V/Hz Control and Variable Frequency Drives

When supplying motor power directly from the utility, the user is limited to the motor’s line speed, uncontrolled torque during acceleration, and uncontrolled deceleration (coasting to a stop). 

Along with these limitations, the motor itself will experience a very high current spike – typically six times the rated current – during startup, which causes unnecessary heating and will damage the motor windings over time. 

An effective, low-cost way to control a motor is to use an open-loop variable frequency drive (VFD) with V/Hz control. This allows the user to select a variable speed or multiple speeds, making for a smoother startup.

 

What Is Volts/Hertz Control?

Volts/Hertz control stems from the voltage-frequency relationship and how it relates to magnetic flux in Faraday’s Law¹. See the equation below.

Where V is the EMF-induced in a coil, N is the number of turns, and φ is the magnetic flux. It can be seen with this equation below.

The K in this equation is a constant and f is the frequency of the supply. If the ratio V/f is constant, the magnetic flux will also be constant. In other words, the voltage-frequency relationship will need to be linear if a constant flux is desired. Furthermore, if flux is constant, so is an induction motor’s torque for a given stator current.

Therefore, constant volts-hertz control has a constant torque characteristic which is attractive for many industrial applications.

V/Hz Control Advantages

A line-fed motor will be exposed to large voltage and current spikes upon startup, limited to its line speed, and will have uncontrolled torque values when spinning up to speed. Adding open-loop V/Hz control to a VFD addresses these issues yet keeps the control system simple.

One big advantage of V/Hz control is the ability to control multiple motors with one VFD. This gives you the benefit of VFD-controlled starts and operations for a fraction of the cost. Each motor will run at the same speed, making this setup ideal for pump and fan applications. The KEB F6 VFD takes this one step further with its set-switching capability.

The F6 has eight sets which can all be programmed for a different move or speed. If you add a motor contactor you can run one motor, then when an input is triggered switch over to another set and motor, and have it do a completely different operation. This saves the user the cost of adding a VFD to each motor and still allows for each motor to operate differently.

Applications for VFDs With V/Hz Control

You’ll find V/Hz control use in applications that require consistent torque and simple speed control. This setup typically performs well in systems with steady loads and predictable operating ranges. 

Common use cases include: 
• Pumps and fans: HVAC systems and water treatment, for example, where flow rates vary. 
• Conveyors: Material handling environments in which multiple motors must run at the same speed. 
• Blowers and compressors: Supports gradual ramp-up and helps reduce mechanical stress. 
• Multi-motor systems: A single VFD may control several motors that run in parallel, thus reducing cost and complications. 

V/Hz control is used in these areas due to its reliable balance of performance and accessibility.

When Not to Use V/Hz Control

In situations where precision, responsiveness, and dynamic load handling are necessary, V/Hz control may not be the best choice. 

Avoid V/Hz control in the following environments: 

• High-precision applications: Such as CNC machines or robotics where exact speed and position control are required. These often rely on servo motors for tighter control. 

• Rapidly changing loads: Systems that demand quick torque adjustments may experience lag due to the lack of feedback.

• Low-speed, high-torque scenarios: V/Hz struggles to maintain torque accuracy at very low speeds without additional tuning.

• Tightly regulated processes: Applications with strict tolerances (e.g., extrusion or winding) require more advanced control methods.

In these cases, what is typically a benefit–the simplicity of V/Hz–becomes a limiting factor, and a closed-loop solution will likely be more effective. However, even advanced control methods can underperform if the drive is not configured correctly—especially during startup and commissioning.

V/Hz vs Vector Control

The key difference between V/Hz and closed-loop is the level of control your application actually needs.

For many systems, V/Hz control can deliver all the performance necessary without the added cost or complexity of vector control. In other words, you get a dependable, efficient option for applications where simplicity and reliability matter most.

V/Hz Control Disadvantages

There are some disadvantages of V/Hz control. Firstly, the speed regulation is about 3% of the maximum frequency. This means for a given speed, there will be a window of roughly 50 rpm on a 1750 rpm-rated motor. A 50 rpm window is unacceptable for some applications, so V/Hz control is not a universal solution. 

Also, the speed response is slower in V/Hz operation than on closed-loop systems because there is no feedback. Changes in load and speed behavior are less easily detected by the VFD, and therefore, it will react more slowly than with encoder feedback.

Possible VFD Adjustments

The V/Hz operating line is ideally linear and intersects at (0,0) and (rated frequency, rated speed). At low speeds, the voltage drop caused by the stator resistance is significant. This drop has a direct effect on the motor flux or torque.

There are a few adjustments you can make to fine-tune your V/Hz operation. The first is called “boost.” Boost adjusts the output voltage at 0Hz and gives the motor a little voltage boost at lower speeds to compensate for the stator resistance losses. In the F6 VFD, rated frequency is parameter uF.0 and boost is uF.1, which can be adjusted up to 25% of the rated voltage.

If a system experiences a large breakaway torque, the KEB F6 has a delta-boost function that further increases the boost for a set period. This is shown by parameters uF.4 (boost) and uF.5 (delta) below.

Implementing the delta-boost function can give you the extra boost to help overcome large breakaway torques during startup. After a set amount of time, this function will drop out, and normal V/Hz operation will take over.

Field Weakening Operation – What Happens?

Another area that is opened up when operating in V/Hz mode is the field weakening range. This is the area above the motor’s rated speed, which is achieved by supplying the motor more than its rated frequency at its rated voltage. A VFD can only supply the line voltage, which no longer follows the ideal V/Hz linear curve.

Based on Faraday’s law, the motor’s flux diminishes as the frequency increases but the voltage is held constant. The motor no longer operates in a constant torque mode but operates in a constant power mode. You need to limit this current to protect the motor and VFD. Limiting the current and voltage but increasing the frequency in the field weakening range will decrease the flux or torque as we saw in the flux equation discussed earlier.

When operating in the field weakening range, it is important to remember the motor’s torque output will diminish and eventually be reduced below the load’s required torque, effectively stalling the motor.

FAQ

What is the main benefit of V/Hz control?
It provides simple, cost-effective speed control with consistent torque for many industrial applications.

Can one VFD control multiple motors using V/Hz?
Yes, as long as the motors operate at the same speed, making it ideal for pumps, fans, and conveyors.

Why is V/Hz control less precise than closed-loop control?
Because it operates without feedback, it cannot adjust as quickly or accurately to changes in load or speed.

Is V/Hz control energy efficient?
Yes, especially in variable-speed applications like fans and pumps, where reducing speed lowers overall energy consumption.

Better Performance at a Lower Cost

Volts/Hertz control is a simple, low-cost way of getting better performance and control out of your motors. You can control multiple motors in V/Hz mode using a single VFD. It is used a lot in pump and fan applications due to its lack of needing very precise speed control. 

Operating in V/Hz mode opens doors that are not available by connecting a motor to line power. Firstly, you can vary the motor speed while maintaining constant torque up to the motor’s rated speed. If desired, you can over-speed the motor into its field weakening range by sacrificing a little torque.

If V/Hz operation sounds like the right choice for your application, contact a KEB Engineer today for help selecting the right VFD for the job.