Frekvenční měniče (VFD)

A Brief History: From Fixed Speed to Total Control

The Dawn of the Speed Problem

Picture this: It's the 1960s, and factory floors everywhere are buzzing with electric motors running at full blast, 24/7. Whether you need 100% or 10% of their power, they're always at maximum speed, gobbling electricity like there's no tomorrow. Need to slow down a pump? You'd install a gearbox, add a valve to throttle the flow, or use a softstarter to ease the startup pain. But once that motor hit full speed, you were stuck with it.

Engineers knew there had to be a better way. After all, car engines don't run at maximum RPM constantly—why should industrial motors?

The Mercury Arc Rectifier Era (1900s-1960s)

The earliest attempts at motor speed control were... interesting. Mercury arc rectifiers—basically glass bottles filled with liquid mercury and electrodes—could convert AC to DC. They looked like something out of a mad scientist's lab, glowing with an eerie blue light. While fascinating, they were:

• Fragile (one good bump and you had mercury everywhere)
• Inefficient (lots of heat, lots of noise)
• High-maintenance (required regular cleaning and care)

They worked, but nobody was calling them elegant.

The Thyristor Revolution (1960s-1970s)

Everything changed in 1957 when Bell Labs developed the thyristor (Silicon Controlled Rectifier, or SCR). This solid-state device could switch high currents on and off electronically—no moving parts, no mercury, no drama.

By the late 1960s, the first commercial Variable Frequency Drives appeared. They were:

• Huge (think refrigerator-sized boxes)
• Expensive (the price of a small car)
• Complex (requiring dedicated engineers to program)

But they could do something magical: change motor speed on the fly. Industries with deep pockets—steel mills, large mining operations, ship propulsion—started adopting them, seeing energy savings of 30-50% in variable-load applications like fans and pumps.

The Microprocessor Age (1980s-1990s)

The 1980s brought microprocessors, and VFDs got a serious upgrade:

• Smarter control algorithms (like PWM - Pulse Width Modulation)
• Smaller footprints (from fridge to briefcase size)
• Better efficiency (95%+ became standard)
• User-friendly interfaces (actual buttons and displays!)

For the first time, smaller businesses could afford VFDs. The technology trickled down from mega-factories to everyday applications—HVAC systems, conveyor belts, water pumps.

The Digital Era (2000s-Present)

Today's VFDs are technological marvels:

• Plug-and-play simplicity (many can auto-configure)
• IoT connectivity (monitor and control from your smartphone)
• Advanced motor protection (real-time diagnostics)
• Energy reporting (see your savings in real-time)
• Compact design (palm-sized drives can control 5 HP motors)

What once cost $50,000 now costs $500. What once required a PhD to program can now be configured by scanning a QR code.

And the best part? The energy crisis and climate concerns have made VFDs more relevant than ever. In many countries, they're now mandatory for new installations above certain power levels.

Practical Use & Selection: When, Why, and How to Choose a VFD

What Exactly Does a VFD Do?

Let's demystify this: A VFD is like a universal remote control for your motor.

Your electric motor wants to spin at a speed determined by two things:

1. Frequency of your electrical supply (50 Hz in Europe, 60 Hz in North America)
2. Number of poles in the motor

For a typical 4-pole motor on 50 Hz power:

• Formula: Speed (RPM) = (120 × Frequency) ÷ Number of Poles
• Result: (120 × 50) ÷ 4 = 1,500 RPM

No more, no less. That motor is locked into 1,500 RPM.

Enter the VFD: It takes your fixed-frequency electricity, converts it to DC, then chops it back up into AC at whatever frequency you want. Want the motor to spin at 750 RPM? Set the VFD to 25 Hz. Want 3,000 RPM? Crank it up to 100 Hz.

It's not magic—it's electronics. But it feels pretty magical.

How Does It Actually Work? (The Simple Version)

A VFD has three main stages:

1. Rectifier (AC → DC)

The incoming AC power (your wall socket) is converted to DC using diodes. Think of diodes as one-way valves for electricity—they only let current flow in one direction.

2. DC Bus (The Smoothing Tank)

The "rough" DC gets stored in large capacitors, which smooth it out into clean, stable DC voltage. This is like a reservoir that keeps everything steady.

3. Inverter (DC → Variable AC)

Here's where the magic happens. Using high-speed transistors called IGBTs (Insulated Gate Bipolar Transistors), the VFD rapidly switches the DC on and off thousands of times per second, creating a simulated AC waveform at the frequency you want.

Result: Your motor gets power at 15 Hz, 50 Hz, 100 Hz—whatever you dial in.

Understanding Motors, Gearboxes, and VFDs: A Quick Primer

Before we dive deeper into VFDs, let's make sure we're all on the same page about how motors, gearboxes, and VFDs actually work—in plain language, no engineering degree required.

If you're fuzzy on the difference between torque, speed, and power, or you're wondering "Why can't a VFD just give me more torque?", stop here and read:

Electric Drives for Dummies: The Stone Carrier Analogy

It's a 5-minute read that explains:

• Why gearboxes are like giants carrying boulders (slow, strong, fixed speed)
• Why motors are like runners with small backpacks (fast, moderate strength)
• Why VFDs are like coaches (they control speed, but can't magically create torque)
• When to use each (or combine them!)

Seriously, go read it. It'll save you from making expensive mistakes like "I'll just use a VFD to get more torque!" (spoiler: that's not how it works).

When Should You Use a VFD?

VFDs are NOT a one-size-fits-all solution. Here's when they make sense:

Perfect Applications (The Sweet Spots)

Let's flip the script: Energy savings are great, but they're not the only reason to use a VFD. In fact, for many businesses, productivity and quality improvements are far more valuable than electricity bills.

ApplicationWhy VFD is IdealEnergy SavingsProductivity/Quality Benefits

Centrifugal Pumps

Flow demand varies throughout the day. With a VFD, pump speed matches demand instead of throttling a valve.

