ranges from 75-90V. Excessively high short-circuit voltage indicates excessive leakage reactance, while excessively low voltage indicates insufficient leakage reactance. Both scenarios are detrimental to normal motor operation.

58.What are the main tasks involved in a major transformer overhaul?

① Remove the core and inspect it (core, windings, tap changers, and leads);
② Inspect the top cover, oil reservoir, safety vent, heat pipes, oil valves, and bushings;
③ Maintain the cooling system and oil filtration system;
④ Filter or replace the oil, and perform drying treatment if necessary;
⑤ Inspect control and measurement instruments, signaling devices, and protective devices;
⑥ Clean the exterior and repaint if necessary;
⑦ Reassemble the transformer and perform specified measurements and tests.

59.What are the advantages of wound-rotor induction motors compared to squirrel-cage induction motors?

Wound-rotor induction motors have the advantage of allowing external resistances to be inserted into the rotor circuit via slip rings and brushes. This improves starting performance and allows speed adjustment within a certain range by changing the external resistance. However, wound-rotor motors are more complex in structure, more expensive, and less reliable in operation compared to squirrel-cage induction motors.

60.If excessive vibration occurs during motor testing after installation, what could be the causes?

① Rotor imbalance was not properly corrected;
② Rotor balancing weights have loosened;
③ The rotor shaft is bent or deformed;
① The coupling alignment was not properly adjusted;
⑤ Mounting bolts are loose;
⑥ The installation base is uneven or unstable.

79 Key Professional Knowledge Areas in Electrical Engineering （41-50）

79 Key Professional Knowledge Areas in Electrical Engineering （61-70）