Low loss permanent magnet excited electric machine

Abstract

A permanent magnet excited electric machine includes a stator and a moving part movable relative to the stator. One of the stator and moving part has an electric winding system in the form of a single or multiple 3-phase system. The other one of the stator and moving part has permanent magnets. The winding system, in the form of a delta connection, has an inductance such that the related total reactance x.sub.B during rated operation according to x.sub.B=ω.sub.B.Math.L.sub.Y.Math.P.sub.B/(Z.sub.3P.Math.U.sub.i,B.sup.2) is at least 0.8, wherein ω.sub.B is the electric angular frequency of the winding system during rated operation, L.sub.Y is the phase inductance of the winding system in star connection, P.sub.B is the power of the machine during rated operation, Z.sub.3P is the number of 3-phase systems of the machine, and U.sub.i,B is the amount of the induced conductor voltage during rated operation.

Claims

1. A permanent magnet excited electric machine, comprising: a stator part; and a moving part that is movable in relation to the stator part, one of said stator and moving parts having an electric winding system in the form of a single or multiple 3-phase system, and the other one of said stator and moving parts having permanent magnets, wherein the permanent magnet excited electric machine is designed for a rated power in the range between 100 kW and 20 MW and the winding system is connected in a delta connection and having an inductance such that the related total reactance x.sub.B in rated operation according to:
x.sub.B=ω.sub.B.Math.L.sub.y.Math.P.sub.B/(Z.sub.3P.Math.U.sub.i,B.sup.2) is at least 0.8, in which ω.sub.B=electric angular frequency of the winding system in rated operation, L.sub.y=phase inductance of the winding system, in star connection or transformed to star connection, P.sub.B=power of the machine in rated operation, Z.sub.3P=number of 3-phase systems of the machine, U.sub.i, B=amount of the induced conductor voltage in rated operation, wherein a short-circuit current of a 3rd harmonic arising in the winding system causes less Ohmic losses in a range between 0.001 and 0.01% of the rated power P.sub.B.

2. The permanent magnet excited electric machine according to claim 1, characterized in that it is designed for a rated power in the range between 1 MW and 3 MW.

3. The permanent magnet excited electric machine according to claim 1, characterized in that it is designed for a rated power in the range between 3 MW and 6 MW.

4. The permanent magnet excited electric machine according to claim 1, characterized in that it is designed for a rated power in the range between 6 MW and 15 MW.

5. The permanent magnet excited electric machine according to claim 1, characterized in that it has a pole number 2p of at least 10.

6. The permanent magnet excited electric machine according to claim 1, characterized in that the winding system is provided in the form of a concentrated winding.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The invention will now be elucidated in more detail with reference to the drawings wherein

(2) FIG. 1 shows a schematic diagram of an embodiment of a permanent magnet excited electric machine with an electric winding system provided in the form of a 3-phase system and connected to a converter unit;

(3) FIG. 2 shows an electric circuit diagram of the delta connection of the winding system of the machine according to FIG. 1; and

(4) FIG. 3 shows a schematic view of an exemplary voltage curve of the three phases of the winding system of the machine with respective fundamental harmonic and third harmonic.

DETAILED DESCRIPTION

(5) The invention will be elucidated in more detail hereinafter by way of FIGS. 1 to 3. FIG. 1 shows a schematic diagram of an embodiment of a permanent magnet excited electric machine (in the following referred to as PM machine). The machine PM comprises a stator part (also stator) ST and a moving part (in the instant case a rotor) R that is movable in relation to the stator part. Stator ST comprises a winding system in the form of a single or multiple electric 3-phase system. Rotor R contains permanent magnets S and N for forming the respective poles. The winding system may also be provided in the form of a 6-phase or multiple 3-phase winding. The machine in the instant case is a PM machine with concentrated winding (i.e. with individual coils in the respective strand or phase, schematically illustrated here as Sp1, Sp2, Sp3). Rotor R is illustrated schematically with two poles 2p (pole pair number p=1). However, the machine may also have a different number of poles, preferably a pole number 2p of at least 10 (p=pole pair number). The PM machine according to the invention may be an electric motor or a generator for generating current or also a combination of both, which selectively operates as motor or as generator. The electric machine according to the invention may be a rotary or a linear machine. In FIG. 1, there is shown a rotary machine to which reference will be made in the following, however, all statements made are applicable analogously to linear machines as well.

