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Motor with a sideways force is a step forward

01 December, 2003

Motor with a sideways force is a step forward

Researchers at Nottingham University are developing a novel motor (shown below) which produces sideways forces, as well as conventional rotational torque, without needing any extra windings. They suggest that possible uses for this technology could include:

• damping motors to avoid vibrations as they pass through critical (resonant) speeds;

• actively balancing rotors, using different strategies either to minimise the rotating force on the stator, or to maintain a significant gap between the rotor and stator;

• reducing motor deflections in grinding and milling operations; and

• injecting small test forces between the rotor and stator to gather information about the mechanical health of a machine for condition monitoring purposes. This information could give a better indication of any motor problems than simple vibration monitoring.

Although motors that can push sideways are not new, these "self-bearing" machines have usually been achieved either by incorporating a secondary set of windings, or by exciting individual coils with dedicated power supplies. Both these approaches are expensive and result in machines with poor specific power ratings.

The Nottingham researchers - working in a team known as the Centre for Magnetic Forces on Rotors and Applications, or MagFoRA - have come up with a new approach which separates the terminals used for torque and power generation from those used to produce a magnetic bearing effect.

The existing machine windings are used to produce the lateral forces, without needing any extra windings. If nothing is connected to the bearing terminals, the motor behaves like a normal machine.

The currents which produce the machine torque are divided into two parallel paths in each phase. A small supply between the mid-points for each path provides the current that produces the sideways forces. This bridge-type connection allows a single set of the windings to carry the currents that produce both the lateral and the rotary forces.

Almost any machine can thus be configured to generate lateral forces, with no changes needed to its main (torque-producing) electrical supply. If there are three or more phases in the machine, this results in some redundancy so that even if one sideways force-inverter fails, it will still be possible to produce lateral forces.

The Nottingham team has rewound a standard permanent magnet brushless motor to the new configuration and shown that it does produce the predicted sideways force. An 0.8A current (about 4% of the motor`s rated peak) is sufficient to lift the 12.6N rotor.




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