V/F vs Vector Control in VFDs: Complete Technical Comparison

Overview

V/F (Volts per Hertz, also written V/Hz) control and vector control represent the two dominant control architectures used in modern variable frequency drives to regulate three-phase squirrel-cage induction motors. The selection between them determines low-speed torque capability, dynamic response to load steps, commissioning effort, and whether a shaft feedback device is required. This reference documents the underlying physics, the field-oriented control (FOC) decoupling that defines vector mode, the closed-loop and sensorless variants, the encoder selection thresholds documented for SINAMICS S120, and the field-proven application boundaries for each mode.

V/F Control: Operating Principle

A V/F drive regulates the air-gap flux implicitly by holding a fixed ratio between the inverter output line-to-line voltage and the output frequency:

V_per_Hz = V_rated / f_rated

For a 230 V, 60 Hz nameplate, the nominal V/Hz ratio is 3.83 (230 / 60 = 3.83). When the drive commands 30 Hz, the inverter scales the line-to-line voltage to 115 V; the per-phase voltage on a wye-wound motor is 115 / sqrt(3) = 66.4 V. The drive does not measure the actual rotor flux; the stator current is the only regulated output variable beyond voltage. The implicit assumption is that stator resistance drop is negligible compared to the back-EMF, which is reasonable above roughly 5-6 Hz and progressively fails below that point.

This mode is functionally a voltage-source inverter operating against a rotating machine whose impedance varies with slip. Because the magnetizing current is established by the applied voltage, low-speed torque degrades once the stator IR drop becomes a significant fraction of the available DC-bus voltage. In practice, V/F drives deliver rated torque only down to roughly 5-6 Hz; below this point the shaft typically exhibits cogging as the rotor slips in and out of synchronism with the rotating field.

V/F Enhancements

Modern V/F implementations add two common compensations that do not change the fundamental control law but recover useful torque margin:

• IR compensation (also called torque boost): adds a fixed or auto-tuned voltage offset proportional to stator current at low frequency to overcome stator winding resistance drop. Configured on SINAMICS G120 as p1310 (continuous boost) and p1311 (acceleration boost).
• Slip compensation: estimates rotor speed from measured current and increases output frequency by the calculated slip value, partially closing the speed regulation loop without an encoder. On SINAMICS this is enabled by p1335 and the slip gain is set in p1336.

With slip compensation enabled, speed regulation typically improves from the open-loop V/F value of approximately +/-2% to about +/-0.5% of nominal speed, still well below vector-mode performance.

Vector Control: Operating Principle (Field-Oriented Control)

Vector control, more precisely called field-oriented control (FOC), does not regulate a V/Hz ratio. Instead the drive decomposes the measured stator current vector into two orthogonal components in a rotating d-q reference frame aligned with the rotor flux:

• Id (flux-producing or magnetizing current): controls the d-axis component and sets the rotor flux magnitude, analogous to the field current in a separately excited DC machine.
• Iq (torque-producing current): controls the q-axis component, oriented 90 electrical degrees ahead of Id in the stator, and produces torque proportional to its magnitude.

The electromagnetic torque equation in the d-q frame is:

T_e = (3/2) x (P/2) x (Lm^2 / Lr) x Id x Iq

where P is the number of poles, Lm is the magnetizing inductance, and Lr is the rotor inductance referred to the stator. Because Id and Iq are decoupled by the Park transform, the drive regulates flux and torque independently, exactly as a DC machine does. The V/Hz profile is the by-product of the flux regulator, not its target.

Coordinate Transforms

FOC requires three transforms computed every PWM cycle:

1. Clarke transform: three-phase currents ia, ib, ic become the stationary alpha-beta frame.
2. Park transform: alpha-beta frame becomes the rotating d-q frame, using rotor flux angle theta from an encoder or a flux observer.
3. Inverse Park and Space Vector Modulation (SVM): d-q voltage commands become three-phase PWM duty cycles.

On SINAMICS S120, these transforms run inside the Control Unit and the Sensor Module; the cycle time is typically 125 microseconds at the current controller and 1 ms at the speed controller, configured via p0115 in the expert parameter list.

Vector Control Variants

Sensorless Vector Control (SLVC, also SVC)

SLVC estimates the rotor flux angle from a mathematical motor model rather than from a feedback device. The drive requires the motor equivalent-circuit parameters: stator resistance Rs, rotor resistance Rr, magnetizing inductance Lm, stator leakage Llsigma, rotor leakage Llrsigma, and the motor inertia. These are entered manually from the nameplate and refined by an automatic motor identification (auto-tune) routine. The drive then runs the current model, integrating the stator voltage equation and updating the flux estimate each cycle.

