Based on the Eaton Wiring Manual 10/23
Basic Concepts of Variable Frequency Drives (VFDs)
- Electronic motor starters and Variable Frequency Drives (VFDs) control motor speed and torque. ⚙️
- VFDs change the frequency of the electric power supplied to a motor. 🔌
- Three main parts make up a VFD: the rectifier, the DC link, and the inverter.
- Rectifiers (A) convert alternating current (AC) into direct current (DC).
- The DC link (B) stabilizes and stores energy. ⚡
- Inverters (C) use Pulse Width Modulation (PWM) to turn DC back into a variable AC.
Internal Control of VFDs
- Central Processing Units (CPUs) manage input/output (I/O), keypad, and bus connections. 🧠
- CPUs monitor values and shut down the system if dangerous limits occur.
Drive System Structure
- Power Drive Systems (PDS) follow the EN 61800-3 standard. ⚙️
- PDS can include Complete Drive Modules (CDMs) and Basic Drive Modules (BDMs).
- BDMs include converters, control units, and protection.
- CDMs also add braking, field supply, and auxiliary tools.
Electrical Connections and Voltage
- VFDs connect easily to star-point-grounded AC mains. ⚡
- Asymmetrical or high-resistance grounding requires extra measures.
- Standard deviations in voltage:
- Voltage: ±10%
- Symmetry: ±3%
- Frequency: ±4%
- North American voltages:
- 120 V ➝ 115 V motors
- 240 V ➝ 230 V motors
- 480 V ➝ 460 V motors
- 600 V ➝ 575 V motors
Components of a Drive Control System
- A complete PDS includes:
- Electrical supply ⚡
- Safety devices ⛔
- Control modules (CMD, BDM)
- Motors and sensors
- Load mechanisms
EMC (Electromagnetic Compatibility) Installation
- Four EMC measures ensure safe operation:
- Earthing ⚠️
- Shielding
- Filtering
- Chokes
- Earthing connects enclosures to the earth with low resistance.
- Shielding prevents electromagnetic interference (EMI). ✨
- Cables must be shielded at both ends and routed separately from signal lines.
Shielding Cables and Connections
- Four-core shielded cables include:
- Copper braids
- Drain wires
- PVC insulation
- Fillers for support
- Shielding must be done using metal plates and terminals. 🧲
- Cables longer than 30 cm inside panels must be shielded.
Residual Current Devices (RCDs)
- Filters and VFDs cause leakage currents. 🌊
- RCDs protect against faults.
- Type B for 3-phase
- Type F for 1-phase
- Protection rules for leakage currents above 3.5 mA:
- Conductor ≥10 mm²
- Monitoring
- Extra protective wire
Motor Control Techniques
- Four methods of VFD control:
- V/Hz control (Volts per Hertz)
- V/f with slip compensation
- Sensorless vector control
- Closed-loop vector control
- V/Hz control matches voltage to frequency. ⚖️
- Sensorless vector control uses rotor/stator fields.
- Closed-loop control adjusts torque with feedback.
Safety Functions in VFDs
- STO (Safe Torque Off) stops torque instantly. ⛔
- Activates by hardware circuit
- Response time ≤1 ms
- SS1 (Safe Stop 1) slows down the motor using a ramp.
- Used when a drive must stop under control
- STO activates after stopping
Two-Channel Safety Schematics
- STO systems can use double-channel logic.
- Connections include:
- Gate logic
- IGBT drivers
- Dual STO inputs
- Emergency stop relays (e.g., ESR5)
Braking Systems for VFDs
- Braking resistors absorb motor-generated energy. ♨️
- Energy converts to heat and prevents overvoltage.
- DC choppers trigger the resistor at voltage peaks.
- Design tips for braking:
- Use correct resistor value
- Match load cycles
- Check chopper availability
Braking Power Calculation Example
- Formula:
- P = U² / R ➝ Power
- ΔE = P × t ➝ Braking energy
- Example:
- 780 V, 40 Ω ➝ 15210 W
- 5 s deceleration ➝ 76050 Ws
- 120 s cycle ➝ 633.75 W power
EMC Compliance and Environment Categories
- PDSs follow standard IEC/EN 61800-3.
- EMC affects both emissions and immunity.
- Europe requires CE compliance.
- Environment types:
- Public low-voltage ➝ C1/C2
- Private mains ➝ C1/C2/C3
- Industrial ➝ C3/C4
PDS Categories
- PDSs group into four categories:
- C1: First environment, ≤1000 V
- C2: Requires warning, technical knowledge
- C3: Second environment, ≥1000 V
- C4: Special industrial setups, >400 A
Key PDS Components
- Fuses protect circuits
- Contactors switch power
- Mains chokes filter harmonics
- EMC filters reduce interference
- Braking resistors handle energy peaks
- Motor chokes stabilize long cables
- Sine filters smooth the waveform
- Shielded cables block emissions
EMC Filters
- EMC filters go on the mains side.
- Internal or external filters reduce leakage currents.
- Special versions reduce leakage below 0.4 mA.
Mounting with EMC Compliance
- Keep motor cables and control cables separate. 🧲
- Avoid parallel routing.
- Use correct shielding for each voltage class.
Chokes for Main and Motor Cables
- Mains chokes reduce feedback and harmonics.
- DC chokes also help inside the DC link.
- Motor chokes protect against short-circuits and fast voltage changes.
Passive Harmonic Filters
- Combine chokes and capacitors.
- Reduce THDi below 8%, or even 5% above 50% speed.
- Comply with EN standards and support central filter use.
Active Harmonic Filters
- Computer-controlled systems detect and cancel harmonics. 🖥️
- They inject opposite current waves.
- The result is clean current with low interference.
Sine Filters
- Sine filters improve motor waveform and lower noise.
- Work best with long cables and fixed frequencies.
- Voltage drop of ~30 V can occur.
- All-pole sine filters also remove common-mode interference.
Variable Speed Starter DE1
- PowerXL DE1 bridges DOL (Direct-On-Line) starters and VFDs.
- Includes:
- External filters
- Chokes
- Communication and memory modules
- External keypads
- DE1 features:
- Set speed with full torque
- Control stopping and reversing
- Adjust acceleration with a screwdriver
Final Thoughts
- Variable Frequency Drives (VFDs) are essential for motor control. ⚙️
- They improve efficiency, reduce noise, and protect systems.
- Correct installation and EMC compliance ensure safe and reliable operation.
My Diary - Leonardo Cardillo 