Electromagnetic interference (EMI) emitted by electric motors and drives is present in any electromagnetic circuit and, if not mitigated, can wreak havoc with other sensitive electronics. Minimize or eliminate EMI noise issues with these eight essential techniques.
1. Use well designed cables: Good cable designs consider a number of key factors to address potential noise. The wire gauge, outer casing material, flex ratings, terminations and shielding all play a role in minimizing noise issues. Our experience shows that customers who use Kollmorgen’s expertly designed cables have significantly fewer problems with EMI than those who build their own cables.
2. Use common-mode chokes on motor leads: Cables longer than 25 m (82 ft) may need motor common-mode chokes. These filters block high-frequency noise common to two or more conductors while allowing the desired signals to pass through. Check your product documentation for details.
3. Separate drive/motor power and signal cables:Bundle and route signal cables separately from motor and power cables. Run cables in separate conduits or maintain at least 100 mm (4") between signal and power bundles for drives below 20 Amps. Use 150 mm (6") for 40 Amp drives and 200 mm (8") for 80 Amp drives.
Newer feedback and cabling technology allows use of a single cable for both power and feedback to minimize overall cable costs and simplify installation. Single motor/feedback cables are specifically designed with appropriate shielding to mitigate any noise issues between the motor power and feedback signals.
If using a separate AC power filter, maintain separation of leads entering and exiting the line power filter. Locate the filter as close as possible to the point where the incoming power enters the cabinet.
If using a drive that contains an internal mains filter, maintain at least 100 mm (4") of separation between line power and motor leads. If input power and motor leads need to cross, cross them at 90 degrees.
4. Splice cables properly: If cables need to be divided, use connectors with metal backshells. Ensure that both shells connect along the full 360 degrees of the shields. No portion of the cabling should be unshielded. Never divide a cable across a terminal strip.
5. Ensure good shield connections: For cables entering a cabinet, connect shields on all 360 degrees of the cable. Never connect a simple “pigtail.”
6. Use differential inputs for analog signals: Noise susceptibility in analog signals is greatly reduced by using differential inputs. Normally, connect the output signal to the plus (+) differential input and the ground of the device that generates the output to the minus (–) differential input. Use twisted-pair, shielded signal lines, and connect shields to grounds on both ends. Many Kollmorgen drives provide internal signal filtering to reduce the effects of electrical noise.
7. Ensure good connections between the cabinet components: Connect the back panel and cabinet door to the cabinet body with several conductive braids. Never rely on hinges or mounting bolts for ground connections. Provide an electrical connection across the entire back surface of the drive panel. Electrically conductive panels, such as aluminum or galvanized steel, are preferred. For painted and other coated metal panels, remove all coating behind the drive.
8. Ensure good ground connection: Connect from the cabinet to proper earth ground. Ground leads should be the same gauge as the leads to mains power or one gauge smaller.
With vigilance, EMI is easy to defeat
Cables that have been poorly designed or improperly installed can act as antennas that transmit EMI, causing feedback devices, controls and other components to malfunction. The result can be unplanned downtime, loss of productivity or even hazardous operating conditions.
Don’t let that happen to you. EMI is easy to mitigate. Make sure you follow these eight key practices in every installation to minimize or eliminate EMI issues, ensuring the safety and productivity of your machine.
