1. The no-load current of a single-phase transformer is out of phase with the main magnetic flux, with a phase angle difference αFe, due to the presence of iron loss current. The no-load current has a peaked waveform because it contains a significant third harmonic.
2. The armature winding of a DC motor also carries alternating current. However, the field winding carries direct current. DC motors can be excited in various ways, including separately excited, shunt excited, series excited, and compound excited.
3. The back electromotive force expression for a DC motor is E=CEFn; while the electromagnetic torque expression is Tem=CTFI.
4. The number of parallel branches in a DC motor is always in pairs. However, the number of parallel branches in an AC winding is not necessarily the same.
5. In a DC motor, the components of a single-lap winding are connected in series, with one component stacked on top of another. Whether it is a single-wave winding or a single-lap winding, the commutator segments connect all the components in series to form a single closed loop.
6. An asynchronous motor is also called an induction motor because the rotor current of an asynchronous motor is generated through electromagnetic induction.
7. When an asynchronous motor is started with reduced voltage, the starting torque decreases, and the starting torque decreases proportionally to the square of the starting current of the winding.
8. When the amplitude and frequency of the primary voltage remain unchanged, the saturation degree of the transformer core remains basically unchanged, and the excitation reactance also remains basically unchanged.
9. The short-circuit characteristic of a synchronous generator is a straight line. When the three-phase symmetrical short circuit occurs, the magnetic circuit is unsaturated. When the three-phase symmetrical steady-state short circuit occurs, the short-circuit circuit is a pure demagnetizing direct-axis component.
10. The current in the excitation winding of a synchronous motor is direct current. The main excitation methods include excitation generator excitation, static rectifier excitation, and rotating rectifier excitation.
11. There are no even harmonics in the three-phase composite magnetomotive force; when a symmetrical three-phase winding carries a symmetrical three-phase current, there are no magnetic harmonics that are multiples of 3 in its composite magnetomotive force.
12. Three-phase transformers generally require one side to be delta-connected or one side to have its midpoint grounded. This is because the winding connections of a three-phase transformer should ideally have a path for the third harmonic current.
13. When a symmetrical three-phase winding carries a symmetrical three-phase current, the 5th harmonic in its composite magnetomotive force is reversed; the 7th harmonic is forward-rotating.
14. Series-wound DC motors have relatively soft mechanical characteristics. Separately excited DC motors have relatively stiff mechanical characteristics.
15. A transformer short-circuit test can measure the leakage impedance of the transformer windings; while a no-load test can measure the excitation impedance parameters of the windings.
16. The turns ratio of a transformer is equal to the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. The turns ratio of a single-phase transformer can also be expressed as the ratio of the rated voltages of the primary and secondary windings.
17. Under normal excitation, the power factor of a synchronous generator is equal to 1. When the output active power is kept constant and the excitation current is less than that under normal excitation (underexcitation), the direct-axis armature reaction is magnetizing. When the output active power is kept constant and the excitation current is greater than that under normal excitation (overexcitation), the direct-axis armature reaction is demagnetizing.
18. In a DC motor, iron loss mainly exists in the rotor core (armature core) because the magnetic field of the stator core remains basically unchanged.
19. In a DC motor, the first pitch y1 is equal to the difference in the number of slots between the first and second sides of the element. The combined pitch y is equal to the difference in the number of slots between the upper sides of two elements connected in series.
20. In a DC motor, when saturation is not considered, the characteristic of quadrature-axis armature reaction is that it shifts the position where the magnetic field is zero, but the magnetic flux per pole remains unchanged. When the brushes are located on the geometric neutral line, the armature reaction is of a cross-magnetic nature.
21. In a DC motor, the component that converts external direct current into internal alternating current is the commutator. The function of the commutator is to convert direct current into alternating current (or vice versa).
22. In a synchronous motor, when the excitation flux F0 linked by the stator winding is at its maximum value, the back electromotive force E0 reaches its minimum value; when F0 reaches zero, E0 reaches its maximum value. The phase relationship between F0 and E0 is that F0 leads E0 by 90°. The expression for the relationship between E0 and F0 is: E0 = 4.44fNkN1F0.
23. In an electric motor, leakage flux refers to the flux that links only the winding itself, and the back electromotive force it generates can often be equivalent to a leakage reactance voltage drop (or negative reactance voltage drop).
24. The rotors of asynchronous motors include: - squirrel cage type and wound-rotor type.
25. The slip s of an asynchronous motor is defined as the ratio of the difference between the synchronous speed and the rotor speed to the synchronous speed. When an asynchronous motor operates in motor mode, its slip s ranges from 1 to 0.
