In recent years, servo motors have been applied across various fields of industrial automation, reaching the peak of industrial automation control systems. We believe that in the near future, we will witness another qualitative leap in the field of industrial control, propelling human civilization forward.
1. The no-load current of a single-phase transformer is out of phase with the main magnetic flux, exhibiting a phase angle difference Fe due to iron loss current. The no-load current exhibits a peaky waveform due to significant third-harmonic content.
2. Alternating current also flows through the armature windings of DC servo motors. However, direct current flows through their field windings. Excitation methods for DC motors include separately excited, shunt excited, series excited, and compound excited.
3. The back-EMF expression for a DC motor is E=CE_F n; while the electromagnetic torque expression is Tem=CTF I.
4. The number of parallel branches in a DC servo motor is always even. The number of parallel branches in an AC winding is not necessarily so.
5. In DC servo motors, single-stack windings are connected in series by stacking one element over another. Whether single-wave or single-stack windings, commutator segments connect all elements in series to form a single closed circuit.
6. Induction motors are also called asynchronous motors because their rotor current is generated through electromagnetic induction.
7. During reduced-voltage starting of an asynchronous motor, the starting torque decreases proportionally to the square of the starting current in the windings.
8. When the primary voltage magnitude and frequency remain constant, the saturation level of a transformer's core remains essentially unchanged, and the excitation reactance also remains largely constant.
9. The short-circuit characteristic of a synchronous generator is a straight line. During a symmetrical three-phase short circuit, the magnetic circuit is unsaturated. In a steady-state symmetrical three-phase short circuit, the short-circuit circuit consists of a purely demagnetizing direct-axis component.
10. The current in the excitation winding of a synchronous motor is direct current. Primary excitation methods include excitation generator excitation, static rectifier excitation, and rotating rectifier excitation.
11. The three-phase composite magnetic field contains no even-order harmonics; when symmetrical three-phase currents flow through symmetrical three-phase windings, the composite magnetic field contains no multiples of the third harmonic.
12. Three-phase transformers typically require one side to be delta-connected or have one neutral point grounded. This ensures a path for third-harmonic currents in the transformer windings.
13. When symmetrical three-phase currents flow through symmetrical three-phase windings, the 5th harmonic in the composite magnetic field is reversed, while the 7th harmonic is in phase.
14. Series-wound DC servo motors exhibit a relatively soft mechanical characteristic curve, whereas separately-excited DC motors display a relatively hard mechanical characteristic curve.
15. Short-circuit testing of transformers measures winding leakage impedance, while no-load testing measures excitation impedance parameters.
16. The transformation ratio of a transformer equals the turns ratio between primary and secondary windings. For single-phase transformers, the transformation ratio can also be expressed as the ratio of rated primary to secondary voltages.
17. Under normal excitation, the power factor of a synchronous generator equals 1. When maintaining constant active power output while reducing the excitation current below normal levels (underexcitation), the direct-axis armature reaction exhibits a magnetizing effect. Conversely, when maintaining constant active power output while increasing the excitation current above normal levels (overexcitation), the direct-axis armature reaction exhibits a demagnetizing effect.
18. In DC motors, iron losses primarily occur in the rotor core (armature core) because the stator core magnetic field remains essentially constant.
19. In DC servo motors, the first pitch y₁ equals the number of slots between the first and second sides of a component. The composite pitch y equals the number of slots between the upper sides of two components connected in series.
20. In DC motors, when saturation is neglected, cross-axis armature reaction shifts the position where the magnetic field is zero, while maintaining constant magnetic flux per pole. When brushes are positioned on the geometric neutral line, armature reaction exhibits cross-magnetic characteristics.
21. In DC servo motors, the component converting external DC to internal AC is the commutator. The commutator's function is to convert DC to AC (or vice versa).
22. In a synchronous motor, when the excitation flux F0 linking the stator windings reaches its maximum value, the counter-electromotive force E0 reaches its minimum value. When F0 reaches zero, E0 reaches its maximum value. The phase relationship between F0 and E0 is such that F0 leads E0 by 90o. The relationship between E₀ and F₀ is expressed as: E₀=4.44 f N kN₁F₀.
23. In motors, leakage flux refers solely to the magnetic flux linking the winding itself. The counter-EMF it generates can often be equivalently represented by a leakage reactance voltage drop (or negative reactance voltage drop).
