MAGNETIC AMPLIFIERS

As we stated earlier in this chapter, various types of servo amplifiers are used to drive servo motors. When the amplifier is required to produce a large amount of power, the conventional electronic amplifier becomes less efficient than some other types. The following is a brief discussion of a typical magnetic amplifier used in a servo system where large amounts of power are required to move a heavy load. If you need to refresh your memory on the theory of the magnetic amplifier, refer to Module 8 of this training series, Introduction to Amplifiers.

Magnetic Amplifiers in a Servo

Figure 2-20 illustrates a magnetic amplifier being used as the output stage of a servo amplifier.

Figure 2-20. - Magnetic amplifier used to drive a servo motor.

The output of the servo amplifier is connected to one of the motor windings (controlled winding W₁). The other winding (uncontrolled winding W₂) is connected across the ac source, in series with a capacitor. The capacitor provides the required 90 phase shift necessary to cause motor rotation. The phase relationship of the current through the two windings determines the direction of rotation of the servo motor.

The magnetic amplifier consists of a transformer (T₁), and two saturable reactors (L₁ and L₂), each having three windings. The key point to the operation of this circuit lies in the fact that the error signal windings are connected in series-opposing while the bias windings are series-aiding.

With the circuit in the quiescent state (no input), the dc bias voltage causes the dc bias current to equally and partially saturate both reactors (L₁ and L₂). The reactances of L₁ and L₂ now being equal result in canceling currents through the W₁ windings of the servo motor. With only one input to the motor, it remains at rest.

Now, let's apply an error signal to the error signal windings. L₂ saturates and L₁ is driven further out of partial saturation, because the error windings are in series-opposition. This results in the error signal aiding the bias current in reactor L₂ and tending to cancel the bias current in reactor L₁. The reactance of L₂ is reduced, causing an increased current through the L₂ circuitry. In the other circuit (L₁), the reverse is true; its current decreases. This imbalance in the L₁ and L₂ circuitry results in current flow through W₁, say from left to right, and causes the motor to turn Reversing the polarity of the error signal causes the direction of motor rotation to change. This is done by saturating reactor L₁ instead of reactor L₂ and causing current to reverse directions through W₁.

In the previous discussion, an ac motor was driven by the output of the magnetic amplifier. If a dc motor is required in the servo to move a heavy load, the ac output from the magnetic amplifier must be rectified.

NOTE: All of the components that have been described as units within a servo system are, in general, the same components used in many other electronic and electrical applications. The theory of these components has been discussed here and in other modules of the Navy Electricity and Electronics Training Series. If you have the desire or a need for an in-depth study of these components, the following are excellent References:

Electronics Installation and Maintenance Books, NAVSEA 0967-LP-000-0130, for synchro and servo subjects. Electronics Installation and Maintenance Books, NAVSEA 0967-LP-000-0120, for the basic components of the servo system.

These References should be available in the technical library of your ship or station.

Q.25 What the three basic components make up the typical magnetic amplifier?

MULTI-LOOP SERVO SYSTEMS

Now that we have gone through the various servo loops and their components, let's continue our discussion with a realistic application of a servo system.

Very seldom will we find applications where one type of servo loop is used by itself. Usually several loops are combined through the use of various types of relays and switches. The many components of a complex system are caused to work together by switching them in and out as necessary.

Figure 2-21 illustrates a practical application of a multi-loop servo system. You should be able to recognize by now the different loops and components that make up this system. Nothing is really new in the system; we discussed all the loops and components earlier in this chapter.

Figure 2-21. - Multi-loop servo system.

As shown by the relay conditions, the system is configured, in its normal state, as a closed-loop position servo. This is indicated by the heavy dark lines in the figure. An alternate configuration positions the load in this system by using the potentiometer. This is done by energizing relay K₂, and switching the system to an open-loop configuration. At the same time, the deenergized contacts of K₂ (1-3) open, thereby breaking the closed loop. The open loop is shown by the medium density lines in the figure. This loop is not as accurate as the closed loop, because the operator must intervene by turning the shaft of the potentiometer back to the zero voltage position to stop the load at the desired position. This type of circuit could be used by maintenance personnel to position the load for easy access to equipments, such as on an antenna or gun mount. The open loop can also function as a basic velocity loop by simply not returning the potentiometer to the zero position. This results in a constant error signal being present at the wiper arm of the potentiometer. With this condition, the load will continue to drive at some speed (rate) determined by the components in the loop.

The last loop we will consider is the closed-loop velocity servo, indicated by the fine density lines.

This loop is switched into operation by energizing K₁. Notice that there are two inputs to the summing network with K₁ energized, the electrical input through contacts 2-4 and the feedback from the tach through contacts 1-3. The two signals are compared in the summing network, and their difference is used as the error signal to drive the load. When a state of equilibrium is reached in the circuit, the load will be moving at the desired velocity.