30-50%

Eliminate water hammer; reduce mechanical stress; precise flow control for mixing processes

HVAC Fans

Building occupancy changes, so does cooling/heating need. VFD adjusts fan speed dynamically.

30-60%

Better temperature control; quieter operation; extended equipment life

Conveyors

Production line speeds fluctuate. Smooth acceleration prevents product spillage.

20-40%

Speed up production 15-20% during peak demand; reduce product damage; sync with upstream/downstream processes

Compressors

Air demand isn't constant. VFD eliminates wasteful load/unload cycles.

25-45%

Stable pressure → consistent product quality; eliminate pressure spikes

Mixers & Agitators

Recipe changes require different mixing speeds. Mechanical precision matters.

15-30%

Find the quality "sweet spot"—eliminate defects by fine-tuning mixing speed; faster recipe changeovers

CNC Machines

Different materials/tools require different spindle speeds.

10-25%

Optimize cutting speed = better surface finish, fewer rejects; longer tool life

Packaging Lines

Product changeovers need different speeds.

15-30%

Run 15% faster without quality loss = 15% more product per shift; quick format changes

The Hidden Value: Productivity & Quality

Forget energy for a moment. Here's where VFDs really shine:

1. Speed Up When It Matters

Your packaging line normally runs at 100 units/minute. But what if you could crank it up to 115 units/minute during rush season—without sacrificing quality? That's a 15% productivity boost with zero change in energy consumption per unit.

• Same motor, same power draw per cycle
• Just more cycles per hour
• More product shipped, more revenue earned

Real Example: A beverage bottling plant increased line speed from 350 to 400 bottles/minute during summer peak season. Result? 14% more output during their most profitable months. The VFD paid for itself in one season.

2. Find the Quality "Sweet Spot"

Sometimes your process isn't quite right. Maybe you're getting too many rejects, or surface finish isn't perfect, or mixing isn't uniform. A VFD lets you experiment and find the exact speed that eliminates defects.

• Too fast? Product defects increase.
• Too slow? Wasting time (and money).
• Just right? Maximum quality at maximum speed.

Real Example: A plastics extruder was getting inconsistent wall thickness. By adjusting the screw speed from 285 RPM to 310 RPM (an adjustment impossible without a VFD), defect rate dropped from 8% to 2%. That's not an energy saving—that's a quality revolution.

3. Total Process Efficiency

Energy is just one piece of the puzzle. VFDs optimize your entire process:

• Less downtime: Soft starts reduce mechanical stress on belts, chains, bearings (equipment lasts longer)
• Faster changeovers: Recipe change = dial in new speed (no mechanical adjustments)
• Better synchronization: Multiple VFDs can coordinate via automation systems, keeping conveyors, pumps, and motors in perfect harmony

The Rule of Thumb: If your application has variable torque (pumps, fans, blowers), VFDs are golden for energy savings. The savings follow the "affinity laws":

• Cut speed by 20% → Power consumption drops by 50%
• Cut speed by 50% → Power consumption drops by 87%

That's not a typo. It's physics.

*But if your application has variable demand (production lines, processing equipment, CNC machines), VFDs are golden for productivity and quality.* Energy savings are just the bonus.

Questionable Applications (Proceed with Caution)

• Constant full-speed operation: If your motor runs at 100% speed 24/7, a VFD just adds cost and complexity. Use a softstarter instead.
• Very high starting torque: Applications like crushers or loaded conveyors starting uphill need serious grunt. A gearbox might be better for torque multiplication.
• Extremely low speeds (under 10 Hz): Most motors overheat below ~20% speed due to poor cooling (the motor fan spins slower too). Consider an external cooling fan or a different motor type.

Bad Ideas (Don't Do This)

• Using a VFD as a phase converter (single-phase input to run a three-phase motor): Possible, but you'll lose ~50% of the VFD's power rating. Just buy the right motor.
• Old motors (pre-1990s): Older motor insulation wasn't designed for the voltage spikes VFDs create. You risk premature failure. If the motor is ancient, upgrade it.
• Super long cable runs (>100m): Long cables between VFD and motor cause voltage reflections that can damage motor insulation. Install output filters or chokes.

How to Choose the Right VFD: A Decision Framework

Step 1: Know Your Motor

You need this info from the motor's nameplate:

• Power (HP or kW): This determines VFD size
• Voltage (V): Must match VFD output (230V, 400V, 480V, etc.)
• Full Load Amps (FLA): The REAL sizing factor (not HP!)
• Frequency (Hz): 50 Hz or 60 Hz

Pro Tip: Size your VFD based on FLA, not HP. A 10 HP motor might draw 14A or 25A depending on efficiency and design. Always check the nameplate.

Step 2: Understand Your Load Type

Load TypeExamplesVFD RequirementsNotes

Variable Torque

Pumps, fans, blowers

Standard duty VFD

Best energy savings

Constant Torque

Conveyors, extruders, cranes

Heavy duty VFD, 110-150% overload

Needs extra headroom

High Inertia

Flywheels, large fans

Oversized VFD + DC braking

Long ramp times

Step 3: Environmental Factors

• Indoor vs. Outdoor: Outdoor = IP65 or NEMA 4X enclosure
• Dusty/Corrosive: Sealed enclosures, conformal coating
• High Altitude (>1000m): Derate VFD by 1% per 100m elevation
• Extreme Temperatures: Most VFDs are rated 0-40°C; outside that, expect derating

Step 4: Special Features You Might Need

• Built-in EMC filter: Reduces electrical noise (required in Europe)
• Harmonic mitigation: Prevents power quality issues (line reactors or active filters)
• Dynamic braking: For fast stops (adds a braking resistor)
• Communication protocol: Modbus, Profinet, EtherNet/IP for automation integration
• Bypass contactor: If VFD fails, motor can still run at fixed speed (critical applications)

