Every variable frequency drive has to decide how much voltage to apply for a given output frequency, and the oldest and simplest answer is to keep the ratio between them fixed. That answer is volts-per-hertz control, often written V/f or V/Hz, and it is the default control mode in nearly every drive shipped. It is a scalar method: it sets voltage as a straight function of frequency rather than modelling the motor's internal magnetic state. This guide explains why holding that ratio constant keeps the motor's magnetic flux steady, why a little extra voltage is added at low speed, and why V/f remains the right choice for the pumps and fans that make up most drive applications.
Volts-per-Hertz (V/f) Control in one line: Volts-per-hertz control is the basic scalar mode of a VFD in which output voltage is raised in proportion to output frequency, holding the volts-per-hertz ratio constant so the motor's magnetic flux stays roughly the same at every speed. Keeping flux constant keeps torque capacity constant, and a small low-speed voltage boost compensates for winding resistance so the motor still produces torque near zero speed. It is open-loop, needs no encoder, and is ideal for pumps, fans, and other variable-torque loads.
The magnetic flux inside an induction motor is set by the voltage applied to its windings divided by the frequency of that voltage. If you were to lower the frequency to slow the motor while leaving the voltage at its rated value, the flux would climb, the iron would saturate, and the motor would draw excessive magnetizing current and overheat. If instead you dropped the voltage without changing the frequency, the flux would collapse and the motor would lose the ability to make torque. The way to slow a motor cleanly is to change both together, and the ratio that keeps flux where the motor was designed to run is the rated voltage divided by the rated frequency.
Volts-per-hertz control does exactly this. As the drive lowers frequency to reduce speed, it lowers voltage in step, holding their ratio at that design value so the flux in the air gap stays constant across the speed range. Because torque capacity in an induction motor depends on flux, keeping flux constant means the motor keeps roughly the same ability to produce torque whether it is running at half speed or full speed. This is why a V/f drive gives a motor a broad constant-torque region below its rated frequency without any complicated modelling of the machine.
Above the motor's rated frequency the drive runs out of voltage, because it cannot output more than the line can supply. Beyond that point the frequency keeps rising but the voltage is pinned at maximum, so the ratio falls, flux weakens, and the motor enters what is called the field-weakening or constant-power region where available torque tapers off with speed. Understanding that the V/f line is flat up to base speed and then flat-topped in voltage above it explains the shape of nearly every drive's torque-versus-speed capability.
The simple straight-line V/f rule works well through most of the speed range, but it breaks down near zero speed. The reason is that the motor's stator windings have resistance, and at low frequency the applied voltage is small, so the fraction of that small voltage lost across the winding resistance becomes significant. What is left over to actually establish flux in the air gap is too little, so the motor's low-speed torque sags just when many loads need it most to break away from standstill.
The fix is a voltage boost: the drive adds an extra increment of voltage at low frequencies, lifting the bottom of the V/f curve above the straight line so that enough voltage remains after the resistive drop to keep flux up. This boost, sometimes called torque boost or IR compensation because it offsets the current-times-resistance loss, is a tunable parameter. Set it too low and the motor struggles to start heavy loads; set it too high and the motor over-fluxes at low speed, drawing extra current and running hot when lightly loaded.
Finding the right boost is one of the few adjustments a V/f setup usually needs. Many drives offer preset V/f patterns - a linear pattern for constant-torque loads and a squared or variable-torque pattern for fans and pumps whose torque demand falls off at low speed - and the squared pattern deliberately reduces voltage more aggressively at low speed to save energy where full flux is not needed. Choosing the right pattern and trimming the boost matches the drive to the load without ever running an auto-tune or fitting a feedback device.
The great strength of V/f control is that it is simple, robust, and requires almost no knowledge of the specific motor. It is open-loop, meaning it needs no encoder or speed feedback, and it does not depend on an accurate model of the machine, so one drive can run a range of motors with minimal setup. It tolerates a motor that is oversized, mismatched, or shared, and it happily drives several motors from one drive at once, which vector modes generally cannot. For the enormous population of centrifugal pumps, fans, and blowers where the load is well behaved and speed does not need to be held to a fraction of a percent, that simplicity is exactly what is wanted.
Those loads also play to the method's other strength: their torque demand rises with the square of speed, so they need very little torque at low speed, which is precisely where plain V/f is weakest. A fan running at a quarter speed asks for only a small fraction of its full torque, so the low-speed torque limitation that would matter for a loaded conveyor or a positive-displacement pump simply never bites. The squared V/f pattern designed for these loads also trims low-speed voltage to save energy, which is why variable-torque V/f is the standard energy-saving control for flow and pressure applications.
In a monitored field operation, V/f drives on pumps and fans are among the most common assets a cloud SCADA platform such as Merobix watches. Because the control mode is open-loop, the useful telemetry is the drive's own data - output frequency, motor current, and any fault codes - trended over time. A rising current at a steady commanded frequency, or nuisance overcurrent trips on start, often points straight at a boost set too high or too low, and being able to see that pattern remotely lets an engineer adjust the parameter without a site visit. For the great majority of variable-speed pumps and fans in the field, V/f is the control mode running underneath, and knowing its behaviour is what makes those drive trends readable.
V/f control is scalar: it sets voltage as a fixed ratio of frequency and does not model the motor's internal magnetic state, so it is simple and needs no motor tuning. Vector control models the motor to control flux and torque separately, giving much better low-speed torque and speed accuracy at the cost of an auto-tune and a good motor match. For well-behaved loads like pumps and fans, V/f is usually enough; loads needing strong torque near zero speed favour vector control.
At low frequency the applied voltage is small, and the portion of it lost across the stator winding resistance becomes a large fraction of the total, leaving too little voltage to establish full flux. The result is weak torque near standstill. A voltage boost adds extra voltage at low frequencies to offset that resistive loss so the motor keeps its torque. The boost is tunable: too little and heavy loads will not start, too much and the motor over-fluxes and runs hot.
Yes, and this is a distinct advantage of V/f control. Because the mode applies a voltage-frequency pattern to its output rather than modelling one specific motor, it can drive several motors in parallel from a single drive, which vector modes generally cannot do because they depend on a single matched motor model. This makes V/f the practical choice for multi-motor applications like banks of fans, provided each motor still has its own overload protection.
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