Variable Frequency Drives: operation, selection and EMC classes

A variable frequency drive (VFD, also called VSD or inverter) is a power-electronic device that regulates the frequency and voltage to an asynchronous or permanent-magnet motor. By varying the frequency, the drive controls motor speed without a mechanical gearbox or belt transmission. This article covers the operation, key selection choices, and the practical pitfalls around EMC and long motor cables.

What is a VFD?

An asynchronous motor runs synchronously with the line frequency: at 50 Hz with a 4-pole motor, the synchronous speed is 1500 rpm. If a different speed is needed, there are classically two options: a pole-changing motor (see our article on multi-pole motors), or mechanical transmission. A VFD adds a third, much more flexible route: the motor no longer receives 50 Hz, but a variable frequency typically between 0 and 60-100 Hz, sometimes higher.

The advantages are clear: stepless speed control, soft-start without the inrush current of 6-8 times rated current, energy recovery during braking with active drives, and, for variable-load applications, sometimes substantial energy savings.

How does a VFD work internally?

A typical VFD consists of three blocks in series:

• Rectifier: converts the 3-phase AC voltage to DC. In standard drives a 6-pulse diode bridge; in regenerative drives an active rectifier that can also feed energy back to the grid.
• DC bus with buffer capacitors: stabilises the DC voltage (typically 540-680 V DC at 400 V line) and buffers brief load variations.
• Inverter (IGBT bridge): switches the DC voltage thousands of times per second according to a PWM (Pulse Width Modulation) pattern. By modulating the pulse width, an average sinusoidal current is created in the motor windings.

IGBT switching frequency typically lies between 2 and 16 kHz. Higher switching frequencies give smoother motor currents (less losses, less audible whine) but more heat in the drive, a typical installation trade-off.

When to use VFD control?

• Variable process: pumps, fans and compressors with variable demand. The cubic law for centrifugal pumps and fans means 80% speed gives roughly 50% power, that is where the largest savings sit.
• Soft-start: for large motors (from approx. 30 kW) where inrush current causes problems on the grid or interferes with other equipment.
• Positioning: CNC machines, packaging lines, conveyors where precise stopping or speed profile is required. Often combined with encoder feedback.
• Energy savings in retrofit: existing pump or fan installations with throttle valve or bypass control can often be replaced by VFD control with payback in 2-4 years.
• Marine bow thrusters: stepless thrust regulation instead of fixed two- or three-speed settings.

However, a VFD also has disadvantages: higher purchase price, more heat in the switchgear panel, EMC emissions requiring filtering, and possibly extra motor insulation or filters with long cables. For applications that always run at nominal speed (continuous cooling pump, simple fan), direct online starting is usually sufficient and cheaper.

Sizing: power and rated current

Variable frequency drives are specified based on output current, not on kW alone. Two 22 kW motors can require different currents depending on IE class, speed and voltage. Rules of thumb for selection:

• Match current, not kW: select a drive whose rated output current equals or exceeds the motor's rated current as stated on the nameplate.
• Heavy-duty applications: for crushers, mixers and hoist motors with overload factor 150-200% briefly: select one frame size larger, or explicitly choose a heavy-duty rating (HD).
• High ambient temperature: above 40 °C derate per manufacturer curves: typically 1-2% per extra °C. In a switchgear panel without air conditioning during summer this adds up quickly.
• Altitude above 1000 m: derate roughly 1% per 100 m extra altitude.
• Parallel motors on one drive: sum of motor currents + 10% reserve, and use V/f control rather than vector control.

EMC classes C1 to C4

VFDs generate electromagnetic emissions through the rapid PWM pulses. The European standard IEC/EN 61800-3 classifies these emissions in four classes that determine which filter is required and in which environment the drive may be used:

In practice, you order a C3 drive for 90% of industrial applications. For installations close to a residential complex or in a light-industrial area, C2 may be required. Note that a C3 drive does not simply meet C1 environment requirements, and conversely, a C1 drive in a C3 environment is unnecessarily expensive.

Cable length, du/dt and sine filters

The PWM pulses at the motor output of the drive have a very steep edge (high du/dt, often 5-10 kV/µs). With long motor cables, these edges interact with the inductance and capacitance of the cable as a transmission line, reflection peaks form at the motor terminals that can overstress the winding insulation.

Rules of thumb for cable lengths and filters at 400 V supply:

• Up to 30 m cable: usually no extra filter needed with standard motor insulation.
• 30 to 100 m cable: du/dt filter at the drive output reduces the voltage rise rate to approximately 500 V/µs.
• Above 100 m cable: sine filter strongly recommended. This fully smooths the PWM output and delivers a near-sinusoidal voltage to the motor, essential for older motors or marine installations with long cable runs.
• Above 300 m cable: always use a sine filter; consider a heavy-duty motor with reinforced insulation (inverter-duty rated, often labelled IEC 60034-25 conformity).

