Electric D.C. motors with a plurality of units, each including a

Details Electric D.C. motors with a plurality of units, each including a Electric D.C. motors with a plurality of units, each including a

1982-09-13

1985-08-13

Wong, Peter S.

Electrical generator or motor structure

Dynamoelectric

Rotary

310 49R, 310156, H02K 720, H02K 3700

Patent

active

045352631

DESCRIPTION:

BRIEF SUMMARY
This invention relates to electric motors comprising a permanent magnet field and a wound armature.
The invention is particularly applicable to a direct drive DC motor, for a servo-actuator, which may be directly attached to the load. Such a motor may have a wound armature and a permanent magnet field which acting together convert electrical currents directly into torque to maintain desired accuracy in a positioning or speed control system. If continuous rotation is required, commutation may be necessary, and this can be provided by semi-conductor switching with timing provided by a magnetic sensor mounted on the rotor. The invention is, however, particularly applicable for a motor capable of a limited angle of rotation only, for example plus and minus 20.degree.. In this case torque is proportional to the coil current and also varies approximately sinusoidally with angle of rotation.
A problem with such an arrangement is that the slots which are provided so that the windings may be inserted provide an interruption in the magnetic circuit so that as the rotor turns (even when the coils are not energized) the changes of reluctance introduce an alternating torque which may be referred to as a cogging torque, which is superposed upon the desired torque, sometimes aiding it and sometimes opposing it. This cogging torque represents a disturbing load which has to be overcome by the applied torque arising from current in the coils. The number of such slots provided for the windings is an integral multiple of the number of poles. The slots are usually equally spaced around the motor.
Various methods have been proposed to overcome this cogging torque, for example by filling the slots with a magnetic metal (such as iron) or skewing them, but in general this involves additional expense.
According to the present invention an electric motor comprises two or more units, each including a permanent magnet field device mounted to rotate relatively to a wound armature having coils producing the same number of poles as the field device but located in a second number of equally spaced slots, the units being mounted with their permanent field devices mechanically connected together and their armatures mechanically connected together in such relative positions that the relative positions of the armature slots in relation to the poles of the permanent magnet field device of different motor units differ by an angle substantially equal to the slot pitch divided by the number of motor units, so that their cogging torques substantially cancel out. The second number of slots is a multiple of the number of poles of the wound armature.
Thus with two motor units the offset will be half the slot pitch, whereas with three motor units it will be one third of the slot pitch.
Thus the relative positions of the parts will generally be such that the relative positions at which different units exert maximum torque (at a given current) differ by an angle equal to the slot pitch divided by the number of units, so that their cogging torques cancel out.
In practice if the motor units are of identical construction their rotors and stators may bear identifying marks which may be brought into alignment or to a given angular spacing, to ensure the desired relative positions.
While the invention is applicable to a continuously rotating stepping motor, it is particularly applicable to oscillating motors moving through a fraction of a revolution. The invention may be applied to a motor in which the permanent magnetic field device is stationary, forming a stator, and the sound armature moves, forming a rotor, particularly in the case of an oscillating motor moving through only a fraction of a revolution, in which case the coils may be supplied through flexible conductors. In a preferred form of the invention, however, the wound armature is the stator and the permanent magnet field device is the rotor.
In the case of a position control system, it will generally be important that the several units should never fight one another.
For example, a feedback signa

REFERENCES:
patent: 3644763 (1972-02-01), Skrobisch
patent: 3693034 (1972-09-01), Inariba
patent: 3895278 (1975-07-01), Picunko
patent: 3979652 (1976-09-01), Faxon
patent: 4009406 (1977-02-01), Inariba
patent: 4246528 (1981-01-01), Nakajima
patent: 4330724 (1982-05-01), Goddijn
patent: 4355249 (1982-10-01), Kenwell

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