rotating amplifier[′rō‚tād·iŋ ′am·plə‚fī·ər]
(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 Ia is generated. The current Ia causes the signal power to be amplified, thus completing the first stage of amplification.
In the second stage of amplification, the current Ia generates a strong transverse magnetic field, which is associated with the magnetic flux Φaq. 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 FCo that is equal to the armature’s magnetizing force Fad, thus eliminating signal distortion.
REFERENCEGoriainov, F. A. Elektromashinnye usiliteli. Moscow-Leningrad, 1962.
M. D. NAKHODKIN