Double-stator squirrel-cage-shaped asynchronous motor based on excitation optimization

Abstract

The invention discloses an asynchronous motor with a double-stator structure. Structurally, the motor is formed by coaxially connecting two squirrel-cage-shaped asynchronous motors and squirrel-cage bars, the squirrel-cage-shaped asynchronous motors are consistent in stator and rotor outer diameters and inconsistent in axial punching sheet length, and the rotors at the opposite end parts are not provided with magnetic conductive punching sheets, so that the two magnetic circuits are not linked; functionally, the double stators are divided into an exciter part and a generator part, the magnitude and frequency of the input voltage of the exciter part are controlled to control the current on the squirrel cage bars, and then the output voltage of the generator part is controlled. The excitation capacity of the asynchronous motor can be reduced to be close to the excitation capacity of a synchronous generator, the reactive capacity of a power generation system is reduced, and then the capacity, the size and the weight of an inversion system of the power generation system are effectively reduced. And meanwhile, excitation is controlled by using an output voltage loop and a capacitor voltage loop, so that the dynamic performance of the power generation system is optimized.

Description

Technical Field

[0001] The present invention belongs to the field of asynchronous motors, and in particular relates to a dual-stator asynchronous motor integrating excitation and power generation. Background Art

[0002] The dual-stator asynchronous motor is a new type of asynchronous motor developed from the traditional squirrel-cage asynchronous motor. It offers advantages such as simple structure, reliability, and high speed. The simplest asynchronous generator structure is generally a squirrel-cage asynchronous motor with a capacitor. The capacitor excites the motor and maintains the rated no-load voltage. However, under load, the motor voltage changes, making it unsuitable for variable speed operation. A dual-winding asynchronous generator: This motor has two sets of windings on the stator: Wex, a three-phase excitation winding, and Wg, a three-phase output winding. The motor is excited by capacitors, and the output voltage is regulated by an inverter. The inverter output voltage fex is slightly lower than the rotor speed fr, resulting in a negative slip s < 0, turning the asynchronous motor into a generator. S = (fex - fr) / fex. As the motor speed n increases, the DCAC output frequency tracks the speed n, but the slip s decreases compared to low speeds, reducing the generator excitation current and maintaining the rated no-load voltage Ug = Ugn. If the motor load increases while the speed remains constant, the generator excitation must be increased to maintain the same motor voltage. This means increasing the slip s, which reduces the frequency of the inverter output voltage. Fex decreases, while Ug increases. Due to rotor armature reaction, the generator load current increases, meaning the output current Wg increases. This increases the rotor current and, consequently, the inverter current. The excitation capacity is approximately 30% to 40% of the generating capacity. When using an inverter, the excitation capacity should be increased to 50% of the generating capacity.

[0003] Compared with the previous two, the present invention has adjustable speed and is suitable for variable speed operation. In addition, the generator excitation current frequency is lower. When running close to synchronous speed, the excitation capacity is also close to the excitation capacity of the synchronous motor, and the excitation capacity accounts for about 10% of the power generation capacity. Summary of the Invention

[0004] The present invention enables the dual-stator asynchronous generator to operate at variable speed, and its excitation capacity is close to that of the synchronous motor (about 10% of the power generation capacity), reducing the reactive capacity of the power generation system, thereby effectively reducing the capacity, volume and weight of its inverter system.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] 1.1 Motor composition:

[0007] The asynchronous excitation generator consists of a permanent magnet auxiliary exciter PMG, a three-phase DCAC inverter, an asynchronous excitation motor Ex and a generator Gen.

[0008] The Pmg internal rotor is a permanent magnet rotor, and the stator features three-phase symmetrical windings with high impedance and a per-unit synchronous reactance close to 1.0. A three-phase DCAC inverter supplies three-phase symmetrical AC power to the exciter stator windings and reactive power to the Ex excitation. The asynchronous exciter stator features three-phase symmetrical windings, with reactive excitation supplied by the DCAC. The rotor is a squirrel cage. When the motor speed n exceeds the speed n0 of the rotating magnetomotive force supplied to the Ex stator by the DCAC inverter, the motor operates as a generator, and the rotor current increases. The rotor current is proportional to the slip s, S = (n0 - n0) / n0. If n = n0 and s = 0, the rotor current is zero; if n > n0 and s < 0, the rotor current increases. Since the rotor of the generator Gen and the exciter Ex are the same rotor, the excitation current of the generator also increases. The stator of the generator is a three-phase or multi-phase winding (6-phase, 9-phase, and 12-phase windings can be used for DC output). When the rotor is excited, the stator generates an electromotive force.

