PM-LINE START MOTOR AND START-UP PROCEDURE FOR THIS

Description

[0001] The present invention relates to a permanent magnet excited line-start motor, hereinafter also referred to as PM line-start motor, and a switching-on method for the same.

[0002] The permanent magnet line-start motor (PM line-start motor), sometimes also referred to as a permanent magnet excited line-start motor, line-start permanent magnet motor, or LSPM motor, is a three-phase asynchronous motor with a squirrel-cage rotor that contains additional permanent magnets in the rotor. After asynchronous start-up, it synchronizes to the supply frequency and then runs in synchronous operation. Due to its design principle, it has minimal rotor and excitation losses, resulting in high efficiency. Thus, it combines the advantages of a robust asynchronous motor with those of a low-loss synchronous motor.

[0003] Such a motor has the ability of an asynchronous motor to generate an asynchronous starting torque as well as the ability of a synchronous motor to run at a speed synchronous to the rotating field.

[0004] From the literature article ALIAKBAR DAMAKI ALIABAD ET AL: "Line-Start Permanent-Magnet Motors: Significant Improvements in Starting Torque, Synchronization, and Steady-State Performance," IEEE TRANSACTION ON MAGNETICS, IEEE SERVICE CENTER, NEW YORK, NY, US, Vol. 46, No. 12, December 1, 2010 (2010-12-01), pages 4066-4072, a concept is known for improving the starting torque and synchronization capability of line-start permanent-magnet motors with a pole-changing winding. Switching of the stator windings is achieved using a centrifugal switch. A continuation of the content is known from the literature article ALIABAD AD ET AL: "Analytic modelling and dynamic analysis of pole-changing line-start permanent-magnet motors, IET ELECTRIC POWER APPLICATION, IET, UK, Vol. 6, No. 3, 8 March 2012 (2012-03-08), pages 149-155.

[0005] US 5,859,513 A discloses a starting circuit designed to prevent large oscillation torques when switching on PM line-start motors after startup in the event of an inrush. CH 435 435 A discloses a method for self-starting a synchronous machine with a damper winding cage and a stator winding that enables pole switching.

[0006] PM-Line-Start motors with a rated power of approximately 4 to 5 kW or higher must be connected to the three-phase network using a suitable switching-on procedure to reduce the switching-on surges in the network and shaft train.

[0007] The starting methods familiar from squirrel-cage motors, such as star-delta starting, starting transformers, or soft starters, initially reduce the motor voltage upon startup. These methods are not practical for PM-Line-Start motors. The reason for this lies in the physics of the PM-Line-Start motor.

[0008] PM-Line-Start motors are a combination of a squirrel-cage motor and a PM synchronous motor. After power-up, the PM-Line-Start motor initially runs up with the starting cage like a squirrel-cage motor. After the run-up process, the final acceleration to synchronous speed occurs, and the PM-Line-Start motor then operates as a PM synchronous motor with a correspondingly high level of efficiency.

[0009] During the asynchronous start-up process, the PM rotor induces a slip-frequency voltage system into the stator winding, for which the three-phase network essentially represents a short circuit. This results in, in addition to the desired asynchronous torque of the starting cage, an additional regenerative braking torque that fundamentally hinders the start-up process. The superposition of both torque components is Fig. 1 outlined.

[0010] The regenerative braking torque curve exhibits a pronounced brake caliper. This regenerative braking caliper, whose magnitude can only be influenced to a very limited extent by design and is also independent of the line voltage, represents the real problem during the switch-on and start-up process with reduced voltage.

[0011] Since the asynchronous torque decreases quadratically with the square of the reduced voltage, but the height of the regenerative brake caliper remains unchanged, the run-up process can end at the regenerative brake caliper with a correspondingly reduced voltage. The asynchronous drive torque with reduced voltage is not large enough to overcome the regenerative brake caliper. Therefore, this may lead to a subsynchronous limit cycle (fluctuation around a speed point in the subsynchronous speed range) of the motor during the switch-on phase with reduced voltage. In practice, this sticking at the regenerative brake caliper already occurs when the voltage is reduced to 80 to 85%. This makes voltage-reducing switch-on methods alone unusable for PM-Line-Start motors. The task therefore arises of developing effective switch-on methods for PM-Line-Start motors that overcome the aforementioned disadvantages.

