Rotor structure of line-start permanent magnet synchronous motor

Abstract

The invention provides a rotor structure of line-start permanent magnet (LSPM) synchronous motor that includes a shaft; four fan-shaped magnetic poles each having a first eccentric circular arcs of the surface of the magnetic poles which has a center O1 that is offset from the center O of the rotor with an offset length OS1 and which makes the maximum thickness of the air gap roughly two to five times as much as the minimum thickness of the air gap; four permanent magnets disposing in the inner loop of each of the fan-shaped magnetic poles□a plurality of pear-shaped conductor slots disposing in equal spaces in the outer loop of the rotor in each of the fan-shaped magnetic poles and orienting in radial direction having O1 as the center for forming a squirrel cage winding; as well as four recesses at the midpoint of the first eccentric circular arcs of the surface of the magnetic poles in each of the fan-shaped magnetic poles.

Description

FIELD OF THE INVENTION [0001] The invention relates to a rotor structure of a line-start permanent magnet synchronous motor, and more particularly, to a rotor structure of a hybrid type motor combining the merits of an induction motor and a synchronous motor, and being able to lower the cogging torque and to improve the starting characteristics of a motor. BACKGROUND OF THE INVENTION [0002] The line-start permanent magnet (LSPM) synchronous motor of the prior art is a hybrid motor with its stator structure being substantially the same as that of an AC induction motor or that of an AC synchronous motor. On the other hand, the rotor structure of this hybrid motor is a combination of the squirrel cage structure in the rotor of the AC induction motor and the permanent magnet structure in the rotor of the AC permanent magnet synchronous motor. When the stator of the LSPM synchronous motor is connected to the power source, a rotating magnetic field is generated by the stator, and an induc...

AI Technical Summary

Benefits of technology

[0012] One of the objectives of the invention described in the first embodiment of the invention is to utilize the conventional LSPM synchronous motor of the hybrid type. This motor of the invention provides four fan-shaped magnetic poles having a central angle 90°. The circular arcs of the surfaces of the magnetic poles are defined as “the first eccentric circular arcs of the surface of the magnetic poles” having four “first eccentric points O1” as their centers that are offset from the rotor center O with offset lengths OS1 and radii R1. If the radius of the rotor is R, then R−R1=OS1. In this way, the air gap thickness of the motor constitutes a gradual and non-uniform distribution along the circumferences between the rotor and the stator. The length of the offset OS1 and the radius of curvature of the circular arc R1 can adjust to accommodate the variation of the range of the air gap thickness. The maximum air gap thickness T of the motor is anywhere from two to five times as much as the minimum air gap thickness t1, that is T=2t1˜5t1. In this way, the fact that the radial component of the magnetic flux density becomes very close to a sinusoidal wave distribution can further lower the cogging torque of the motor and reduce the operating vibration and noise of the motor. As the motor employs the first eccentric circular arcs of the surface of the magnetic poles, when the loading of the motor increases, even though the magnetic flux formed by the stator winding increases accordingly, the motor can still attenuate the effect of demagnetization of the magnetic field of the stator with respect to the permanent magnet. This is because that the air gap is relatively large at both ends of each of the fan-shaped magnetic poles.

[0014] One other objective of the invention described in the first embodiment of the invention is to provide a semi-circle recess at the mid-way of the first eccentric circular arcs of the surface of the magnetic poles. The diameter of the semi-circle recess is almost equal to the width of the tooth part of the stator of the motor. The recess can adequately attenuate the magnetic flux of the magnetic pole of the motor. In addition, the recess can also reduce the self-retaining torque caused by the magnetic flux of the magnetic pole of the motor so that the motor can improve its starting characteristic.

[0016] One further objective of the invention described in the third embodiment of the invention is to provide a smooth curve of “the second eccentric circular arcs of the surface of the magnetic pole” to substitute the above-mentioned semi-circle recesses. In this way, the motor can adequately reduce the radial component distribution of the magnetic flux density of the air gap at the mid-way of the second eccentric circular arcs of the surface of the magnetic poles. Moreover, the motor can further reduce the cogging torque as well as the vibration and noise during the operation. What is more, the LSPM synchronous motor can slightly attenuate the magnetic flux of the magnetic pole to reduce the transient state during the operation from stop to start due to the self-retaining torque caused by the fan-shaped magnetic pole of the motor. In this third embodiment, one can adjust the radius of the second eccentric circular arcs and the length of offset OS2 of the center of curvature O2 of the second eccentric circular arcs of the surface of the magnetic poles to accommodate the variation of the range of the air gap thickness. In this way, not only the motor can adequately reduce the self-retaining torque, but also the radial component of the magnetic flux density becomes very close to a sinusoidal wave distribution.

Problems solved by technology

However, the cogging torque of the permanent magnet synchronous motor of the prior art becomes very large since the permanent magnet with high magnetic energy product is used.

As a result, the motor is subject to occurrence of operating vibration and noise.

Generally, the stator provides skew channels to improve the demerit, but the winding work of the stator becomes rather difficult. FIG. 1 is the U.S. Pat. No. 5,925,727, Boyd et al. of the prior art.

However, since the rotor does not have squirrel cage structure provided, the rotor cannot be line-started without utilizing some other devices.

But, as described before, since the rotor 13 is round in shape without special designed configuration, therefore, the cogging torque as well as the vibration and noise of the motor while the motor is in operation are still relatively large.

However, just how the curvature of the surface of the rotor varies is not explained anywhere in their specifications.

However, generally, among all the permanent magnets of rare earth metals having high magnetic energy products, the permanent demagnetization temperatures are less than 200° C. Placing the permanent magnets in the permanent magnet containing slots beforehand can cause the permanent magnets to be subject to demagnetization during the casting manufacturing process.

However, no explanation is given anywhere in the specification as to what kind of the curve of the arc it belongs to.

What is more, the prior art does not provide any means to adequately attenuate the magnetic flux of the magnetic pole to reduce the self-retaining torque caused by the magnetic flux of the magnetic poles.

Therefore, the prior art cannot improve the starting characteristic of the motor at the transient state during the operation from “stop” to “start” of the LSPM synchronous motor.

Furthermore, as pointed out in the preceding discussion, since the rotor does not have squirrel cage structure provided, the rotor cannot be line-started without utilizing some other devices.

The foregoing statements are the disadvantages of the prior art.

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