(6) The PM machine according to the invention is operated on a converter UR connected upstream of the 3-phase voltage system of the machine. The converter UR generates an alternating voltage appropriate for the machine PM and, both in operation as a motor and in operation as a generator, can produce the corresponding voltages and currents between machine PM and mains N in terms of amount and phase. Converter UR serves for electronic commutation of the machine (optionally via a resolver system or the like), i.e. at the terminals of phases U, V, W of the winding system, the converter UR provides a 3-phase system by means of which the machine is operated. The machine may also be operated with several converter units that have the function to provide a 3-phase system or a multiple 3-phase system.

(7) As illustrated in FIG. 3, the sinusoidal voltages and currents of the fundamental harmonic (designated 1. H. in FIG. 3) of the three phases U, V, W (which are offset from each other by 120° each) in a symmetric 3-phase system at all times add up to zero. The also sinusoidal voltages and currents of the 3rd harmonic (designated 3. H. in FIG. 3) in contrast thereto add up or accumulate, as they are each offset from each other by 120°.Math.3=360°. Thus, with the delta connection of the winding system, a resulting circulating current is created due to the voltages of the 3rd harmonic.

(8) According to the present invention, a high power density PM machine with concentrated winding (i.e. single coils) is operated on one or more active converter units having their (3-phase or 6-phase or multiple 3-phase) winding connected in a delta connection. The connection especially of larger electric machines in a delta connection so far is avoided when the induced voltage (U.sub.i, “EMF”) contains a significant 3rd (and other) harmonic, as the winding for the 3rd (and other) harmonic is short-circuited due to the delta connection. Due to this, there are created additional ohmic losses, temperature increases and torques (the maximum values thereof being present at very low speeds n, e.g. n.sub.B/100 . . . n.sub.B/1000). With high power density electric machines, in particular PM machines, however, the inductance can be set so high that the resulting short-circuit current of the 3rd harmonic remains low and causes only little additional losses (e.g. 0.001 . . . 0.01% of the rated power P.sub.B). The additional iron losses saved by this short-circuit current (as the 3rd harmonic of the iron B-field is attenuated thereby) may be a multiple of these additional ohmic losses (especially at higher speeds n). Moreover, the idle-operation sound components caused by the 3rd harmonic (and others) are compensated to a very large extent, except at very low speeds n (e.g. up to a speed n.sub.B/100, with a very rapid continuous decrease with increasing n).

(9) The PM machine according to the invention is designed for a rated power in the range between 100 kW and 20 MW. The winding system has such a high inductance that the related total reactance x.sub.B during rated operation (inclusive of all counter and leakage reactances, related to the induced voltage thereof) according to x.sub.B=ω.sub.B.Math.L.sub.Y.Math.P.sub.B/(Z.sub.3P.Math.U.sub.i,B.sup.2) is at least 0.8.

(10) In said equation: ω.sub.B=the electric angular frequency of the winding system in rated operation, L.sub.Y=the phase or strand inductance of the winding system in star connection (Y-connection), P.sub.B=the power of the machine in rated operation (rated power), Z.sub.3P=the number of 3-phase systems of the machine, U.sub.i,B=the amount of the induced conductor voltage in rated operation.

(11) According to the invention, the winding system is connected in the form of a delta connection, as shown in FIG. 2, in order to use this ring connection for attenuation (iron losses, sound) of upper harmonics (mainly the 3rd harmonic) while tolerating one or more circulating or ring currents, as described hereinbefore. By providing a high total reactance of the machine, the ring current remains limited, which thus limits the ohmic losses caused by the same as well.

(12) The PM machine preferably is a high power density PM machine in an enhanced performance range, with respect to which, for the afore-mentioned reasons of the high short-circuit current, it was avoided before to connect the winding system in a delta connection. According to an advantageous embodiment of the invention, the PM machine is designed for a rated power in the range between 1 MW and 3 MW. In accordance with another advantageous embodiment of the invention, the PM machine is designed for a rated power in the range between 3 MW and 6 MW. In accordance with still another advantageous embodiment of the invention, the PM machine is designed for a rated power in the range between 6 MW and 15 MW. According to the concept according to the invention, a PM machine with high total reactance can be operated in a delta connection in these power ranges in particularly advantageous manner, so as to exploit the resulting ring connection for attenuation (in particular iron losses, sound) of upper harmonics (mainly the 3rd harmonic).