Rotating Amplifier

(also rotary amplifier or dynamoelec-tric amplifier), an electric machine that is used to amplify, by means of the energy of a prime mover (usually an electric motor), the power of a signal supplied to a field winding. Rotating amplifiers are used in automatic control systems. The power rating of such amplifiers ranges from fractions of a watt to tens of kilowatts; the gain—that is, the ratio of the output power to the input power—is 10⁴–10⁵. In a rotating amplifier, a small change in the power supplied to the field excitation circuit results in a much larger change in the output power.

A distinction is made between direct-field rotating amplifiers, which have one amplifier stage, and cross-field rotating amplifiers, which have two amplifier stages. Two-stage rotating amplifiers, called amplidynes, are more widely used.

An amplidyne (Figure 1) is a DC generator, usually a bipolar generator, with two pairs of brushes on the commutator. One or more field windings, which are more often called control windings, are located on the stator poles. When a signal to be amplified is supplied to the control winding (CW), a magnetic flux Φ₁ is produced; the flux is parallel to the axis d-d. An electromotive force (emf) is induced in the armature winding. The emf reaches its maximum value between the brushes a-a and is equal to zero between the brushes b-b. The armature is short-circuited by the brushes a-a. Therefore, even when the emf in the armature winding is not substantial, a sufficiently large current I_a is generated. The current I_a causes the signal power to be amplified, thus completing the first stage of amplification.

Figure 1. Circuit diagram of an amplidyne: (1) and (2) armature brushes, (CW) control winding, (Co) compensating winding, (Φ₁,) magnetic flux along the axis d-d, (Φ_a,g) magnetic flux associated with the transverse field, (U₁) control winding voltage, (I₁) control winding current, (U₂) output voltage, (I₂) output current, (F_ad) magnetizing force of armature, (F_co) magnetizing force of compensating winding

In the second stage of amplification, the current I_a generates a strong transverse magnetic field, which is associated with the magnetic flux Φ_aq. When the armature rotates in the transverse field, a voltage U₂ is induced between the brushes b-b, which are connected to an external circuit. As a result, a large current I₂ is generated in the external circuit. The current I₂ gives rise to a high output power.

An additional winding called a compensating winding produces a magnetizing force F_Co that is equal to the armature’s magnetizing force F_ad, thus eliminating signal distortion.