Understanding (VFD) Variable Frequency Drives
Understanding How a Variable Frequency Drive Works
A Variable Frequency Drive (VFD), also called an AC drive or frequency inverter, controls an AC motor by creating the voltage, frequency and current conditions required by the motor and the driven machine.
The easiest way to understand a VFD is to stop thinking of it as a simple electronic speed knob. It is actually a power-conversion system that processes the incoming electrical energy in several stages.
AC Supply → Rectifier → DC Bus → IGBT Inverter → PWM Output → Motor
The VFD does not simply “reduce” the 50 or 60 Hz mains waveform. It first converts incoming AC into DC, stores and smooths that energy in the DC bus, and then rapidly switches power semiconductors to create a new motor output at the required fundamental frequency and voltage.
For a wider introduction, see What is an AC drive?.
See the energy path inside a VFD
Change the frequency reference in the educational model below. The mains supply remains the same, while the VFD changes its motor-side output frequency and the estimated shaft speed.
1. Why is AC motor speed related to frequency?
The stator of an AC motor creates a rotating magnetic field. The speed of that field depends on electrical frequency and the number of motor poles.
The basic synchronous-speed relationship is:
ns = 120 × f / p
where f is electrical frequency and p is the number of poles.
A four-pole motor supplied at 50 Hz therefore has a synchronous field speed of:
120 × 50 / 4 = 1500 rpm
The shaft of an induction motor normally runs slightly below this value under load. The difference is called slip.
2. How can a VFD produce 25 Hz from a 50 Hz supply?
The VFD does not change the frequency of the utility network itself.
Instead, it converts the incoming AC supply into DC and then electronically creates a new AC output for the motor.
The mains can therefore remain at 50 Hz while the motor receives 8 Hz, 25 Hz, 40 Hz or another commanded fundamental frequency.
3. What does the rectifier do?
The rectifier is the input power stage that converts AC supply voltage into DC.
Many conventional drives use a diode bridge, while other drive topologies can use controlled or active rectifier systems.
The purpose of rectification is not to operate the AC motor directly from DC. It creates an intermediate electrical energy source from which the inverter can synthesize a new motor supply.
4. What is the DC bus?
After rectification, the electrical energy enters the DC bus or DC link.
Capacitors and, depending on drive design, inductive components smooth and support the DC-link voltage.
A useful beginner's analogy is to think of the DC bus as the energy highway between the mains rectifier and the motor inverter.
The DC bus also becomes important during braking. A decelerating mechanical load can return energy through the motor and raise DC-link voltage.
5. What does the inverter stage do?
The DC bus contains direct voltage with no motor operating frequency of its own. The AC motor needs controlled three-phase power.
The inverter stage uses power semiconductors such as IGBTs to switch the DC-link voltage at high speed.
By controlling this switching pattern, the drive creates the electrical conditions required for the motor.
6. What is PWM?
PWM – Pulse Width Modulation is one of the key methods used to control the inverter switching pattern.
If the motor terminals are observed with suitable instrumentation, the voltage is normally not a clean utility-style sine wave. It consists of fast voltage pulses.
The timing and width of those pulses are controlled so that the motor current and magnetic field respond to the required fundamental voltage and frequency.
7. Motor frequency and carrier frequency are different
A VFD may operate a motor at a fundamental frequency of 40 or 50 Hz while its IGBTs switch several thousand times per second.
The high switching rate is commonly referred to as the carrier or switching frequency.
Increasing carrier frequency can reduce audible motor switching noise, but it also generally increases semiconductor switching losses and drive heating.
It therefore should not be confused with the frequency that determines motor speed.
8. Does a potentiometer directly control motor power?
No. In a typical VFD application a potentiometer sends a low-power reference signal to the drive's control electronics.
The drive interprets this reference and its power stage then creates the required motor output.
This is explained in more detail in What is a frequency reference?.
A VFD can receive its reference from a keypad, potentiometer, 0–10 V signal, 4–20 mA signal, preset digital inputs, PID controller, PLC or industrial network.
9. Does 50 Hz always mean 1500 rpm?
No. Motor speed also depends on the number of poles.
At 50 Hz, a two-pole motor has a synchronous field speed of 3000 rpm, a four-pole motor 1500 rpm and a six-pole motor approximately 1000 rpm.
An induction motor shaft then runs slightly below synchronous speed because it requires slip to develop torque.
10. Why does a VFD vary voltage as well as frequency?
Changing only frequency without managing voltage would produce poor magnetic-flux control in an induction motor.
In basic V/F control, voltage and frequency are changed in a defined relationship to maintain suitable motor flux through much of the operating region.
For a simplified 400 V, 50 Hz example:
400 / 50 = 8 V/Hz
Real drives use more sophisticated compensation, voltage limits, motor models and control algorithms, so actual behaviour is more complex than this simple ratio.
11. What is the difference between V/F and Vector Control?
V/F control is a relatively simple method that primarily manages the voltage-frequency relationship.
Vector control uses a more detailed motor model and current information to achieve better control of flux and torque.
This can provide advantages in applications requiring high starting torque, stronger low-speed performance and better speed regulation when load changes.
Vector systems may operate sensorless or use encoder feedback in higher-performance closed-loop applications.
