Electric power conversion systems – Current conversion – Including automatic or integral protection means
Patent
1988-11-29
1989-11-28
Wong, Peter S.
Electric power conversion systems
Current conversion
Including automatic or integral protection means
363 37, 323222, H02M 544
Patent
active
048841854
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an electric energy converter, more particularly a frequency converter, operating by pulse width modulation--abbreviated to PWM--and connectable to a load for varying the energy delivered thereby.
Known frequency converters for synchronous or asynchronous motors generally comprise a rectifier bridge connected to an inverter through an intermediate circuit having a smoothing capacitor. The inverter must be connected to the phase windings of the motor and it operates by pulse width modulation. The inverter has several arms each of which is equipped with two static switches including respective recovery diodes. The middle point of each pair of switches of the same arm is connected to a respective phase winding of the motor. A control circuit enables or disables the switches at chosen times. By static switches is meant in the present description bipolar or MOS transistors, bipolar MOS components such as those known under the name IGT, thyristors of the GTO type which can be disabled or other similar components able to be disabled.
If we assume that the first switch of an arm is disabled and that the second switch of the same arm is not yet conducting, the current which transmits through the phase winding of the motor connected to the middle point of the arm passes through the recovery diode associated with the first switch. When the second switch is enabled and during the recovery time of the first diode, this switch is subjected to the passage of the reverse current of the first diode at the same time as the passage of the current of a winding of the motor and, as long as the voltage at the terminals of the switch has not dropped to a conduction value V.sub.CESAT, high switching losses can be observed.
Of course, the switching losses increase with the operating frequency of the converter. Now, this frequency is related to the value of the inductances of the motor and it must be all the higher the smaller the inductances. In the case of a synchronous motor, this is precisely the case and it is therefore necessary to use a high switching frequency with correspondingly not inconsiderable switching losses. On the other hand, in the case of an asynchronous motor, and although the inductances of this latter are relatively high, a tendency can be observed for increasing the frequency of the converter so as to reduce the pulsating couple and the noise, which again results in an increase of the switching losses.
In a known arrangement for reducing the switching losses, the converter comprises in series with each switch an inductance and in parallel with each switch a resistance-capacity-diode network RCD. Each inductance contributes to limiiting the transient current passage previously mentioned on enabling the switch whereas, when this latter is disabled, the energy stored in the inductance charges the capacity through the diode, the capacity then being discharged through the resistance. This arrangement has the drawback of requiring for each switch an inductance and an auxiliary RCD network. In addition, it does not allow integrated bridges to be used including switches interconnected with their recovery diodes.
The present invention aims at overcoming these drawbacks by efficiently reducing the switching losses of a pulse width modulation converter--more particularly a frequency converter--by means of a simple device, with a small number of components and a reduced cost.
The expression PWM converter designates any variable voltage generator with modulated pulse width such as a frequency converter for a synchronous or an asynchronous motor, a converter for a DC motor, an inverter delivering an AC voltage etc.
The object of the invention is in addition to make it possible in some cases to recover the residual switching losses occurring in the modulator bridge of the converter.
According to the invention, the converter includes, in one of the conductors connecting the intermediate circuit to the modulator bridge, an inductance for assisting in causing conduction of the static switches o
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patent: 3569819 (1971-03-01), Martzloff et al.
patent: 4378533 (1983-03-01), Carnel
patent: 4670830 (1987-06-01), Peppel
patent: 4763239 (1988-08-01), Ball
S. Manias et al., "An SMR Topology with Suppressed DC Link Components and Predictive Line Current Wave Shaping", Conference Record of 1986 IEEE Industry Applications Society Annual Meeting, Part I, Denver, Col., (U.S.), Sep. 28-Oct. 3, pp. 630-639.
Drucker William A.
La Telemecanique Electrique
Wong Peter S.
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