[0009] 1.2 Motor structure:

[0010] The stator and rotor laminations of the asynchronous exciter and generator are of the same size, with the Ex laminations being shorter and the Gen laminations being longer. The Ex and Gen rotors are coaxial and share a cage. If the Ex is on the left and the Gen is on the right, then the Ex cage has an end ring at the left end and the Gen cage has an end ring at the right end, thus forming an imaginary cage. There are no laminations at the opposite ends of the Ex and Gen, but they are connected by cage bars. Therefore, the Ex and Gen rotors have the same outer diameter, are coaxial, and have the same speed n. The structural diagram is shown in the figure below. Figure 1 .

[0011] 1.3 Motor low speed and no load:

[0012] Assume this motor is a variable-speed motor with a minimum operating speed of Nl and a maximum operating speed of Nm. When n = Nl, the PMG outputs a three-phase AC voltage, which is rectified and filtered before being fed to the DCAC inverter. The DCAC inverter outputs three-phase sinusoidal AC power to the stator excitation winding of Ex. Since Nl > n0 and s < 0, Ex generates power, increasing the Ex rotor current and the Gen no-load potential. Ex's stator also outputs active power, which is returned to the DC side through the DCAC, increasing Udc until Udc exceeds the PMG DC voltage, and PMG no longer supplies power to the DCAC. The DCAC digital controller maintains a constant DC voltage, indicating that the DCAC only provides excitation reactive power to Ex and does not provide active power. Ex compensates for the DCAC's own losses. Ex generates power, increasing the rotor current and, consequently, the Gen rotor current and the Gen no-load potential. If Gen's no-load voltage is less than the rated value, the DCAC-side output voltage frequency f0 should be reduced, increasing the slip s so that s = Sl and the rotor current Ex increases until Eg = Egn. Egn is Gen's rated no-load potential, Ugn = Egn, and Ugn is Gen's rated voltage.

[0013] 1.4 No-load condition after the motor speed increases:

[0014] If the motor speed n > Nl, since n increases, if the excitation of Gen remains unchanged, the no-load electromotive force Eg of Gen will surely increase. To make Eg = Egn = Ugn, obviously the output frequency of dcac must track the motor speed n, adjust the slip s, and the slip Sm at high-speed no-load is < S1.

[0015] 1.5 The generator is connected with a load:

[0016] After the motor is loaded, the stator current of Gen becomes larger, and the demagnetizing effect of the armature reaction becomes larger. At a constant speed, the voltage Ug < Ugn. Therefore, the rotor excitation must be increased, that is, the output voltage frequency f0 of dcac is decreased, s becomes larger, the rotor current of Ex increases, and Ug approaches Ugn. It can be seen from this that the output voltage of Gen can be adjusted to maintain the rated value by adjusting the dcac frequency and then adjusting the slip.

[0017] 1.6 Voltage regulation closed-loop:

[0018] The asynchronous excitation generator has two voltage regulation closed-loops as Figure 2 . The regulation of the generator's output voltage is achieved by changing the motor speed and the speed difference between the rotating magnetic motive forces of the exciter, that is, changing s = (n0 - n) / n0 = (f0 - f) / f0. The DC voltage Udc is achieved by changing the phase difference between the output voltage of dcac and the induced electromotive force of Ex. The phase difference is φ.

[0019] The voltage difference and phase difference between two three-phase power supplies

[0020] 1.7 The main circuit of two three-phase ac power supplies

[0021] Figure 3 is the main circuit diagram of two three-phase ac power supplies. The dcac on the left is a three-phase power supply with terminal voltages UaUbUc, and the right side is the exciter Ex with terminal voltages UA UB UC. The inductance L is between the two power supplies. Figure 3 The single-phase equivalent circuit of Figure 4 is as shown. Figure 3 The voltage frequencies of the two power supplies in

[0022] 1.8 Reactive power transmission

[0023] To make dcac supply excitation current to Ex, it is necessary to make Ua > UA. ΔUa = Ua - UA. The larger the voltage difference, the larger the excitation current. Most of the excitation current is reactive current, and a small part is iron loss current and copper loss current. As Figure 4 , Ua is the DCAC output voltage, and UA is the voltage at the ends of the Ex coil. When Ua is greater than UA, the DCAC provides reactive excitation to Ex.

[0024] 1.9 Active Power Transmission

[0025] When Ua = UA, if Ua leads UAφ, the DCAC side provides active power to the Ex side, and vice versa, Ex provides active power to the DCAC side, thus keeping the Udc on the DCAC side constant. Figure 4 Figure 5(a) shows the voltage waveforms of the two power supplies, where Ua leads UA by an angle of φ. Figure 5(b) shows that at t = t0-t1, Ua+, UA-, and IL flow from the dcac side to the Ex side. Ua and UA simultaneously output active power, causing iL to increase from 0 and increasing the energy stored in the inductor L. Figure 5(c) shows that at t = t1-t2, Ua's polarity remains unchanged, UA changes from negative to positive, and iL's direction remains unchanged. iL is the active current from the dcac side to the Ex side. Figure 5(d) shows that at t = t2-t3, UA is greater than Ua, but iL continues to flow in the same direction. iL begins to decrease, but the dcac side still outputs active power. Figure 5(e) shows that at t = t3-t4, Ua reverses, UA's polarity remains unchanged, and iL increases from 0 under the combined action of the dcac and Ex sides. Active power is still transmitted from the dcac side to the Ex side during the negative half-cycle of Ua and UA.

