Three-Phase Power and Drives
Inside a Variable-Frequency Drive
Follow the rectifier, DC link and PWM inverter in one chain.
You need to understand the internal functions in “Inside a Variable-Frequency Drive” before selecting, connecting or troubleshooting the real equipment.
Follow energy through a typical voltage-source VFD from rectification and DC-link storage to high-speed inverter switching, then distinguish pulsed terminal voltage from its fundamental component and motor current.
- Select or check first
- Separate conversion stages from control decisions. The rectifier, DC link and inverter are power stages. A controller decides the switching pattern from commands, limits, measurements and a motor-control algorithm. Knowing the requested frequency does not by itself reveal the instantaneous terminal voltage.
- Controlling assumption
- Circuit state, waveform, source impedance, ratings, temperature and protection topology match the equipment being analysed.
- Top risk
- PWM output is not ordinary mains. Fast edges can produce reflected-voltage peaks, common-mode current and instrument errors. Do not place power-factor capacitors, ordinary contact switching or an unevaluated filter between a drive and motor.
- Proof
- Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.
Start with the basics Optional foundation before “Inside a Variable-Frequency Drive”
Electrical-engineering basics
Electrical behaviour becomes easier to reason about when every voltage has a reference and every current has a complete return path.
- Voltage
- Voltage is potential difference between two points. A voltage statement is incomplete unless both points or its reference are known.
- Current
- Current is charge flow through a path. It does not disappear after a component; in a complete circuit it returns to the source.
- Resistance and impedance
- Resistance opposes steady current. Impedance extends that idea to changing signals and can depend on frequency through capacitance and inductance.
- Power and heat
- Electrical power is the rate of energy transfer. Some becomes useful output and some becomes heat that must stay within component and enclosure limits.
- Rating versus operating point
- A rating is a limit or qualified condition, not a target. The operating point must remain inside all relevant voltage, current, power, temperature and timing limits.
How to use this article: Draw the source, load, reference and return path before calculating. Treat exposed or stored electrical energy as hazardous until isolation is proved.
On this page
A common variable-frequency drive does not turn fixed-frequency AC directly into a slower sine wave. It first rectifies the input, stores and filters energy on an intermediate DC link, and then switches that DC link across the motor phases in a controlled pattern. The motor responds to the fundamental voltage and current created by that pulse pattern, while its insulation and cable also experience the switching edges.
A typical voltage-source drive has three power-conversion stages
Diagram description
The illustration shows functional blocks rather than a complete circuit. It omits input filtering, precharge, protection, control power, sensors, electromagnetic-compatibility parts and topology variations such as active front ends.
| Stage | Main job | What the simplified picture omits |
|---|---|---|
| Rectifier | Creates a unidirectional DC-link supply from the AC input | Diode, thyristor or active-front-end options; input harmonics and regeneration capability |
| DC link | Buffers energy and reduces rectified-voltage ripple | Capacitor ripple current, precharge, discharge, inductors, protection and dangerous retained charge |
| Inverter | Connects motor phases to DC-link rails in timed switching states | Dead time, semiconductor voltage drop, switching loss, common-mode voltage and protection |
| Motor and cable | Convert the commanded fundamental electrical quantities into flux and torque | Edge reflections, parasitic current, insulation stress, acoustic noise and thermal limits |
Rectification does not guarantee bidirectional power flow
A basic diode input bridge naturally transfers average power from the supply into the DC link. It normally cannot return sustained braking energy to the AC source. An active front end or another regenerative arrangement can be designed for bidirectional flow, but that is a declared product feature—not an assumption to make from the letters VFD. During deceleration, this difference determines whether energy can return to the mains, must be dissipated, or forces the drive to limit braking.
Energy stored on the DC-link capacitor
E_C = ½·C·V_DC²
The square relationship explains why even a modest DC-link voltage rise adds significant stored energy and why a disconnected drive can remain hazardous. The drive’s documented discharge time and voltage-verification procedure govern access; this equation is not a discharge-time estimate.
