1. Field of the Invention
This invention relates to control circuits for supplying current to a load in general and more particularly to a reversible control circuit for a shunt field winding of a direct current electric motor.
2. Description of the Prior Art
Several systems for controlling the speed of a direct current electric motor have been disclosed in the prior art. Probably the most simple method of speed control is the use of an adjustable resistor in series with the armature circuit. The resistance is increased for starting or for short-time or intermittent slowdowns in its most common usage, but this control has the disadvantage of power loss in the resistor which decreases the system efficiency. A second type of speed control involves a constant armature voltage and a variable voltage applied to the field or an adjustable resistor in series with the field to achieve control over a speed range of approximately four or five to one. The maximum torque is limited by the permissible armature current and the maximum flux which, in turn, is limited by magnetic saturation or by heating of the field winding. This type of control has the disadvantage of slow response when the field current polarity is reversed to change the direction of rotation since larger horsepower motors generally have relatively large field time constants.
Another form of motor speed control involves the use of constant armature current and a variable field excitation. In addition to the disadvantage of the slow response time of the field, this type of control is less common than other controls since constant voltage sources are more readily available than constant current sources. Still another form of speed control is the constant field current and controlled armature voltage type. The armature may be supplied from a controlled rectifier voltage supply or a separately excited direct current generator which is commonly known as a Ward Leonard system. In another form of speed control, both the armature and the field supply voltages may be varied to control speed in response to a control circuit which defines a relationship between these two voltages as a function of the speed error signal.
In any of the types of speed control devices described above wherein the field is controlled, it is desirable to provide as efficient and as economical a control device as possible. Generally, where controlled rectifiers are utilized, the current flow is controlled by blocking a portion of each alternating current voltage cycle and rectifying the portion of the voltage cycle passed to provide an average direct current to the field winding. Therefore, more expensive circuit components having higher peak current ratings must be utilized than in a circuit where the polarity of a direct voltage is alternately reversed to provide the same average current at a lower input voltage level.
U.S. Pat. No. 3,421,065 issued to Lucio Stabile and entitled "Apparatus For Controlling The Speed Of Direct Current Electric Motors" discloses a motor control which provides a variable voltage to the armature and reverses the polarity of the field current in response to a speed error signal. The field is supplied with a relatively constant voltage of one polarity when the speed of the motor is equal to or less than a reference speed and the polarity is reversed to slow the motor if the speed of the motor becomes greater than the reference speed. However, this system has the disadvantage that the field current polarity reversal is only utilized for decreasing the motor speed while a more expensive armature controller is utlized to maintain the desired speed.
U.S. Pat. No. 3,593,077 issued to Richard C. Loshbough and entitled "Electrical Circuit For Pulse Fed Inductive Load" discloses a circuit for applying current pulses of opposite polarity to a shunt generator field in a Ward Leonard speed control system. This circuit applies portions of an alternating current wave form as current pulses of opposed polarity and controllable magnitude for each polarity to a capacitor across the field winding to obtain a net current. However, this control provides only a portion of each input voltage cycle to the capacitor so that there are discontinuities between the pulses and the capacitance must be large enough to store electrical energy to supply current to the field during those discontinuities. Furthermore, the magnitude of each current pulse in this type of control must e greater than the magnitude of more closely spaced current pulse of the same duration to achieve the same net current flow.