VFD Control Modes: V/Hz, Sensorless and Closed-Loop
Short answer
- A VFD control mode is the method the drive uses to decide what voltage and frequency to put on the motor, and the three common ones are V/Hz (scalar), sensorless vector, and closed-loop vector with an encoder.
- They differ mostly in how much torque is available at low speed and how tightly speed holds under load, with V/Hz suiting fans and pumps, sensorless vector suiting conveyors and mixers, and closed-loop vector suiting hoists, spindles and winders.
- Vector modes depend on a motor model, so the nameplate data must be entered exactly and an autotune run, while V/Hz is the only mode that allows several motors on one drive.
A VFD control mode is the method the drive uses to decide what voltage and frequency to put on the motor. There are three common ones: V/Hz (scalar), sensorless vector, and closed-loop vector with an encoder. They differ mostly in how much torque you get at low speed and how tightly speed holds under load.
What are the three VFD control modes?
V/Hz, also called scalar or volts per hertz. The drive keeps voltage in proportion to frequency, so the motor's magnetic flux stays about constant. It does not measure the motor at all. Many drives add a boost at low frequency to make up for stator resistance, and some add slip compensation. It is simple, forgiving and does not much care whether the motor data is exact.
Sensorless vector. The drive keeps a mathematical model of the motor and estimates rotor flux and torque current from the measured output current and the voltage it is commanding. It then controls those separately, which gives better torque at low speed and faster response to a load change. It needs reasonably accurate motor data and usually an autotune.
Closed-loop vector, also called flux vector with feedback. An encoder on the motor shaft tells the drive the real speed and position. The drive can hold full torque down to zero speed, and speed regulation is very tight. It needs the encoder, its wiring and a feedback card or input on the drive.
How do V/Hz, sensorless and closed-loop vector compare?
| Point | V/Hz | Sensorless vector | Closed-loop vector |
|---|---|---|---|
| Feedback device | None | None | Encoder required |
| Torque at very low speed | Limited, depends on boost | Good, down to a few hertz typically | Full torque, even at standstill |
| Speed regulation under load | Drifts with load unless slip compensated | Good | Tight |
| Motor data accuracy needed | Low | Fairly high | High |
| Autotune | Rarely needed | Needed | Needed |
| Multiple motors on one drive | Yes | No, single motor only | No |
| Can run without a motor connected for testing | Often yes | Often not | Often not |
| Typical fit | Fans, pumps, simple conveyors | Conveyors, mixers, machine tools | Hoists, spindles, winders, test stands |
Which VFD control mode fits which load?
- Centrifugal fans and pumps. The torque demand falls as speed drops, so there is little need for low-speed torque. V/Hz is the usual choice and often the best one, because it is stable and tolerant. Many drives have a variable-torque V/Hz curve for this.
- Conveyors and general constant-torque machinery. They need decent torque at low speed to start a loaded belt. Sensorless vector is the common answer, or V/Hz with the boost set carefully.
- Mixers, extruders, crushers. Heavy starting torque and big load swings. Sensorless vector, closed-loop if you need the speed held.
- Hoists and cranes. They need full torque at zero speed to hold the load while the brake releases, and controlled behaviour through that moment. That points to closed-loop vector, or a drive built and approved for lifting. This is safety-critical work, so the design and commissioning belong to a qualified person.
- Spindles, winders, test stands. They need accurate speed or torque over a wide range, and often torque control. Closed-loop vector.
What is the difference between speed mode and torque mode?
In speed mode, the drive is told how fast to turn and it varies torque to hold that speed. This is what almost every application uses. In torque mode, the drive is told how much torque to produce and the speed is whatever results from that torque and the load. A winder that has to hold constant tension on a web is a classic torque-mode job, and so is a load-sharing second drive on a shared shaft.
Torque mode on a load that can run away, such as a conveyor with the product removed, will accelerate without limit until a speed limit or fault catches it. Make sure the drive has a speed limit set. A drive in torque mode also normally needs the vector control and feedback to be meaningful at low speed.
Why must a VFD autotune be done properly?
