Motor chokes

Motor (output) chokes are connected between the frequency converter and the electric motor. Motor chokes provide:

Suppression of high-frequency harmonics in the motor current. The frequency converter generates a sinusoidal current in the motor windings using pulse-width modulation (PWM) of the voltage according to a sinusoidal law. At low PWM frequencies, high-frequency motor current pulsations can reach up to 5-10%. High-frequency harmonics of the current cause additional motor heating.
Limitation of the short-circuit current amplitude. In the event of a sudden short circuit at the frequency converter output, the short-circuit current does not increase suddenly, since there are inductances in the short-circuit current loop (L of the motor choke + parasitic inductances of the motor cable). When the short-circuit current reaches the frequency converter protection threshold, the motor is de-energized. Since the protection response time is non-zero, the maximum short-circuit current value is When using a motor choke, the actual maximum current is much lower than the maximum value without a choke. Without a motor choke, many VFDs are unable to protect the VFD transistors from one or more sudden short-circuits at the VFD output.
They reduce the rate of rise of emergency short-circuit currents and delay the moment the maximum short-circuit current is reached, thereby providing the necessary time for the VFD's electronic protection circuits to operate.
They compensate for capacitive currents in long motor cables, preventing the development of large capacitive currents and, accordingly, preventing false tripping of the VFD's overcurrent protection.
They reduce voltage surges on the motor windings. When an asynchronous motor is powered from a frequency converter, a pulsed voltage with significant overvoltage peaks is applied to the motor windings, the total magnitude of which exceeds the amplitude of the rated supply voltage of the asynchronous motor. This can cause a breakdown of the insulation of the motor windings, especially during long-term operation, when the insulation of the winding wires and windings loses its original insulating properties.