An amplidyne comprises an electric motor which turns a generator on the same shaft. Unlike an ordinary motor-generator, the purpose of an amplidyne is not to generate a steady voltage but to generate a voltage proportional to an input current, to amplify the input. The motor provides the power, turning the generator at a constant speed, and the signal to be amplified is applied to the generator's field winding. The higher the current applied to the winding, the stronger the magnetic field and thus the higher the output voltage of the generator. So the output voltage of the generator is an amplified copy of the current waveform applied to the field winding. In a typical generator the load brushes are positioned perpendicular to the magnetic field flux. To convert a generator to an amplidyne, what would normally be the load brushes are connected together and the output is taken from another set of brushes that are parallel with the field. The perpendicular brushes are now called the 'quadrature' brushes. This simple change can increase the gain by a factor of 10,000 or more. The amplidyne's frequency response is limited to low frequencies, it cannot even handle audio frequencies, so its use is limited to amplifying low frequency control signals in industrial processes. Historically, amplidynes were one of the first amplifiers to generate very high power, allowing precise feedback control of heavy machinery. Vacuum tubes of reasonable size were unable to deliver enough power to control large motors, but vacuum tube circuits driving the input of an amplidyne could be used to boost small signals up to the power needed to drive large motors. Early gun tracking and radar systems used this approach. Amplidynes are now obsolete technology, replaced by modern power semiconductor electronic devices such as MOSFETs and IGBTs which can produce output power in the kilowatt range.
The amplidyne was first used in the US Navy in servo systems to control the electric motors rotating naval gun mounts, to aim the gun at a target. The system is a feedback control system in which a feedback signal from a sensor representing the current position of the gun is compared with the control signal which represents the desired position, and the difference is amplified by the amplidyne generator to turn the gun mount motor. The components are:
the amplidyne motor-generator, which is similar to a Ward Leonard control drive;
and the follow-up DC motor which drives the load to be positioned.
Chapter 10 of the U.S. Navy manual Naval Ordnance and Gunnery, Volume 1 explains the operation of the amplidyne: Specifically, the phase of the control transformer's output provided the polarity of the error signal. A phase-sensitive demodulator, with the synchroAC power as its reference, created the DC error signal of the required polarity.
