Overview
This lecture explains the basics of Variable Frequency Drives (VFDs), how they control AC motors, and their role in improving efficiency in electrical systems like HVAC.
What is a Variable Frequency Drive (VFD)?
- VFD stands for Variable Frequency Drive, also called AC drives or variable speed drives.
- VFDs control the rotational speed of AC motors by varying the frequency of the supplied electricity.
- Used in many industries, especially HVAC, to match motor speed to demand and save energy.
Types of Electricity: DC vs. AC
- DC (Direct Current) flows in one direction, produced by sources like batteries and solar panels.
- AC (Alternating Current) reverses direction periodically, as found in household electrical outlets.
- DC appears as a constant voltage (flat line) on an oscilloscope, while AC forms a sine wave.
- Frequency is the number of AC cycles per second: 60 Hz in North America, 50 Hz elsewhere.
Phases of AC Electricity
- Single-phase systems use one sine wave; three-phase systems use three sine waves, each offset by 120°.
- Three-phase systems can deliver more power with less fluctuation than single-phase.
- Split-phase systems, used in North America, provide two hot wires and a neutral.
How a VFD Works: Main Components
- VFDs are typically installed in the power supply line of a three-phase AC motor.
- The VFD consists of three main parts: rectifier, DC bus (filter), and inverter.
Rectifier
- Converts AC from three phases into rough DC using diodes (which only allow current in one direction).
- Each phase is paired with diodes to manage current flow direction depending on the phase cycle.
DC Bus (Filter)
- Uses capacitors and/or inductors to smooth out ripples in the rectified DC, providing steady DC voltage.
Inverter
- Uses electronic switches (IGBTs) to convert DC back to AC at a controlled frequency and voltage.
- Switches are opened and closed in pairs to simulate AC phases for the connected motor.
Pulse Width Modulation (PWM)
- The inverter uses PWM to rapidly open and close switches, creating a waveform that closely mimics an AC sine wave.
- Adjusting the duration and frequency of pulses controls both output voltage and frequency, thus controlling motor speed.
Key Terms & Definitions
- Variable Frequency Drive (VFD) — A device that controls the speed of AC motors by varying supplied frequency and voltage.
- Rectifier — Converts AC electricity to DC using diodes.
- DC Bus/Filter — Smooths DC output using capacitors or inductors.
- Inverter — Converts DC back to controlled AC using electronic switches (IGBTs).
- Pulse Width Modulation (PWM) — A technique for controlling power and wave shape by varying the width and timing of electrical pulses.
- Single-phase/Three-phase — Types of AC electricity; three-phase delivers more constant power.
Action Items / Next Steps
- Review recommended videos or case studies on VFDs and their applications.
- Learn more about capacitors, rectifiers, and inverters for deeper understanding.
- If working with electricity, ensure you are qualified and follow all safety guidelines.