18, in DC motor, iron loss mainly exists in the rotor core (armature core), because the stator core magnetic field is basically unchanged.
19. In a DC motor, the first pitch y1 is equal to the number of slots that differ between the first side and the second side of the element. The composite pitch y is equal to the number of slots that are different between the upper element sides of two elements connected in series.
20. In DC motor, when saturation is not considered, the characteristic of quadrature-axis armature reaction is to shift the position where the magnetic field is zero, but the magnetic flux of each pole remains unchanged. When the brush is located on the geometric neutral line, the armature reaction is cross-magnetic.
21. In a DC motor, the component that converts external DC into internal AC is a commutator. The function of commutator is to convert DC into AC (or vice versa).
22. In a synchronous motor, when the excitation flux F0 of the stator winding is the maximum, the back electromotive force E0 reaches the minimum, and when F0 reaches zero, E0 reaches the maximum. The phase relationship between F0 and E0 is that F0 leads E0 90o. And the expression of the relationship between E0 and F0 is: E0=4.44 f N kN1F0.
23, in the motor, magnetic leakage refers to only the magnetic flux of the chain winding itself, and the back electromotive force generated by it can often be equivalent to a leakage voltage drop (or negative voltage drop).
24. The rotors of asynchronous motors are:-squirrel-cage type and winding type.
25. The slip rate S of asynchronous motor is defined as the ratio of the difference between synchronous speed and rotor speed to synchronous speed. When the asynchronous motor works in the motor state, its slip ratio s ranges from 1>s>0.
26. Relationship between electromagnetic torque Tem of asynchronous motor and slip rate S The Tem-s curve has three key points, namely starting point (s=1), maximum electromagnetic torque point (s=sm) and synchronization point (s=0). When the rotor resistance of asynchronous motor changes, the magnitude of the maximum electromagnetic torque Tem and the slip rate sm are characterized by the same magnitude and the change of S position.
27. The asynchronous motor must absorb the lagging reactive power from the power grid for excitation.
28. A coil group is energized with alternating current, and its magnetomotive force changes with time and has the property of pulse vibration. When a single coil is energized with alternating current, its magnetomotive force changes with time and also has the property of pulse vibration.
29. When the synchronous generator is connected to the grid, its three-phase terminal voltage is required to be the same as the three-phase voltage of the grid: frequency, amplitude, waveform, phase sequence (and phase), etc.
30, synchronous motor rotor has hidden pole type and salient pole type.
31. The equivalent number of phases of squirrel cage rotor is equal to the number of slots, and the equivalent number of turns of each phase is 1/2.
32. Three-phase symmetrical AC winding is connected with symmetrical three-phase AC current, and its fundamental synthetic magnetomotive force is a circular rotating magnetomotive force, and its rotating direction 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 connection methods between three-phase windings of three-phase transformer, such as star and triangle. The magnetic circuit has two structures: group structure and heart structure.
34, three-phase transformer six odd connection group number is 1, 3, 5, 7, 9, 11. And the numbers of the six even-numbered connection groups are 0, 2, 4, 6, 8 and 10.
35. In AC winding, the number of slots per pole and phase is q = q = z/2p/m (assuming that the number of slots is z, the number of pole pairs is p and the number of phases is m). In the AC winding, there are both 120o phase band and 60o phase band. Among them, the fundamental winding coefficient and back electromotive force of 60o phase band are higher.
36, symmetrical component method can be used to analyze the asymmetric operation of transformer and synchronous motor, and its application premise is that the system is linear, so the superposition principle can be applied to decompose the asymmetric three-phase electric power system into three groups of symmetrical three-phase systems: positive sequence, negative sequence and zero sequence.
37. The calculation formula of short-distance coefficient is ky1=sin(p/2×y1/t), and its physical meaning is the discount (or reduced coefficient) of back emf (or magnetomotive force) caused by short-distance compared with the whole distance. The formula for calculating the distribution coefficient is kq1=sin(qa1 /2)/q/sin(a1/2), and its physical meaning is the reduced coefficient (or discount) when the back electromotive force (or magnetomotive force) is relatively concentrated when the Q coils differ by A1 electrical angle in turn.
38, current transformer is used to measure current, its secondary side can’t open. The voltage transformer is used to measure the voltage, and its secondary side cannot be short-circuited.
39. A machine 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, motors can be divided into three categories: transformers, motors and generators.
40. The formula for calculating the electric angle a1 between slots is a1 = p× 360o/z.. It can be seen that the electrical angle a1 of the slot spacing is equal to p times of the mechanical angle am of the slot spacing.
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 characteristic curve of transformer efficiency is that there is a maximum value, that is, when the variable loss equals the constant loss, it reaches the maximum value.
43, no-load test of transformer is usually in the low voltage side voltage and measurement. The short-circuit test of transformer usually applies voltage on the high-voltage side and measures it.
44. When transformers run in parallel, the conditions of no-load and no circulation are: the same transformation ratio and the same connection group number.
45. When transformers run in parallel, the principle of load distribution is that the nominal value of transformer load current is inversely proportional to the nominal value of short-circuit impedance. The condition that the capacity of transformers can be fully utilized in parallel operation is that the nominal value of short-circuit impedance should be equal and their impedance angles should be equal.