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In
principle, the rotating anode is the moving part of an electric
motor, running in a vacuum. The rotor carries the anode.
An external electromagnetic field,
produced by a winding (stator) outside the glass envelope, drives
the rotor. Both together work as an asynchronous motor.
The air gap between rotor and stator isolates both from each other,
since the winding is electrically close to ground and the anode lays
on high potential during operation. On the other hand the gap
reduces the efficiency of the rotating anode motor significantly.
Due to this distance, the power supply for the motor must be
relatively high, in order to speed up the anode in an acceptable
short time
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The
Rotor consists of a copper cylinder and rests in ball
bearings for smooth movement.
The bearings cannot be lubricated with ordinary grease because it
would affect the vacuum and, as a consequence, the high tension
characteristics of the tube.
Soft metals such as lead and silver are applied to separate the ball
bearings and the running surfaces, in order to prevent the
possibility of "jamming" in the vacuum.
This form of lubrication limits the life time of the bearings in the
x-ray tube to about 1000 hours.
Therefore, the running time needs to be as short as possible, which
does not allow continuous rotation.
The rotation is controlled when a radiography is started. |
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The stator consist of
several windings which are equally spread out around the neck of the
tube. They induce a rotating electro-magnetic field which interacts
with the rotor, causing it to rotate synchronously.
The simpliest power supply is a 220 V AC source. It was used in old
generators for the normal speed anode.
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A capacitor C provides the stator with a second phase.
The current in the two phases I and II have a phase shift of 120° to
each other, which produces the rotating field.
The value of the capacitor depends on the type of stator coil.
This stator is called "two-phase stator".
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Diagram showing anode rotation speed from prep to expose to braking
period
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www.siemens.com |
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