By utilizing specific combinations of transistors, current can be induced in either direction between phases.įor example, if T1 and T6 are open, current will flow from the DC bus positive through the U to V phase of the motor and then to the DC bus negative. In order for the current to flow between two phases of the motor above, at least one transistor in the top portion of the diagram and one in the bottom of the diagram must be activated. The goal of the PWM control is to create a sine wave current waveform output to produce torque in the motor. Thus the ‘pulse’ refers to the switching on and off of the transistors producing a pulse of voltage with an amplitude of approximately 678VDC. A VFD with a 480VAC input will have a DC bus of approximately 678VDC. The transistors act as a switch connecting the DC bus across the windings of the motor. The typical configuration of the IGBTs in the inverter section of a VFD is shown below in figure 2.įigure 2 – Schematic of output IGBT stage of a VFD. Most VFDs today utilize insulated gate bipolar transistors or IGBTs. Pulse width modulation uses transistors that switch the DC voltage on and off in a defined sequence to produce the AC output voltage and frequency. The process involved in inverting the DC voltage to the variable voltage variable frequency (VVVF) AC voltage in the inverter section of the VFD is called pulse width modulation or PWM. The inverter section of the VFD takes the DC voltage from the DC bus and inverts it back to a variable voltage and variable frequency AC voltage used for the motor control. The DC bus section consists of a capacitor bank, which is used to smooth out the DC voltage from the converter section and provide some voltage storage capacity. The converter section uses a diode bridge rectifier to convert the AC input voltage to a DC voltage. The VFD consists of three main parts: The input converter section, the intermediate DC bus, and the output inverter section.įigure 1 – A VFD is made up of 3 main parts. Variable frequency drives (VFDs) used on industrial applications provide an efficient way to vary the speed and torque of the connected motor.
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