Electrical generator or motor structure – Dynamoelectric – Rotary
Patent
1991-04-12
1992-10-13
Stephan, Steven L.
Electrical generator or motor structure
Dynamoelectric
Rotary
310190, 310214, H02K 126, H02K 1716, H02K 3493
Patent
active
051554046
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a squirrel-cage induction motor for frequency convertor operation comprising a rotor having a rotor core with a plurality of substantially axially extending rotor slots, in which rotor conductors are arranged, each rotor slot having a main portion in which a rotor conductor is arranged and a slot opening arranged between the main portion and that surface of the rotor which faces the air gap of the motor.
BACKGROUND ART
It is known that an asynchronous motor can be supplied from a static frequency convertor, for example for achieving a high speed of the motor or for making possible control of the speed of the motor. However, in frequency convertor operation additional losses harmonic losses, will arise because of harmonics in the motor current. By designing the motor so that its leakage reactance (short-circuit reactance) is high, a reduction of the harmonic currents, and hence of the additional losses, is obtained. However, an increase of the leakage reactance entails a reduction of the maximum torque of the motor and of its power factor, and it is therefore possible only to a limited extent to reduce the additional losses in this way.
From Swedish published patent application 433 995 it is known that the additional losses in an asynchronous motor supplied by a frequency convertor may be reduced by providing each winding bar with an axially extending channel facing the slot opening. However, this solution has several drawbacks. It is not possible to use it in rotor windings of a cast design. It gives a relatively low value of the differential leakage reactance of the motor and hence still relatively high harmonic currents and additional losses. In the solution mentioned it is not possible to effectively prevent the field in the vicinity of the slot opening from penetrating into the rotor conductor and generating additional losses there. Further, in this known solution it is the eddy currents in the rotor conductor that reduce the field penetrating into the conductor, and the effect achieved is therefore inseparably connected with a certain degree of additional losses.
SUMMARY OF THE INVENTION
The present invention aims to provide a motor of the kind described in the introductory part of the description, which
The above is achieved by providing the rotor core, according to the invention, with a magnetic bridge which separates the slot opening from the main portion of the slot, where the rotor conductor is arranged, and by giving the bridge a curved shape in the direction towards the rotor conductor.
As will be described more closely below, this provides a possibility of designing the rotor winding as a cast squirrel-cage winding, a considerable reduction of the additional losses of the motor and improved cooling of the rotor winding.
According to a preferred embodiment of the present invention, the bridge is provided with a recess, constituting an extension of the slot opening in the direction towards the interior of the rotor conductor. This results in an increase of the effective area exhibited by the slot opening for the leakage flux of the rotor conductor and hence an increase of the differential leakage reactance and a reduction of the harmonic currents and the additional losses.
Such recess is preferably designated as tapering from the slot opening in the direction of the rotor conductor and, in a similar manner, the width of the bridge tapers from the ends of the bridge towards the part thereof located centrally and directed towards the interior of the rotor conductor. This results in a very effective shielding of the leakage flux from the rotor conductor and, in addition, as linear a variation as possible of the leakage reactance of the rotor with the load of the motor, which for a given maximum leakage reactance gives a differential leakage reactance as high as possible at the working point and hence a very effective reduction of harmonic currents and of additional losses.
BRIEF DESCRIPTION OF THE DRAWING
The present invention will be descr
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Asea Brown Boveri
Haszko D. R.
Stephan Steven L.
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