Step 5: Budget Reality Check

VFD SizeTypical Cost (USD)Notes

1-5 HP

$200-600

Entry-level, basic features

7.5-20 HP

$600-2,000

Mid-range, most common

25-50 HP

$2,000-5,000

Industrial, advanced features

75-200 HP

$5,000-20,000

Heavy industrial, custom

Remember: The VFD is just part of the total cost. Budget for:

• VFD-rated cables (shielded)
• Installation labor
• Programming/commissioning
• Harmonic filters (if needed)
• Output chokes/reactors (if cable >50m)

VFD vs. Gearbox vs. Softstarter: The Eternal Debate

Let's settle this once and for all with a real-world example:

Scenario: You have a 15 HP motor driving a centrifugal pump. The pump needs to run at different speeds throughout the day.

Option 1: Gearbox

• Pros: Simple, reliable, mechanical torque multiplication
• Cons: Fixed ratio (e.g., 5:1 means output is always 1/5 input speed). No flexibility. If demand changes, you're stuck.
• Cost: ~$1,500 for the gearbox + larger motor might be needed
• Verdict: Not suitable for variable speed needs

Learn more about gearboxes

Option 2: Softstarter

• Pros: Smooth startup, reduces inrush current from 600% to ~200%, protects motor
• Cons: No speed control once running. Motor goes to full speed and stays there.
• Cost: ~$400
• Verdict: Good if you only need startup protection; useless for speed control

When to use a softstarter instead

Option 3: VFD

• Pros: Infinite speed control, huge energy savings, soft start included, precise process control
• Cons: Higher upfront cost, generates harmonics, requires proper installation
• Cost: ~$800-1,200
• Verdict: Best solution for this application

The Winner? For variable-speed applications, VFD wins. But if you just need torque multiplication, use a gearbox. If you only want soft starting, a softstarter is cheaper.

Control Methods: How VFDs Actually Control Your Motor

Not all VFDs are created equal. The control method (also called control algorithm) determines how precisely the VFD can regulate speed and torque. Choosing the right method is critical—use the wrong one, and your application might not work at all.

1. Scalar V/Hz Control (Linear)

What it is: The simplest method. The VFD maintains a constant Voltage-to-Frequency ratio (V/Hz). For example:

• 50 Hz → 400V
• 25 Hz → 200V
• 75 Hz → 600V (if motor can handle it)

How it works: Open-loop control (no feedback from the motor). The VFD assumes the motor follows the commanded frequency.

Pros:

• Simple and reliable
• Works with any standard motor (no encoder needed)
• Cheap (entry-level VFDs use this)

Cons:

• Poor torque control at low speeds (below 15-20 Hz)
• No compensation for load changes (if load increases, speed drops slightly)
• Not suitable for precise speed or torque control

Best for:

• Fans and pumps (light loads, not critical)
• Conveyors with consistent loads
• Applications where ±5% speed variation is acceptable

2. Quadratic V/Hz Control

What it is: A variation of scalar V/Hz, but the voltage increases quadratically (as the square of frequency) instead of linearly.

Why it matters: Centrifugal pumps and fans have variable torque loads—torque increases with the square of speed. This control method matches the V/Hz curve to the load characteristic.

Voltage curve:

• 50 Hz → 400V
• 25 Hz → 100V (not 200V like linear V/Hz)
• 75 Hz → 900V (if motor can handle it—usually capped)

Pros:

• Massive energy savings for centrifugal pumps and fans (optimized V/Hz curve reduces losses)
• Better efficiency at partial speeds
• Still open-loop (no encoder needed)

Cons:

• Only works for variable-torque applications (fans, pumps, blowers)
• Terrible for constant-torque applications (conveyors, extruders, hoists)

Best for:

• HVAC fans
• Centrifugal pumps
• Blowers and air handling units

Pro Tip: If your VFD has a "pump/fan mode" or "energy saving mode," it's probably using quadratic V/Hz. Use it—your electricity bill will thank you.

3. Sensorless Vector Control (SVC / Open-Loop Vector)

What it is: Advanced algorithm that estimates motor torque and flux in real-time without an encoder. The VFD monitors motor current and voltage, then calculates what the motor is doing using mathematical models.

How it works: The VFD separates motor current into two components:

• Torque-producing current (makes the motor spin)
• Magnetizing current (creates the magnetic field)

By controlling these independently, the VFD achieves much better torque control.

Pros:

• Excellent torque control down to ~5-10 Hz (much better than scalar V/Hz)
• Faster response to load changes (the VFD "senses" the load and compensates)
• No encoder required (cheaper than closed-loop vector)
• Suitable for most industrial applications

Cons:

• Not as precise as closed-loop vector (±1-2% speed variation under load)
• Limited torque at zero speed (can't hold a load stationary without drift)
• Requires proper motor parameter configuration (auto-tuning recommended)

Best for:

• Extruders (need consistent torque)
• Mixers and agitators (load varies)
• Conveyors with variable loads
• CNC machines (moderate precision)

4. Closed-Loop Vector Control (FOC - Field-Oriented Control)

What it is: The gold standard. Uses a motor encoder (or resolver) to measure actual motor position and speed. The VFD continuously adjusts torque and flux to match the setpoint exactly.

How it works: Real-time feedback loop—the VFD knows the exact rotor position at all times and adjusts current within milliseconds.

