Harmonics & Power Quality Trainer

Interactive Whitebridge Marine - CSMART Systems Trainers

Power Conversion Topology Click any block for details

AC Input Current 3-Phase

DC Link Voltage 3100V

Motor Current Filtered

🎯
KEY POINT: Harmonics come from the RECTIFIER
The input side (rectifier) generates harmonics back to the ship's network — this is where THD matters for generators. The motor sees clean sinusoidal current because PWM switching + motor inductance acts as a filter.

PWM Inverter Output Before motor inductance filtering

Switching: 2.0 kHz │ Output: 8.0 Hz
Running
Speed: 1×

Input Current Harmonics THD: 28.5%

Output Current Harmonics THD: 4.2%

Rectifier Configuration

Affects input harmonics to network

Inverter Output

Motor Frequency (Speed) 8.0 Hz
PWM Switching Frequency 2.0 kHz

Higher switching = cleaner output, more losses

Motor Load

Load 75%

Affects current magnitude

💡 Teaching Point

6-pulse rectifier: Current only flows when AC voltage exceeds DC link voltage, creating characteristic "twin-humped" pulses. This non-sinusoidal current contains strong 5th, 7th, 11th, and 13th harmonics, resulting in ~25-30% THD on the input. The motor sees much cleaner current because motor inductance filters the PWM switching.

Switching Frequency — Set at Commissioning

↑ Higher Switching Frequency (4-8 kHz)
  • Cleaner motor current (lower THD)
  • Less audible motor noise
  • Better torque control at low speeds
  • More switching losses → heat in IGBTs
  • Higher dV/dt stress on motor insulation
↓ Lower Switching Frequency (1-2 kHz)
  • Lower switching losses → cooler drive
  • Less stress on motor insulation
  • Higher drive efficiency
  • More motor current ripple
  • Audible "singing" from motor

Commissioning decision: Set by the drive engineer based on motor type, cable length, and application requirements — not adjusted during normal operation. Long cable runs may require lower switching frequency or output filters to protect motor insulation from voltage reflections. Some drives automatically reduce switching frequency at high loads to manage IGBT temperatures.

Power Conversion Topology - Direct AC to AC

How the Output is Constructed Output "rides" the input phases

━━ Phase A ━━ Phase B ━━ Phase C ┅┅ Desired output
Output: 8.0 Hz │ 3 segments per half-cycle
Running
Speed: 1×

Network Impact ← This is why cycloconverters need massive filters

INPUT CURRENT DRAWN FROM SHIP'S GRID
Show:
FILTER: No filtering applied
HARMONIC CONTENT
THD: 35%

Output Frequency

Motor Speed Command 8.0 Hz
1 Hz Max ~⅓ of input (20 Hz) 20 Hz

Operating Condition

Load 75%

Frequency vs Quality

Output Freq: 8.0 Hz (13%)
Segments/half-cycle: 3
Waveform quality: Good

💡 Teaching Point

No DC Link: The cycloconverter directly synthesizes low-frequency AC from high-frequency AC using SCR switching. Each output phase requires a positive and negative converter bank (36 SCRs total for 3-phase). Output frequency is limited to approximately ⅓ of input frequency (max ~20Hz from 60Hz supply).

The problem: Both input AND output waveforms are distorted. Input harmonics pollute the ship's grid, while output harmonics cause motor heating and torque ripple. THD gets WORSE at higher output frequencies.

Power Conversion Topology Click any block for details

Harmonic Generation Harmonics at both rectifier (input) and inverter (output) stages

Output: 8.0 Hz │ U+V−
Running
Speed: 1×

Rectifier Configuration

SCR rectifier with firing angle control

Output Frequency

Frequency: 8.0 Hz

No ⅓ limit — can run up to and beyond input frequency

Motor Load

Load: 75%

Network Impact ← Harmonics fed back to generators

Input Current Waveform (Grid Side)
Input Harmonic Spectrum
THD: 28%
6-pulse: 5th, 7th, 11th, 13th harmonics (6n±1 rule)
12-pulse: 5th & 7th cancelled, only 11th, 13th remain

Motor Current ← 6-step quasi-square waveform

Output Current Waveform (Motor Side)
Output Harmonic Spectrum
THD: 31%
6-step inverter: Dominant 5th (20%) and 7th (14%) harmonics
Sync motors tolerate this — designed for high THD operation

LCI vs Cycloconverter vs VSI PWM

VSI PWM
Diodes + DC Link + IGCTs
Clean output, any frequency
Motor THD: ~5%
Cycloconverter
SCRs only, no DC link
Direct AC-AC, max ⅓ input freq
Motor THD: 10-15%
LCI / Synchro
SCRs + DC Link + SCRs
High power, any frequency
Motor THD: 25-35%

Understanding the Load Commutated Inverter

How it works: The rectifier converts AC to smooth DC current (via a large inductor). The inverter then "steers" this DC current into the motor phases in a 6-step sequence — it doesn't create the waveform, it just switches where the current flows.

Why "Load Commutated"? The inverter SCRs turn off naturally when the motor's back-EMF reverses the current through them. This only works with synchronous motors running at leading power factor, which is why LCI drives always use synchronous machines.

The trade-off: Simple, robust, handles megawatts easily — but the motor sees a stepped quasi-square voltage with ~31% THD. Motor windings must be designed to handle this (extra insulation, derating).