The present invention relates generally to the control of direct current (d.c.) motors and more particularly to the control of d.c. shunt motors operable in both the constant torque and constant horsepower modes.
It is well known in the art that a d.c. motor having separately excited armature and field windings can be operated in what are commonly called the constant torque and constant horsepower modes. In the constant torque mode, assuming the armature current is maintained at rated value, the maximum torque available from the motor is constant and the horsepower output is proportional to speed. In the constant horsepower mode, again assuming the armature current is maintained at rated value, the maximum horsepower output is constant while the torque varies as a function of the speed. The latter type of operation is achieved, in the case of a d.c. shunt motor, by what is commonly called field weakening. That is, the power supplied to the field winding is reduced to increase speed and the relationship existing is, essentially, that if the field is reduced to produce one-half the original amount of flux, the speed will double.
It is also recognized by those skilled in the art that a closely regulated d.c. motor can become unstable when rapid regulation changes are required as will be more fully described in the detailed specification which follows. This is because certain of the circuit components used in the control are inherently frequency responsive and the motor-load itself, when viewed as a system, is also responsive to frequency. The problem of stability becomes even more acute when motor operation is in the constant horsepower range. This is because the addition of the field control into the system adds an additional gain to the system which multiplies the remaining system gain such that relatively small changes now become larger in total effect.
These instability problems are recognized and numerous solutions have been proposed. Many of these proposed solutions are complex and require a number of adjustments. In U.S. Pat. No. 3,935,520, "DC Motor Regulator" by Chandler A. Johnson, III, which patent is assigned to the assignee of the present invention and was issued on Jan. 27, 1976 and which patent is specifically incorporated hereinto by reference, there is shown a scheme to increase the stability of a motor system in the constant torque region. This patent shows, describes and claims a d.c. motor control circuit including a speed control having a specified transfer characteristic resulting in extremely high gains in response to low frequency signals and a constant gain (e.g., 1) at higher frequencies.
The aforementioned patented invention performs quite adequately within the constant torque range and only a single adjustment is required. That invention does not, however, provide corresponding control when the motor is operative in the constant horsepower range, i.e., when field weakening occurs. The stability problems in the constant horsepower range can be better understood with the following explanation. In the constant torque region, motor speed will vary from zero to base speed by varying the armature voltage from zero to rated volts. Typically, speed might vary from 0 to 1150 rpm with a corresponding variation in armature terminal voltage of from 0 to 240 volts. In the field weakening region, however, it is possible to achieve an increase of approximately 200% in speed with a relatively small increase in armature terminal voltage. That is, the motor speed could rise from 1150 rpm to approximately 3450 rpm with an increase of armature terminal voltage of only from 240 to 245 volts. Thus, it is seen that maintenance of stability and accurate speed control in this latter range can present serious problems.