[0012] Until now, this problem has not been solved because PM Line-Start motors, contrary to their name "Line-Start," were initially developed specifically for variable-speed operation with a Vf converter. Only in recent years has the Line-Start functionality been rediscovered in connection with the demand for efficiency-class motors. PM Line-Start motors currently on the market only cover the lower power range up to approximately 5 kW. However, no starting procedures are yet required for this power range, so such motors are not exposed to the problems described above.

[0013] The present invention solves this problem by means of a PM line start motor having the features of claim 1.

[0014] Accordingly, the PM-Line-Start motor, which preferably has a power of at least 5 kW, comprises a rotor and a stator with a stator winding, wherein the stator winding is designed as a pole-changing rotating field winding.

[0015] This makes it possible to avoid the generator brake calliper, which inhibits the motor from running up, in a switch-on process.

[0016] According to the invention, the pole-switchable rotating field winding comprises a first winding stage and a second winding stage which can be operated separately from one another, wherein the first winding stage has a number of pole pairs, referred to below as the starting pole pair number p 1 , which is not equal to a number of pole pairs, referred to below as the working pole pair number p 2 , of the second winding stage.

[0017] Typically, the first winding stage is used for asynchronous start-up of the motor, and the second winding stage is used for synchronous operation. It is advantageous if the first winding stage is dimensioned such that no regenerative brake calliper can form.

[0018] Suppression of the regenerative brake caliper during the start-up phase is possible if the first winding stage, which is used for asynchronous start-up of the motor, has a starting pole pair number p1 that does not match the number of pole pairs of the PM rotor p3. During the start-up phase, the stator winding and starting cage operate as a pure asynchronous motor with the starting pole pair number p1. The PM rotor is decoupled due to the different pole pair numbers. This decoupling prevents the formation of the regenerative brake caliper. Therefore, the PM rotor does not generate a braking torque that counteracts the starting torque during the start-up process.

[0019] After start-up, the system switches to the second winding stage, which preferably has the same number of pole pairs p 3 of the PM rotor. Therefore, in an advantageous embodiment of the invention, the working pole pair number p 2 of the second winding stage is equal to the number of pole pairs p 3 of the PM rotor. It will be clear to those skilled in the art that the second winding stage is intended for the synchronous operation of the PM line-start motor.

[0020] According to a further advantageous modification of the invention, the number of starting pole pairs p 1 of the first winding stage is smaller than the number of working pole pairs p 2 of the second winding stage. For example, the number of pole pairs of the first and second winding stages of the stator winding can assume a ratio of 1:2. However, this does not preclude the variation of the invention in which the number of starting pole pairs of the first winding stage p 1 can also be greater than the number of working pole pairs p 2 of the second winding.

[0021] According to the invention, the motor further comprises a sequence control designed to perform a targeted winding changeover of the pole-changing rotating field winding during a motor start-up process. This involves a targeted winding changeover from the first winding stage for motor start-up to the second winding stage for synchronous operation of the motor.

[0022] According to a further advantageous embodiment of the invention, the sequence control is further designed to provide a phase or a time period between the switching from the first winding stage to the second winding stage in which neither the first nor the second winding stage is supplied with energy, wherein the time of switching to the second winding stage preferably takes place as a function of the phase position of a mains voltage.

[0023] The switching time has a significant influence on a reclosing current surge, which can occur when considering the phase position of a mains voltage with respect to the induced voltage (pole wheel voltage).

[0024] The motor according to the invention can further comprise a first switch connected to the first winding stage and a second switch connected to the second winding stage, for selectively closing or interrupting a line arranged between the respective winding stage and the mains voltage line. Preferably, the first and second switches can be actuated by a sequence control. Contactors are particularly suitable as switches.

[0025] According to an optional development of the invention, the respective ends of the first and second switches that are not connected to the associated winding are connected to each other and connected to a supply line via a power switch. The power switch is also preferably designed to be operable by the sequence control.

