1. Field of the Invention
This invention is related generally to regulated DC power supplies and more specifically to the control circuit of such a power supply.
2. Description of the Prior Art
A power supply producing a regulated DC load current from an AC power source typically has a converter having input terminals adapted for connection to the AC source and output terminals adapted for connection to a load. The converter may be one of several conventional designs ranging from a three-phase converter to a singlephase semiconverter. Whichever design is chosen, the converter will have switches, such as silicon controlled rectifiers, which, upon proper regulation of when the switches are conductive, rectify the AC current thus providing the DC load, or output, current. The conduction of the switches is regulated by firing signals produced by a control circuit.
There are several known methods for producing the firing signals. They basically involve the production of a signal representative of the AC source, or input, voltage. This signal is compared with a reference signal and the firing signal is produced in response to a predetermined relationship existing therebetween. The user may effectively control the production of the firing signals by controlling the reference signal.
In one particular application the DC power supply is used to provide a DC load current to the stator winding of an elevator motor. In this application several reference signals are produced. A full field reference signal is representative of the load current used to overexcite the stator winding to produce the torque necessary to accelerate an elevator car. A running field reference signal is representative of the load current used to maintain the torque required to maintain the speed of the elevator car. Finally, a standby reference signal is representative of the load current used to maintain the field of the stator winding while the elevator car is not moving.
The full field, running field, and standby load currents are presently produced by an unregulated DC power supply and power resistors connected in series with the field winding. Power relays may be used to shunt some of the resistors thereby causing the value of the load current to change. Because of AC input voltage variations and changes in the field resistance due to temperature variations the power supply is unregulated. Without some type of feedback loop to compensate for these variations, the resulting load current will be unregulated.
The present invention eliminates the need for relays and allows operation over a wide range of input voltages. The present invention also provides a negative current feedback loop to regulate the load current. These and other features of the present invention are discussed in detail hereinbelow.