- E_C
- stored electric-field energy, J
- C
- effective DC-link capacitance, F
- V_DC
- DC-link voltage, V
The inverter synthesizes a fundamental component with pulses
Diagram description
The three traces are conceptual and use no common amplitude or time scale. The yellow sine curve is a component or reference, not the literal instantaneous voltage at a two-level inverter terminal.
In a two-level inverter, each phase terminal is switched among states derived from the DC-link rails. The controller changes the fraction of each switching interval spent in those states. Over an electrical cycle, the pulse pattern contains a fundamental component with the requested frequency and usable amplitude plus switching-frequency components and harmonics. Calling the output ‘variable AC’ is correct at system level, but a normal oscilloscope view of terminal voltage is not a clean sine wave.
| Quantity | Expected character | Interpretation boundary |
|---|---|---|
| Supply voltage | Near-sinusoidal at fixed system frequency | Distortion and unbalance depend on the installation |
| Rectifier output | Pulsating unidirectional voltage/current | Topology, input phase count and load change the ripple |
| DC-link voltage | DC with ripple and transients | It is neither perfectly flat nor safe after input disconnection |
| Inverter terminal voltage | Fast PWM pulses with high edge rate | Ordinary AC meters can misread it |
| Fundamental voltage component | Sinusoidal mathematical component | Useful for flux and V/f reasoning; not the full stress waveform |
| Motor phase current | Often smoother and approximately sinusoidal | Still contains ripple and harmonics; depends on winding, load and control |
PWM is a method, not the whole control strategy
Scalar V/f control, vector control and direct-torque control can all command an inverter, but they estimate or regulate different quantities. A basic V/f model schedules voltage with frequency and lets the load determine slip. A vector scheme uses a motor model and measurements to control flux- and torque-producing current components. The power-stage drawing alone does not tell which algorithm is active.
Why motor current looks smoother than terminal voltage
A winding resists rapid current change because its magnetic field stores energy. During each short switching state, current changes by an amount related to applied voltage, inductance and interval length. Successive positive and negative changes accumulate into a lower-frequency current waveform. Winding resistance, back EMF, leakage inductance, switching frequency and control action all matter, so the current is not an automatically perfect sine wave.
Review From AC to DC With a Diode Bridge for rectification and Real Capacitors and Rectifier Smoothing for ripple and capacitor limits. The motor ratings entering the control model come from How to Read a Three-Phase Motor Nameplate.
One-minute questions
Questions to answer before you apply this lesson
What should I select or check first for Inside a Variable-Frequency Drive?
Separate conversion stages from control decisions. The rectifier, DC link and inverter are power stages. A controller decides the switching pattern from commands, limits, measurements and a motor-control algorithm. Knowing the requested frequency does not by itself reveal the instantaneous terminal voltage.
Which assumption can change the conclusion?
Circuit state, waveform, source impedance, ratings, temperature and protection topology match the equipment being analysed.
What can go wrong if I copy the example directly?
PWM output is not ordinary mains. Fast edges can produce reflected-voltage peaks, common-mode current and instrument errors. Do not place power-factor capacitors, ordinary contact switching or an unevaluated filter between a drive and motor.
How do I verify the result in the real system?
Check the exact manufacturer data, units and limits, then verify the result by calculation, measurement or a controlled acceptance test.
Sources and further reading
- Technical Guide No. 4: Guide to Variable Speed DrivesABB · 2026-10-13Drive-manufacturer guide reviewed for the rectifier, DC-bus and inverter chain and for switching states that create rotating motor flux.
- ABB Drives Technical Guide BookABB · 2026-10-13Reviewed for PWM frequency control, constant V/f scope, motor-current response and distinctions among control methods.
- Troubleshooting Transient Voltages in VFDs with a MultimeterFluke Corporation · 2026-10-13Instrument-manufacturer source reviewed for six-switch PWM output, fast voltage edges, cable reflections and the boundary between multimeter and oscilloscope analysis.