Magnetic Amplifiers – Custom Built
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.
- highly reliable
- high isolation voltages
- two identical coils
A properly manufactured mag amp is highly reliable, hence they are still used in some applications which 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.