Drive technology

AC drives are used to adjust the speed of motors

Because it is difficult to change the frequency of an AC sine wave, the first job of an AC drive is to convert the wave to DC. It is relatively easy to manipulate DC to make it look like AC. The first component of all frequency converters is the rectifier, shown to the left of the figure below.

The rectifier circuit converts AC to DC. The result is a fully rectified AC wave form that is interpreted by a DC Link circuit (LC) as a native DC wave form. The converter circuit manipulates the DC pulse using high-speed semiconductors and width modulation to make the DC pulse look like an adjusted AC.

AC Drives are found in virtually every area of industry, in applications as diverse as pumps, air conditioning systems, elevators and cranes, conveyors, machine tools, renewable electricity production and in a vast array of other industrial and domestic automation. The most common uses of AC drives are for control of fans, pumps, and compressors. These applications account for 75% of all AC drives operating globally.


There are many reasons why we may want to adjust motor speed:

  • Save energy
  • Improve efficiency
  • Make it suitable for digitization
  • Reduce noise from fans and pumps -> improve working environment
  • Reduce mechanical stress -> extend lifetime

Chokes and transformers inside AC drives

Kohsel have been a leading supplier of transformers and chokes for use inside AC drives for more than 15 years. Today millions of AC drives around the world are equipped with Kohsel transformers and chokes. We can benefit from expertise gained through the many development projects that we have participated in to find the optimum solution in terms of efficiency, cost, compact design and maximum output.

DC link chokes

Operating in the DC Link between the rectifier and capacitor.

Common mode chokes (CM)

On the common mode choke both line and neutral windings are wound on the same core. Common mode chokes suppress high frequency noise (common mode noise).

RFI chokes

RFI filters suppress the radio frequency interference in the AC Drive.

PFC chokes

The Power Factor Correction choke correct a leading power factor (current leads voltage) caused by a capacitive load.

Gate drive transformers

Used to drive a gate on a switching device (transistor/IGBT/FET).

External reactors, chokes and filters

AC drives are known sources of interference and are therefore usually equipped with an external input filter.

Unfortunately, the use of high-speed semiconductor (IGBT) switches and pulse width modulation (PWM) techniques in the converter in order to generate fast rise time voltage pulses of the appropriate duration and polarity creates a considerable number of problems, from purely functional difficulties to very severe motor damage. AC drives are therefore usually also equipped with an external output filter.

For 3-phase AC drives the following filter combinations can be considered:

Line reactors

Reduce low frequency interference (harmonics) and relieve the strain on the grid network by compensating for the distortion reactive power induced.

Harmonics filters

Reduce current harmonics (THDi) to comply with the limit values required under the standards. This protects the connected grid network from circuit feedback and increases the system immunity from interference. Our LCL filters are specially designed to reduce harmonics of current absorbed by the rectifiers in the input converter.

Motor chokes

Reduce the voltage rise and the voltage spikes between the conductors which become more pronounced as cable length increases.

dU/dt filters

Protect motors against high voltage increases and voltage peaks at the motor terminal.

Sine wave filters

Effectively protect the motor insulation and reduce supplementary losses and noise emissions from the motor. All-pole sine filters also enable the use of unshielded motor lines.

Rectangular wire

Rectangular wire provides compact windings with better electrical efficiency, high performance and low cost. Kohsel has many years of experience with supplying transformers and chokes with rectangular wire.

Every day, we strive to unleash the full potential in transformer design. Through our proven process, dedicated engineering and close collaboration, we detect untapped savings and optimization potential throughout your entire value chain.