Pros:

• Full torque at zero speed (motor can hold position without drifting)
• Ultra-precise speed control (±0.01% accuracy)
• Instant response to load changes (dynamic performance)
• Perfect for servo-like applications

Cons:

• Requires an encoder (adds cost—$200-1,000 depending on type)
• More complex installation (encoder wiring, calibration)
• Higher initial cost (VFD + encoder + installation)

Best for:

• Elevators and hoists (need precise positioning)
• Cranes (hold loads stationary on a slope)
• Winding/unwinding machines (tension control)
• High-precision CNC spindles
• Robotics and pick-and-place systems

Quick Comparison Table

FeatureScalar V/HzQuadratic V/HzSensorless VectorClosed-Loop Vector

Torque at low speed (<10 Hz)

Poor

Poor

Good

Excellent

Torque at zero speed

None

None

Minimal

Full (100%)

Speed accuracy

±5%

±5%

±1-2%

±0.01%

Response time

Slow

Slow

Fast

Very fast

Encoder required?

No

No

No

Yes

Cost

Lowest

Low

Medium

Highest

Best for

Simple loads

Fans/pumps

Industrial general

Precision control

How to Choose the Right Control Method

Ask yourself three questions:

1. Do I need torque at low speeds (<10 Hz)?

• No → Scalar V/Hz is fine
• Yes → Sensorless or closed-loop vector

1. Is my load variable-torque (fan/pump) or constant-torque (conveyor/extruder)?

• Variable-torque → Quadratic V/Hz
• Constant-torque → Scalar or vector

1. Do I need the motor to hold position (zero speed with load)?

• No → Sensorless vector is enough
• Yes → Closed-loop vector (requires encoder)

Braking Methods: How to Stop (Or Slow Down) Safely

When you reduce VFD frequency, the motor slows down—but how quickly it stops depends on the braking method. Choose wrong, and you'll either wait forever for the motor to stop, or trip the VFD on overvoltage.

The Problem: Regenerative Energy

When a motor decelerates, it acts as a generator—the rotating mass (flywheel effect) forces the motor to keep spinning, converting kinetic energy back into electrical energy. This energy flows back into the VFD's DC bus, increasing voltage.

If the DC bus voltage exceeds the VFD's safe limit (typically 800-850V for a 400V system), the VFD trips on "overvoltage fault."

1. Natural Deceleration (No Active Braking)

What it is: The VFD simply reduces frequency slowly, relying on mechanical friction and load resistance to stop the motor.

Pros:

• Free (no extra components)
• Simple

Cons:

• Extremely slow (can take 30-60 seconds or more for high-inertia loads)
• Motor "coasts" and may overshoot the target speed
• Not suitable for fast stops or emergency stops

Best for:

• Light loads with low inertia (small fans, pumps)
• Applications where stop time doesn't matter

2. DC Injection Braking (Built into VFD)

What it is: The VFD injects DC current into the motor windings, creating a stationary magnetic field. This acts like a "magnetic brake," forcing the rotor to stop.

How it works: When you command a stop, the VFD:

1. Ramps down frequency normally
2. At a certain low frequency (e.g., 5 Hz), switches to DC injection mode
3. Applies DC current for a set time (e.g., 2-5 seconds)
4. Motor stops

Pros:

• Built into most VFDs (no extra hardware)
• Faster than natural deceleration
• Simple to configure (just set DC current level and duration)

Cons:

• Heats the motor (DC current = no rotation = no cooling)
• Not effective at high speeds (only kicks in at low frequency)
• Limited braking torque (~20-30% of motor rated torque)

Best for:

• Moderate-inertia loads (conveyors, small centrifuges)
• Controlled stops (not emergency stops)

3. Dynamic Braking (Braking Resistor)

What it is: An external braking resistor connected to the VFD's DC bus. When the motor regenerates energy (acts as a generator during deceleration), the VFD dumps excess energy into the resistor, converting it to heat.

How it works:

1. Motor decelerates → generates energy → DC bus voltage rises
2. VFD detects rising voltage (e.g., > 750V)
3. VFD activates an internal braking chopper (high-power transistor)
4. Chopper switches the resistor on/off rapidly, dissipating energy as heat

Pros:

• Much faster stops (can stop in 1-3 seconds depending on inertia)
• Works at all speeds (not just low speeds like DC injection)
• Prevents overvoltage trips
• Relatively cheap (resistor costs $50-500 depending on power)

Cons:

• Energy is wasted (converted to heat, not recovered)
• Resistor gets extremely hot (must be properly ventilated or liquid-cooled)
• Duty cycle limits (resistor can't brake continuously—typically 10-30% duty cycle)

Best for:

• High-inertia loads (flywheels, large fans, centrifuges)
• Applications needing fast stops (conveyors, cranes, hoists)
• Emergency stop situations

Sizing tip: Braking resistor power rating (in watts) should be at least:

• P_brake ≈ Motor power (kW) × 1,000 × Duty cycle
• Example: 10 kW motor, 20% duty cycle → 10 × 1,000 × 0.2 = 2,000W resistor minimum

4. Regenerative Braking (Energy Recovery)

What it is: Instead of wasting braking energy in a resistor, the VFD feeds it back into the electrical supply (the grid or a DC bus shared with other drives).

How it works: The VFD has an active front end (AFE) or regenerative unit that can reverse power flow. During deceleration:

1. Motor generates energy → DC bus voltage rises
2. VFD's regenerative unit converts DC back to AC
3. Energy flows back to the grid or other equipment

Pros:

• Energy savings (can recover 15-40% of braking energy)
• No heat dissipation (no resistor)
• Continuous braking (no duty cycle limits)
• Ideal for frequent start/stop applications

Cons:

• Expensive (regenerative VFDs cost 2-4× more than standard VFDs)
• Grid must accept regenerated power (check with utility)
• Complex installation (may require grid-side filters, permits)

Best for:

• Elevators (constant up/down cycles—huge energy recovery)
• Cranes and hoists (lowering heavy loads generates massive energy)
• Test benches (frequent acceleration/deceleration cycles)
• Large installations with multiple VFDs (energy shared via DC bus)

Payback calculation:

• High-rise elevator: 20-40% energy savings → payback in 1-3 years
• Crane with 50 cycles/day: 30% savings → payback in 2-4 years

5. Mechanical Brake (Electromagnetic or Spring-Loaded)

What it is: A physical brake mounted on the motor shaft—typically electromagnetic (energized to release) or spring-loaded (spring applies brake, power releases it).

