MAGNETIC AMPLIFIERS

You have now been shown various ways that electron tubes (NEETS, module 6) and transistors (NEETS, module 7) can be used to amplify signals. You have also been shown the way in which this is done. There is another type of amplifier in use - the MAGNETIC AMPLIFIER, sometimes called the MAG AMP.

The magnetic amplifier has certain advantages over other types of amplifiers. These include (1) high efficiency (up to 90 percent); (2) reliability (long life, freedom from maintenance, reduction of spare parts inventory); (3) ruggedness (shock and vibration resistance, high overload capability, freedom from effects of moisture); and (4) no warm-up time. The magnetic amplifier has no moving parts and can be hermetically sealed within a case similar to the conventional dry-type transformer.

However, the magnetic amplifier has a few disadvantages. For example, it cannot handle low-level signals; it is not useful at high frequencies; it has a time delay associated with the magnetic effects; and the output waveform is not an exact reproduction of the input waveform (poor fidelity).

The magnetic amplifier is important, however, to many phases of naval engineering because it provides a rugged, trouble-free device that has many applications aboard ship and in aircraft. These applications include throttle controls on the main engines of ships; speed, frequency, voltage, current, and temperature controls on auxiliary equipment; and fire control, servomechanisms, and stabilizers for guns, radar, and sonar equipment.

As stated earlier, the magnetic amplifier does not amplify magnetism, but uses electromagnetism to amplify a signal. It is a power amplifier with a very limited Frequency response. Technically, it falls into the classification of an audio amplifier; but, since the Frequency response is normally limited to 100 hertz and below, the magnetic amplifier is more correctly called a low-frequency amplifier.

The basic principle of a magnetic amplifier is very simple. (Remember, all amplifiers are current-control devices.) A magnetic amplifier uses a changing inductance to control the power delivered to a load.

BASIC OPERATION OF A MAGNETIC AMPLIFIER

Figure 3-29 shows a simple circuit with a variable inductor in series with a resistor (representing a load). The voltage source is 100 volts at 60 hertz.

Figure 3-29. - Variable inductor in series with a load.

What happens when the inductance decreases? The end result is that the power in the load (true power) increases. Why? Look at the following formulas and see how each is affected by a decrease in inductance.

(True power is covered in NEETS, Module 2 - Introduction to Alternating Current and Transformers.)

As inductance (L) decreases, X_L decreases. As X_L decreases, Z decreases. As Z decreases, I increases. Finally, as I increases, true power increases.

This general conclusion can be confirmed by using some actual values of inductance in the formulas along with other values from figure 3-29.

If the value of inductance is 23 millihenries, the formulas yield the following values:

Now, if the value of inductance is decreased to 11.7 millihenries, the formulas yield the following values:

So a decrease in inductance of 11.3 millihenries (23 mH - 11.7 mH) causes an increase in power to the load (true power) of 625 watts (1125 W - 500 W). If it took 1 watt of power to change the inductance by 11.3 millihenries (by some electrical or mechanical means), figure 3-29 would represent a power amplifier with a gain of 625.

Q.39 What is the frequency classification of a magnetic amplifier?
Q.40 What is the basic principle of a magnetic amplifier?
Q.41 If inductance increases in a series LR circuit, what happens to true power?