Servo drives and robot joint actuators share a magnetic problem: a PWM inverter feeding a motor through a cable, inside a cabinet full of switching noise, with an encoder that must not see any of it. The magnetic components that solve this are small, cheap and frequently chosen last — which is why EMC retests are so common in this segment.
1. Where the magnetics sit in a servo drive
| Location | Component | Function |
|---|---|---|
| AC input | Common mode choke + DC-link choke | Conducted EMI suppression, inrush limiting |
| DC link | DC reactor (larger drives) | Reduce capacitor ripple current, limit harmonics |
| Motor output | Output reactor / dv/dt choke | Limit cable-reflected overvoltage, reduce bearing current |
| Control supply | Flyback transformer | Isolated gate-drive and logic power |
| Encoder / signal | Common mode choke, small | Protect differential signal from inverter noise |
2. The output reactor is a motor-protection device
A long motor cable behaves as a transmission line. With a fast IGBT or SiC edge, the reflected wave at the motor terminals can reach nearly twice the DC-link voltage. At 400 V DC that is 800 V spikes on a motor rated for 500 V. The output reactor slows the dv/dt and reduces the peak. Design points:
- Inductance is chosen against cable length and edge rate, not against a generic catalogue value; 1–2% voltage drop at rated current is a typical starting point.
- The core must tolerate the full motor current plus ripple at the switching frequency. Iron powder or gapped ferrite; the bias is heavy and mostly DC.
- Insulation must withstand the reflected-wave voltage, not the nominal DC link.
- Thermal design must account for the ripple current at 8–16 kHz, which is where the copper loss concentrates.
3. Common mode choke selection for a drive cabinet
Conducted emissions on the motor cable are dominated by common mode current driven by the inverter’s dv/dt. A nanocrystalline or high-permeability ferrite core gives high impedance with few turns, which keeps the choke small and the copper loss low. The trap is saturation from the DC component of the motor current when the choke is placed on the output — a common mode choke on the output must be specified for the DC bias or replaced with a current-compensated arrangement.
4. Encoder and signal integrity
A small common mode choke on the encoder pair, plus adequate shielding, is usually the difference between a clean position signal and occasional lost counts at high acceleration. The choke must not distort the differential waveform: keep the impedance low at the signal bandwidth and high at the switching frequency and its harmonics.
5. Gate drive supply transformer
Isolated gate drive for SiC and GaN needs a transformer that holds isolation while delivering a fast, high-current pulse. Leakage inductance and interwinding capacitance are the critical parameters, not inductance. A ferrite core with a small window and a tightly coupled, single-layer winding is the usual solution; the isolation barrier must meet the drive’s reinforced requirement.
6. Derating and reliability in a cabinet
Servo cabinets run 50–55 °C internally and are often sealed to IP54. Magnetics are derated for that ambient, and for the 40 °C rise allowed by the insulation class rather than for a bench measurement. Vibration on a robot arm is continuous but low amplitude; the failure mode to watch is fretting of the winding against the core, which varnish impregnation prevents.
Shaanxi SiRui Technology supplies Magnetic Core series components for servo and robotics platforms — nanocrystalline common mode chokes, iron powder DC-link and output reactors, and gate-drive transformers with controlled leakage inductance.
Send your drive rating and cable length to sales@sirui-tech.com.