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In EMC testing of multi‑axis robots, there are cases where adding a noise filter provides no noticeable improvement. What could be the possible causes?
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The choke coil inside the noise filter may be saturating due to the following factors.
- ・Using shielded motor cables
- ・Long motor cable length
Cause
- Choke coil saturation is more likely to occur when the following conditions overlap:
-Use of shielded motor cables
-Operation of multiple servo axes
-Long distance between the controller and motor, resulting in long shielded cables
These factors increase the stray capacitance to ground of the shielded cables. As a result, common‑mode current increases—especially as the number of axes increases—ultimately causing the choke coil in the noise filter to saturate. Once saturation occurs, the filter’s common‑mode attenuation decreases significantly.
Solution
In environments where multiple shielded cables are routed in parallel, replacing the existing filter with a high‑performance model designed for improved saturation characteristics is effective.
A two‑stage filter with enhanced saturation resistance is particularly recommended.
COSEL’s noise filters, such as the FSB Series and FTB Series*, may help resolve these issues.
*For the FTB Series, select the ultra‑high attenuation specification (model code: FTB‑□□□‑355‑L).
Improved Saturation Resistance
Filters with enhanced saturation performance maintain effective noise reduction even when multiple shielded cables are installed together.
Improved Electric Field Strength
The attenuation provided by the 2‑stage filter also contributes to reduced radiated noise.
Commentary
The mechanism behind the above example is explained below.
- (1)Increasing the number of shielded cables increases stray capacitance to ground.
- (2)More common‑mode current flows through this capacitance, increasing further with additional axes.
- (3)This increased current flows through the choke coil, causing it to saturate.
- (4)A saturated choke coil can no longer provide sufficient common‑mode attenuation.
Case Study: Mitsubishi Electric Corporation - General-Purpose AC Servo
COSEL's noise filters are recommended for Mitsubishi Electric Corporation's general-purpose AC servos.
For details, please refer to the Mitsubishi Electric Corporation website.
