Operating principle in three steps
A frequency inverter (FI) converts the fixed mains frequency into a variable frequency, thereby continuously controlling the motor speed. It operates in three stages: rectification (the mains voltage is converted into DC voltage), intermediate circuit (a capacitor buffers and smooths the DC voltage), and inverter (IGBT transistors generate an approximately sinusoidal output voltage with a variable frequency, typically 0–200 Hz).
Two principles are available for controlling voltage and frequency: The U/f characteristic curve is a simple, open-loop control system (output voltage proportional to frequency) and is sufficient for straightforward applications such as fans and pumps without positioning requirements. Vector control is a closed-loop control system with current feedback that regulates torque and speed independently of one another—a necessity for more precise applications and constant load profiles.
Energy savings according to the law of affinity
The main advantage of an inverter lies in applications with quadratic load profiles (pumps, fans, compressors): In such cases, the required power decreases with the cube of the speed. The law of affinity (also known as the law of proportionality) applies in an idealized scenario, without accounting for the static head component—for pumps with high static back pressure, the actual energy savings are lower.
P₂ / P₁ = (n₂ / n₁)³
At 80% speed: P = 0.8³ = 0.512 = 51.2% of the rated power
- Marked point: Knowledge Check Example: 80% speed → 51.2% power
- Marked point 1: Knowledge Check Example: 80% speed → 51.2% power
Description and values of the figure
The curve illustrates the cubic law P₂/P₁=(n₂/n₁)³ for pumps, fans, and compressors with a quadratic load profile. Even a small reduction in speed significantly lowers power consumption: at 90% speed, approximately 73% of the power is still required; at 80%, only about 51%; and at 60%, as little as 22%. This explains why a frequency inverter is particularly cost-effective in terms of energy consumption for these load profiles.
| Speed ratio n₂/n₁ | Leistungsbedarf P₂/P₁ (%) |
|---|---|
| 0.4 | 6.4 |
| 0.5 | 12.5 |
| 0.6 | 21.6 |
| 0.7 | 34.3 |
| 0.8 | 51.2 |
| 0.9 | 72.9 |
| 1.0 | 100.0 |
Additional benefits: Soft start (start-up current limited to approximately one-1.5th of the rated current, rather than 5 to 8 times the rated current with a direct start) reduces the load on the power grid; precise speed control adapts the drive to process requirements; with the appropriate equipment, regenerative braking is also possible.
When is it worth using—and what are its limitations?
As a rule of thumb, a frequency inverter typically pays for itself within 2 to 4 years for motors over 5 kW with operating times exceeding 4,000 hours per year and a quadratic load profile —provided that the speed is actually reduced during operation (e.g., instead of using a throttle valve or a bypass valve). If the drive runs continuously at full speed, the energy savings are negated, and the inverter will not pay for itself through energy cost savings. It is also rarely cost-effective for smaller motors, short operating times, or constant loads.
Practical limitations: PWM-controlled inverters generate high-frequency switching noise, which may require EMC filters in accordance with EN 61800-3. Voltage spikes between the shaft and the housing can cause bearing damage via micro-discharges (EDM)—for larger motors, insulated bearings or a shaft grounding ring are recommended. PWM modulation causes the motor to heat up by an additional 10–15 K. Furthermore, the electrolytic capacitors in the DC link age: after approximately 10 years at an ambient temperature of 70 °C, their capacitance decreases by 20–30%, requiring replacement.
Mnemonic
Knowledge check
Answer all three questions, then click "Check". From 2 of 3 correct answers, the unit counts as completed. You can retry at any time.
This feature needs JavaScript. You can still read the learning content without JavaScript.