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45 motor knowledge that electricians need to master

45 motor knowledge that electricians need to master

13:49 source: SiGe education

original title: 45 motor knowledge that electricians need to master

1 The current of single-phase transformer at no-load is different from the main flux, and there is a phase angle difference AFE, because there is iron consumption current. The no-load current is a sharp waveform, because there is a large third harmonic

2 . AC current also flows in the armature winding of DC motor. However, DC current flows in the excitation winding. The excitation modes of DC motor include separate excitation, parallel excitation, series excitation, compound excitation, etc

3 . The expression of back EMF of DC motor is e =ce f n; The expression of electromagnetic torque is TEM =ctfi

4 . The number of parallel branches of DC motor is always in pairs. The number of parallel branches of AC winding is not necessarily

5 . In a DC motor, the components of a single stack of windings are made in series in a way of stacking one on top of the other. Whether it is a single wave winding or a single stack winding, the commutator blade 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 starts with step-down 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 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 the three-phase symmetrical short-circuit occurs; When the three-phase symmetrical steady-state short circuit, the short circuit is a direct axis component of pure demagnetization

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

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

12 . Three phase transformers generally want to have a triangular connection on one side or a neutral grounding on one side. Because the winding connection of three-phase transformer hopes to have the path of the third harmonic current

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

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

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

16 . The transformation ratio of transformer is equal to the turn ratio of primary side winding and secondary side winding. The transformation ratio of single-phase transformer can also be expressed as the ratio of the rated voltage on the primary and secondary sides

17 . During normal excitation, the power factor of synchronous generator is equal to 1; Keep the output active power unchanged, so that the excitation current is less than the normal excitation (under excitation), then the nature of the direct axis armature reaction is magnetizing; When the output active power is kept constant and the excitation current is greater than the normal excitation (overexcitation), the nature of the direct axis armature reaction is demagnetization

18 . In DC motors, the 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 composite pitch y is equal to the number of slots between the upper element edges of the two elements in series

20 . In DC motor, when saturation is not considered, the characteristic of AC 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 on the geometric neutral line, the armature reaction is AC magnetic

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 DC to AC (or vice versa)

22 . In synchronous machines, 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, and the phase relationship between F0 and E0 is F0 leading E0 90o. And the relationship expression between E0 and F0 is: E0 = 4.44 f n kn1f0

23 . In motors, leakage flux refers to the flux of the AC winding itself only, and the back EMF generated by it can often be equivalent to a leakage pressure drop (or negative pressure drop)

24 . The rotors of asynchronous motors are: - squirrel cage type, wound type, etc

25 . Slip ratio 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, the slip ratio S is 1>; s>; 0。

26 . The electromagnetic torque of asynchronous motor TEM facilitates the installation of the tooling at the force point of the stabilizer bar and the relationship between the slip rate s. The tem-s curve has three key points, namely, the starting point (s = 1), the maximum electromagnetic torque point (s=sm), and the synchronization point (s=0). When the rotor resistance of asynchronous motor changes, the characteristics of the maximum electromagnetic torque TEM and slip SM are: the size remains unchanged, and the s position changes

27 . Asynchronous motors must absorb reactive power with hysteresis from electricity for excitation

28 . When a coil group is connected with alternating current, the change of its magnetomotive force with time has the nature of pulsation. When a single coil is connected with AC, the change of its magnetomotive force with time also has the property of pulsation

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

30 . There are two types of rotor of synchronous motor: 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 per phase is 1/2

32 . Three phase symmetrical AC winding is connected with symmetrical three-phase AC current, and its fundamental wave synthetic magnetomotive force is a circular rotating magnetomotive force that teaches you how to choose a single arm electronic tensile testing machine. Its rotation direction is from the leading phase winding axis to the lagging phase axis, and then to the next lagging phase axis

33 . There are two connection methods between the three-phase windings of three-phase transformer, such as star and triangle; The magnetic circuit has two kinds of structures: group type and heart type

34 . No matter what kind of pressure machine is adopted, the six odd connection groups of three-phase transformer are 1, 3, 5, 7, 9 and 11. The six even connection group numbers are 0, 2, 4, 6, 8, 10

35 . In AC winding, the number of slots per pole and phase q = q = z/2p/m (assuming that the number of slots is Z, the number of poles is p, and the number of phases is m) In AC windings, both 120o phase band and 60o phase band are used. The fundamental winding coefficient and back EMF of 60o phase band are higher

36 . The symmetrical 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 three groups of symmetrical three-phase systems, such as positive sequence, negative sequence and zero sequence

37 . The formula for calculating the short distance coefficient is KY1 = sin (p/2 × Y1/t), whose physical meaning is the discount (or reduced coefficient) of the back EMF (or magnetomotive force) caused by the short distance compared with the whole distance. The calculation formula of the distribution coefficient makes the misalignment of the upper and lower jaws kq1 = sin (QA1/2)/Q/sin (A1/2), and its physical meaning is the coefficient (or discount) reduced by the relative concentration of the back EMF (or magnetomotive force) when Q coils differ by A1 electrical angle in turn

38 . Current transformer is used to measure current, and the secondary side cannot be open circuit. The voltage transformer is used to measure the voltage, and the 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 transformers, motors, generators and other three categories

40 . The calculation formula of the electric angle A1 of the slot distance is A1 = P × 360o/z。 It can be seen that the electrical angle A1 of the slot distance is equal to p times of 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 the reduction

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

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

44 . When the transformer operates in parallel, the conditions for no-load circulation are the same transformation ratio and the same connection group number

45 . When the transformer operates in parallel, the load distribution principle is that the unit value of transformer load current is inversely proportional to the unit value of short-circuit impedance. The condition that the capacity of transformers can be fully utilized in parallel operation is that the unit value of short-circuit impedance should be equal, and their impedance angles should also be equal

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