24. Asynchronous motors have two types of rotors: squirrel-cage and wound.
25. The slip rate s of an asynchronous motor is defined as the ratio of the difference between synchronous speed and rotor speed to the synchronous speed. When operating as a servo motor, the slip rate s ranges from 1 > s > 0.
26. The electromagnetic torque Tem of an asynchronous motor has three critical points on its Tem-s curve: the starting point (s=1), the maximum electromagnetic torque point (s=sm), and the synchronous point (s=0). When the rotor resistance of an asynchronous motor changes, the characteristics of its maximum electromagnetic torque Tem and slip rate sm are: the magnitude remains constant, while the position of s changes.
27. Asynchronous servo motors must draw lagging reactive power from the grid for excitation.
28. When an alternating current flows through a coil group, its magnetic potential exhibits pulsating characteristics over time. Similarly, when an alternating current flows through a single coil, its magnetic potential also exhibits pulsating characteristics over time.
29. When synchronous generators are connected to the grid, their three-phase terminal voltages must match the grid's three-phase voltage in terms of: frequency, amplitude, waveform, phase sequence (and phase angle), etc.
30. Synchronous motor rotors come in two types: shaded-pole and salient-pole.
31. The equivalent number of phases for a squirrel-cage rotor equals its number of slots, while the equivalent number of turns per phase is 1/2.
32. When symmetrical three-phase alternating current flows through symmetrical three-phase AC windings, the fundamental composite magnetic field is a circular rotating magnetic field. Its rotation direction shifts from the axis of the leading-phase winding to the axis of the lagging-phase winding, then to the axis of the next lagging phase.
33. Three-phase windings of a transformer can be connected in either a star or delta configuration; magnetic circuits may adopt either a group-type or core-type structure.
34. The odd-numbered connection group designations for a three-phase transformer are 1, 3, 5, 7, 9, 11. The even-numbered connection group designations are 0, 2, 4, 6, 8, 10.
35. In AC windings, the number of slots per pole per phase is q=Z/2p/m (assuming Z slots, p pole pairs, and m phases). AC windings may employ either 120℃ phase bands or 60℃ phase bands. The fundamental winding coefficient and counter-EMF are higher in 60℃ phase bands.
36. The symmetrical components method can be used to analyze the asymmetrical operation of transformers and synchronous motors. Its application requires the system to be linear, allowing the superposition principle to decompose the asymmetrical three-phase electrical system into three symmetrical three-phase systems: positive sequence, negative sequence, and zero sequence.
37. The short-circuit coefficient is calculated as k_y₁=sin(π/2 × y₁/t). Its physical significance represents the reduction factor applied to the counter-EMF (or magnetic force) due to short-circuit conditions relative to full-circuit conditions. The formula for the distributed coefficient is kq1=sin(qa1/2) / q / sin(a1/2). Its physical meaning is the reduction factor (or discount) applied to the counter-EMF (or magnetomotive force) when q coils are sequentially spaced by a1 electrical angles, resulting in a relatively concentrated distribution.
38. Current transformers measure current, and their secondary side must not be open-circuited. Voltage transformers measure voltage, and their secondary side must not be short-circuited.
39. An electric motor is a device that converts mechanical energy into electrical energy (or vice versa), or transforms one AC voltage level into another. From an energy conversion perspective, servo motors can be categorized into three types: transformers, motors, and generators.
40. The formula for calculating slot pitch electrical angle a₁ is a₁=p × 360℃/Z. It is evident that the slot pitch electrical angle a₁ equals p times the slot pitch mechanical angle a_m.
41. The principle of transformer winding conversion is: before and after conversion, ensure that the magnetic motive force of the winding remains unchanged, and that the active and reactive power of the winding remain unchanged.
42. The efficiency characteristic curve of a transformer features a maximum point where variable losses equal fixed losses.
43. No-load tests on transformers typically involve applying voltage and taking measurements on the low-voltage side. Short-circuit tests usually apply voltage and take measurements on the high-voltage side.
44. For transformers operating in parallel, the condition for no circulating current at no-load is: identical turns ratio and identical connection group designation.
45. The principle for load distribution in parallel-connected transformers is: the square root of the transformer load current is inversely proportional to the square root of the short-circuit impedance. The conditions for fully utilizing the capacity of transformers in parallel operation are: the square roots of the short-circuit impedances must be equal, and their impedance angles must also be equal.