26. The relationship between the electromagnetic torque Tem and slip s of an asynchronous motor: The Tem-s curve has three key points: the starting point (s=1), the maximum electromagnetic torque point (s=sm), and the synchronization point (s=0). When the rotor resistance of the asynchronous motor changes, the characteristics of its maximum electromagnetic torque Tem and slip sm are: the magnitude remains unchanged, but the position of s changes.
27. Asynchronous motors must absorb the reactive power with lagging characteristics from the power grid for excitation.
28. When an alternating current is applied to a coil group, its magnetomotive force exhibits pulsating characteristics as it changes over time. Similarly, when an alternating current is applied to a single coil, its magnetomotive force also exhibits pulsating characteristics as it changes over time.
29. When a synchronous generator is connected to the grid, its three-phase terminal voltages must have the same characteristics as the grid's three-phase voltages: frequency, amplitude, waveform, phase sequence (and phase), etc.
30. Synchronous motors have two types of rotors: salient pole and non-salient pole.
31. The equivalent number of phases of a squirrel-cage rotor is equal to the number of its slots, and the equivalent number of turns per phase is 1/2.
32. A three-phase symmetrical AC winding carries a symmetrical three-phase AC current. Its fundamental wave composite magnetomotive force is a circular rotating magnetomotive force. The direction of rotation is from the axis of the leading phase winding to the axis of the lagging phase winding, and then to the axis of the next lagging phase winding.
33. There are two connection methods between the three-phase windings of a three-phase transformer: star and delta; and two structures for the magnetic circuit: group type and core type.
34. The six odd-numbered connection groups of a three-phase transformer are 1, 3, 5, 7, 9, and 11. The six even-numbered connection groups are 0, 2, 4, 6, 8, and 10.
35. In AC windings, the number of slots per pole per phase is q = q = Z/2p/m (assuming the number of slots is Z, the number of pole pairs is p, and the number of phases is m). AC windings can use either 120° phase bands or 60° phase bands. The 60° phase band has a higher fundamental winding coefficient and back electromotive force.
36. The symmetrical component method can be used to analyze the asymmetrical operation of transformers and synchronous motors. Its application is based on the premise that the system is linear. Therefore, the superposition principle can be applied to decompose the asymmetrical three-phase electrical system into three sets of symmetrical three-phase systems: positive sequence, negative sequence, and zero sequence.
37. The formula for calculating the short-pitch coefficient is ky1=sin(p/2×y1/t), which means that the short pitch causes a discount (or reduction) in the back electromotive force (or magnetomotive force) compared to the full pitch. The formula for calculating the distribution coefficient is kq1=sin(qa1/2)/q/sin(a1/2), which means that when q coils are successively separated by an electrical angle a1, the back electromotive force (or magnetomotive force) is relatively concentrated, resulting in a reduction (or discount).
38. Current transformers are used to measure current, and their secondary side must not be open-circuited. Voltage transformers, on the other hand, are used to measure voltage, and their secondary side must not be short-circuited.
39. An electric motor is a device that converts mechanical energy into electrical energy (or vice versa), or changes one AC voltage level to another. From the perspective of energy conversion, electric motors can be divided into three categories: transformers, electric motors, and generators.
40. The formula for calculating the electrical angle a1 of the slot pitch is a1 = p × 360° / Z. It can be seen that the electrical angle a1 of the slot pitch is equal to p times the mechanical angle am of the slot pitch.
41. The principle of transformer winding reduction is to ensure that the magnetomotive force of the winding remains unchanged before and after reduction, and to ensure that the active and reactive power of the winding remains unchanged.
42. The characteristic of the efficiency characteristic curve of a transformer is that there is a maximum value, that is, the maximum value is reached when the variable loss equals the constant loss.
43. The no-load test of a transformer is usually performed by applying voltage and taking measurements on the low-voltage side. The short-circuit test of a transformer is usually performed by applying voltage and taking measurements on the high-voltage side.
44. When transformers are connected in parallel, the conditions for no-load circulating current are: the turns ratio is the same and the connection group number is the same.
45. When transformers are connected in parallel, the load distribution principle is that the per-unit value of the transformer load current is inversely proportional to the per-unit value of the short-circuit impedance. The condition for fully utilizing the capacity of transformers during parallel operation is that the per-unit values of the short-circuit impedances must be equal, and their impedance angles must also be equal.