[0026] According to the invention, the motor comprises a series circuit for reducing a voltage between a mains voltage line and the stator winding in order to limit an inrush current or a restart current that may occur, for example, when switching from the first winding to the second winding. The series circuit can be implemented by series impedances or a soft starter.

[0027] According to another optional feature, the stator winding is designed in the form of a Dahlander winding.

[0028] The implementation of the stator winding in the form of a Dahlander winding is advantageous because it enables better utilization of the active part.

[0029] As an alternative to the one-piece design of the stator winding in the form of a Dahlander winding, the stator winding can also have two separate windings, each designed for a different number of poles.

[0030] According to an advantageous embodiment of the invention, the motor is designed to carry out a switchover from the first winding to the second winding with respect to the synchronous rated speed n 1 of the PM line start motor supersynchronously.

[0031] The rated speed n 1 is the ratio of the mains frequency f mains to the number of working pole pairs p 2 of the second winding stage.

[0032] This ensures that the restart current surge is as small as possible when switching from the first winding stage to the second winding stage.

[0033] Furthermore, the present invention relates to a switching-on method of a PM line-start motor having the features of one of the above-mentioned embodiments, the method comprising the steps: (i) switching on a first winding stage to start an asynchronous run-up phase, (ii) switching off the first winding stage to end the asynchronous run-up phase, and (iii) switching on the second winding stage, which is designed to operate the motor synchronously.

[0034] According to the invention, the above switching-on method further comprises, between steps (ii) and (iii), the step: (iv) monitoring the grid voltage and an induced rotor voltage in order to determine a switching time which allows a grid connection which is as smooth as possible, wherein preferably a switching time allows a grid connection which is as smooth as possible when there is approximately the same frequency between the rotor voltage and the grid voltage and the difference between the grid voltage and the same frequency is small, i.e. falls below a predetermined threshold value.

[0035] The invention and its further developments are described in more detail below with reference to the accompanying drawings of preferred embodiments. They show: Fig. 1: a quasi-stationary torque curve during asynchronous start-up of a conventional PM-Line-Start motor, Fig. 2: a structural diagram of a connected PM-Line-Start motor, Fig. 3: a switching sequence of the Fig. 2 illustrated switch during start-up of the motor, Fig. 4: an M(n) diagram of a start-up process of a PM-Line-Start motor according to the invention with winding switching, Fig. 5: a wiring variant of a PM-Line-Start motor according to the invention, Fig. 6: a structural circuit diagram of a wired PM-Line-Start motor according to the invention with an additional soft starter, Fig. 7: a circuit diagram of a wired PM-Line-Start motor whose pole-changing stator winding is a Dahlander winding, Fig. 8: a stationary M(n) diagram of a start-up process of a PM-Line-Start motor according to the invention with supersynchronous winding switching, and Fig. 9: a structural circuit diagram of a wired PM-Line-Start motor according to the invention.

[0036] Fig. 1 was already explained in more detail in the introductory part of the description and shows the braking torque curve versus motor speed. The problem is that the asynchronous torque decreases quadratically with a reduction in voltage, but the braking torque induced by the PM rotor remains unchanged. Therefore, if one now wants to provide a switch-on procedure that initially provides a reduced voltage, there is a risk that the braking torque will be greater than the asynchronous torque and counteract the acceleration of the rotor.

[0037] Fig. 2 shows the schematic diagram of a PM-Line-Start motor with winding switching.

[0038] A total of three switches can be seen, which are designed as contactors, in the form of semiconductor and / or mechanical contactors, or as switching elements. Switches S1 and S2 supply the motor winding in such a way that two winding stages are created for the PM Line Start motor. The third switch, labeled "Mains," disconnects the entire circuit from the three-phase mains during the switching phase and when the drive is not in operation. The first winding stage, connected to switch S1, is intended for asynchronous start-up and is dimensioned so that virtually no regenerative brake calliper can form. After the asynchronous start-up in the first winding stage, the system switches to the second winding stage, which is specifically designed for synchronous operation of the motor. All switches, namely S1, S2, and "Mains," are connected to a sequence control that controls the operation of the individual switches.

