A variable frequency drive controls the speed of an AC induction motor by changing the frequency of the voltage it supplies, because synchronous speed follows the formula N = 120 × f / P. It does this in three stages: a rectifier converts incoming AC to DC, a DC bus with capacitors smooths and stores it, and an inverter of IGBTs switches that DC into pulses whose width varies to synthesise an AC waveform at the required frequency. Voltage must rise and fall with frequency — the V/f ratio — so the motor's magnetic flux stays constant and full torque is available across the speed range.
- Speed comes from frequency; torque comes from flux, and flux is preserved by holding the voltage-to-frequency ratio constant.
- Below base speed a drive gives constant torque; above base speed voltage cannot rise further, so the machine enters constant-power field weakening and torque falls.
- Almost every trip code — overvoltage, overcurrent, undervoltage — is a statement about the DC bus. Learn to read faults in terms of bus voltage.
- A self-cooled motor run continuously at low speed can overheat at perfectly legal current, because its shaft fan is turning slowly.
Why Frequency Controls Motor Speed
An AC induction motor has no mechanism for setting its own speed. The rotating magnetic field in the stator turns at the synchronous speed determined entirely by supply frequency and pole count:
Ns = 120 × f / P — where Ns is synchronous speed in RPM, f is frequency in Hz, and P is the number of poles.
A 4-pole motor on a 50 Hz supply has a synchronous speed of 1500 RPM. The rotor always turns slightly slower — this difference is slip, and it is what generates torque. A typical loaded 4-pole motor therefore runs near 1440 RPM.
Since pole count is fixed by the motor's construction, frequency is the only variable available. Feed the motor 25 Hz and synchronous speed becomes 750 RPM; feed it 40 Hz and it becomes 1200 RPM. That is the entire premise of a VFD. Everything else inside the drive exists to produce that variable frequency cleanly, and to keep the motor's magnetics healthy while doing so.
Older methods of speed control worked by wasting energy — throttling a valve, using a damper, slipping a coupling. A VFD instead reduces the energy delivered in the first place, which is why centrifugal pump and fan applications show such dramatic savings: their power demand falls roughly with the cube of speed, so a modest speed reduction cuts consumption sharply.
The Three Power Stages Inside Every VFD
| Stage | Main components | What it does | What goes wrong here |
|---|---|---|---|
| Rectifier (converter) | Diode bridge, six diodes for a three-phase input | Converts incoming fixed-frequency AC into unregulated DC | Input phase loss, blown input fuse, failed diode causing DC bus ripple |
| DC bus (link) | Electrolytic capacitors, pre-charge circuit, sometimes a DC choke | Smooths the rectified DC and stores energy to buffer load transients | Overvoltage during deceleration, undervoltage on supply dips, capacitor ageing |
| Inverter | Six IGBTs with anti-parallel diodes, gate drive circuitry | Switches the DC bus into pulses that synthesise variable-frequency, variable-voltage AC | Overcurrent, output short circuit, earth fault, IGBT thermal trip |
| Control section | Microprocessor, current sensors, keypad, fieldbus card | Runs the control algorithm, ramps, limits and protection | Parameter errors, communication loss, incorrect motor data |
The pre-charge circuit deserves a mention because it explains a common field observation. When a drive powers up, a resistor limits the inrush current into the empty capacitors, then a relay or thyristor shorts it out once the bus is charged. Repeatedly power-cycling a drive rapidly stresses this circuit, which is why frequent mains switching is discouraged and why the drive's own start/stop command should be used for cycling the motor.
The DC bus also explains why a drive is dangerous after power-off. Those capacitors hold a lethal charge for several minutes. Every drive manual specifies a wait time — commonly five minutes — before opening the enclosure, and the bus voltage should be verified as discharged 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.
PWM: How the Inverter Fakes a Sine Wave
The inverter cannot produce a smooth sine wave; IGBTs are switches, either fully on or fully off. Instead it uses pulse width modulation: it switches the DC bus onto each motor phase in rapid pulses, making the pulses wider where the desired sine wave is at its peak and narrower where it approaches zero.
The motor's own winding inductance does the rest. Inductance resists rapid current change, so the current in the winding — which is what actually produces torque — comes out close to a genuine sinusoid even though the voltage is a train of sharp-edged pulses. The output voltage on an oscilloscope looks nothing like a sine wave; the output current largely does.
The switching rate is the carrier frequency, typically 2 to 16 kHz, and it is one of the few genuine trade-offs an engineer sets deliberately:
| Higher carrier frequency | Lower carrier frequency |
|---|---|
| Quieter motor — audible whine moves above hearing range | Audible motor whine, sometimes objectionable |
| Smoother current waveform, slightly less motor heating | More current ripple in the motor |
| Higher IGBT switching losses, more drive heat, often requiring derating | Cooler drive, full current rating available |
| Worse EMC emissions and higher stress on motor insulation with long cables | Gentler on cable and motor insulation |
The default carrier frequency is usually correct. Raise it only to solve a noise complaint, and check the drive's derating table when you do — many drives reduce their continuous current rating substantially at the highest carrier settings.
