A VFD controls AC motor speed by changing supply frequency (N = 120 × f / P). A rectifier turns AC into DC, the DC bus smooths it, and an IGBT inverter uses PWM to rebuild AC at any frequency — while keeping the V/f ratio constant so torque stays available.
- Speed comes from frequency; torque comes from flux, held by a constant V/f ratio.
- Overvoltage, overcurrent and undervoltage trips are all read through the DC bus.
- Self-cooled motors can overheat at low speed even within nameplate current.
Rotating Field: Why Frequency Controls Motor Speed
Stator field speed is fixed by frequency and poles: Ns = 120 × f / P. A 4-pole motor runs 1500 rpm synchronous at 50 Hz (about 1440 rpm loaded, due to slip). Poles are fixed, so frequency is the only lever — and on pumps and fans, power falls with the cube of speed, which is where VFD energy savings come from.
VFD Block Diagram: The Three Power Stages
| Stage | What it does | Typical fault |
|---|---|---|
| Rectifier | Diode bridge converts AC to DC | Input phase loss, DC ripple |
| DC bus | Capacitors smooth and store energy | Overvoltage on stopping, undervoltage on dips |
| Inverter | IGBTs switch DC into variable-frequency AC | Overcurrent, earth fault, IGBT overtemp |
Safety: bus capacitors stay charged for minutes after power-off — wait the manual's discharge time (commonly 5 min) and verify zero volts before touching terminals.
See the DC bus, PWM and V/f curve on real hardware
Practical VFD training with SINAMICS and other drives — commissioning, parameter setup, fault analysis. Pune classroom or live online.
Rectifier and DC Bus: Turning AC into Smooth DC
The six-diode bridge always conducts the two phases with the largest voltage difference. The result is a 6-pulse DC close to the line peak, which the DC bus capacitors smooth to about 1.41 × line voltage.
PWM Inverter: How DC Becomes Variable-Frequency AC
IGBTs are only fully on or off, so the inverter uses pulse width modulation. The motor's winding inductance smooths the pulses into a near-sine current.
A higher carrier frequency means a quieter motor but more drive heat and derating; keep the default unless you are solving a noise problem.
V/f Ratio: Why Voltage Follows Frequency
Flux ≈ V / f. Cut frequency without cutting voltage and the core saturates, so the drive holds about 8 V/Hz on a 400 V, 50 Hz motor (200 V at 25 Hz). At very low speed, voltage boost adds extra volts — too little and the motor won't start under load.
Variable Torque, Constant Torque and Constant Horsepower Loads (HP = T × S / 5252)
Every load is linked by one formula: HP = T × S / 5252 (T in lb-ft, S in rpm). The metric form is kW = T (N·m) × n / 9550. The constant 5252 comes from 33,000 ÷ 2π. The animation below runs a 10 HP, 1500 rpm motor (35 lb-ft rated torque) through each load type.
Variable Torque
Pumps, fans, blowers
T ∝ S² · HP ∝ S³ — half speed needs only ⅛ power
HP = T × S / 5252Constant Torque
Conveyors, extruders, hoists, PD pumps
T constant · HP rises in step with speed
HP = T × S / 5252Constant Horsepower
Winders, spindles, machine tools (above base speed)
HP constant · T falls as speed rises
HP = T × S / 52525 VFD Control Types: V/f, FCC, Sensorless Vector, Closed-Loop Vector and DTC
The power stage is the same in every drive — the control type decides how the drive calculates voltage and frequency. Tap each type to watch its control loop.
1. V/f Control
Open-loop scalar control: the drive follows a fixed voltage-to-frequency curve and never measures motor speed.
2. Flux Current Control (FCC)
V/f with a current loop: measured current trims voltage so motor flux stays constant as load changes.
3. Sensorless Vector
Splits current into flux (Id) and torque (Iq) parts; a motor model estimates speed, so no encoder is needed.
4. Closed-Loop Vector
Vector control with an encoder: true rotor speed and position give full torque at zero speed and holding.
5. Direct Torque Control (DTC)
Controls torque and flux directly: a fast motor model and switching table pick IGBT states with no PWM modulator.
| Control type | Feedback | Low-speed torque | Best for |
|---|---|---|---|
| V/f | None | Limited | Fans, pumps, multi-motor |
| FCC | Motor current | Moderate | Conveyors, general machines |
| Sensorless vector | Current + motor model | Good | Mixers, extruders, conveyors |
| Closed-loop vector | Encoder | Full, even at 0 rpm | Hoists, positioning, winders |
| DTC | Motor model (~25 µs) | Good to full | Fast torque response, rolling mills |
Braking and DC Bus Overvoltage
A fast stop turns the motor into a generator that pumps energy into the DC bus, causing an overvoltage trip — a motor-side fault, not a supply fault. Fix it in this order: a longer decel ramp, the DC bus voltage controller, a braking resistor, or a regenerative front end for continuous regeneration.
Which Siemens or ABB VFD? Selection by Application and Control Type
Pick the control type your load needs, then choose the matching drive family. Use the filter to see which drives suit each control type.
Fans, blowers & HVAC
V/f quadratic / FCC- Siemens
- SINAMICS G120X (V20 for small, simple fans)
- ABB
- ACH580 (HVAC drive)
Pumps & water / wastewater
V/f quadratic / FCC- Siemens
- SINAMICS G120X (deragging, multi-pump)
- ABB
- ACQ580 (water drive)
Simple OEM machines, small conveyors
V/f / FCC- Siemens
- SINAMICS V20
- ABB
- ACS180
Conveyors, packaging, compressors
Sensorless vector- Siemens
- SINAMICS G120C
- ABB
- ACS380 · ACS580 (DTC)
Mixers, extruders, crushers (high starting torque)
Sensorless vector / DTC- Siemens
- SINAMICS G120 (CU240E-2)
- ABB
- ACS580 (DTC)
Hoists, cranes, winders, multi-axis lines
Closed-loop vector / DTC + encoder- Siemens
- SINAMICS G120 (CU250S-2 + encoder) · S120
- ABB
- ACS880 (+ FEN encoder module)
Final model depends on kW, overload duty, supply voltage, IP rating and fieldbus. Size with Siemens TIA Selection Tool / SIZER or ABB DriveSize.