Important: This is NOT for normal stopping! It's a holding brake—designed to keep the motor stationary after it has stopped.

How it works:

1. VFD decelerates motor to near-zero speed (using DC injection or dynamic braking)
2. Once motor is stopped (or near-stopped), VFD signals the brake to engage
3. Mechanical brake clamps the shaft, holding it in position

Pros:

• Safety (motor can't move even if power is lost)
• Infinite holding torque (shaft is physically locked)
• Required by safety regulations (elevators, cranes, hoists)

Cons:

• Not for dynamic braking (brake can't stop a spinning motor—it'll burn out)
• Adds cost ($100-500 depending on size)
• Requires coordination with VFD (timing is critical)

Best for:

• Elevators (holding position when stopped)
• Hoists and cranes (prevent load from dropping)
• Vertical conveyors (prevent rollback)
• Any application where safety requires a physical lock

Braking Method Decision Matrix

ApplicationBest MethodWhy

Small fan/pump (low inertia)

Natural deceleration

Simple, no extra cost

Conveyor (moderate inertia)

DC injection or dynamic braking

Fast enough, cost-effective

Centrifuge (high inertia)

Dynamic braking (resistor)

Fast stop, prevents overvoltage

Elevator

Regenerative + mechanical brake

Energy recovery + safety holding

Crane lowering heavy load

Regenerative (if budget allows) or dynamic braking

Energy recovery or fast dissipation

Test bench (frequent stops)

Regenerative

Continuous operation, energy savings

Accessories and Additional Modules: Enhance Your VFD

VFDs are modular. Beyond the basic drive, you can add components to improve performance, reliability, and safety. Here's what's available:

Input-Side Accessories (Between Power Supply and VFD)

1. Line Reactors (Input Chokes)

• What: Inductors (coils) that smooth incoming power
• Why: Reduce harmonics (distortion) fed back into the electrical system, protect VFD from voltage spikes
• Typical value: 3-5% impedance
• Cost: $50-300 depending on VFD size
• When to use: Almost always—especially if you have other sensitive equipment on the same circuit

2. EMC/RFI Filters

• What: Electromagnetic compatibility filters that reduce high-frequency noise
• Why: VFDs generate electrical "noise" that can interfere with radios, PLCs, computers
• Required: Often mandatory in Europe (CE marking) and industrial environments
• Cost: $100-500
• When to use: If you have communication issues, radio interference, or need CE compliance

3. Harmonic Filters (Active or Passive)

• What: Specialized filters that eliminate harmonics (distorted waveforms)
• Why: Large VFDs (>50 HP) create significant harmonics that can damage transformers, capacitors, and neutral wires
• Types:
• Passive: Tuned LC filters (cheap, but bulky)
• Active: Electronic filters (expensive, but compact and effective)
• Cost: $500-5,000+
• When to use: Large VFD installations, shared transformers, sensitive equipment nearby

4. Disconnect Switches / Circuit Breakers

• What: Manual or automatic disconnect for safety and maintenance
• Why: Required by electrical code, allows safe VFD servicing
• Must have: Lockout/tagout capability
• Cost: $50-300

Output-Side Accessories (Between VFD and Motor)

5. Output Chokes / dv/dt Filters

• What: Inductors that slow down VFD voltage rise time (dv/dt = change in voltage / change in time)
• Why: Reduce voltage spikes at motor terminals, protect motor insulation, reduce bearing currents
• Cost: $100-500
• When to use: Long cables (>30m), older motors, or if you hear excessive motor noise

6. Sine Wave Filters

• What: Filters that convert VFD's "chopped" output into smooth, near-perfect sine waves
• Why: Eliminate motor noise, reduce bearing currents, extend motor life
• Cons: Expensive, bulky, reduce VFD efficiency slightly
• Cost: $300-2,000+
• When to use: Ultra-quiet operation needed, very long cables (>100m), or precious/old motors

7. Motor Thermistors / RTDs (Temperature Sensors)

• What: PTC thermistors or PT100 RTD sensors embedded in motor windings
• Why: VFD monitors actual motor temperature and trips before damage occurs
• Cost: $20-100 (often built into "inverter-duty" motors)
• When to use: High-value motors, continuous low-speed operation, harsh environments

Control & Communication Modules

8. Communication Modules (Fieldbus)

• What: Add-on cards for industrial protocols (Modbus RTU/TCP, Profinet, EtherNet/IP, CANopen, etc.)
• Why: Integrate VFD into automation systems (PLCs, SCADA)
• Cost: $100-500 per module
• When to use: Any automated system, remote monitoring, Industry 4.0 applications

9. Remote Keypads / HMI Panels

• What: External control panels mounted on cabinet doors or operator stations
• Why: Easier access to controls (don't have to open electrical panel)
• Cost: $50-300
• When to use: Wall-mounted VFDs, VFDs inside locked panels

10. Analog/Digital I/O Expansion

• What: Extra analog inputs (0-10V, 4-20mA), digital inputs/outputs (relay contacts)
• Why: Connect additional sensors (pressure, temperature), control external devices
• Cost: $100-400
• When to use: Complex control logic, multi-sensor applications

Safety & Redundancy Modules

11. Bypass Contactors (Soft Bypass)

• What: Electromechanical contactors that bypass the VFD and connect motor directly to line power
• Why: If VFD fails, motor can still run at full speed (critical applications)
• Cons: Lose all VFD features (speed control, soft start, protection)
• Cost: $200-1,000
• When to use: Critical processes where downtime = disaster (water treatment, HVAC in hospitals)