[0039] Fig. 3 shows the entire switch-on and start-up process, which is essentially divided into four switching phases. As already briefly explained above, the sequence control, for example in the form of a programmable logic controller or a conventional relay controller, ensures the desired switching sequence of the switches. The suppression of the regenerative brake caliper, which typically forms in switching phase 1, is possible if the first winding stage of the stator winding has a different number of pole pairs than the PM rotor. In phase 2, the stator winding is disconnected from the mains. The mains switch or switch S1 can be moved to their respective open positions simultaneously or sequentially. In phase 3, the stator winding is switched to the second winding stage with S2, but the mains switch is still held in its open position.The second winding stage now has the number of pole pairs of the PM rotor to enable synchronous operation of the PM line-start motor. To minimize the inrush current, it is advisable to compare the mains voltage and the induced voltage in phase 3. Even if both voltages have different frequencies, a transition to phase 4, in which the mains switch is closed, should only occur when the phase relationship between the two voltages is as favorable as possible—i.e., when both three-phase voltage systems have the same phase relationship.

[0040] In phase 4, the second winding is therefore connected to the mains voltage via S2, so to speak, it has taken over the rotor started by the first winding.

[0041] Fig. 4 shows this start-up process in a quasi-stationary manner in the M(n) diagram for the case where the number of pole pairs of the two winding stages of the stator winding are in a ratio of 2:1. In this case, the number of starting pole pairs p 1 of the first winding is twice as large as the number of working pole pairs p 2 of the second winding. Two curves can essentially be seen, with the motor speed shown along the abscissa and the torque generated by the motor shown along the ordinate. The curve labeled S1 corresponds to the characteristic curve of the first winding stage, and the curve labeled S2 to the characteristic curve of the second winding stage. The curve shown in dashed form and labeled W is the load characteristic curve.

[0042] In a switching-on process of the motor according to the invention with winding switching, the characteristic curve S1 is followed starting from the speed 0 until the switching process represented by a double line and a transparent triangle pointing upwards results in a change to the characteristic curve of the second winding.

[0043] As a result, the generator brake caliper of the characteristic curve S2 of the second winding stage, which is designed for synchronous operation of the motor, is skipped and only used in a section of the characteristic curve that no longer has a brake caliper.

[0044] The described switching-on procedure with the winding switching now also enables a reduction of the voltage in the switching-on and run-up phase without the risk of the generator brake calliper stalling the run-up process, especially a run-up process with reduced voltage.

[0045] Fig. 5 shows another representation of an embodiment of the invention, which has a series circuit controllable by the sequence control instead of a mains switch. The series circuit is connected to both switch S1 and switch S2. Just like the Fig. 2 The aim of the circuit shown is to prevent the regenerative brake calliper that occurs during the start-up phase during the start-up process. To achieve this, the switchable stator winding is used during start-up. The first winding stage is switched via switch S1 and is specifically designed for asynchronous start-up and dimensioned so that no regenerative brake calliper can form. To limit the inrush current, a voltage-reducing series circuit can be arranged between the mains and the stator winding. In the simplest case, this involves series impedances or a soft starter.

[0046] As already explained above, after asynchronous start-up, the first winding stage is switched off via switch S1, and the second and final winding stage is switched on via switch S2. This is specifically designed for synchronous operation. The voltage-limiting series connection between the mains and the stator winding can also be used to limit the restart current when switching to the second winding stage.

[0047] The suppression of the regenerative brake caliper during the start-up phase is possible if the stator winding in the first winding stage has a starting pole pair number p 1 that does not match the pole pair number of the PM rotor p 3. During the start-up phase, the stator winding and starting cage operate as a pure asynchronous motor with the starting pole pair number p 1. The PM rotor is decoupled due to the different pole pair numbers. After start-up, the motor switches to the second winding stage, where the stator winding now has the working pole pair number p 2 of the PM rotor p 3. Accordingly, the stator winding of the second winding stage has the pole pair number p 2 that is intended for synchronous operation of the PM line-start motor. To avoid the regenerative brake caliper, the stator winding of the PM line-start motor is designed as a pole-changing rotating field winding.