A common mistake in retrofit projects is connecting an existing motor with standard insulation to a new VFD via a long cable run without filter. The motor then fails within months due to winding breakdown at the first few turns.

VFD with PMSM and SynRM motors

IE5 and IE6 motors are usually Permanent Magnet Synchronous Motors (PMSM) or Synchronous Reluctance Motors (SynRM). These have fundamentally different magnetic characteristics from asynchronous motors and require a drive that supports this technology. Not every standard VFD can correctly control a PM motor.

• Specific PMSM parameter set with motor characteristics (back-EMF, pole pair count, inductance L_d and L_q).
• Sensorless vector control or encoder feedback for rotor position.
• Active magnet damping at standstill to prevent unwanted rotation due to external forces.

When ordering a new IE5 motor, we recommend motor and drive as a matched set, with manufacturer-verified parameter sets. This prevents commissioning problems on site.

Retrofit and electrification, the VFD as connecting element

VFD technology plays a key role in two parallel transitions in the maritime and industrial sector: retrofit of existing installations and electrification of drives that were historically mechanical or hydraulic. We see both tracks daily with our customers.

Retrofit: existing installation, new drive

Most industrial and shipboard installations have a service life of 25 to 40 years. The drive motors usually survive that term without trouble, it is mostly the drive that hits end-of-life or obsolescence first. Old Danfoss, Siemens Master Drive or early ABB ACS-series drives are no longer maintainable or supported, while the connected motor can last for years. Retrofit means: keep the existing motor, replace only the drive with a current ACS580/ACS880 or Yilmaz model.

A proper retrofit takes more than a 1:1 box swap. We run a short audit for every retrofit:

• Re-establish motor parameters from the nameplate and, if needed, resistance and insulation measurement of the winding.
• Assess cable and filter compatibility: on older installations the existing cables are often not EMC-suited for modern fast-switching IGBT drives. A sine or du/dt filter is then unavoidable to protect the motor insulation.
• Drive enclosure: an IP55 stand-alone retrofit kit is usually faster to fit than an IP20 panel build-in, because no cabinet modification is needed.
• Process integration: communication interfaces (Modbus, Profibus, Profinet, EtherCAT) must connect to the existing PLC, otherwise a drive replacement turns into a complete controls project.

A retrofit typically costs 30-50% of a complete motor plus drive replacement, and is faster (1 day of downtime instead of 1 week). For customers with several identical drives, think of a rolling mill with 8 mill motors or a ship with 6 pump motors, we prepare standard retrofit packages with identical parameter sets, so maintenance and spares stay simple.

Electrification: from mechanical/hydraulic to VFD-driven

In the maritime sector we have seen a clear shift from traditional drives to electric since 2020. Hybrid and fully electric vessels, shore power converters and peak-shaving battery systems demand drives that are fast, accurate and bidirectionally controllable. VFD technology is the connecting component between the DC bus (battery or generator) and the AC drive motor.

Specific electrification tracks we contribute to:

• Bow thruster electrification: replacing a mechanical or hydraulic bow thruster drive with a VFD-controlled electric motor, often coupled to a PMSM for high torque at standstill and fast response during manoeuvres.
• Hydraulic deck crane modernisation: replacing a hydraulically driven deck crane with electric DC or VFD-AC drive, with regenerative braking to feed hoisting energy back into the ship's grid.
• Pump electrification: converting traditional direct-online or star-delta driven pumps to VFD control, for flow-on-demand operation and considerable energy savings (see also our article on IE efficiency classes and what a VFD adds to them).
• Shore power integration: drives and control technology for ships that must switch from their own generators to shore power (shore connection) in port without interrupting critical load.

In electrification projects we supply not just the motor and drive, but think along on the total energy balance: peak powers, recovered braking power, drive cooling on a ship with limited ventilation, EMC compatibility with ship navigation and communication equipment, and class certification (BV, LR, DNV, ABS) when the vessel falls under a flag state.

Delivery and stock at Van Bodegraven

Van Bodegraven supplies VFDs from the full ABB ACS portfolio (ACS180/380/480/580/880, plus ACQ580 water, ACH580 HVAC and ACS580MV medium voltage) and Yilmaz, from 0.18 kW to 5600 kW. Standard IP20 (panel-mount) and IP55 (stand-alone) configurations up to about 250 kW are in stock in Dordrecht with delivery time 1-3 working days. Larger cabinet units (ACS880, ACS580MV) are delivered directly via ABB EU with indicative lead time 4-12 weeks. EMC C2 versions, du/dt filters and sine filters are delivered on request within 1-2 weeks. For complete projects with motor and drive bundled, we also provide the parameter set and a brief commissioning guide.

No rights can be derived from the information in this article. Contact us for a concrete quote and current delivery status.