Application Problem:

Three 250 HP variable frequency drives are generating EMI that adversely affected a lonworks system. The power to the drives and system is derived from a 480 VAC Delta transformer secondary.


EMI/RFI that is generated by the variable frequency drives enters the ground system through the parasitic capacitances within the motor. Therefore, the motor frames must be bonded to the conduits and to the bond wires that may be within the conduits. The motor frames must not be directly grounded, i.e., they must not be bonded to facility structural I-beams or bonded to grounding rods that are driven into the ground in the vicinity of the motor. There must be only one earth ground in the facility and that is the one that satisfies the National Electric Code requirement at the upstream transformer or service entrance panel.

The motor and drive conduits must not come in contact metallic with water pipes, other conduits, BX cables, or structural steel.

The suggested equipment to reduce EMI/RFI in the system is listed below. There is no guarantee that any of the equipment suggested will solve the problem. Thus, the application of the equipment is not warranteed. Return of the items will be for warranty failures only and subject to factory approval.

The ferrite cores are the lowest cost remedy. The purpose of the cores is to introduce impedance to the high frequencies of EMI/RFI. The ideal application is to have all three power conductors pass through one core. However, since the inside diameters of the cores are limited in size, it may be necessary to have only one wire pass through a core. Thus, three cores may be required for the three drive output conductors. see Figure 2 of AC Drives and EMI/RFI Mitigation.

Two or more cores may be placed on the three drive output conductors or an each individual conductor, allowing a series additon of the core impedances.

The cores may be used on single or three phase systems. Neutral and ground conductors should never pass through the cores since the neutral and ground conductors must be allowed to return the EMI/RFI to source and, therefore, impedance must not be added to those conductors. Adding impedance to the neutral and ground conductors will create a greater voltage gradient on the ground plane and, therefore, increase the EMI/RFI problems.

The insulated conductors must pass through the inside diameter (ID) of the ferrite core.

You may find that the ferite cores offered on this website are too small for the 250 HP motor conductors.

Distribution systems with high resistance grounding will have EMI/RFI problems due to the voltage drop created by the EMI/RFI that is travelling on the ground plane. High frequency bypass capacitors must be used at the grounding resistor.

If the problem still exists after application of the ferrite cores, examine the controller to determine if there are EMI filters, input line reactors, output load reactors, long lead filters (also known as dv/dt filters), or harmonic filters. Note the manufacturer and manufacturer’s model numbers and then add or replace equipment as necessary.

The most effective item will be an EMI filter which has high frequency capacitors located on the drive end of the filter. Refer to Figure 3 of AC Drives and EMI/RFI Mitigation. Note the flow of EMI that is shown in the figure.

Use the below filter when an upstream transformer is used with either a wye or delta connected secondary.

The filter is rated 150 amps. Three filters will be needed to be used in parrallel for a 250 HP drive. Each unit, when used in parallel, must be derated by 20%. Thus, a 150 amp unit may be used at 120 amps. Three units will be required if 360 amps maximum is flowing into the drive.

If the drive input amperage exceeds 360 then a fourth filter will be required.

The filters are designed for panel mounting. You must supply an enclosure and mount the filters in the enclosure.

If the problem persists, remove the grounding from the delta secondary and use grounding transformers to create a wye system with a grounded neutral. The created neutral will allow a low impedance path for the EMI/RFI to return to the drive input.