[0048] In principle, an embodiment is conceivable that has two separate stator windings, each designed for a different number of pole pairs.

[0049] Fig. 6 shows a variation of the present invention with a soft starter that reduces the motor voltage and inrush currents and increases them after a voltage-time ramp up to the direct line voltage. After start-up, such a soft starter is bypassed.

[0050] Fig. 7 shows the wiring of a PM line-start motor according to the invention, in which the stator winding is designed as a Dahlander winding. This has the advantage of better utilization of the active component.

[0051] After Fig. 7 The stator winding consists of two partial windings, the winding ends U 1,2,3 , V 1,2,3 and W 1,2,3 of which can be connected by means of a contactor circuit to form symmetrical three-phase circuits (star or delta circuits, double star or double delta circuits). Fig. 7 It also shows a voltage-limiting series connection between the mains and the stator winding for additional limitation of the inrush and restart current. However, this is optional.

[0052] To minimize the reclosing current surge when switching from the first winding stage to the second winding stage, the switchover is performed supersynchronously, relative to the synchronous rated speed n 1 = f mains / p 2 of the PM-Line-Start motor. This variant of the switch-on procedure is therefore only possible if the number of starting pole pairs p 1 of the first winding stage is smaller than the number of working pole pairs p 2 of the second winding stage. This variation of the switch-on procedure can therefore be used for PM-Line-Start motors with a pole pair number p 2 ≥ 2.

[0053] Fig. 8 shows a start-up process in the stationary M(n) diagram, in which the characteristic curve S1 represents the characteristic curve of the first winding stage, the characteristic curve S2 represents the characteristic curve of the second winding stage and the dashed characteristic curve represents the load characteristic.

[0054] After Fig. 8 The stator winding is initially connected to the three-phase mains in the first winding stage, either directly or via an optional series connection. The stator winding must be designed with respect to the first winding stage so that a steady-state operating point with a load characteristic curve above the synchronous rated speed n 1 = f mains / p 2 is achieved. In the next step, the stator winding is disconnected from the mains and initially switched to the second winding stage using the sequence control. The stator winding now has the same number of pole pairs as the PM rotor, and the PM rotor induces a rotor voltage in the stator winding with a frequency p 2 n that depends on the current speed.

[0055] Since the motor is not yet connected to the mains, the drive is decelerated according to the load characteristic curve. This can be seen by the sliding of the operating point, which is located at the intersection of the S1 characteristic curve and the load characteristic curve W load and moves towards lower speeds along the load characteristic curve W load.

[0056] Typically, the task of the sequence control is to monitor the grid voltage and the induced rotor voltage to ensure the smoothest possible grid connection. For example, if the frequency is equal and the difference between the grid voltage and rotor voltage is as small as possible, the stator winding of the PM Line Start motor is connected to the three-phase grid in the second winding stage, either directly or via a series connection. The operating point thus transitions from the characteristic curve S1 of the first winding to a region of the characteristic curve S2 of the second winding without having to pass through the generator brake caliper of the characteristic curve S2.

[0057] Will be in Fig. 8 If we assume a centrifugal pump drive and a stator winding according to the Dahlander principle, the first winding stage can be designed in a delta connection and the second winding stage in a double star connection.

[0058] Fig. 9 shows the contactor circuit for the main circuit. Apart from a possible upstream circuit for additional reduction of inrush current and re-inrush current, which is shown in the Fig. 8 As shown optionally, only three main contactors are required to implement this circuit variant.

[0059] The sequence control, which can be implemented, for example, by a PLC (programmable logic controller), then ensures the necessary switching sequence of the switching-on process.

[0060] In a first step, the stator winding in the first winding stage is switched on. If a series circuit is necessary to limit the inrush current, this is then switched off.

[0061] In a subsequent second step, the asynchronous start-up phase is shut down. This is typically done by interrupting the power supply to the first winding stage.

[0062] In a third step, the stator winding is switched from the first winding stage to the second winding stage. The switches or contactors S1 and S2 are switched accordingly. The electrical star point for the second winding stage is established by switch or contactor S3.

