What Is a Frequency Inverter? Drive Selection Guide
A frequency inverter is a drive that adjusts the speed and torque of an induction motor by changing the output frequency; see the selection criteria and drive types.
What is a frequency inverter and where is it used
A frequency inverter, in short a variable frequency drive (VFD) or AC motor drive, is an electronic device that adjusts the speed and torque of an AC motor by changing the motor's supply frequency. In pumps, fans, conveyors, compressors and the main motions of machines, it is used to slow down and speed up the motor according to the process instead of running it at constant speed.
Some people searching for the word "inverter" mean a solar inverter. A solar inverter converts the direct current of a photovoltaic panel into alternating current at mains frequency and is a separate device from the motor drives on this page. This guide covers motor drives only.
In loads such as centrifugal pumps and fans, the power demand changes approximately with the cube of the speed, so reducing the speed can reduce energy consumption; the gain depends on system conditions such as static head. The drive also provides soft start and stop. A drive does not by itself replace a motor protection circuit breaker or a contactor; protection and isolating devices are selected separately according to project data.
Types of drives
In this category, the product types seen are frequency inverter, frequency converter and DC motor drive; the frequency principle above applies to AC drives, and a DC motor drive is a separate device group. By input supply there are single-phase and three-phase models. In terms of control method there are two main approaches:
- V/f (scalar) control: The voltage-to-frequency ratio is kept constant. It is used for loads that do not require precise torque, such as pumps and fans; it may fall short at low speed and in applications that require high performance.
- Vector control: The magnetic flux and torque of the motor are managed more directly. It is preferred in applications that need torque at low speed and dynamic response.
Subcategories:
- Braking Resistors: Convert the energy fed back while the motor slows down into heat.
- Solar Pump Drives: Special drives that supply a pump from a photovoltaic source.
- Drive Accessories: Products such as memory cards, communication and access modules.
If position or motion control is required, servo drives or stepper drives are the more suitable group.
What to look at when choosing a frequency inverter
- Motor power and rated current: Look at the kW and current values on the motor nameplate. The drive's output current must cover the motor current; matching kW alone is not enough. The current and fuse calculation from active power tool can be used for the current calculation.
- Supply and number of phases: Single-phase or three-phase input is determined by the mains voltage.
- Load type: Variable-torque loads such as pumps and fans and constant-torque loads such as conveyors, presses and hoisting require different current margins. Heavy-duty and light-duty rating values are read from the manufacturer's catalog.
- Control method: If the load requires precise torque or rotation at low speed, look for vector control support.
- Degree of protection: The list includes IP00, IP20, IP21 and UL Type 1 values. IP2x means protection against objects such as fingers by the first digit, and IP21 also means protection against vertically falling drops; IP00 offers no protection. For mounting inside a panel, the enclosure and ventilation are considered separately.
- Communication and I/O: PLC connections such as Modbus and fieldbus, and analog or digital inputs, are checked according to the application.
- Braking need: If there is large inertia or a fast stop is required, a braking resistor or a similar solution is planned according to the manufacturer data.
- Output frequency: The products list maximum output frequencies such as 500, 598 and 599 Hz and a value of 3,000 Hz in V/f mode. For high-speed motors, this value is verified with the manufacturer data.
For the cable length and cross-section between the motor and the drive, see the cable current carrying capacity tool; cable and protection selection is made according to the project and the manufacturer data. For products that will additionally protect the motor, you can review motor protection circuit breakers.
Brands in this category
- ABB: Drive, solar pump drive and accessory products.
- Schneider Electric: Altivar series variable speed drives such as ATV630 and ATV930.
- Delta Electronics: Three-phase variable frequency drives such as CP2000.
- GMT: AC motor drives and solar pump drives.
- Isıso: Three-phase variable frequency drives and braking resistors.
- Veichi: AC310 series single-phase drives.
- Siemens: Mainly drive accessories in this category.
Order notes
Products are sold by the piece. Some drives are listed with a lead time; for these products, the supply time is confirmed at the time of ordering. If you know the model or stock code, you can search directly by code in the site search.
Variable frequency drive (VFD) selection table
| Load Profile | Typical Application | Overload Capacity | Control Mode | Product Group |
|---|---|---|---|---|
| Variable torque (Pump & fan) | Centrifugal pumps, HVAC fans, blowers | 120% for 60 s | V/f curve with automatic energy optimization | Variable Frequency Drives |
| Constant torque (Conveyor, mixer) | Belt conveyors, extruders, compressors | 150% for 60 s | Sensorless flux vector control | IP20 Drives |
| High starting torque & dynamic braking | Cranes, hoists, vertical lifts, winders | 150% – 200% + braking resistor | Closed-loop vector control with braking chopper | IP21 Drives |
Frequently asked questions
Is a frequency inverter the same as a solar inverter?
No. A frequency inverter takes the mains voltage and changes the output frequency to adjust the speed and torque of a motor. A solar inverter, on the other hand, converts the direct current of a photovoltaic panel into alternating current. Although the names are similar, their purposes, inputs and places of use are different; a frequency inverter is needed for motor speed control.
What does a frequency inverter do?
It adjusts the speed and torque of an induction motor by changing the supply frequency. This way, loads such as pumps, fans and conveyors run at the desired speed according to the process, and starting and stopping are softened. In centrifugal pumps and fans, power changes approximately with the cube of the speed, so reducing the speed can reduce energy consumption.
Is there a difference between a frequency inverter and a frequency converter?
In everyday use the two names are often used for the same device; both are seen as product types in this category. To avoid confusion, instead of the name look at the input voltage, number of phases, output current and output frequency data in the catalog, and compare them with the motor nameplate.
How is a drive selected according to motor power?
The kW, rated current and voltage on the motor nameplate are read, and then a drive is chosen whose output current covers the motor current and which offers the duty rating suited to the load type. Matching kW alone is not enough, because motors of the same power can have different currents. For heavy-duty and light-duty values, see the manufacturer's catalog.
When is a braking resistor needed for a frequency inverter?
When the motor slows down, it returns energy to the drive; in dynamic braking, this energy is converted into heat in the resistor. A braking resistor comes into question for loads with large inertia or machines that must stop in a short time. The resistance value and power are selected according to the load inertia and the deceleration need, using the drive manufacturer's data.