Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1995-10-30
1998-03-31
Martin, David S.
Electricity: motive power systems
Induction motor systems
Primary circuit control
318808, H02P 534
Patent
active
057342504
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention refers to induction motors and concerns more exactly a method and arrangement for controlling a three-phase induction motor driven by an inverter coupling to be supplied with full-wave rectified mains alternating voltage and to provide the various phases of the motor with drive voltage consisting mainly of square shaped, in relation to a virtual zero reference level, positive and negative half periods that alternate with a fixed or adjustable drive frequency corresponding to the required speed.
The speed of induction motors is determined in the first instance by the frequency of the feed alternating voltage supplied. To drive such a motor at a higher speed than that which can be achieved using the ordinary mains frequency it is usual to employ an inverter coupling by means of which the motor can be frequency-controlled so that it can be run at the required speed. To achieve a large degree of simplicity in the motor's control device it is desirable to drive the motor using square voltage pulses. At power levels exceeding about 300 W, this results in inconvenience as the motor losses will be large and the motor sound level will be high. In inverter operation of induction motors for power levels higher than about 300 W, the inverter is therefore generally designed so that it gives off a voltage that is shaped like a sine wave. The frequency of the alternating voltage supplied is only changed if the speed is to be changed or to compensate for greater or lesser slippage. It is also common that the amplitude of the alternating voltage supplied is changed so that it increases with increasing frequency.
Inverters of the type described often become complicated and expensive to make, which results in inverter-controlled induction motors not being used in conjunction with domestic appliances of various types even if they would provide purely technical advantages.
Another problem with inverter-driven induction motors for powers above about 300 W is that they can give rise to considerable mains distortion. The reason for this is that a typical inverter coupling is fed with rectified and smoothed mains alternating voltage, which implies that, seen from the mains, current flows to the inverter only during a short period in the region round the mains alternating voltage peak value, when a reservoir condenser included in the coupling is charged. As the energy fed to the motor load is to be taken from the mains by current that flows only during short periods, current transients occur that give rise to the mains distortion mentioned above. At higher powers this distortion reaches unacceptable levels and various types of filter must therefore be provided, which further increases the expense of the device.
The invention is in the first instance envisaged for applications in conjunction with induction motors of small sizes and for very high speeds, exceeding 50,000 revolutions per minute, preferably in the speed range around 100,000 revolutions per minute. At these high speeds, the leakage inductance in the motor assumes a higher value due to the small physical dimensions of the motor. This leak inductance is in the first instance concentrated to the air gap between the rotor and stator. If the motor is now driven using square voltage, the leak inductance will affect the current so that it will more or less adopt a sine-wave form. The desired curve form of current can thus here be achieved without the supply voltage being sine shaped.
If, to obtain an inexpensive and simple inverter and also avoid the occurrence of mains distortion, such a motor is supplied with full-wave rectified unsmoothed mains alternating voltage, the higher drive frequency will be modulated by a frequency that is twice the mains alternating voltage frequency. Such a low-frequency modulation of the drive frequency results in the rotor, if it is braked by a load, changing its speed up and down with a frequency of 100 Hz (at a grid frequency of 50 Hz). If, for example, a rotor rotates at 90,000 revolutions per minu
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Aktiebolaget Electrolux
Martin David S.
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