NFO Sinus technology: the frequency converter with a pure sine-wave output
In short: NFO Sinus is a frequency converter (frequency inverter, variable speed drive) developed and manufactured in Sweden by NFO Drives AB that feeds the electric motor with a pure sine-wave voltage instead of the pulses of a conventional PWM drive. The result: no electromagnetic interference, no shielded cables, no EMC filters, no bearing currents and an earth leakage current below 2 mA, so 30 mA residual current devices can be used for personal protection.
What is a sine-wave frequency converter?
A frequency converter controls the speed of an induction motor by changing the frequency and voltage of its supply. Matching the speed to the actual demand – for example on pumps and fans – can save up to 70% of the energy, according to NFO Drives. Conventional converters use PWM (pulse-width modulation): the voltage is chopped into short, high-frequency pulses, which generates electromagnetic interference, both conducted through the cables and radiated through the air.
The NFO Sinus converter transforms the mains frequency into a new frequency with a pure sine curve. The motor receives the same voltage waveform as when it is connected directly to the mains, only with frequency and amplitude adapted to the required speed. Since 1996, NFO Drives has been the first manufacturer to deliver interference-free motor control with a pure sine wave.
How NFO Sinus technology works
In each motor phase, the NFO switching circuit ("Sinus switch") consists of two transistors, a coil and two capacitors, together with logic and regulators for control:
- the transistors switch so that the current in the coil is triangular; they turn on at zero current (soft switching) and off at full current;
- the capacitors act as buffers and ensure a smooth current to the motor;
- a regulator calculates the switching times so that the voltage applied to the motor follows its sinusoidal set value exactly.
Because the switching pulses never reach the motor or the cable, the converter has intrinsic EMC: it is interference-free in itself and meets the EMC Directive 2014/30/EU without filters and without shielded cables.
NFO Sinus compared with a conventional PWM drive
| Criterion | Conventional PWM drive | NFO Sinus converter |
|---|---|---|
| Voltage waveform at the motor | high-frequency pulses | pure sine wave |
| Motor cable | EMC-shielded cable and EMC-rated accessories | ordinary unshielded cable; existing cables can be re-used |
| EMC filters | required | not required |
| Distance between drive and motor | often limited to a few tens of metres | practically unlimited – several hundred metres, limited only by cable resistance |
| Bearing currents | discharges through the bearings, premature wear, sometimes within months | none |
| Earth leakage current | typically 50 – 500 mA | below 2 mA |
| Residual current devices (RCD) | usually cannot be used | can be used, including 30 mA devices for personal protection |
| Motor noise | high-frequency switching noise | silent operation |
| Installer requirements | strict cabling rules, EMC competence | no special requirements – plug and play |
Advantages of sine-wave technology
- Simple: quick and cost-effective installation without shielded cables, EMC filters or EMC-rated accessories. The converter can be installed centrally in the electrical room, even if the motor is hundreds of metres away.
- Silent: no interference with other equipment (sensors, identification systems, communications) and no switching noise in the motor – important for both people and animals.
- Safe: no bearing currents, so motors last longer; no earth currents, so residual current protection for people and against fire can be used – often required in hospitals, farms, fountains or by insurance companies.
- Precise and efficient: full torque from standstill and at low speed, frequency, speed or process (PI) control, and an energy-save function at low load.
Where NFO Sinus is used
- Water systems: borehole pumps driven by NFO converters installed in the water treatment building, 460 m from the wells, using the existing unshielded three-phase cables.
- Fountains: the fountains in Bucharest, Romania (design and installation by Oase), with long motor cables and mandatory residual current protection.
- Healthcare: Oslo University Hospital (Rikshospitalet) replaced more than 300 PWM drives with NFO Sinus converters for ventilation and pumps.
- Defence: NFO Sinus was the first frequency converter approved by SAAB Kockums for naval vessels, where ordinary drives disturbed sensitive on-board equipment.
- Agriculture: DeLaval (Tetra Laval Group) uses NFO Sinus to avoid interference with automatic animal identification and earth currents that affect animals.
- Industry and food: at Guldfågeln a single NFO converter drives 8 conveyor motors through 360 m of existing unshielded cable; at Kährs (flooring) NFO converters control the polishing machines with full torque at 15–18 Hz.
The NFO Sinus Optimal range
The current NFO Sinus Optimal range covers motors from 1.5 to 18.5 kW: 1.5 kW (single-phase 230 V supply) and 2.2 kW, 5.5 kW, 11 kW and 18.5 kW (three-phase 380–480 V supply). All models have an IP55 enclosure, 0–150 Hz sine-wave output, Safe Torque Off, built-in Modbus and RS-485 and IE2 efficiency class. See the models in the Sine wave frequency converters category.
Facts about NFO Drives
- Manufacturer: NFO Drives AB, Karlshamn, Sweden – a Swedish cleantech company listed on the Spotlight market.
- Founded in 1996, based on research by engineer Ragnar Jönsson (Ideon, Lund) started in the mid-1980s; the first product – a 1.5 kW single-phase converter – was launched in 1998.
- Technology protected by patents in the field of electric motor control.
- Advice, sizing and quotations in Romania: Edwards.
Frequently asked questions about sine-wave frequency converters
Can an NFO Sinus converter be installed without shielded cable and EMC filters?
Yes. NFO Sinus meets the EMC Directive 2014/30/EU without filters and without shielded cables and is certified for this use. Existing unshielded cables can also be used.
How far can the converter be from the motor?
The only limit is the resistance of the cable. In practice NFO converters run motors several hundred metres away – for example 460 m to borehole pumps.
Can a residual current device (RCD) be used with a frequency converter?
With NFO Sinus, yes. The converter has an earth leakage current of at most 2 mA, while RCDs for personal protection trip at 30 mA. PWM drives typically have 50–500 mA, so they usually cannot be used with residual current protection.
Why do motor bearings fail on PWM drives?
PWM pulses cause electrical discharges through the motor bearings, which erode the races and balls; the damage can appear within months. NFO Sinus feeds the motor with a sine-wave voltage, so no bearing currents are generated.
Can one NFO Sinus converter drive several motors?
Yes, if the motors are of the same size and equally loaded. During tuning, the sum of the motors' currents and powers is entered.
What does "intrinsic EMC" mean?
The equipment is interference-free in itself and can be installed without EMC filters, shielded cables or other EMC-rated installation accessories.
Want to know whether NFO Sinus suits your application? Contact us for sizing and a quotation: we help you choose the right model from the motor data (power, rated current, voltage) and the installation conditions.
Source of technical data: NFO Drives AB (www.nfodrives.se) – NFO Sinus Optimal technical data sheets, versions 2605/2619, and the company presentation.