Eddy Current Coupling
Presently Dr Brown is working on the configuration of a monitoring device to alert of abnormal circumstances in the operation of an Eddy Current coupling device referred to commercially as a ‘Magna Drive’.
What is a Magna Drive?
In a typical Adjustable Speed Drive (ASD), Magna Drive application, the ASD is installed between a motor and the load the motor is driving, generally a pump, fan or blower. The drive consists of two independent components that have no physical contact. One component, a precision rotor assembly containing high-energy permanent magnets, is mounted on the load shaft. The second component, a copper conductor assembly, is connected to the motor shaft. Relative motion between the magnets and the copper creates a magnetic field that transmits torque from motor side to load side. In effect, the magnetic “pull” between the magnets on the load side of the drive and the copper on the motor side creates the coupling force; varying the width of the air gap changes the coupling force, so the amount of torque transmitted between the motor and load can be continuously adjusted. This allows precise and efficient speed control for optimum performance, including “cushioned starts.” The result is a much more efficient use of energy.
The latter explanation of operation is all well and good yet to those inquisitive mind it provokes the question how does a Magna Drive functions if the second component of the drive, is a copper conductor assembly and relative motion generates magnetic pull and torque since copper is non-magnetic? Well fundamentally the drive function by way of the electromagnetic induction.
The complicated ‘physics’ stuff!
Faraday’s law of induction (briefly, Faraday’s law) is a basic law of electromagnetism predicting how a magnetic field will interact with an electric circuit to produce an electromotive force (EMF)—a phenomenon known as electromagnetic induction. It is the fundamental operating principle of transformers, inductors, and many types of electrical motors, generators and solenoids.
The Maxwell–Faraday equation (listed as one of Maxwell’s equations) describes the fact that a spatially varying (and also possibly time-varying, depending on how a magnetic field varies in time) electric field always accompanies a time-varying magnetic field, while Faraday’s law states that there is EMF (electromotive force, defined as electromagnetic work done on a unit charge when it has traveled one round of a conductive loop) on the conductive loop when the magnetic flux through the surface enclosed by the loop varies in time.
In simple terms
A conductive surface (copper disc) moving past an array of rotating magnets develops many small ‘circular’ electric currents referred to as ‘eddy currents’ in the copper disc. The eddy currents are induced in the disc by the radiating magnetic field of the magnets, as described by Faraday’s law of induction.
By Lenz’s law, the circulating ‘eddy currents’ thus create their own magnetic field that opposes the field of the magnets, i.e. repels. Recall school, playtime physics experiments, unlike poles of magnets (red/black) attract whereas like poles (red/red,../black/black) repel.
Thus the moving copper disc conductor experiences a drag force from the magnet that opposes its motion, proportional to its velocity.
The kinetic energy of the rotating copper disc is dissipated as ‘heat’ generated by the eddy currents flowing through the electrical resistance of the copper disc. In simple terms if the speed of the copper disc becomes too great (over speed circumstance) the copper disc will rise in temperature to a point of state/phase change, i.e. it will melt!!, hence the need for a monitoring device to alert of abnormal circumstances, i.e. SensCoupling.
Eur Ing Dr Robert Brown (Robert) is a Consultant Engineer in the fields Electrical Electronic and Control Engineering. Robert is also an accomplished professional Expert Witness having prepared and presented many court compliant reports and presented oral evidence within the High Court, Crown Courts and County Courts.
For further information please contact Robert via;
Email,…robert.brown@frasergeorge.com or robertbrown@robertbrown.uk.com
Tel Land: +44 (0)1777 709175
Tel Mobile: +44 (0) 7976250624