V/f Ratio and Why Voltage Must Follow Frequency
Magnetic flux in the stator is approximately proportional to voltage divided by frequency. If the drive reduced frequency while holding voltage constant, flux would rise sharply, the core would saturate, and magnetising current would climb until the motor overheated or the drive tripped on overcurrent.
So the drive reduces voltage in proportion to frequency, maintaining a constant V/f ratio. For a 400 V, 50 Hz motor that ratio is 8 V per Hz: at 25 Hz the drive supplies about 200 V, at 10 Hz about 80 V. Flux stays constant, and so does available torque.
This holds only up to base frequency, the motor's nameplate frequency. Above it the drive cannot produce more voltage than its DC bus allows, so frequency keeps rising while voltage stays at maximum. The V/f ratio falls, flux weakens, and available torque drops roughly in inverse proportion to speed. The result is the classic two-region characteristic:
| Region | Frequency range | Voltage | Torque available | Typical use |
|---|---|---|---|---|
| Constant torque | 0 Hz to base frequency | Rises with frequency | Full rated torque throughout | Conveyors, extruders, positive displacement pumps, hoists |
| Constant power (field weakening) | Above base frequency | Fixed at maximum | Falls as speed rises | Spindles, centrifuges, machine tools |
At very low frequencies the stator winding's own resistance consumes a meaningful share of the small applied voltage, leaving too little to establish flux. Drives compensate with a voltage boost parameter that adds extra volts at low speed. Too little boost and the motor will not break away from standstill under load; too much and it draws excess magnetising current and overheats at low speed. This single parameter accounts for a large share of "the motor won't start" complaints on V/f-controlled drives.
Control Modes: V/f, Sensorless Vector, Closed Loop
| Mode | How it works | Low-speed torque | Needs | Suits |
|---|---|---|---|---|
| V/f (scalar) | Sets voltage and frequency from a fixed curve, no model of the motor | Limited; depends on boost setting | Nothing beyond basic nameplate data | Fans, centrifugal pumps, multiple motors on one drive |
| Sensorless vector | Estimates rotor flux position from measured current and a motor model | Good — often full torque near zero speed | Accurate motor data and a motor identification run | Conveyors, mixers, extruders, most machine applications |
| Closed-loop vector | Vector control with an encoder giving true rotor position | Excellent, with full torque at zero speed and holding | Encoder plus an encoder interface option | Hoists, positioning axes, tension control, test rigs |
The practical rule: use V/f when the load is a fan or centrifugal pump, or when one drive feeds several motors in parallel — vector control assumes a single known motor and cannot model a group. Use sensorless vector for anything needing torque at low speed, and accept that it requires a proper motor identification run to work well. Use closed loop only when the application genuinely needs position or zero-speed holding torque, since the encoder adds cost, cabling and a new failure mode.
Braking, Regeneration and DC Bus Overvoltage
When a drive decelerates a load faster than friction would, the motor becomes a generator. Mechanical energy from the rotating mass flows back through the inverter's anti-parallel diodes and into the DC bus, pushing bus voltage up. A standard diode-rectifier drive cannot send that energy back to the mains, so the bus voltage climbs until the drive trips on overvoltage.
| Method | What it does | When to use |
|---|---|---|
| Longer deceleration ramp | Lets friction absorb the energy instead of the bus | Always try this first — it is free |
| DC bus voltage controller | Drive automatically extends the ramp when the bus rises | Enabled by default on most modern drives; ideal where stop time is flexible |
| Braking resistor with chopper | Dumps excess bus energy as heat in an external resistor | High-inertia loads, frequent stops, required stop times |
| Regenerative / active front end | Returns energy to the supply instead of wasting it | Continuous regeneration — hoists, downhill conveyors, test benches |
Overvoltage trips during deceleration are among the most frequently misdiagnosed drive faults. The instinct is to suspect the supply, but the bus is being pushed up from the motor side, not the mains side. If a drive trips on overvoltage only when stopping, the deceleration ramp or the braking arrangement is the place to look.
Practical Consequences on Real Machines
Motor cooling at low speed. A totally enclosed fan-cooled motor is cooled by a fan on its own shaft. At 20 Hz that fan turns at 40 % speed and moves far less air, so the motor can overheat while drawing current well within its nameplate rating. Continuous low-speed operation needs either a separately powered forced-vent fan or a motor rated for inverter duty.
Cable length and reflected waves. The steep-edged PWM pulses travel down the motor cable and reflect at the motor terminals, where the reflected and incident waves can add to nearly double the intended voltage. On long cable runs this stresses the first turns of the winding. Manufacturers publish maximum cable lengths; exceeding them calls for an output reactor or dV/dt filter.
EMC and earthing. Fast switching makes a drive a significant source of electrical noise. Screened motor cable with the screen bonded through 360 degrees at both ends, separation from signal cabling, and a proper earthing arrangement are not optional refinements — they are the difference between a stable installation and one where analogue signals drift and nearby devices behave erratically.
Bearing currents. Common-mode voltage from PWM switching can drive small currents through the motor bearings, gradually eroding the raceways. On larger motors or long-life applications this justifies insulated bearings or a shaft grounding ring.
Step-by-Step 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
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.