Electricity: motive power systems – Automatic and/or with time-delay means – Rate-of-change of a condition
Patent
1995-11-01
1998-12-29
Cabeca, John W.
Electricity: motive power systems
Automatic and/or with time-delay means
Rate-of-change of a condition
318629, 318623, 318611, 310 51, 257665, 32 58, H02H 0708
Patent
active
058545468
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
This invention relates to a method for suppression of the emission of radio frequency energy from an electrical drive comprising the combination of an electronic controller and an electric motor, and to an electrical drive adapted to operate in accordance with the method of the invention.
BACKGROUND OF INVENTION
The control of operating characteristics such as the speed or torque of an electric motor commonly is achieved by use of an electronic controller which incorporates a semiconductor or other solid state device arranged to operate in a high speed switching mode. An example of an electronic controller of this type is a pulse width modulation (PWM) inverter.
Said high speed switching mode devices are employed in a range of types of electrical drive, e.g. for the control of AC inverter drives with permanent-magnet motors, induction and reluctance motors, servo drives having AC or DC motors and also switched reluctance motors. By the use of a high speed switching mode type of controller it is possible to achieve various desired operating features such as smooth control of the output power or torque of an electric motor without a significant loss of electrical energy.
The benefit of the flexibility in control of an electric motor which can be achieved by the use of a high speed switching mode electronic device does, however, result in the generation of high levels of radio frequency energy which potentially can enter the surrounding environment to cause unacceptable disturbance to neighbouring equipment. In some cases that disturbance can have potentially dangerous consequences. This has resulted in stringent regulations, such as the Electromagnetic Compatibility Directive 89/336/EEC, which are now established to ensure that new equipment, and its installation, will not result in potentially hazardous levels of radio frequency emissions.
The reduction of radio frequency emissions from electrical drives so that they lie within acceptable limits may be achieved by the use of an electrical filter, but a suitable filter commonly is unduly expensive or bulky. Also, with an AC power supply the filter causes some earth leakage current, which presents an electrical safety problem and may cause protective devices to operate prematurely. Other conventional techniques such as arranging that circuits liable to generate radio frequency emissions should be deliberately coupled to earth or provided with an earthed screen are either not possible or not effective in power circuits where the earth and the power system are required for safety reasons to be electrically separate.
The present invention seeks to provide an improved method for the suppression of radio frequency emissions from an electrical drive, and an electrical drive adapted to operate in accordance with the method of the invention.
SUMMARY OF INVENTION
In accordance with one of its aspects the present invention provides a method for the suppression of radio frequency emissions from an electrical drive of the kind comprising an electronic controller comprising high speed switching means which generates high frequency energy and an electric motor to which power from a source of electrical power is supplied via said electronic controller, said method comprising providing an electrical screen to receive radio frequency energy which, in use, is emitted from at least a part of the electrical drive and electrically connecting said electrical screen back to the input side of said switching means.
The method may comprise use of an electrical screen of a kind which substantially wholly surrounds at least a part of the electrical drive from which, in use, radio frequency energy is emitted. Preferably use is made of an electrical screen which, at the least, substantially wholly surrounds at least a part of the electric motor.
The invention provides also an electrical drive comprising an electronic controller comprising high speed switching means which generates high frequency energy, and an electric motor to which power from a source of electr
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Cabeca John W.
Control Techniques PLC
Hochberg D. Peter
LandOfFree
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