VFD Motor Control: V/F, SVC and FVC Explained

V/F control: the fixed curve

V/F control is the first of the three methods the series documentation names. Its published envelope on this site's series reaches 0–320 Hz (AU1000 V/F), 0–400 Hz (AU320 table) and 0–500 Hz (EV510, AU8000). The EV510's V/F documentation publishes the curve options for the method — straight-line, multi-point and N-power V/F curves, automatic and manual torque boost (manual 0.1%–30.0%), and linear or S-curve acceleration profiles. The AU1000's documentation lists four independent S-ramps in its standard-function list.

Sensorless vector control (SVC): the model-based step

SVC works without a shaft sensor: the AUD32's documentation describes its sensorless vector control as built on motor parameter identification, online estimation and flux estimation. The published results on the AU720 are the benchmark figures for the three methods: in SVC, starting torque of 150% at 0.25 Hz, a speed regulation range of 1:200 and steady-speed precision of ±0.5%; and automatic torque limiting in vector mode. The AUD32's documentation publishes its own ratings alongside its technology description, including overload capacity of 150% of rated output current for 60 s. The EV510 publishes the same structure at slightly softer numbers (for its G type in SVC: 150% starting torque at 0.5 Hz, a 1:100 speed range and ±0.5% steady precision). The figures are the comparison points — starting torque at a stated frequency, speed range, steady precision; which figures a given duty works to is an enquiry item: state the duty's requirements with the enquiry and ask for the applicable figures to be addressed in the quotation.

Closed-loop vector control (FVC): the measured step

FVC closes the loop with an encoder on the motor shaft. The published FVC figures are: the AU720, and the EV510 for its G type, publish FVC starting torque of 180% at 0 Hz, a 1:1000 speed range and ±0.02% steady precision, and the EV510 documentation adds ±5% torque control accuracy. The AU720's documentation publishes the same FVC structure — 180% at 0 Hz and 1:1000 — in its own specification table. Closed-loop operation uses an encoder and its card; the EV510 documents three PG card options — differential input, rotating transformer (resolver) and OC input. See the closed-loop page for the series and encoder options.

The supporting cast: PID, multi-step speed, torque control

Built on top of the control core, three documented functions recur across the application pages. Built-in PID is documented on the EV510, described as realising a process control close-loop system; a built-in PID software function is named in the AU320's product item; the pump lines document PID constant-pressure control. Multi-step speed (16 steps on the AU720, through its simple PLC function or digital inputs; up to 16 settings on the AUD32) is a documented function set on both series. Torque control and limiting: torque limit and control are documented functions of the vector modes on the AU720 and EV510. The AU400 documentation also publishes a wobble (traverse) frequency function for textile winding, and the AU720 documentation an AVR function. The pump documentation publishes its own PID arrangement alongside the function: the AU620's material describes PID constant-pressure control with master and auxiliary pumps coordinated over RS485, one drive controlling up to four auxiliaries.

One documented data point on expectations: the AU320's product material states that its open-loop vector function can exert more than 80% of the control performance without motor-parameter learning. The AU1000's V/F envelope, published in elevator documentation alongside its vector modes, shows the same series carrying all three levels.

Choosing the level

Level	Published figures
V/F	Published output envelopes: 0–320 Hz (AU1000 V/F), 0–400 Hz (AU320), 0–500 Hz (EV510, AU8000); the AU400 documentation publishes a V/F starting-torque figure of 0.5 Hz / 150%
SVC	150% starting torque at 0.25 Hz, 1:200 range, ±0.5% precision (AU720 figures)
FVC	180% starting torque at 0 Hz, 1:1000, ±0.02% (AU720; EV510 G type); ±5% torque accuracy (EV510); with an encoder

Which level a given application takes is not printed in the figures: it is an enquiry item — state the application data with the enquiry and ask for the applicable level to be addressed in the quotation.

Torque: the published figures

The published figures are per series: the AU720's SVC documentation prints 150% starting torque at 0.25 Hz, and the EV510's FVC documentation prints ±5% torque control accuracy. The AU720's specification prints the speed ranges beside its starting-torque figures — 1:200 under SVC and 1:1000 under FVC — and the EV510's specification prints the same 1:1000 FVC range.

Motor parameter identification

The AUD32's documentation lists motor parameter identification and flux estimation among its functions. The published figures are ratings with their conditions attached, and which control level a duty takes is an enquiry item: state the application data with the enquiry and ask for the level to be addressed against those figures in the quotation.

A note on what the numbers leave out

The AU1000's elevator documentation prints the same per-mode set in its own specification — ±0.5% (SVC) and ±0.02% (FVC) speed stability accuracy, and ±5% torque control accuracy under FVC.

Frequently asked questions

Does SVC need motor tuning?

The series documentation includes motor parameter identification functions (the AUD32 documents parameter identification and flux estimation).

Can a drive switch between modes?

Control mode is a parameter choice on the multi-mode series (the AU720 publishes V/F, SVC and FVC in one specification). The encoder-dependent mode obviously requires the encoder fitted.