In brief
A frequency inverter is an electronic device that converts the frequency and voltage of the mains supply to control the speed of three-phase motors steplessly.
Contents
What is Frequency inverter?
On the mains, three-phase motors run at a fixed speed that depends on the mains frequency and the number of poles. A frequency inverter makes the speed variable by supplying the motor with a voltage of adjustable frequency and amplitude. It enables soft starting, speed control, positioning tasks and braking operation.
The usual design comprises three stages: a rectifier converts the AC mains voltage into DC voltage, the DC link with capacitors smooths and stores it, and an inverter with power semiconductors uses pulse width modulation to generate the required output voltage. Particularly with pumps and fans, considerable energy can be saved by adjusting the speed instead of throttling. Parameters are used to set motor data, ramps, limit values, control methods and communication interfaces.
As an electronic component, the frequency inverter is subject to comparatively rapid obsolescence: device generations are discontinued, and successors differ in dimensions, connections and parameterisation. DC link capacitors and fans age and are among the typical causes of failure. For critical drives it is therefore advisable to record the installed device types and check their availability status regularly.
In practice: maintenance and procurement
The parameter settings should be backed up regularly and filed with the plant documentation; without them, a replacement is considerably more laborious. An enquiry requires nameplate data, power, mains voltage, communication interface and options used. For discontinued devices, it pays to look at successors and their differences early on.
Synonyms and abbreviations: variable frequency drive (VFD) · VFD · inverter · AC drive · variable speed drive (VSD)
Frequently asked questions
How does a frequency inverter save energy?
With pumps and fans, the power required falls much more sharply than the speed. If the flow rate is adjusted by lowering the speed instead of using throttle flaps or valves, power consumption falls accordingly. How large the saving is depends on the plant's load profile; with continuous full load, the effect is small.
What is the difference between a frequency inverter and a soft starter?
A soft starter limits current and torque only during starting and stopping; after that, the motor runs at mains speed. A frequency inverter, by contrast, controls the speed throughout operation. Soft starters are simpler and less expensive but are not suitable for applications that require variable speed.
What should be backed up before replacing a frequency inverter?
Before replacement, all parameters should be read out and saved using the relevant software or an operator panel. These include motor data, ramps, limit values, input and output assignments and communication settings. Photos of the wiring are also helpful. When changing to a successor device, the parameters often have to be transferred or reassigned.
Why do frequency inverters sometimes fail after long storage?
The electrolytic capacitors in the DC link age even when not in use. After prolonged storage, it may be necessary to reform them gently before first commissioning, i.e. to apply voltage gradually. For stored spare devices, it is advisable to follow the manufacturer's instructions on storage time and reforming.