12. Redundant VFD Systems

• What: Two VFDs in parallel (one active, one standby)
• Why: If one fails, automatic switchover to backup
• Cost: Double the VFD cost + switchover logic
• When to use: Mission-critical applications (life safety, continuous process)

Braking Components (Already covered in detail above)

13. Braking Resistors

• See "Dynamic Braking" section

14. Braking Choppers (usually built into VFD)

• High-power transistor that switches braking resistor on/off

15. Electromagnetic Brakes (motor-mounted)

• See "Mechanical Brake" section

Quick Recommendation Matrix

Your SituationRecommended Accessories

Basic installation, short cable (<30m), new motor

Line reactor, EMC filter, disconnect switch

Long cable (30-100m)

Add output choke or dv/dt filter

Very long cable (>100m) or old motor

Add sine wave filter

Large VFD (>50 HP) or shared transformer

Add harmonic filter (active or passive)

Automated system with PLC

Communication module (Modbus, Profinet, etc.)

High-inertia load needing fast stops

Braking resistor + braking chopper

Elevator, crane, or hoist

Regenerative VFD + electromagnetic brake

Critical process (can't afford downtime)

Bypass contactor or redundant VFD

VFD Reality Check: Losses, Stress, and Motor Protection

Before we get into the common mistakes, let's talk about some hard truths that VFD salespeople don't always mention.

Truth #1: VFDs Consume Energy (They're Not 100% Efficient)

A VFD is an electronic device with power losses. Energy gets converted (and wasted) in three stages:

1. Rectifier losses (~1%)
2. DC bus capacitor losses (~0.5%)
3. Inverter switching losses (~2-3%)

Total efficiency: Modern VFDs are ~95-97% efficient. That means 3-5% of your input power becomes heat, not mechanical work.

What this means for you:

• A 20 kW motor running through a VFD actually draws ~21 kW from the grid
• The VFD itself generates heat (that's why they have heat sinks and fans)
• If you're borderline on electrical capacity, factor in VFD losses

The good news: For variable-speed applications (pumps, fans), the 30-60% energy savings from running slower far outweigh the 3-5% VFD losses. But for constant-speed applications, the VFD just adds inefficiency.

Truth #2: VFDs Stress the Motor (Especially Without Filters)

VFD output isn't smooth sine-wave AC—it's a chopped, high-frequency approximation created by rapidly switching DC on and off (PWM - Pulse Width Modulation).

This creates problems:

1. Voltage Spikes

VFD switching creates voltage peaks that can be 2× the nominal voltage. For a 400V motor, that's 800V spikes hitting the motor windings thousands of times per second.

• Older motors (pre-2000) weren't designed for this → insulation degrades faster, leading to premature failure
• Solution: Use "inverter-duty" motors (enhanced insulation), or add output filters/chokes to smooth the waveform

2. Bearing Currents

High-frequency switching induces electrical currents in the motor shaft and bearings, causing:

• Pitting and fluting (microscopic arc welding damage)
• Premature bearing failure (can reduce bearing life by 50-80%)

Solution:

• Use insulated bearings (one end of the motor shaft is electrically isolated)
• Install shaft grounding brushes
• Add output chokes or dv/dt filters

3. Audible Noise

The high-frequency PWM switching (typically 4-12 kHz) causes motor magnetic fields to vibrate, creating a distinctive "whining" or "singing" sound.

Solution:

• Increase carrier frequency (8-16 kHz) for quieter operation (but VFD generates more heat)
• Add sine wave filters for near-silent operation (expensive)

Truth #3: Motor Overload Capacity May Be Reduced

Here's a sneaky one: A motor that worked fine at "full load" without a VFD might struggle with a VFD.

Why?

1. VFD waveform is less efficient: The chopped PWM waveform causes extra heating in the motor (due to harmonic losses). A motor rated for 100% load on clean AC might only handle 90-95% load on VFD power.
2. Reduced cooling at low speeds: Most motors have shaft-mounted cooling fans. At 50% speed, the fan moves 50% less air → motor runs hotter. If you run at low speeds continuously, you may need to derate the motor (e.g., use a 15 kW motor for a 12 kW application).
3. VFD overload protection is different: VFDs typically offer 150% overload for 60 seconds. A motor's built-in thermal protection might allow brief overloads that the VFD won't tolerate.

Real-world scenario:

• Your 15 HP motor drives a conveyor that occasionally hits 16 HP during startup (107% load).
• Without VFD: Motor handles it fine (thermal mass absorbs the brief overload).
• With VFD: VFD sees 107% load, gets nervous, and trips on overcurrent after 30 seconds.

Solution:

• Size the VFD (and possibly the motor) with extra margin (110-120% of continuous load)
• Use "heavy-duty" or "constant torque" VFDs for demanding applications
• Add an external cooling fan if running at low speeds continuously
• Install a motor thermistor (PTC or PT100) connected to the VFD for real-time temperature monitoring

Truth #4: Cable Length Matters (A Lot)

Long cables between the VFD and motor act like antennas, causing:

• Voltage reflections (can double the voltage spikes at motor terminals)
• Electromagnetic interference (EMI) (messes with nearby equipment)
• Ground loop currents (bearing damage)

Cable Length Guidelines:

Cable LengthRequired Action

< 30m

Standard VFD-rated cable, properly grounded

30-100m

Add output chokes or dv/dt filters

> 100m

Add sine wave filters or use output transformers

Always use shielded VFD-rated cable, grounded at both ends (VFD and motor).

