Mag-Amp Magnetic Amplifiers

Magnetic amplifiers, also called mag amps for short, provide an electro-magnetic method of amplification.  Mag amps were quite common prior to the development of solid state transistors.  As advances in semiconductor technology progressed, magnetic amplifiers because a relatively expensive component.  Consequently the use of mag amps declined.  A properly made mag amp is highly reliable, hence they are still used in some applications with demand the reliability performance criteria that a mag amp can meet.  Another feature of mag amps is the high isolation voltages that can be achieved between windings with proper design.  Mag amps may still be preferred over semiconductor devices in safety critical applications.

A typical simple mag amp contains two identical coils, each having identical high permeability square loop magnetic cores and each wound with an identical winding not shared with the other coil.  An alternating voltage source is connected to one end of these windings and a load is connected to the other end.  The windings are either connected in series or in parallel such that the cores’ magnetic flux generated by the alternating voltage are out of phase (in opposite directions).  Alternating current (A.C.) will flow through these windings.  Either a shared second winding is wound on both coils or each coil is wound with a second identical winding.  In the latter case the windings are series connected such that a direct current (D.C.) flowing through these windings generate magnetic flux in the cores, which are in phase (in the same direction).  These windings are connected to a variable D.C. current source (which might consist of series connected D.C. voltage source and a variable resistor).  The D.C. winding(s) is (are) referred to as the control winding(s).  Schematic representations of two typical mag amps are given in Figures 1 and 2 further below.  The mag amps shown may also be referred to in literature as a type of saturable reactor.  A mag amp may also be referred to in literature as a type of transductor.

 

Air gaps within a mag amp’s core structure are detrimental to mag amp performance.  Proper mag amp performance requires nearly identical symmetry in core flux excursions; hence leakage flux should be minimized.  Toroidal cores have essentially zero air gaps and the toroidal geometry maximizes magnetic coupling and minimizes leakage flux.  Consequently, toroids are the core shape of choice.

Other variations of mag amps exist, including a single core version that has three core legs.  The middle leg has a D.C. control winding.  The outer legs have identical A.C. windings.  In theory D.C. flux generated in the center leg divides equally and flows through both outer legs.  The A.C. windings are connected such that their phases do not permit any A.C. flux flow through the center leg (in theory).  There are practical difficulties (in the form of magnetic tolerances) with this type of mag amp design.  More advanced mag amp circuits use rectifying elements to isolate the load from the mag amp during core reset.  Core reset refers to the volt-second transition from saturation flux (top flat portion of the B-H loop) to the flux value at the opposite side of the B-H loop (bottom flat portion of the loop).

Butler winding can make (and has made) mag amps.  Butler winding has several types of toroid winding machines that can be used to wind a variety of mag amp core sizes. This includes toroid-taping machines.  For toroids, we can (and have done) sector winding, progressive winding, bank winding, and progressive bank winding.  Butler winding also has other types of winding machines. That includes two programmable automated machines.  We can wind and assemble various standard types of  “core with bobbin” structures (E, EP, EFD, PQ, POT, U and others), and some custom designs.  Our upper limits are 40 pounds of weight and 2 kilowatts of power.  We have experience with foil windings, litz wire windings, and perfect layering.  Butler winding has vacuum chamber(s) for vacuum impregnation and can also encapsulate.  To ensure quality, Butler Winding purchased two programmable automated testing machines.  Most of our production is 100% tested on these machines.  For more information on Butler Winding’s capabilities, click on our “capabilities” link.

 

Mag Amp Theory

The following discussion is not intended to give a detailed understanding of mag amp operation.  It is not intended to describe all the variations of mag amp designs or applications.  It is intended to give a basic insight to how a typical simple mag amp functions.  Rectifier aided mag amp circuits are not discussed.  Butler Winding has some but limited experience with mag amps.  If you require more information than the following discussion supplies, please contact Butler Winding and ask to speak to an engineer about mag amps.  Butler Winding will provide whatever help we reasonably can.

Refer to the schematic of Figure 1 bearing in mind (in theory) that the two coils have identical windings and identical cores.  Because of transformer action, the A.C. voltage impressed across the mag amp’s A.C. windings will induce a voltage across each control winding.  Because of the opposite phasing of the A.C. windings, the induced voltages in the D.C. windings will buck each other and exactly cancel each other (in theory) resulting in zero A.C. voltage induced across the D.C. source.  Consequently, low impedance D.C. source will not load down the A.C. windings.

Consider the impedance of the A.C. windings with no D.C. current supplied.  The core and windings are designed such that; 1) the core does not saturate at the maximum intended A.C. voltage, and 2) each A.C. winding has a relatively much higher impedance than the intended load.  Because of the high impedance, very little A.C. current flows.  Consequently, there is very little voltage drop across the load.

 

Now consider the impedance of the A.C. windings with a D.C. current flowing through the control winding.  Both cores have a D.C. biasing flux of equal value and the same phasing.  The A.C. windings of Figure 1 are connected in parallel but with opposite phasing.  The total flux in a core is the sum of the D.C. flux and the A.C. flux.  Because of the opposite A.C. winding phasing, the A.C. voltage increases the core flux of one core while decreasing the core flux of the other core until saturation occurs.  Eventually the alternating fashion of the A.C. voltage causes the changing flux to reverse the direction of flux change of both cores.  Now apply enough D.C. current to cause one core to enter saturation.  The core’s flux reaches its maximum values and does not change (ideal theory) while in saturation; hence no induced voltage will oppose the applied A.C. voltage.  The impedance of that core’s A.C. winding drops to near zero value.  There can be very little voltage drop across that A.C. winding.  The other A.C. winding is connected in parallel to this A.C. winding.  This A.C. winding shunts the current around the other A.C. winding hence the other A.C. winding also sees very little voltage impressed across it.  Consequently the flux of the other core changes very little (essentially stays where it is).  While a core is saturated there is very little impedance between the A.C. voltage source and the load impedance.  Consequently significant load current flows during saturation and produces a relatively large voltage drop across the load.  Because of the eventual A.C. voltage reversal, the saturated core will eventually come out of saturation, high A.C. winding impedance will occur again, and the load current will again drop to near zero value.  Eventually the other core saturates resulting in high load current until the core leaves saturation.  The mag amp has seen a complete A.C. cycle and will proceed to the next cycle.  For mag amps, entering saturation is like closing a switch.  The time spent in saturation is the “turn-on” time of the mag amp switch.

The amount of time spent in saturation is determined by the amount of D.C. biasing current.  A larger D.C. bias current causes the cores to enter saturation earlier and exit saturation later, thereby increasing the length of time current is delivered to the load, thereby increasing the average amount of current delivered to the load in a given period of time.  Once a steady state condition is reached in an idealized mag amp, it can be shown that the averaged ampere-turns of the load current are proportional to the ampere-turns of the control current.  With appropriate choices of turns ratio, windings, and cores, one can achieve significant power amplification gain.

The schematic in Figure 2 shows the A.C. windings connected in series.  When one core saturates both of its winding have relatively very low impedance and can be ignored.  The core’s A.C. winding does not shunt the other A.C. winding, but the other A.C. winding will not maintain its high impedance level if the D.C. source has a sufficiently low impedance.  With one core saturated the low impedance D.C. source becomes a transformer-coupled load to the unsaturated A.C. winding.  The impedance on the unsaturated A.C. winding drops to the transformer coupled reflected value of the low impedance D.C. source.  A load current flows which produces a significant load voltage.

Mag-Amp Diagram Fig1 & Fig 2

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