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عربى1. When the single-phase transformer is no-load, the current and the main magnetic flux are in different phases, and there is a phase angle difference because there is an iron consumption current. The no-load current is a peak waveform because there is a large third harmonic in it.
2. AC current flows in the armature winding of a DC motor. But the DC current flows in its excitation winding. The excitation modes of DC motors include separate excitation, shunt excitation, series excitation, compound excitation, etc.
3. The expression of the back electromotive force of the DC motor is E=CEFn, and the expression of electromagnetic torque is Tem=CTFI.
4. The number of parallel branches of DC motors is always in pairs. The number of parallel branches of the AC winding is not certain.
5. In a DC motor, the components of a single stack winding are stacked one on top of the other and are connected in series. Whether it is a single-wave winding or a single-stack winding, the commutator connects all 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 by electromagnetic induction.
7. When the asynchronous motor is started with reduced voltage, the starting torque decreases, and the starting torque decreases in 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 core of the transformer remains unchanged, and the excitation reactance also remains unchanged.
9. The short-circuit characteristic of the 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 direct-axis component of pure demagnetization.
10. The current in the excitation winding of the synchronous motor is DC 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 synthetic magnetomotive force; symmetrical three-phase windings pass symmetrical three-phase currents, and there are no multiples of 3 magnetic harmonics in the synthetic magnetomotive force.
12. It is generally expected that one side of a three-phase transformer has a delta connection or that the midpoint of one side is grounded. Because the winding connections of three-phase transformers hope to have a path for a third harmonic current.
13. When a symmetrical three-phase winding passes a symmetrical three-phase current, the 5th harmonic in the resultant magnetomotive force is reversed; the 7th harmonic is forwardly rotated.
14. The mechanical characteristics of series DC motors are relatively soft. The mechanical characteristics of separately excited DC motors 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 the transformer is equal to the turn ratio of the primary winding to 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. During normal excitation, the power factor of the synchronous generator is equal to 1; keep the output active power unchanged and make the excitation current smaller than normal excitation (under excitation), then the nature of the direct-axis armature reaction is magnetizing; keep the output active power without When the excitation current changes and the excitation current is larger than the normal excitation (overexcitation), the nature of the direct-axis armature reaction is demagnetization.
18. In DC motors, 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 pitch y1 is equal to the number of slots between one side of the component sequence and the second side of the sequence. The resulting pitch y is equal to the number of grooves between the upper part sides of two parts connected in series.
20. In a DC motor, when saturation is not considered, the characteristic of the quadrature armature reaction is that the position where the magnetic field is zero is shifted, 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 power into internal AC power is the commutator. The purpose of a commutator is to convert DC to AC (or vice versa).
22. In a synchronous motor, when the excitation flux F0 interlinked by the stator winding is a large value, the back electromotive force E0 reaches a small value. When F0 reaches zero, E0 reaches a large value. The phase relationship between F0 and E0 is F0 over E090o. The relationship between E0 and F0 is E0=4.44fN·kN1F0.
23. In motors, leakage flux refers to the magnetic flux that only cross-links the winding itself. The counter-electromotive force generated by it can often be equivalent to a leakage resistance voltage drop (or negative resistance voltage drop).
24. There are two types of rotors for asynchronous motors: - squirrel cage type and wound type.
25. The slip ratio of an asynchronous motor is defined as the ratio of the difference between the synchronous speed and the rotor speed and the synchronous speed. When the asynchronous motor works in the motor state, the range of its slip s is 1>s>0.
26. The relationship between the electromagnetic torque Tem and slip rate of the asynchronous motor. The Tem-s curve has three key points, namely the starting point (s=1), the electromagnetic torque point (s=sm), and the synchronization point (s=0). When the rotor resistance of an asynchronous motor changes, the characteristics of its electromagnetic torque Tem and slip rate sm are: the magnitude remains unchanged, but the position of s changes.
27. The asynchronous motor must absorb hysteretic reactive power from the power grid for excitation.
28. When a coil group is supplied with alternating current, its magnetomotive force changes with time in a pulsating nature. A single coil is supplied with alternating current, and its magnetomotive force changes with time and also has pulsating properties.
29. When a 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. There are two types of rotors of synchronous motors: hidden pole type and salient pole type.
31. The equivalent number of phases of the 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 flows through symmetrical three-phase AC current. Its fundamental wave synthetic magnetomotive force is a circular rotation magnetomotive force. The direction of rotation is from the forward phase winding axis to the lagging phase axis and then to the downward phase axis. The axis of the lagging phase.
33. There are two connection methods between the three-phase windings of a three-phase transformer: star type and delta type; the magnetic circuit has two structures: group type and core type.
34. The six odd-numbered connection group numbers of the three-phase transformer are 1, 3, 5, 7, 9, and 11. The six even-numbered connection group numbers are 0, 2, 4, 6, 8, and 10.
35. In the 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 AC windings, there are those that use a 120o phase belt and some that use a 60o phase belt. Among them, the fundamental winding coefficient and back electromotive force of the 60-phase zone are relatively high.
36. The symmetric component method can be used to analyze the asymmetric 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 asymmetric three-phase power system into positive sequence, negative sequence, and Three groups of symmetrical three-phase systems such as zero sequence.
37. The calculation formula of the short distance coefficient is ky1= sin(p/2×y1/t). Its physical meaning is the discount (or reduction) of the back electromotive force (or magnetomotive force) caused by the short distance compared with the whole distance. coefficient). The calculation formula of the distribution coefficient is kq1= sin(qa1/2) /q/ sin(a1/2). Its physical meaning is that when q coils are separated by an electrical angle of a1, the back electromotive force (or magnetomotive force) is relatively concentrated. The coefficient is reduced (or discounted) by the situation.
38. The current transformer is used to measure current, and its secondary side cannot be open-circuit. 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 AC voltage level. From the perspective of energy conversion, motors can be divided into three categories: transformers, motors, and generators.
40. The calculation formula of the electrical angle a1 from the slot is a1= p×360o/Z. It can be seen that the electrical angle a1 of the slot distance is equal to p times the mechanical angle am of the slot distance.
41. The principle of transformer winding calculation is to ensure that the magnetomotive force of the winding remains unchanged before and after calculation and that the active and reactive power of the winding remains unchanged.
42. The transformer efficiency characteristic curve is characterized by a high value, which reaches a low value when the variable loss is equal to the constant loss.
43. The no-load test of the transformer usually applies voltage and measurements on the low-voltage side. Short-circuit tests of transformers usually apply voltage and make measurements on the high-voltage side.
44. When the transformers are running in parallel, the conditions for no-load circulating current are the same transformation ratio and the same connection group number.
45. When transformers are operated 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 conditions for the capacity of the transformer to be fully utilized during parallel operation are: the unit values of the short-circuit impedances must be equal, and their impedance angles must also be equal.
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