Practical Tips for Real Machines
Low-speed cooling
Use a forced-vent fan or an inverter-duty motor for continuous low speed.
Long motor cables
Reflected PWM waves can nearly double terminal voltage, so fit a dV/dt filter beyond the rated length.
EMC and earthing
Use screened motor cable with 360° bonding at both ends, routed apart from signal cables.
Bearing currents
Fit insulated bearings or a shaft grounding ring on large motors.
Hands-On Lab: See the Principle on a Live Drive
Hands-on- Any VFD with a display that can show DC bus voltage, output frequency, output voltage and output current — most modern drives can.
- A small three-phase motor, uncoupled or on a light test load.
- Correct isolation procedure and a five-minute wait before opening any enclosure. Do not open a drive that has just been powered down.
- Estimated time: 30 minutes.
Confirm the DC bus voltage matches theory
With the drive powered but the motor stopped, display the DC bus voltage. Compare it with the incoming line voltage multiplied by approximately 1.41.
Watch the V/f ratio hold as you change speed
Run the motor at 50 Hz and record output voltage. Reduce to 25 Hz, then 10 Hz, recording output voltage at each point.
Verify the speed formula against the shaft
Using a tachometer or the drive's own speed display, measure actual shaft speed at 50 Hz and at 25 Hz on a 4-pole motor.
Make the DC bus rise on deceleration
Set a short deceleration ramp — one second or less — run to 50 Hz, then stop while watching the DC bus voltage.
Hear the carrier frequency
Note the motor's audible tone at the default carrier setting, then raise the carrier frequency and listen again. Check the drive's derating table for the new setting.
You understand the working principle if you can explain, from the drive display alone, why bus voltage is 1.41 times the line voltage, why output voltage fell when you reduced frequency, and why the bus rose during a fast stop. Those three observations cover rectifier, V/f control and regeneration — the whole machine. The natural next step is the parameter setting sequence that turns this understanding into a correctly commissioned drive.
Frequently asked questions
What do variable torque, constant torque and constant horsepower mean for a VFD?
They describe how a load's torque changes with speed, linked by HP = T × S / 5252 (T in lb-ft, S in rpm). Variable torque loads such as pumps and fans need torque that rises with speed squared, so horsepower rises with speed cubed and big energy savings come from running slower. Constant torque loads such as conveyors and extruders need the same torque at every speed, so horsepower rises in proportion to speed. Constant horsepower loads such as winders and spindles hold horsepower steady, so torque falls as speed rises, which matches a motor running above base speed in field weakening.
Which Siemens and ABB VFD should I choose for pumps, conveyors and hoists?
For fans and pumps (V/f or FCC), use SINAMICS G120X, or ABB ACH580 for HVAC and ACQ580 for water. For conveyors and general machines (sensorless vector), use SINAMICS G120C or G120, or ABB ACS380 or ACS580. For hoists, cranes and winders that need closed-loop control, use SINAMICS G120 with CU250S-2 and an encoder, or S120, or ABB ACS880 with an encoder module. Confirm the final size with SIZER or DriveSize.
What are the main types of VFD control?
Five are common: V/f (open-loop scalar), Flux Current Control (V/f with current feedback), sensorless vector control (motor model estimates speed), closed-loop vector control (encoder feedback) and Direct Torque Control (DTC, used by ABB). Moving down that list gives more low-speed torque and faster response, but needs more accurate motor data and setup.
How does a VFD actually change motor speed?
By changing the frequency of the AC it supplies. Synchronous speed follows N = 120 × f / P, so with pole count fixed by the motor's construction, frequency is the only variable available. The drive rectifies incoming AC to DC, then uses PWM switching in the inverter stage to synthesise AC at whatever frequency the application needs.
Why does the output voltage change along with frequency?
Magnetic flux is roughly proportional to voltage divided by frequency. Holding voltage constant while reducing frequency would saturate the motor core and cause excessive magnetising current. Keeping the V/f ratio constant — about 8 V per Hz for a 400 V, 50 Hz motor — maintains flux and keeps full torque available.
Can a VFD run a motor above its rated frequency?
Yes, but with reduced torque. Above base frequency the drive cannot raise voltage further, so the V/f ratio falls and the motor enters field weakening. Power stays roughly constant while torque drops in inverse proportion to speed. Check the mechanical speed limit of the motor and driven equipment before doing this.
Why does my drive trip on overvoltage when stopping?
During deceleration the motor acts as a generator and pushes energy back into the DC bus, raising its voltage until the protection trips. Lengthen the deceleration ramp, enable the drive's DC bus voltage controller, or fit a braking resistor for high-inertia loads. The fault comes from the motor side, not the supply.
What does carrier frequency change?
It sets how fast the IGBTs switch. Higher carrier frequency gives quieter motor operation and a smoother current waveform but increases switching losses in the drive, often requiring a reduced current rating, and raises EMC emissions and insulation stress with long cables. Leave it at default unless solving a specific noise problem.
Can I run a standard motor at very low speed continuously?
Not safely on a self-cooled motor. A shaft-mounted fan moves far less air at low speed, so the motor can overheat even at nameplate current. Continuous low-speed duty needs a separately powered forced-ventilation fan or a motor specifically rated for inverter duty.