Because the flux estimate degrades at very low speed (back-EMF approaches zero and the integration becomes noise-sensitive), SLVC delivers full torque down to approximately 1 Hz and useful torque to about 0.5 Hz. On SINAMICS the sensorless mode is selected via p1300 = 20; the speed controller sample time is set in p0115 and the current model gain in p1750.

Closed-Loop Vector Control (FVC)

FVC adds an encoder on the motor shaft, typically supplying TTL, HTL, sin/cos 1 Vpp, EnDat 2.1/2.2, or resolver feedback through a Siemens SMCxx or SME25 Sensor Module. The encoder angle directly feeds the Park transform, eliminating the flux observer at low speed. Torque is regulated down to and including zero speed, making FVC the only viable mode for hoists, vertical lifts, and high-torque hold-at-zero applications.

The encoder selection threshold for SINAMICS S120 is documented in Function Manual FH1: an encoder is required if torque control is needed in a speed range greater than 1:10, or if a defined torque must be maintained below approximately 10% of the rated motor frequency (p0310).

Performance Comparison

Parameter	V/F open loop	V/F with slip comp.	Sensorless Vector (SLVC)	Closed-Loop Vector (FVC)
Speed regulation	+/-2 to +/-3% of nominal	+/-0.5% of nominal	+/-0.1% of nominal	+/-0.01% of nominal
Full torque low-speed limit	approx. 5-6 Hz	approx. 5-6 Hz	approx. 1 Hz (0.5 Hz useful)	0 Hz (full torque at standstill)
Constant-torque speed range	10:1	10:1	100:1 typical	1000:1 or wider
Dynamic response (bandwidth)	5-10 Hz	5-10 Hz	30-60 Hz	100-200 Hz
Starting torque	150% with boost	150% with boost	200% typical, up to 300% with oversize drive	200-300% for 60 s
Encoder required	No	No	No	Yes (TTL / HTL / sin-cos / EnDat / resolver)
Auto-tune required	Optional	Recommended	Mandatory for rated performance	Mandatory
Commissioning time	Low	Low	Medium	High
Typical application	Fans, pumps, conveyors	Conveyors, mixers	Extruders, machine tools, non-hoist cranes	Hoists, servohydraulic pumps, winders

When Vector Control Requires an Encoder

The SINAMICS S120 Function Manual FH1, section "Selecting the encoder," lists the following criteria; an encoder is required when at least one is binding:

• High speed accuracy is required (typically better than +/-0.1% of nominal speed).
• High dynamic response is required: command behavior and disturbance rejection both must be tighter than sensorless vector.
• Torque control is required across a control range greater than 1:10.
• A defined or variable torque must be maintained at speeds below approximately 10% of the rated motor frequency (parameter p0310).

If none of these criteria apply, sensorless vector control is generally sufficient. If at least one criterion is binding, an encoder plus FVC is the recommended configuration.

Setpoint Input Modes for Vector Control

Vector control is divided into two setpoint branches:

1. Speed control (default): the speed controller output becomes the torque setpoint, which becomes the q-axis current command Iq. Flux command Id is held at the rated magnetizing value and reduced above base speed for field weakening.
2. Torque control: the torque setpoint is taken directly from a higher-level controller, PLC, or winder tension loop, and the speed controller is bypassed. The drive regulates Iq against the torque reference and uses Id to maintain the flux reference.

Switching between speed and torque control on SINAMICS is performed by setting p1501 and configuring the master/slave or tension controller using function blocks from the technology library or PROFIdrive telegrams 110/111.

Motor Parameters Required for Vector Mode

Vector control requires the equivalent-circuit data. On SINAMICS the following parameters must be populated, either from the motor nameplate or from the automatic motor identification routine (p1910 = rotating measurement, p1900 = static measurement):

Parameter	Meaning	Typical source
p0304	Rated motor voltage	Nameplate
p0305	Rated motor current	Nameplate
p0307	Rated motor power	Nameplate
p0308	Rated power factor (cos phi)	Nameplate
p0310	Rated motor frequency	Nameplate
p0311	Rated motor speed	Nameplate
p0314	Number of motor pole pairs	Calculated from p0310 / p0311
p0335	Motor cooling method	Nameplate or frame
p0340	Trigger automatic calculation of motor model	Set to 1 after entering nameplate
p0350 / p0354 / p0356 / p0358 / p0360	Stator / rotor resistance, leakage reactances, magnetizing reactance	Auto-tune (p1900 / p1910)
p0341 / p0342	Total inertia, load / motor inertia ratio	Mechanical specification, or measured by p1959 inertia identification

If nameplate data is incomplete (a common situation when motors are resold or repurposed), V/F control remains the only feasible option, because the auto-tune routine cannot identify parameters it cannot measure.