[0063] The mains voltage and the induced rotor voltage are then monitored to enable reconnection as smoothly as possible.

[0064] The switch-on then occurs at a favorable time. The second winding stage is connected to the mains voltage. If a series circuit is necessary to dampen the reclosing current or the inrush current, this is removed in a subsequent step.

Claims

1. Permanent-magnet line-start motor, preferably having a power of at least 5 kW, comprising: a rotor, and a stator having a stator winding, wherein the stator winding is embodied as a changeable-pole rotating field winding, wherein the changeable-pole rotating field winding comprises a first winding stage and a second winding stage, which winding stages can be operated separately from one another, and the first winding stage has a number of start-up pole pairs p1, which is not equal to a number of operating pole pairs p2 of the second winding stage, and the motor comprises a sequence controller, which is designed to perform a targeted winding changeover of the changeable-pole rotating field winding during a switch-on process of the motor, wherein the targeted winding changeover takes place from the first winding stage for the run-up of the motor to the second winding stage for the synchronous operation of the motor, characterized in that a ballast circuit for reducing a voltage is arranged between a supply system voltage line and the stator winding in order to limit an inrush current or a restart current.

2. Motor according to Claim 1, wherein the first winding stage serves for the asynchronous run-up of the motor and the second winding stage serves for the synchronous operation of the motor.

3. Motor according to Claim 1 or 2, wherein the number of start-up pole pairs p1 of the first winding stage does not correspond to a number of poles p3 of the PM rotor and the number of operating pole pairs p2 of the second winding stage is preferably equal to the number of poles p3 of the PM rotor.

4. Motor according to any one of Claims 1 to 3, wherein the number of start-up pole pairs p1 of the first winding stage is lower than the number of operating pole pairs p2 of the second winding stage.

5. Motor according to any one of the preceding claims, wherein the sequence controller is further designed to provide a phase between the changeover from the first winding stage to the second winding stage, in which phase neither the first nor the second winding stage is supplied with power, wherein the time of the changeover to the second winding stage preferably takes place depending on the phasing of a supply system voltage.

6. Motor according to any one of the preceding claims, further comprising a first switch, which is connected to the first winding stage, and a second switch, which is connected to the second winding stage, wherein the first and the second switch can be actuated by the sequence controller.

7. Motor according to Claim 6, wherein the ends of the first and second switch that are remote from the windings are connected to one another and connected to a supply line by means of a power switch, wherein the power switch can be actuated by the sequence controller.

8. Motor according to any one of the preceding claims, wherein the ballast circuit is implemented by ballast impedances or a soft starter.

9. Motor according to any one of the preceding claims, wherein the stator winding is embodied in the form of a Dahlander winding.

10. Motor according to any one of preceding Claims 1 to 8, wherein the stator winding has two windings that are isolated from one another and that are each designed for different numbers of poles.

11. Motor according to any one of preceding Claims 1 to 10, wherein the motor or the sequence controller is designed to execute a changeover based on the synchronous rated rotational speed n1 of the PM line-start motor in oversynchronous fashion and the rated rotational speed n1 is: n 1 = f supplysystem / p 2 ,, wherein fsupply system is the supply system frequency, and P2 is the number of operating pole pairs of the second winding stage.

12. Method for switching on a permanent-magnet line-start motor, which has the features of one of the preceding claims, wherein the method comprises the following steps: (i) switching on a first winding stage to start an asynchronous run-up phase, (ii) disconnecting the first winding stage in order to terminate the asynchronous run-up phase, by isolating the stator winding from the three-phase supply system and (iii) connecting a second winding stage, which is designed for the synchronous operation of the motor, wherein, between step (ii) and (iii), the method further comprises the following step: (iv) monitoring the supply system voltage and an induced rotor voltage in order to determine a changeover time that permits the smoothest possible supply system connection, wherein a changeover time preferably permits the smoothest possible supply system connection when there is approximate frequency equality between the rotor voltage and the supply system voltage and the difference between the supply system voltage and the rotor voltage is small, that is to say undershoots a predetermined threshold value.