This invention generally relates to a new and improved control circuit for DC machines and, more particularly, relates to such a circuit which is well suited for use with an electric motor used to propel a vehicle such as an automobile.
DC motors are widely used as traction motors for various types of vehicles because of the relative simplicity of operation and ability to provide a high torque. A DC motor having its field winding in series with its armature winding (a "series-wound" motor) provides high torque at low rpm but its power output decreases substantially at high rpm. A DC motor having its field winding in parallel with its armature winding (a "parallel-wound" or "shunt-wound" DC motor) can be designed to provide high power output at higher rpm but only within a rather narrow range of rpm, outside of which range power and/or efficiency decrease significantly. This range of rpm for optimum efficiency can be relocated about different rpm values by well-known techniques. For many applications, such as electric automobiles, however, the amount of output power for such conventional motors is significantly limited at the higher cruising or passing speeds if the motor is designed to achieve optimum efficiency at medium speeds such as those used in city driving, and vice versa.
Attempts to at least partially solve this problem include "field weakening", employing a compound-wound motor, or utilizing a separately-excited shunt motor. None of these techniques has solved the problem to the fullest.
The ratio of field current or excitation to armature current in a DC machine is referred to as the coefficient of field excitation and is commonly designated as "k". In a separately-excited shunt-wound motor, field excitation is independent of the armature current; hence, k equals zero. In a series-wound motor, the field current and armature current are the same; thus, k equals unity. For a compound-wound motor, a part of the field excitation can be varied independently of the armature current so that the k coefficient can vary between zero and one.
In general, for a particular rpm, the coefficient k can be optimized for maximum motor efficiency. The problem is that the optimum value of k differs as the rpm differs. Consequently, DC motors are generally optimized for a particular rpm at or near which the motor will operate for the intended application. In a shunt-wound motor, for example, the peak performance is obtained only in a very narrow range where the armature back EMF becomes slightly (i.e., 5-15%) lower than the input voltage. In a series-wound motor, the maximum efficiency is reached at a given input voltage and rpm under a given torque load. In order to increase rpm, load torque has to be reduced, as a result of which the back EMF prevents further energy from being accepted by the armature.
The peak operating range of a series-wound motor can be expanded somewhat by a technique known as "field weakening" wherein a resistance is placed in parallel with the field winding so that a part of the armature current that would otherwise pass through the field is by-passed around the field, thereby reducing the amount of back EMF generated by the armature. A compound-wound motor has a field excitation, a part of which is independent of armature current and another part of which is directly related to the armature current which varies with the load torque applied to the motor. The shunt portion of the field can of course be reduced or even turned off completely to effect field weakening so that the motor has two ranges wherein its efficiency may be peaked.
It is therefore a general object of the invention to provide a new and improved control circuit for DC machines.
A more specific object is to provide a new and improved control system for a DC machine which substantially widens the range of optimum efficiency of operation of the DC machine.
A more specific object is to provide a new and improved control circuit, in accordance with the foregoing object, for a DC motor which substantially expands the power band of the motor.
A related object is to provide a control system in accordance with the foregoing object which also substantially increases the maximum power acceptance of a DC motor.
It is another object of the invention to provide such a new and improved control circuit which is particularly well suited for use in a vehicle such as an automobile.