Selection Criteria: Choosing V/F or Vector

Use V/F control when:

• The load is a quadratic torque application (fan, pump, centrifugal compressor) where power scales with the cube of speed and high low-speed torque is not required.
• Multiple different motors are connected to the same drive and motor parameters are not known or not stable.
• Cost per drive is the dominant constraint and incremental torque performance is not required.
• Speed range does not exceed 10:1 and +/-2% speed regulation is acceptable.

Use sensorless vector (SLVC) when:

• Constant torque loads (conveyors, extruders, mixers, machine tool spindles) require full torque at low speed without the cost of an encoder.
• Speed regulation better than +/-0.5% is required.
• Motor nameplate data is available and the operator can run an auto-tune.
• Dynamic response in the 30-60 Hz bandwidth range is sufficient for the application.

Use closed-loop vector (FVC) when:

• The application is a hoist, lift, winder, or any vertical load that must hold position at zero speed with full torque.
• Torque control is required across a 1:10 range or wider.
• High dynamic response (>100 Hz bandwidth) is required, for example in servo-hydraulic pumps or web tension loops.
• Speed accuracy must be better than +/-0.1%.

Commissioning and Verification

1. Verify the motor nameplate data has been entered exactly: p0304, p0305, p0307, p0308, p0310, p0311. A missing power factor is the most common commissioning error.
2. Set p0340 = 1 to allow the drive to calculate motor model data from the nameplate. This produces a coarse model and is not a substitute for the auto-tune.
3. Run the static motor identification (p1900 = 1 in STARTER / SIMOTION Scout) to identify the stator resistance and leakage reactances with the rotor locked.
4. If the motor can be uncoupled from the load, run the rotating identification (p1910 = 1) to capture the magnetizing curve, rotor time constant, and friction.
5. Enable the encoder (p0400, p0404, p0408, p0420) before selecting FVC. For SLVC, leave the encoder unconfigured.
6. Select the control mode on SINAMICS G120 / S120: p1300 = 0 (V/F linear), 1 (V/F with FCC), 2 (V/F quadratic), 20 (sensorless vector), 21 (vector with encoder).
7. Verify the speed controller tuning: trigger the speed-controller optimization (p1960) and confirm the gain Kp (p1460) and integral time Tn (p1462) are within the recommended range.
8. Step the speed setpoint from 0% to 100% nominal and observe the acceleration ramp in STARTER trace. Confirm no F07900 (motor overloaded), F07901 (motor overspeed), or F07902 (motor stall) fault occurs.
9. For FVC only: with the motor at standstill, command a small torque setpoint (10% rated) and verify the shaft holds position with no rotation. This is the canonical zero-speed holding test.

Faults and Diagnostics Specific to Control Mode

Fault / Alarm	Meaning	Likely cause in V/F	Likely cause in vector
F07900 (motor blocked)	Speed at torque limit for too long	Load exceeds V/F low-speed torque	Encoder failure, wrong p0311, incorrect motor model
F07901 (motor overspeed)	Speed exceeded p2162 overspeed threshold	Slip comp not active, or wrong p0311	Speed controller gain too high, encoder wiring reversed (A/R swapped)
F07902 (motor stall)	Model-based stall detection	Excessive load, low V/Hz	Flux estimate lost, Id reference dropped, encoder noise
F31117 (encoder signal error)	Encoder amplitude or quadrature fault	N/A (no encoder)	Shielding, cable length, encoder supply voltage
A07910 (motor overtemperature)	Thermal model I-squared-t alarm	Long-term overload in V/F	Reduced cooling, encoder-less operation at zero speed

Fault numbers above are SINAMICS S120 / G120 conventions and may differ on other platforms; consult the manufacturer-specific fault list when applying this guide to other drives.

Notes on Platform Variants

Although the underlying physics is the same, parameter numbers and control-mode select codes differ across platforms. Allen-Bradley PowerFlex 525 / 755 selects V/F as "V/Hz" and vector as "FVC" or "SVC" in parameter 4 (Motor Control Mode). Schneider Electric Altivar Process uses the configuration menu to choose "Standard" (V/F), "Performance" (SVC), and "Expert" (FVC) motor control. Yaskawa A1000 uses A1-02 = 0 (V/F), 2 (open-loop vector, OLV), 3 (closed-loop vector, CLV). Always verify the control-mode parameter and the auto-tune procedure against the manufacturer's programming manual before commissioning.

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