Common Mistakes to Avoid

1. Oversizing "Just in Case"

Many people buy a 20 HP VFD for a 10 HP motor "for safety." Bad idea:

• VFDs are most efficient at 60-100% load
• Oversized VFDs cost more and can have control issues at low speeds
• Rule: Size for 110% of motor FLA, no more

2. Ignoring Cable Requirements

Standard motor cables aren't designed for VFD output. Use:

• Shielded VFD-rated cable
• Grounded at both ends
• Separate from signal/control wires

If you don't, you'll get:

• Electrical noise interfering with other equipment
• Premature motor bearing failure
• Tripped VFD faults

3. Skipping the Manual (Or: "I'll Just Use the Defaults")

Modern VFDs have hundreds of parameters. The factory defaults are "safe" but rarely optimal for your specific application. You need to configure at least two categories of parameters:

Motor Parameters (from the nameplate):

• Motor rated voltage (e.g., 400V)
• Motor rated frequency (e.g., 50 Hz or 60 Hz)
• Motor rated power (kW or HP)
• Motor rated current (full load amps - FLA)
• Motor rated speed (RPM)
• Motor power factor (cos φ) - if available

Why this matters: The VFD uses this data to optimize its control algorithm, protect the motor from overcurrent, and adjust voltage/frequency ratio (V/Hz curve) correctly. If you skip this, the VFD is "flying blind."

Operating Parameters (your application):

• Acceleration time (how fast the motor ramps up from 0 to max speed)
• Too fast → VFD trips on overcurrent (motor can't accelerate that quickly under load)
• Too slow → wasted time, reduced productivity
• Deceleration time (how fast the motor ramps down from max to 0 speed)
• Too fast → VFD trips on overvoltage (motor acts as a generator, pumps energy back into the DC bus)
• Too slow → can be OK, but consider braking methods if you need fast stops
• Minimum frequency (lowest speed the motor will run)
• Below ~15-20 Hz, motor cooling becomes an issue (shaft fan spins slower)
• For continuous low-speed operation, add an external cooling fan
• Maximum frequency (highest speed the motor will run)
• This does NOT have to match the motor nameplate frequency!
• You can run a 50 Hz motor at 60 Hz, 75 Hz, or even 100 Hz if the mechanical system can handle it and you understand the risks (reduced torque at higher speeds, bearing stress, etc.)
• Common practice: Set max frequency 10-20% above nameplate for "boost" capability

Pro Tip: Spend 30 minutes with the manual during commissioning. Configure these basics, test under load, and fine-tune. The difference between "factory defaults" and "properly tuned" is night and day—smoother operation, fewer nuisance trips, better performance.

4. No Harmonic Mitigation

VFDs chop up electricity, creating harmonics—distorted waveforms that can:

• Damage transformers and capacitor banks
• Cause neutral conductor overheating
• Trip breakers randomly

Solution: Add a line reactor (5% impedance) or harmonic filter upstream.

5. Assuming "It Worked Without a VFD, So It'll Work With One"

This is a huge mistake. A motor running at the edge of its capacity on direct-on-line (DOL) power might fail with a VFD due to:

• Extra heating from PWM harmonics
• Voltage spikes stressing insulation
• Reduced overload tolerance

Solution: If your motor was already running hot or near its limit, consider upgrading to a larger motor or a proper "inverter-duty" motor when adding a VFD.

FAQ: Your Questions, Answered

1. What does "VFD" stand for, and is it the same as a frequency inverter?

VFD = Variable Frequency Drive. Yes, it's the same thing as a frequency inverter, also called an AC drive, inverter drive, or adjustable-speed drive (ASD). Different names, same magic box.

2. Can I use a VFD on any motor?

Mostly yes, but with caveats:

• Modern motors (post-2000): Usually "inverter-duty rated"—designed for VFD use
• Older motors (1980s-1990s): Check the insulation class. If it's below Class F, you might have issues
• Ancient motors (pre-1980): Insulation wasn't designed for VFD voltage spikes. You're risking premature failure

If your motor is old, consider upgrading to an IE3/IE4 efficient motor—you'll save energy and avoid headaches.

3. Will a VFD really save me money?

Short answer: If you have variable loads (pumps, fans), absolutely.

The Math:

• A 20 HP fan running 24/7 at full speed uses: 20 HP × 0.746 kW/HP × 8,760 hours/year = 130,632 kWh/year
• At $0.12/kWh, that's $15,676/year
• With a VFD running 30% slower on average, power drops to ~36% of full (remember the cubic law!)
• New consumption: 47,027 kWh/year = $5,643/year
• Savings: $10,033/year
• VFD cost: ~$1,500
• Payback: 1.8 months 🎉

For constant-speed applications, savings are minimal. But for anything with variable demand, VFDs are money-printing machines.

4. VFD vs. Softstarter: Which one do I need?

FeatureVFDSoftstarter

Speed Control

Full control, 0-100%

No control after startup

Soft Start

Included

Main purpose

Energy Savings

Huge (variable loads)

Minimal

Cost

Higher (~$800+)

Lower (~$300-500)

Complexity

Moderate (needs programming)

Simple (plug-and-play)

Choose a Softstarter if:

• Motor runs at full speed 95%+ of the time
• You just want to eliminate startup current spikes
• Budget is tight

Choose a VFD if:

• You need speed control or speed variation
• Energy savings are a priority
• Process control precision matters

5. Can a VFD replace my gearbox?

Sometimes, but not always.

VFD is great for:

• Speed reduction (run a 1,500 RPM motor at 500 RPM electronically)
• Variable speed applications (adjust on the fly)

Gearbox is better for:

• High torque at low speed (VFDs reduce torque as speed drops; gearboxes multiply it)
• Constant speed (mechanical = no electronics to fail)
• Extreme environments (dust, moisture, explosions—gearboxes don't care)

Ideal combo: Use both! A VFD + gearbox gives you the best of both worlds—electronic flexibility AND mechanical torque.

6. Why does my motor make a humming/whining noise with a VFD?

That's the sound of PWM (Pulse Width Modulation)—the rapid on/off switching (thousands of times per second) that creates the variable frequency. The motor's magnetic fields vibrate in response, creating audible noise.

