1. Field of Invention
This invention relates to a constant speed direct current motor and particularly but not exclusively to a motor for use in driving tape recorders and cartridge machines.
2. Description of Prior Art
Hysteresis synchronous motors are currently used in tape recorder and cartridge machine capstan drives. Where there is a constant frequency A.C. mains supply such drives exhibit excellent constant speed characteristics.
The use of D.C. motors for this purpose is not new, however, speed control is poor leading to serious problems with timing stability. Nevertheless D.C. motors have inherent advantages over A.C. synchronous motors:
(i) The D.C. motor is considerably more efficient than its A.C. counterpart thereby avoiding overheating problems.
(ii) The standard set speed of the D.C. machine is variable within narrow limits whereas the synchronous machine is fixed. Thus small speed changes must be achieved by changing the diameter of the drive capstan (as applied to a tape machine).
(iii) The D.C. machine is unaffected by frequency changes in the mains supply and voltage differences are easily catered for in the control circuitry.
Applicants are aware of U.S. Pat. Nos. 3,663,877 and 3,651,266 both to Clark relating to a brushless D.C. motor including a tachometer commutation circuit and a digitalization circuit. The digitalization circuit is used to digitalize analog signals generated by Hall effect switches in the winding of the motor. The Hall effect switches do not have any effective direct control of the coil windings in the Clark motor as disclosed. This digitalization of the analog signals creates eight digital signals which are used to identify the instantaneous angular position of the rotor. Simultaneously, a tachometer generates a set of pulses of constant frequency which are correlated with the digital signals to produce a set of phase related switching signals. Each of the phase related switching signals has a period corresponding to a full revolution of the rotor, and this defines a unique phase relationship therewith.
This appears to constitute the basis of operation of the Clark invention. The Clark motor is primarily designed for use on machines where it is necessary to have both accurate positional and speed information, for example in editing machines where information concerning the precise position of the tape is required. The Clark motor could be used in tape recorder and cartridge machines but this would not make sense technically because the motor would produce unnecessary positional information, and also because the cost would be increased significantly due to the digital circuitry.