Vector modes depend on the motor model. Enter the nameplate data exactly (voltage, current, frequency, rated rpm, power) and run the autotune. A standstill tune measures resistance and some inductance with the shaft locked. A rotating tune is better and measures more, but it needs the motor uncoupled or at least free to spin, which is not always possible once it is in the machine. Do the autotune with the motor at normal temperature if the drive allows it.
What goes wrong when the VFD control mode is wrong?
- Trips on acceleration at low speed. V/Hz boost set too high drives the motor into saturation, or too low and it stalls. Either way the current limit trips.
- Hunting or oscillating speed. Common in V/Hz with a lightly loaded motor, or in vector control with poor motor data. Check the autotune, then slip compensation and gain settings.
- Weak start on a loaded conveyor. V/Hz without enough boost. Move to sensorless vector.
- Overcurrent fault when the motor starts. Wrong motor data entered, a tune that was skipped, or a motor much smaller or larger than the drive.
- Encoder fault in closed-loop. Lost feedback, wrong pulse count, reversed channels or noise on the cable. The drive usually trips because it can no longer trust the speed.
- Motor overheats at low speed. Any mode: a self-cooled motor has less airflow below base speed. This is a motor cooling issue and not a control one.
How do you choose a VFD control mode?
- Is the torque demand falling with speed (fan, pump)? Start with V/Hz.
- Does it need to start under load, or hold speed as load changes? Sensorless vector.
- Does it need full torque at standstill, torque control or precise speed? Closed-loop with an encoder.
- Are several motors on one drive? V/Hz is the only mode that allows it.
- Check the motor can take it: inverter-duty insulation and a suitable encoder mount if closed-loop.
Common questions
Is sensorless vector always better than V/Hz?
No. For a fan or pump, V/Hz is usually just as good and easier to commission. Vector earns its place where low-speed torque or load response matters.
Do I need an encoder for vector control?
Not for sensorless vector. You need one for closed-loop, and also for any application that needs torque at zero speed.
Why does my drive say it needs an autotune?
Vector control is built on a model of your motor. The tune measures the real values so the model matches the machine in front of you.
Can I switch modes on an existing machine?
Often yes, but re-enter the motor data, run the tune and retest the acceleration, braking and holding behaviour. Do not assume the old gains still apply.
Further reading on this site: how a VFD works; why VFD-driven motors run hot; the cost of oversizing a VFD; encoders explained; soft starter vs VFD; motor selector.
If you are replacing a drive or motor and unsure what control the application needs, send us the nameplate details and what it drives, and we can help find a suitable equivalent.
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Request a QuoteFrequently asked questions
What is the difference between V/Hz and sensorless vector control on a VFD?
V/Hz keeps voltage in proportion to frequency so the motor's magnetic flux stays about constant, and it does not measure the motor at all. Sensorless vector keeps a mathematical model of the motor and estimates rotor flux and torque current from measured output current and commanded voltage. That gives better torque at low speed and faster response to a load change, but it needs reasonably accurate motor data and usually an autotune.
Do I need an encoder for vector control?
Not for sensorless vector, which runs without a feedback device. An encoder is required for closed-loop vector, and also for any application that needs torque at zero speed. With an encoder on the motor shaft, the drive knows the real speed and position, can hold full torque down to zero speed, and regulates speed very tightly. It also needs the encoder wiring and a feedback card or input on the drive.
Which VFD control mode suits centrifugal fans and pumps?
V/Hz is the usual choice for centrifugal fans and pumps. Torque demand falls as speed drops, so there is little need for low-speed torque. V/Hz is stable and tolerant, and many drives have a variable-torque V/Hz curve for this. Sensorless vector is not always better than V/Hz, because vector earns its place where low-speed torque or load response matters.
Why does a VFD need an autotune for vector control?
Vector control is built on a model of the motor, and the autotune measures the real values so the model matches the machine. Enter the nameplate data exactly (voltage, current, frequency, rated rpm, power) first. A standstill tune measures resistance and some inductance with the shaft locked, while a rotating tune measures more but needs the motor uncoupled or free to spin.