Page 100 - Electric Drives and Electromechanical Systems

Chapter 3 Power transmission and sizing 93

FIG. 3.15 A Radial magnetic bearing, showing the control system required. The four position sensors accurately
measure the shafts position relative to the bearing, which is controlled by the magnetic fields generated at the
four poles.

The magnetic bearing stator has a similar construction to a brushless d.c. motor and
consists of a stack of laminations. The current is supplied to each winding will produce
an attractive force that levitates the shaft inside the bearing. The controller controls the
current applied to the coils by monitoring the position signal from the positioning
sensors in order to keep the shaft at the desired position throughout the operating range
of the machine. Usually there is 0.5e2 mm gap between the rotor and stator depending
on the application. In addition to operation as a bearing, the magnetic field can be used
to influence the motion of the shaft and therefore have the inherent capability to
precisely control the position of the shaft to within microns and additionally virtually
eliminate vibrations.

3.5 Couplings

The purpose of a coupling is to connect two shafts, end-to-end, to transmit power.
Depending on the application speed and power requirements a wide range of couplings
are commercially available, and this section summarises the couplings commonly found
in servo type applications.
A flexible coupling is capable of compensating for minor amounts of misalignment
and random movement between the two shafts. Such compensation is vital because
perfect alignment of two shafts is extremely difficult and rarely attained. The coupling