Magnetic Amplifier - Miller 330A/BP

A magnetic amplifier or MagAmp is not so much a means of amplifying as it is a means of controlling AC power. These devices were used extensively in welders before high power SCRs became available. They consist of two or three coils wound around an iron core. The main coil caries the AC power that is to be controlled while another coil of finer wire carries DC control current. A third coil is often used to carry bias current for calibration.
When AC current is passed through the main coil it induces a fluctuating magnetic field in the iron core that resists or impedes the flow of the AC current through inductive reactance. The welder's output is significantly restricted.
If DC current is passed through the control coil at the same time the AC current is flowing in the main coil, The DC control current will begin to build-up a magnetic field of its own. This DC induced magnetic field will use up some of the magnetic capacity of the iron, leaving less magnetic capacity to oppose the flow of AC current. The output of the welder is increased.
The DC control current can be increased to the point where the iron is saturated, leaving virtually no magnetic capacity to oppose the AC current flow. This would result in maximum output of the welder, or maximum output within the range the welder is set for.
This is my best recollection of the function of a MagAmp. There are many very smart folks here, so please feel free to offer any corrections or additions.
Bob

I'd say you did a magnificant job of explaining how a magnetic amplifier works. Billh

Thanks Billh. Bob
calibration. significantly

Bob, very helpful. Does the magnetic amplifier also control the power when the welder is in DC mode? Thanks.

Yes, This device controls the power before it is converted to DC by the rectifier.
Bob