12. Why is Auto-Tuning used?
A high-performance VFD needs more information than just the motor's nominal kW rating.
Auto-tuning helps the drive identify important electrical characteristics of the connected motor so that the internal motor model can be improved.
Depending on the drive, tuning can be performed while the motor is stationary or during a controlled rotation.
Correct motor nameplate data must still be entered. Auto-tuning cannot turn incorrect input data or incorrect wiring into a reliable motor model.
13. Open-loop and closed-loop control
In an open-loop system, the VFD may control the motor without directly measuring actual shaft speed.
A closed-loop system uses feedback, commonly from an encoder, to tell the drive what the shaft is actually doing.
Feedback becomes particularly useful where high speed accuracy, low-speed torque or dynamic control performance is required.
In suitable induction-motor applications, slip compensation can help reduce the speed drop caused by increasing mechanical load even without a shaft encoder.
14. Where does energy go when a VFD slows a motor?
During acceleration, electrical energy is converted into mechanical energy in the rotating system.
During rapid deceleration, the power flow can reverse. The motor may temporarily act as a generator and return energy to the DC bus.
If this regenerated energy raises the DC-link voltage too far, the drive can trip on overvoltage.
This is why high-inertia and frequently braking machines may require dynamic braking, a braking resistor or a regenerative solution.
DC injection braking is a different braking method and should not be confused with resistor-based dynamic braking.
15. Why can a VFD save energy?
A VFD does not automatically guarantee energy savings.
The benefit comes when the process allows motor speed to be reduced instead of operating continuously at full speed.
This is particularly important with variable-demand pumps and fans. Other machine types, such as conveyors or constant-torque processes, can have very different energy behaviour.
16. Is VFD output a pure sine wave?
On a conventional PWM drive, the motor-terminal voltage is normally made from high-speed voltage pulses rather than a clean utility sine wave.
The inductance of the motor windings smooths the current so that motor current is generally much closer to a sinusoidal waveform than the terminal voltage.
This PWM output also explains why motor-cable length, insulation stress, EMC, bearing currents and output filters can matter in real installations.
17. Does increasing VFD frequency always increase current?
No simple rule can describe motor current from frequency alone.
Motor current is strongly influenced by mechanical load torque, acceleration, motor characteristics, control mode and operating point.
The drive supplies controlled current and voltage so the motor can produce the torque required by the machine.
18. Why does a VFD produce heat?
A drive is not lossless. The rectifier, DC-link components, IGBT power stage, control electronics and cooling system all contribute to electrical losses.
Semiconductor conduction and switching losses are especially important in the inverter stage.
This is one reason unnecessarily high switching frequency can increase drive temperature.
19. Where do RUN and speed commands come from?
For troubleshooting, it helps to think of two separate commands:
1. Run command: RUN, STOP, Forward or Reverse.
2. Speed reference: the value that tells the drive how fast the motor should run.
The RUN command may come from terminals while the frequency reference comes from a potentiometer. Both may instead come from a PLC or fieldbus.
A drive can therefore show no fault and still refuse to run simply because it is listening to a different command source.
20. Are “VFD”, “inverter” and “AC drive” the same thing?
In industrial motor-control discussions, VFD, frequency inverter and AC drive are often used for the same general class of product.
Technically, however, the word inverter can also mean only the DC-to-AC power-conversion stage.
The same word is also used in solar power systems, UPS systems and other power-electronic equipment, so context matters.
VFD FAQ
How can a VFD create 30 Hz from a 50 Hz supply?
It first rectifies the incoming AC to DC and then uses the inverter stage to synthesize a new motor output with a 30 Hz fundamental frequency.
Does a potentiometer reduce motor voltage directly?
No. It normally provides a reference to the drive's control electronics.
Will a motor at 25 Hz run at exactly half its rated rpm?
Approximately in many basic cases, but actual shaft speed also depends on pole count, load, slip and the control method.
Why does a 50 Hz four-pole motor nameplate show 1450 or 1470 rpm instead of 1500 rpm?
An induction motor requires slip between synchronous field speed and rotor speed to produce torque.
Why can an ordinary multimeter show unusual readings on VFD output?
PWM output is not a simple sine waveform. Measurement equipment and methods should be suitable for variable-frequency drive outputs.
Can a VFD produce torque at zero speed?
Capability depends strongly on motor type and control method. Simple V/F and high-performance closed-loop vector systems have very different zero- and low-speed capabilities.
Can I simply set a standard motor to 100 Hz and double its speed?
Not safely without checking motor mechanical limits, bearings, driven-machine limits, voltage availability and the field-weakening operating region.
Can every AC motor be used with a VFD?
No. Voltage, current, insulation, cooling, motor construction and the application all need to be compatible.
Does raising carrier frequency make the motor run faster?
No. Carrier frequency controls semiconductor switching. Motor fundamental frequency is the frequency related to motor speed.
Can an induction motor be speed-controlled without a VFD?
Other mechanical and electrical techniques exist, but VFDs are among the most widely used methods for continuous electronic speed control of industrial AC motors.
Related topics: inside a VFD
What Is an AC Drive? · DC Bus · IGBT · PWM · Carrier Frequency · Frequency Reference
Related topics: motor control
V/F Control · Vector Control · Slip Compensation · Auto-Tuning · Torque Control