The power saving principle of the inverter!

I've often heard people say that inverters can save energy, and those who make the claim usually believe it without fully understanding the reasoning behind it. Why do inverters save power? And why is there a difference in energy savings between high-frequency and low-frequency applications?

Here are some key questions:

1. If two identical motors are operating at 50Hz, one using an inverter and the other not, with both running at their rated speed and torque, will the inverter save energy? And how much exactly?

2. What if the motor’s torque doesn’t reach its rated value under the same frequency and speed conditions? How much energy could the inverter save then?

3. In no-load conditions, how much energy can be saved, and which scenarios offer the most savings among these three cases?

The answer is that inverters can indeed save energy, but this depends on the application. In certain situations, energy savings can exceed 40%, while in others, they might even lead to higher power consumption than not using an inverter.

Inverters save energy by reducing voltage under light loads. For constant-torque loads, such as pumps or compressors, the energy savings are minimal because the torque remains relatively stable even when the speed decreases. However, for variable-torque loads like fans, where energy consumption is proportional to the 1.7th power of speed, the savings can be significant. Using an inverter on oil well pumps, for example, may actually increase energy use due to braking resistors during deceleration.

If the system requires speed adjustment, inverters still provide clear energy-saving benefits. But in fixed-speed applications, inverters mainly improve the power factor without saving energy.

1. Under rated load conditions, does an inverter save energy? No—its main role is improving the power factor, not reducing energy usage.

2. When the motor operates below its rated torque, but at the same frequency, inverters can save some energy if automatic energy-saving mode is enabled, though the savings are not dramatic.

3. In no-load conditions, drag-type loads don’t see much energy saving from inverters.

Regarding closed-loop control, the concept is broader than just speed feedback. Vector control, for instance, uses internal closed-loop mechanisms, while V/F control is open-loop. PID-based control systems also fall under closed-loop, and these can be managed via inverters. The term should not be overly restricted.

Braking concepts are sometimes misunderstood, leading to confusion rather than clarity.

To summarize:

1. Inverters don't always save energy. There are many cases where they don't.

2. Inverters themselves consume about 3–5% of the rated power.

3. Energy savings are only guaranteed under specific conditions: high-power fan/pump loads, built-in energy-saving features, and long-term continuous operation.

Without these conditions, inverters may not save energy at all. Claiming otherwise is misleading or driven by commercial interests. Understanding the context allows for more effective and accurate use.

What about starting current and torque when using an inverter? The inverter gradually increases frequency and voltage, keeping the starting current around 150% of the rated value (between 125% and 200% depending on the model). Direct start from the grid causes a 6–7x surge, causing mechanical stress. With an inverter, the start is smooth, with a current of 1.2–1.5x and a torque of 70–120% of the rated value. Some models with torque enhancement can even start under full load.

Why do inverters stop when large motors are running together in the same factory? Large motor startups cause voltage drops, which can trigger undervoltage protection in connected inverters, stopping the system.

Is there a limit on inverter installation direction? Yes. Proper ventilation and cooling must be considered, especially for vertical mounting.

Can a soft starter replace a fixed-frequency inverter? At very low frequencies, it's possible, but at high frequencies, the current surge may cause overcurrent tripping.

What should be considered when running a motor above 60Hz? Mechanical strength, noise, vibration, torque, and bearing life are all factors. High-speed operation may require special motor designs.

Can an inverter drive a gear motor? It depends on the gear structure and lubrication method. Low-speed operation with oil-lubricated gears may cause damage.

Can an inverter drive a single-phase motor? Generally not. Single-phase motors may suffer from overheating or capacitor failure when used with inverters.

How much power does the inverter itself consume? It varies by model, but efficiency is typically 94–96% below 60Hz. Regenerative braking models may consume more.

Why can't inverters be used continuously across the entire 6–60Hz range? Motor cooling decreases at lower speeds, so load must be reduced or a dedicated motor used.

What should you watch out for when using a motor with a brake? The brake power should come from the input side of the inverter to avoid overcurrent trips.

Trying to use a capacitor with an inverter may cause overcurrent issues. Removing the capacitor or adding an AC reactor can help.

How long does an inverter last? With proper maintenance, inverters can last over 10 years, though components like capacitors and fans may need replacement.