Electricity: motive power systems – Synchronous motor systems
Patent
1997-03-10
1998-10-20
Sircus, Brian
Electricity: motive power systems
Synchronous motor systems
318803, 318254, 307 83, 363 37, H02P 146, H02K 2300, H02J 100
Patent
active
058251529
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a load-commutated synchronous motor drive, which comprise
a line-commutated network-side converter with dc terminals, and with ac terminals for connection to an alternating voltage network, and with a natural pulse number, to a synchronous machine, and with dc terminals which are connected to the dc terminals of the network-side converter via a current-source dc intermediate link.
BACKGROUND ART
Synchronous motor drives of the kind described above are previously known from, for example, ABB Handbok Industri, Asea Brown Boveri AB Vasteras 1993 (referred to in the following as "ABB Handbok"), pages 276-284, from IEEE Transactions on Industry Applications Vol. IA-19, No.2, March/April 1983 (referred to in the following as "IEEE"), pages 217-222, and from H. Buhler:"Einfuhrung in die Theorie geregelter Drehstromantriebe", Birkhauser, Basel-Stuttgart 1977 (referred to in the following as "Buhler"), e.g. Band I, pages 21-23; Band II, pages 88-158.
In a synchronous motor drive of the above type, the machine-side converter is of a line-commutated type, and the induced alternating voltage of the synchronous machine constitutes the commutating voltage of the converter. This voltage decreases with the frequency, that is, with the speed of the machine, but since the resistance in the commutating circuit is negligible in comparison with the reactance down to relatively low speeds, the commutating capability of the machine is not influenced in any practical sense. At very low speeds, however, the effect of the resistance is no longer negligible, and the commutating capability of the machine is then not sufficient for commutation of intermediate-link currents which are greater than the limiting current for discontinuous current.
To achieve commutation, also at very low machine speeds, of currents larger than the above-mentioned limiting current, that is, to make it possible to maintain the current and torque of the machine also at a very low speed, it is previously known from ABB Handbok, page 282, from IEEE, page 219, and from Buhler, Band 2, pages 92-93, to use so-called forced commutation operation. In this case, the intermediate-link current is reduced to zero with the aid of the network-side converter in connection with each commutation of the machine-side converter. Each such reduction, however, entails a loss of current-time area and hence a loss of torque--a torque disturbance (see, e.g. ABB Handbok, page 284, last paragraph). Moreover, these torque disturbances are reinforced by the speed regulator of the system, which increases its output in order to compensate for the loss of the torque-time area which is equivalent to the current-time area caused by the intermediate-link discontinuous current operation.
The torque disturbances occur with a frequency which is proportional to the machine frequency. It has proved that the frequency of the torque disturbances may typically coincide with a mechanical natural oscillation frquency of the drive system. This means that the torque disturbances will excite mechanical natural oscillations in the drive system. In such drive systems as are operating with a high current and a high torque at low speeds, these oscillations may become considerable, and the oscillations entail considerable disadvantages in the form of, for example, a high sound level, increased wear and a risk of mechanical damage. These disadvantages are particularly great in drive systems which are operating for lengthy periods at a critical speed, or which often pass through a critical speed range.
The sudden reduction and increase, respectively, of the current-time area when changing to and from, respectively, intermediate-link discontinuous current operation further entails an unfavourable discontinuity in the control properties of the system.
To ensure current commutation in a synchronous motor drive at low speeds, it has been proposed, as an alternative to intermediate-link discontinuous current operation, to short-circuit the smoothing inductor of the
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ABB Industry OY
Sircus Brian
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