Introduction
PWM inverters are ubiquitous in variable-speed motor drives due to their efficiency and controllability. However, their high-frequency switching generates steep voltage transitions (dV/dt) that propagate through motor cables, causing reflected waves, CM currents, and electromagnetic interference (EMI). These issues manifest as:
- Differential Mode (DM) Challenges: High THVD and voltage overshoots degrade motor insulation and increase winding losses.
- Common Mode (CM) Challenges: CM currents flow through parasitic capacitances (motor windings-to-frame, cable-to-ground), leading to bearing currents, electromagnetic interference (EMI), and erratic control system behavior.
While existing solutions like dV/dt filters (for short cables) and isolation transformers (for long cables) address these issues separately, they incur high costs and complexity. Multi-stage filters require separate CM and DM components, increasing footprint and design intricacy. This newsletter speaks about an integrated filter combining CM and DM mitigation into a single magnetic structure, reducing cost and space while improving performance.
The Problem: PWM Drives Are Quietly Killing Your Motors
Pulse-width modulation (PWM) drives are the backbone of modern industry, powering everything from HVAC systems to electric vehicles. But their high-frequency switching creates two stealthy enemies:
- Common Mode (CM) Noise:
- Leaks through parasitic capacitances (motor windings → frame, cables → ground).
- Causes bearing currents (premature motor failure), EMI (interference with sensors/controls), and erratic shutdowns.
- Differential Mode (DM) Noise:
- Generates voltage spikes (up to 1,660V!) and harmonic distortion (THVD >20%).
- Accelerates insulation breakdown and increases energy losses.
Traditional fixes like dV/dt filters (limited to short cables) or isolation transformers (bulky, costly) only address half the problem. Multi-stage filters add complexity and cost.
The Innovation: One Filter to Rule Both CM and DM
MTE’s SineWave Filter combines CM and DM mitigation into a single, compact device. Here’s how it works:
Core Technology: The Tricore Inductor
- Six Mutually Coupled Windings: A patented laminated core with three legs hosts two windings per leg (line + shunt coils).
- CM Suppression: Mutual coupling creates high impedance to block ground-loop currents.
- DM Smoothing: Reduces voltage spikes and harmonics by acting as a low-pass filter.
Capacitor Network
- DM Capacitors (48 µF): Absorb high-frequency voltage spikes across phases.
- CM Capacitors (0.57 µF): Wye-connected to ground, divert residual CM currents.
Band-Stop Magic
- Low-Pass Pole (700 Hz): Attenuates high-frequency noise.
- Band-Stop Zero (2 kHz): Targets PWM switching frequencies for 90% CM noise reduction.
Real-World Results: From Theory to Practice
Simulation & Lab Validation
- DM Performance:
- THVD slashed to 4.6% (vs. >20% unfiltered).
- Peak voltage reduced by 57% (1,660V → 720V).
- CM Performance:
- CM voltage at 2 kHz cut by 90% (125V → 46V RMS).
- Peak CM current dropped from 16.7A to 3.5A (→ less bearing stress).
Cost & Space Savings
- No isolation transformer needed (saves ≈50% system cost).
- Single device replaces multi-stage filters, cutting footprint and design time.
Why This Matters for Your Industry
- Manufacturing: Extend motor life in CNC machines, conveyors, and pumps.
- Renewables: Reduce EMI in wind turbine pitch controls and solar inverters.
- Oil/Gas: Reliable operation in remote, long-cable applications (up to 305 meters tested).
- HVAC: Quieter, more efficient compressor drives.
The Bottom Line
MTE’s filter isn’t just a technical breakthrough, it’s a cost-saving revolution. By merging CM and DM mitigation into one robust package, it eliminates the need for expensive transformers and complex multi-stage setups. With 4.6% THVD and 90% CM attenuation, motors run cooler, last longer, and cause fewer headaches.
Source: https://mtecorp.com/blog/2017/04/04/using-passive-sinewave-filter-eliminate-common-mode-differential-mode/
Do you want to learn more about MTE’s SineWave Filter line? Contact us!
vancoelectrical.com
sales@vancoelectrical.com
(905) 305-7702