Common sense that BLDC electrical engineers must master

Many engineers who have just contacted BLDC have some doubts about it. Today, I have sorted out 44 common sense of BLDC for everyone to learn. I hope the sorting of my editor can help you. If you have other problems related to electronic technology, you can leave a message at the end of the article.

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

    2. AC current also flows in armature winding of DC motor. But the current in its excitation winding is DC current. The excitation methods of DC motor include separate excitation, parallel excitation, series excitation, compound excitation, etc.

    3. The back EMF expression of DC motor is E=CE F n; The electromagnetic torque expression is Tem=CT FI.

    4. The number of parallel branches of DC motor is always in pairs. However, the number of parallel branches of AC winding is not certain.

    5. In the DC motor, the elements of the single winding are connected in series in a way that one is stacked on the other. 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. Asynchronous motor is also called induction motor, because the rotor current of 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 direct proportion to the square of the starting current of the winding.

    8. When the amplitude and frequency of the primary side voltage are unchanged, the saturation degree of the transformer core is basically unchanged, and the excitation reactance is also basically unchanged.

    9. The short circuit characteristic of synchronous generator is a straight line, and the magnetic circuit is unsaturated when three-phase symmetrical short circuit occurs; When three-phase symmetrical steady state short circuit occurs, the short circuit circuit is the direct axis component of pure demagnetization.

    10. The current in the excitation winding of synchronous motor is DC current, and the excitation mode mainly includes excitation of excitation generator, static rectifier and rotating rectifier.

    11. There is no even harmonic in the three-phase composite magnetomotive force; Symmetrical three-phase winding is connected with symmetrical three-phase current, and there is no magnetic harmonic multiple of 3 in its resultant magnetomotive force.

    12. Generally, three-phase transformers are expected to have triangle connection on one side or neutral grounding on one side. Because the winding connection of three-phase transformer is expected to have a third harmonic current path.

    13. When symmetrical three-phase winding is connected with symmetrical three-phase current, the 5th harmonic of its resultant magnetomotive force is reversed; The 7th harmonic is in forward rotation.

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

    15. The transformer short-circuit test can measure the leakage impedance of the transformer winding; 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 side winding and the secondary side winding. The transformation ratio of single-phase transformer can also be expressed as the ratio of rated voltage of primary and secondary sides.

    17. Under normal excitation, the power factor of synchronous generator is equal to 1; If the output active power is kept unchanged and the excitation current is less than the normal excitation (under excitation), the nature of the direct axis armature reaction is magnetic assistance; If the output active power is kept unchanged and the excitation current is greater than the normal excitation (overexcitation), the nature of the direct axis armature reaction is demagnetized.

    18. In DC motors, iron loss mainly exists in the rotor core (armature core), because the magnetic field of the stator core 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 resultant pitch y is equal to the number of slots in the difference between the upper element edges of two elements in series.

    20. In DC motors, 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 is unchanged. When the brush is on the geometric neutral line, the armature reaction is of alternating magnetic nature.

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

    22. In a synchronous machine, when the excitation flux F0 of the stator winding cross link is the maximum value, the back EMF E0 reaches the minimum value. When F0 reaches zero, E0 reaches the maximum value. The phase relationship between F0 and E0 is E0 90 ° ahead of F0?. And the relationship between E0 and F0 is: E0=4.44 f N kN1F0.

    23. In the motor, the magnetic flux leakage refers to the magnetic flux of only the cross link winding itself, and the back electromotive force generated by it can often be equivalent to a leakage voltage drop (or negative voltage voltage drop).

    24. The rotor of asynchronous motor includes squirrel cage type and wound type.

    25. The slip rate s of asynchronous motor is defined as the ratio between the difference between synchronous speed and rotor speed and synchronous speed. When the asynchronous motor works in the motor state, the range of its slip s is 1>s>0.

    26. There are three key points in the relationship between electromagnetic torque Tem and slip s of asynchronous motor Tem-s curve, 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 maximum electromagnetic torque Tem and the slip rate sm are characterized by unchanged size and changing s position.

    27. The asynchronous motor must absorb the reactive power with hysteresis property from the power grid for excitation.

    28. When a coil group is energized with alternating current, its magnetomotive force has pulsating property with time. When a single coil is energized with alternating current, its magnetomotive force also has pulsating property with time.

    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 rotor of synchronous motor: hidden pole type and salient pole type.

    31. The equivalent phase number of squirrel cage rotor is equal to its slot number, and the equivalent turns of each phase is 1/2.

    32. Three phase symmetrical AC winding is connected with symmetrical three-phase AC current, and its fundamental wave resultant magnetomotive force is a circular rotating magnetomotive force. The direction of rotation is from the leading phase winding axis to the lagging phase axis, and then to the axis of the next lagging phase.

    33. There are two connection methods between three-phase windings of three-phase transformer, namely star and triangle; The magnetic circuit can be divided into two types: group type and core type.

    34. The six odd connection groups of three-phase transformer are 1, 3, 5, 7, 9 and 11. The number of six even number connection groups is 0, 2, 4, 6, 8, 10.

    35. In the AC winding, the number of slots per pole and phase 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 the AC winding, the existing 120? For phase belt, 60? Is also used? Phased. 60? The fundamental winding coefficient and back EMF of the phase band are high.

    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 principle of superposition can be applied to decompose the asymmetrical three-phase power system into three groups of symmetrical three-phase systems, including positive sequence, negative sequence and zero sequence.

    37. The calculation formula of short distance coefficient is ky1=sin (p/2 × Y1/t), its physical meaning is the discount (or reduced coefficient) between the back EMF (or magnetomotive force) and the whole range caused by short distance. The calculation formula of distribution coefficient is kq1=sin (qa1/2)/q/sin (a1/2). Its physical meaning is the coefficient (or discount) that is reduced when q coils are in sequence different from a1 electrical angle and the back EMF (or magnetomotive force) is relatively concentrated.

    38. The current transformer is used to measure the current, and its secondary side cannot be open circuited. The voltage transformer is used to measure the 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 to another. From the perspective of energy conversion, motors can be divided into three categories: transformer, motor and generator.

    40. The calculation formula of slot distance electrical angle a1 is a1=p × 360?/ 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 reduction is to ensure that the magnetomotive force of the winding and the active and reactive power of the winding remain unchanged before and after reduction.

    42. The characteristic of the efficiency characteristic curve of the transformer is that there is a maximum value, that is, the maximum value is reached when the variable loss is equal to the constant loss.

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

    44. When the transformers operate in parallel, the conditions for no load circulating current are the same transformation ratio and the same connection group number.