To reduce noise:

• Increase the carrier frequency (default is ~4 kHz; try 8-12 kHz)
• Higher frequency = more heat in VFD (might need derating)
• Install output chokes to smooth the waveform
• Use a sine wave filter for near-silent operation (expensive but effective)

7. Can I run a 50 Hz motor on 60 Hz with a VFD?

Yes! That's one of the cool tricks VFDs enable.

A 50 Hz motor rated for 1,500 RPM can run at 1,800 RPM on 60 Hz. BUT:

• The motor runs 20% faster → check if your mechanical system can handle it
• Power consumption increases (might exceed motor rating)
• Cooling is adequate since the internal fan spins faster too

Pro Tip: You can also "over-speed" a motor (e.g., 50 Hz motor at 75 Hz) for short bursts, but it's risky—bearings, insulation, and mechanical balance aren't designed for it.

8. What are "harmonics," and should I care?

Harmonics are electrical distortions created by VFDs chopping up the AC waveform. Think of them as "noise" in your electrical system.

Why you should care:

• They can overheat transformers and neutral wires
• They mess with power factor, increasing electricity costs
• They can trip breakers or damage sensitive equipment (PLCs, computers)

Solution:

• Install a line reactor (5% impedance) between the power source and VFD—cheap and effective
• For large VFDs (>50 HP), consider active harmonic filters

9. Can I install a VFD myself?

If you're electrically competent, yes. But there are rules:

You CAN:

• Mount the VFD in a clean, ventilated location
• Wire power input and motor output per the manual
• Configure basic parameters

You SHOULD NOT (without an electrician):

• Modify electrical panels or breaker boxes
• Work on live circuits (obvious, but worth saying)
• Ignore local electrical codes

Best practice: Hire a qualified electrician for installation, then learn to adjust parameters yourself.

10. How long do VFDs last?

Typical lifespan: 10-15 years with proper care.

The weak link: Electrolytic capacitors in the DC bus (they dry out over time). After 10-12 years, consider replacing capacitors preemptively.

To maximize lifespan:

• Keep it cool (clean dust from heat sinks annually)
• Avoid moisture (use the right enclosure rating)
• Don't overload (stay within rated current)
• Use proper cables and grounding

11. Do I need a "VFD-rated" motor?

For new installations: Yes, get an inverter-duty motor. They have:

• Enhanced insulation (to handle voltage spikes)
• Improved bearings (to resist VFD-induced currents)
• Often an external cooling fan (for low-speed operation)

For existing setups: Most motors built after 2000 work fine. If it's old or you're running below 30% speed continuously, upgrade.

12. Can I control multiple motors with one VFD?

Technically yes, but practically no.

Here's why:

• If one motor has different load than another, they fight each other
• Protection is compromised (overload on one motor won't trip the VFD if others are fine)
• Speed control becomes unpredictable

Better solution: One VFD per motor. Or use a mechanical shaft connection + single motor + single VFD.

13. What's the difference between cheap and expensive VFDs?

FeatureCheap VFD (~$200)Premium VFD (~$1,500)

Build Quality

Basic components

Industrial-grade

Control Algorithm

Simple V/Hz

Advanced vector control, sensorless

Overload Capacity

110% for 1 min

150% for 1 min

Diagnostics

Minimal (LED codes)

Full display, event logs

Protection

Basic

Comprehensive (motor thermistor, ground fault, etc.)

Warranty

1 year

3-5 years

Support

Good luck

Phone/email/onsite support

When to buy cheap:

• Hobby projects
• Non-critical applications
• You're willing to replace it

When to buy premium:

• Industrial production (downtime = $$$)
• Mission-critical systems
• You want it to last 15+ years

14. My VFD keeps tripping—what's wrong?

Common causes & fixes:

Fault CodeMeaningFix

OC (Overcurrent)

Motor drawing too much current

Increase accel time; check for mechanical binding; verify motor FLA

OV (Overvoltage)

DC bus voltage too high

Increase decel time; add braking resistor; check supply voltage

OH (Overheating)

VFD too hot

Clean heat sinks; improve ventilation; reduce carrier frequency; check ambient temp

GF (Ground Fault)

Current leaking to ground

Check motor insulation; verify cable shielding; ensure proper grounding

OL (Overload)

Motor running too long at high current

Reduce load; check motor for issues; verify VFD is properly sized

Pro Tip: Modern VFDs log fault history. Check the fault log to see what happened before the trip.

15. Can I use a VFD outdoors?

Yes, with the right enclosure.

LocationRequired RatingNotes

Indoor (dry)

IP20 / NEMA 1

Standard

Indoor (dust/moisture)

IP54 / NEMA 12

Sealed enclosure

Outdoor (protected)

IP65 / NEMA 4X

Weatherproof, corrosion-resistant

Outdoor (extreme)

IP66/IP67

Full submersion protection (rare)

Temperature matters too: Most VFDs are rated 0-40°C. Below 0°C, electronics can fail. Above 40°C, expect derating (reduced output).

Final Thoughts: Is a VFD Right for You?

VFDs are one of the most versatile tools in industrial automation. They offer:

Flexibility (speed control on demand)

Energy savings (up to 50% in the right applications)

Motor protection (soft start, overload, diagnostics)

Process control (precise speed for quality)

But they're not magic bullets. If your motor runs full-speed 24/7, you don't need one. If you need torque at low speeds, a gearbox is your friend.

The bottom line: For pumps, fans, and variable loads, a VFD pays for itself in months. For everything else, think carefully and maybe consult an expert (or just explore our kWiki for more insights).

Related Topics:

• Electric Motors 101 - Understand what's spinning inside
• Gearboxes Explained - When mechanical beats electronic
• Softstarters - The simpler (but limited) cousin
• Automation Basics - Integrate VFDs into your process