Electricity: motive power systems – Induction motor systems
Patent
1996-12-19
1998-11-24
Ip, Paul
Electricity: motive power systems
Induction motor systems
318800, 318803, H02P 534, H02P 7622
Patent
active
058412646
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT/FR95/00817 filed on Jun. 20, 1995.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a method of stepwise voltage control using thyristors for supplying an induction motor at a fixed frequency and at a variable voltage, more particularly in the situation in which the load that it drives has a quadratic torque-speed law, noteworthy in that it combines great simplicity of the equipment with a level of performance in terms of progressive stopping and starting that has hitherto been unattainable using more complex equipments such as frequency converters.
2. Discussion of the Background
Electronic starters for induction motors have achieved significant penetration of the industrial market in the last ten years, replacing older generation electrotechnical hardware such as star-delta switches, autotransformers and liquid rheostats. Their success is related to advances in power semiconductors (increased reliability and economic competitivity), on the one hand, and to advances in digital control circuits having an increasing processing capacity, on the other hand. Among the various electronic speed variation systems, the electronic starter represents the simplest means of controlling the speed of induction motors: in a system of this kind, the power circuit or three-phase stepwise voltage controller typically comprises a set of three alternating current switches each in series with one phase between the AC line voltage and the motor, each switch typically comprising two thyristors connected in anti-parallel. The fact that these switches change state at a low frequency, namely that of the AC line voltage, and do not require any turn-off control, as turn-off occurs naturally when the current passes through zero, also simplifies the control circuits. However, these starters can only control the amplitude of the motor voltage, at the fixed frequency of the AC line voltage. By comparison, frequency converters have a more complex structure, as much from the point of view of the power circuit, typically comprising a three-phase power transistor bridge with the rupture capacity needed to switch the motor current at a frequency of at least a few kilohertz, as from that of the control circuit, which has to generate variable voltage and frequency waves by pulse width modulation. To compensate this, they have a second control quantity, namely the frequency, which can be varied independently of the magnetic flux and the torque to optimize the operation of the motor at each point, in particular with regard to slip and losses. As a result, applications are divided between frequency converters and electronic starters, the former offering high performance and the latter moderate cost.
Given the trend for increasingly higher performance of digital control circuits, it has now become possible to expand the field of applications of electronic starters into that of frequency converters. For example, starting and stopping pumps represents a particular problem due to the existence of mechanical resonance that is manifested on the occasion of a rapid variation in flowrate by oscillation of the fluid in the pipe, known as "water hammer". This phenomenon is harmful as much through its reduction of the service life of the installation as through the accompanying acoustic noise. This noise is particularly unacceptable in water distribution installations in urban areas. Previous means of solving this problem tend to eliminate all sudden variation in the flow, and therefore in the speed, during stopping or starting. They use two techniques: consisting of a frequency converter and an alternating current motor.
Both methods have the same drawback, namely high cost, increasingly so with increasing power levels, the latter being routinely between 10 kW and 500 kW. Efforts to date to eliminate "water hammer" when stopping a pump, by regular deceleration commanded by a basic electronic starter, have failed. It is well known that varying the voltage at constant frequency introduces
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Caen Claude
Ip Paul
Kohen Cecile
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