1. The current of a single-phase transformer is out of phase with the main magnetic flux when it is no-load, and there is a phase angle difference aFe because of the iron loss current. The no-load current is a peak waveform because it contains a large third harmonic.

2. The armature winding of a DC motor also flows with AC current. However, the field winding of a DC motor flows with DC current. The excitation methods of a DC motor include separate excitation, parallel excitation, series excitation, and compound excitation.

3. The back electromotive force expression of a DC motor is E = CE F n; and the electromagnetic torque expression is Tem = CTFI.

4. The number of parallel branches of a DC motor is always in pairs, but the number of parallel branches of an AC winding is not necessarily the same.

5. In a DC motor, the components of a single-layer winding are connected in series one on top of the other. Whether it is a single-wave winding or a single-layer winding, the commutator connects all the components in series to form a single closed loop.

6. Asynchronous motors are also called induction motors because the rotor current of asynchronous motors is generated by electromagnetic induction.

7. When the asynchronous motor is started at reduced voltage, the starting torque decreases, and the starting torque decreases in direct proportion to the square of the starting current of the winding.

8. When the amplitude and frequency of the primary side 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 short circuit occurs symmetrically, the magnetic circuit is unsaturated . When the three-phase short circuit occurs symmetrically in steady-state, the short-circuit circuit is a pure demagnetizing direct-axis component.

10. The current in the excitation winding of the synchronous motor is direct current. The main excitation methods include excitation generator excitation, static rectifier excitation, rotating rectifier excitation, etc.

11. There are no even harmonics in the three-phase composite magnetomotive force; when a symmetrical three-phase winding is connected to 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 want one side to be connected in a triangle or have one side midpoint grounded, because the winding connection of the three-phase transformer wants to have a path for the third harmonic current.

13. When a symmetrical three-phase winding is passed through a symmetrical three-phase current, the fifth harmonic in its synthetic magnetomotive force is reversed, and the seventh harmonic is forward.

14. The mechanical characteristics of a series-excited DC motor are relatively soft. The mechanical characteristics of a separately-excited DC motor are relatively hard.

15. The transformer short-circuit test can measure the leakage impedance of the transformer winding; while the no-load test can measure the excitation impedance parameters of the winding.

16. The transformation ratio of a transformer is equal to the ratio of the number of turns of the primary winding to the number of turns of the secondary winding. The transformation ratio of a single-phase transformer can also be expressed as the ratio of the rated voltages of the primary and secondary sides.

17. Under normal excitation, the power factor of the synchronous generator is equal to 1; when the output active power is kept unchanged and the excitation current is less than normal excitation (under-excitation), the nature of the direct-axis armature reaction is magnetizing; when the output active power is kept unchanged and the excitation current is greater than normal excitation (over-excitation), the nature of 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 number of slots between the first and second sides of the element. The composite pitch y is equal to the number of slots between the upper element sides of the two elements connected in series.

20. In a DC motor, when saturation is not considered, the characteristic of the quadrature-axis armature reaction is that the position where the magnetic field is zero is offset, but the flux per pole remains unchanged. When the brush is located on the geometric neutral line, the armature reaction is of a quadrature nature.

21. In a DC motor, the component that converts external DC power into internal AC power is the commutator. The function of the commutator is to convert DC into AC (or vice versa).

22. In a synchronous motor, when the stator winding interlinked excitation flux F0 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 90o. The relationship between E0 and F0 is expressed as: E0 = 4.44 f N kN1F0.

23. In a motor, leakage flux refers to the magnetic flux that only links the winding itself. 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 are of two types: squirrel cage type and winding type.

25. The slip rate 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 the asynchronous motor works in the motor state, the range of its slip rate s is 1>s>0.

26. The relationship between the electromagnetic torque Tem and the slip rate s of the asynchronous motor. The Tem-s curve has three key points, namely the starting point (s = 1), the maximum electromagnetic torque point (s = sm), and the synchronous point (s = 0). When the rotor resistance of the asynchronous motor changes, the size of its maximum electromagnetic torque Tem and the slip rate sm are characterized by: the size remains unchanged, and the position of s changes.

27. Asynchronous motors must absorb lagging reactive power from the power grid for excitation.

28. When an alternating current is passed through a coil group, its magnetomotive force changes with time and has a pulsating nature. When an alternating current is passed through a single coil, its magnetomotive force also changes with time and has a pulsating nature.

29. When a synchronous generator is connected to the grid, its three-phase terminal voltage is required to have the same frequency, amplitude, waveform, phase sequence (and phase), etc. as the three-phase voltage of the grid.

30. The rotors of synchronous motors are of two types: hidden pole type and salient pole type.

31. The equivalent number of phases of a squirrel cage rotor is equal to its number of slots, and the equivalent number of turns per phase is 1/2.

32. A three-phase symmetrical AC winding passes a symmetrical three-phase AC current, and its fundamental wave synthetic magnetomotive force is a circular rotating magnetomotive force, and its direction of rotation is from the axis of the leading phase winding to the axis of the lagging phase, and then to the axis of the next lagging phase.

33. There are two ways to connect the three-phase windings of a three-phase transformer: star and triangle; there are two structures of 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). In AC windings, there are both 120o phase belts and 60o phase belts. Among them, the fundamental wave winding coefficient and back electromotive force of the 60o phase belt are higher.

36. The symmetrical component method can be used to analyze the asymmetrical operation of transformers and synchronous motors. The premise of its application is that the system is linear. Therefore, the superposition principle can be applied to decompose the asymmetrical three-phase electrical quantity system into three groups of symmetrical three-phase systems, namely positive sequence, negative sequence, and zero sequence.

37. The calculation formula of the short-distance coefficient is ky1 = sin(p/2×y1/t), and its physical meaning is the discount (or reduction coefficient) of the back electromotive force (or magnetomotive force) caused by the short distance compared with the full distance. The calculation formula of the distribution coefficient is kq1 = sin(qa1/2)/q/sin(a1/2), and its physical meaning is the reduction coefficient (or discount) of the relative concentration of the back electromotive force (or magnetomotive force) when q coils are separated by an electrical angle of a1 in sequence.

38. The current transformer is used to measure current, and its secondary side cannot be open-circuited. The voltage transformer is used to measure voltage, and its secondary side cannot be short-circuited.

39. A motor is a device that converts mechanical energy into electrical energy (or vice versa), or changes one AC voltage level into another. From the perspective of energy conversion, motors can be divided into three categories: transformers, motors, and generators.

40. The calculation formula of the slot electrical angle a1 is a1 = p×360o/Z. It can be seen that the slot electrical angle a1 is equal to p times the slot mechanical angle am.

41. The principle of transformer winding reduction is: before and after the reduction, ensure that the magnetomotive force of the winding remains unchanged, and ensure that the active and reactive power of the winding remain unchanged.

42. The efficiency characteristic curve of the transformer is characterized by the existence of a maximum value, which is reached when the variable loss is equal to the constant loss.

43. The no-load test of the transformer is usually carried out by applying voltage and measuring on the low voltage side. The short-circuit test of the transformer is usually carried out by applying voltage and measuring on the high voltage side.

44. When transformers operate in parallel, the condition for no-load circulating current is: the same transformation ratio and the same connection group number.

45. When transformers are operated in parallel, the load distribution principle is: 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 the transformer capacity to be fully utilized 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.