A power supply to control the flow of energy between a first alternating current (AC) system and a second AC system is required in a variety of commercial and industrial applications, such as the control of AC motor operation. Some such power supplies convert the energy from a first frequency and voltage to a second frequency and voltage. One way to implement such a system is with a drive containing one or more power cells that include two solid state converters with an intermediate direct current (DC) link. A power cell is an electrical device that has a three-phase alternating current input and a single-phase alternating current output. One such system is described in U.S. Pat. No. 5,625,545 to Hammond, the disclosure of which is incorporated herein by reference in its entirety.
Power cells may be either non-regenerative or regenerative. Non-regenerative power cells can drive a motor but cannot control the flow of current to brake a motor by absorbing regenerative power. Regenerative power cells have the capability of absorbing regenerative power and optionally returning it to the source, thus, allowing the cell to participating in braking the motor. Recently, power cells have been developed that use switching devices, such as insulated gate bipolar transistors (IGBTs) to allow the cell to assist in both driving and braking a motor. For example, referring to FIG. 1 herein, a power cell 160 includes an active front end 162 that serves as a three-phase bridge as it receives power from dedicated three-phase secondary windings of the transformer via an input 142. The cell 160 also includes a plurality of input switches 170-175, which may be bidirectional current-controlling devices such as IGBTs or other transistors, thyristors, or other switching devices. Although six transistors in a bridge format—in this example, three pairs of two transistors each connected in parallel across the DC terminals—are illustrated in FIG. 1, other numbers and types of input switches may be used. The input switches control the DC bus voltage in the cell. A DC filter section 166 includes one or more capacitors 168. The remainder of the cell may include an output stage 164 such as an H-bridge inverter made up of four output switches 131-134, each connected across the output or DC terminals of the active front end 162 and the DC filter 166, to deliver AC power to the output 144.
A circuit including power cells such as 160 in FIG. 1, when connected to a load such as a motor, can provide power from an input source to the motor when operating in the motoring mode. When input converter sections 162 such as those shown in FIG. 1 are used, drive switching events can product harmonic currents. Harmonic currents may also be produced when a single-phase inverter section is connected to a motor. The harmonics can be compounded when multiple cells have switches that operate at the same time.
U.S. Pat. No. 5,625,545 discloses that a multi-winding transformer may be used to reduce harmonic currents created by a unidirectional, or non-regenerative, power cell. However, this method may not always reduce harmonics in regenerative power cells to a desired level. Other methods, such as using a master clock to generate synchronized carriers and applying the carriers to multiple converters connected to a transformer having non-phase shifted secondary windings, have not been entirely effective, and many increase drive complexity and cost.
The disclosure contained herein describes attempts to reduce harmonics created by operating a power cell having bidirectional switching devices.