[0026] 1.10dcac input voltage dc control

[0027] When the exciter Ex is generating electricity, active energy is transferred in two directions: to the rotor and to the stator. This energy is then transferred to the DC side via the DCAC, increasing UDC. At this point, the DCAC output voltage should be slightly ahead of the stator winding terminal voltage of the exciter Ex to allow the DC side energy to be transferred to the exciter Ex.

[0028] 4. Beneficial effects of this technical solution

[0029] Simple structure:

[0030] Ex and Gen laminations are the same, only the lamination length is different.

[0031] The Ex and Gen rotors are squirrel cages, consisting of copper bars and copper bar end rings. If Ex is on the left and Gen is on the right, the left end of Ex is the left end ring and the right end of Gen is the right end ring.

[0032] Power generation status:

[0033] The Ex operating slip is less than 5%. The Gen excitation current is a low-frequency AC current. Due to its low frequency, it produces less reactive power and more active power. The Gen asynchronous machine has low armature reaction, and the generator armature frequency is equal to the DCAC output frequency. If the Gen is a DC generator, it should be designed with multiple three-phase windings, a small phase shift overlap angle of the three-phase rectifier, a high cosΨ, and minimal armature reaction demagnetization. Therefore, the Gen rotor excitation power is low, and the excitation reactive power is also low. The Ex capacity is only approximately 5% of the Gen capacity, and the DCAC capacity is approximately 50% of the Ex capacity, a 30% reduction in excitation capacity compared to traditional asynchronous generators.

[0034] The output voltage of Gen is adjusted by changing the slip s, s = (n0-n) / n0, n0 is the stator rotating magnetomotive force speed of Ex, and n is the motor speed. The control is simple.

[0035] The DC voltage of the DCAC is kept constant, and the DCAC mainly outputs Ex excitation reactive power. The DCAC loss is compensated by Ex.

[0036] If the generator fails, the DCAC output can be reduced to 0, which can demagnetize it and prevent the fault from expanding.

Claims

1. An asynchronous motor with a dual-stator structure. Structurally, the motor consists of two squirrel-cage asynchronous motors with identical stator and rotor outer diameters but unequal axial lamination lengths, connected coaxially with cage bars. The rotors at opposite ends lack magnetically conductive laminations, so the magnetic circuits of the two parts do not interlink. Functionally, the dual stator is divided into an exciter and a generator. Controlling the input voltage and frequency of the exciter controls the current in the cage bars, thereby controlling the output voltage of the generator.

2. According to the dual-stator asynchronous motor of claim 1, the air gap of the exciter part is smaller than that of the generator part. The small air gap means small magnetic resistance, large magnetic permeance, strong ability to establish a magnetic field, and large induced current on the squirrel cage bars. This current flows into the generator part to provide excitation current for the generator.

3. The dual-stator asynchronous motor according to claim 1 has a shorter axial length of the exciter part and a longer axial length of the generator part, because the induced potential is positively correlated with the length of the iron core.

4. The dual-stator asynchronous motor according to claim 1, wherein the exciter and the generator are coaxial and rotate at the same speed, and there are no magnetic conductive punchings at the opposite ends thereof, so the two parts do not share a magnetic circuit, but only share the current on the squirrel cage bars.

5. The dual-stator asynchronous motor according to claim 1, wherein the output voltage of the exciter is provided by a DC source through an inverter, so the input voltage and frequency of the exciter are controllable, and the output voltage of the generator is controllable.

6. The dual-stator asynchronous motor according to claim 5, wherein the DC source is initially powered by a permanent magnet auxiliary exciter, and when the voltage fed back by the exciter is greater than that of the auxiliary exciter, the auxiliary exciter exits the power generation system.

7. The dual-stator asynchronous motor according to claims 5 and 6, when the exciter input voltage amplitude is greater than its back electromotive force amplitude and the phases are the same, the DC source provides reactive power to the generator via the exciter; when the exciter input voltage amplitude is equal to its back electromotive force amplitude and the phase leads by an angle φ (less than 90°), the DC source provides active power to the generator via the exciter; when the exciter input voltage amplitude is equal to its back electromotive force amplitude and the phase lags by an angle φ (less than 90°), the generator provides active power to the DC source via the exciter.

8. The dual-stator asynchronous motor according to claim 1, wherein the stator winding of the generator is a three-phase or multi-phase winding, and the DC output can be a 6-phase, 9-phase, or 12-phase winding to obtain a more stable DC voltage.

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