Motor Chokes (Output Reactors) for VFDs: Reliable Protection for Your Electric Motor

A variable frequency drive (VFD) generates output voltage using pulse-width modulation (PWM). This means that the motor does not receive a smooth sine wave, but rather a series of sharp, high-frequency rectangular pulses. This specific type of operation causes the motor to heat up significantly, accelerates bearing wear, and breaks down the winding insulation. To mitigate these negative effects, a motor choke (output reactor) is installed between the inverter and the motor.

In the «DETZ» online store, you can buy motor chokes to protect three-phase electric motors of any power capacity. A minimal investment in an output filter will pay off in the very first year of operation by preventing accidents and the costs associated with rewinding a burnt-out stator.

What tasks does an output motor choke solve?

The motor reactor is installed directly at the output of the frequency converter (terminals U, V, W). By passing the pulsed current through itself, it performs several critically important functions simultaneously:

• Limiting the rate of voltage rise (dV/dt): The choke smooths the steep fronts of the PWM pulses, protecting the electric motor's winding insulation from breakdown and premature aging.
• Compensating for capacitive currents: The cable connecting the VFD and the motor has its own parasitic capacitance. The longer the cable, the higher the capacitive leakage currents, which overload the inverter and cause false short-circuit protection trips. The choke compensates for this capacitance.
• Reducing motor temperature: By smoothing the current waveform and reducing higher harmonics, the motor runs cooler (temperature drops by 10–15%), which significantly extends the life of its end shields and insulation.
• Eliminating acoustic noise: The reactor removes the characteristic unpleasant high-frequency "squeal" or "whistle" of a motor running on a VFD.

When is the installation of a motor choke strictly mandatory?

Not every electric drive system requires the installation of an output reactor. However, equipment manufacturers insist on its installation in the following cases:

1. Long cable runs: If the distance from the frequency converter to the electric motor exceeds 30–50 meters (for unshielded cables) or 20–30 meters (for shielded cables).
2. Connecting old motors: If you are upgrading a system and connecting an older motor (or a motor without special «Inverter Duty» reinforced insulation) to a modern VFD.
3. Group drives: When several electric motors are powered in parallel from a single frequency converter (e.g., fans in a drying kiln). In this case, the total cable capacitance increases exponentially.

How to correctly select an output choke?

The most common mistake when choosing is to select the choke by power (kW). The correct selection is made exclusively by nominal current (A).

The operating current of the motor choke must be equal to or slightly exceed the maximum nominal current of the electric motor. If several motors operate in parallel in the system, the choke current must be greater than the sum of their currents.

Why should you buy a motor choke at «DETZ»?

We offer comprehensive solutions for industrial automation and power electronics. By choosing us, you get:

• Free calculation: Our engineers will analyze your network parameters, cable length, and VFD switching frequency to select the optimal motor choke.
• Reliable components: We supply chokes made with high-quality electrical steel and reliable varnish impregnation, which eliminates buzzing during operation.
• Prompt dispatch: Popular ratings of output reactors (from 1.5 to 315 kW) are always in stock at our warehouse.
• Favorable prices: Honest pricing with no hidden markups, as well as discounts for regular B2B partners.

Do not risk the life of your electric motors! Choose the required motor choke from our catalog or contact our specialists — we will help ensure safe and long-lasting operation for your equipment.