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A magnetic amplifier is a device used for controlling the flow of power to a load by means of saturating a magnetic core. They are widely used in recent years for both high and low power applications.
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Simple Circuit for a Magnetic Amplifier
Magnetic materials suitable for magnetic amplifiers have a rectangual hysteresis loop with sharp saturation and low hysterisis loses. The excitation level is adjusted such that the flux swing in the core occurs between the knees of the B-H curve.
The typical magnetic amplifier circuit shown above has the following components:
- R1 - load resistance
- G - gate winding
- C - control winding
- Ec - excitation
- Es - input from AC source
- Is - amplified AC current
- Ic - control DC current
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Working of a Magnetic Amplifier
When DC is applied to the second winding, which is called the control winding, the core will saturate during half of the AC cycle due to the additional magnetization.
When no DC is applied to the control winding, the core flux variation is within the saturation limit. During this time, the flux swing is very large, so the gate winding offers very large amount of inductance and the current drawn from the AC source is only the magnetizing current.
Suppose that a current is passed through the control winding on the DC side. The operating point is suddenly shifted. Now the core becomes saturated during the positive half of the cycle. Under this condition, the change of flux is very small compared to when there is no control current. The inductance of the gate winding is reduced to a very small value, and now the entire supply voltage appears across the load resistance R1.
With increasing DC control current, the core saturates earlier in the positive half cycle, thus delivering more current to the load. In this way, the saturable reactor works much like a gas-filled Thyraton, where the voltage shifts the firing angle. Once the tube fires, the grid loses its control, and the plate current is controlled by the load resistance.
Similarly in a magnetic amplifier, if the control current is such that the core can be saturated, then the magnetic amplifier will fire at an angle α, and once it fires, the load current is determined by the load resistance alone. By varying the control current, the firing angle α can be varied and hence the load current can be controlled. Thus with an expense of a very small amount of power in the control side, a large amount of power in the AC side is controlled in the saturable reactor.
- EMF is induced in the control winding by transformer action
- Output is delivered in one half cycle only