There are many reasons for using frequency converters. They make process control and machine operation much easier; they reduce inrush current and make starting more controlled; they reduce unnecessary noise and wear. But one of the main benefits is that they often reduce your electricity bill.

Before we get into the powertrain savings, it's worth remembering that engine efficiency is also important. European legislation now basically says you can't buy an IE1 motor (the least efficient one); If you buy an IE2 motor, you must use it with a frequency converter.

Now the cost of the motor is only a percent or two of the total cost of ownership due to the energy it consumes over its lifetime, so alongside legislation it makes sense to look at the IE3 and IE4 motors that are now in widespread use. More exotic motors, like permanent magnet motors, offer even higher efficiencies when used with a drive, but more on that in a later article. And as you choose your motor, remember that most motors are more efficient when running close to their rated power, so don't choose a larger motor than you need.

Back to frequency converters. If you reduce the speed of a motor with a drive you will almost certainly reduce the energy you use. When the load is a constant torque load, power is proportional to speed. A constant torque load is one where the load doesn't change too much with speed. Conveyor belts, mixers, many small machines. In these applications, however, process control is often more important than energy saving.

Incidentally, operating the drive in Vector Control often results in a reduction in energy consumption, as the drive continuously adjusts the voltage and frequency to optimize motor operation. This is particularly useful with constant torque loads as it also offers the best performance. Invertek E3 now works in Vector Control by default and the P2 range is easy to set up in Vector mode. Try it and check the reduction in motor current by looking at the display.

However, AC drives really start to pay for themselves with variable torque loads. that is, loads that increase with speed. Most fans and many types of pumps are variable torque loads - in fact, torque increases as the square of the speed. This means that power is proportional to the cube of speed (see Figure 1).

Fig. 1: Power and torque curves for a pump or fan

So if you reduce the speed of a pump or fan by 10%, the power consumption is reduced by about 25%, even taking into account the losses of the drive (typically about 3%). Reduce the speed by 20% and you will be close to halving the energy consumption. Note that some pumps, e.g. B. positive displacement pumps, represent constant torque loads and work better with an industrial or constant torque setting with an E3 or P2 drive.

Of course, if you put in a variable speed drive now and just run the pump or fan at full speed, you won't make any savings, but if you reduce the speed - even a little - the payback time will likely be months, not years. Drive suppliers offer you a nice calculation sheet to show these savings. If one particular drive shows better results than others, be suspicious - it's the physics of the pump or fan that allows for the saving and should be pretty much the same for all drives. Figure 2 shows a typical calculation sheet. The journey usually runs at 70-90% of full speed and the payback period is eleven months. You can download this calculator here

Fig. 2. Calculation of payback for a typical application

If you want to get the most out of the drive, don't just turn the speed down a bit and hope it doesn't slow down the process. You could put a sensor in, reconnect that to the drive, and use the closed-loop control in the drive to automatically maintain the flow rate, temperature, or pressure, or whatever. We will look at how to do this in detail in a later article. You can now remove those clumsy throttle bodies and dampers and watch the energy savings increase.

Or you can just use a timer and ride on that fan that runs all the time and choose a slower speed at night and on weekends.

Because mounting an AC drive on a pump or fan is so inexpensive and easy, it's not surprising that about half of AC drive applications involve these variable torque loads. Remember that not only are you saving energy and improving control, but starting currents are reduced and shock and vibration are largely eliminated. Actually better all around.

Most drive manufacturers offer a specific range of drives optimized for pump and fan applications. These drives do not have the sophisticated control and overload capabilities of an industrial drive (i.e. constant torque), but they often have additional features that are useful for pumps and fans. For example, the Invertek Eco drive has special software to control multiple pumps in cascade operation. it has a pump cleaning function that detects and removes blockages; It saves energy by going into standby mode when there is no need etc. Some eco drive variants could also save money due to their low input harmonics; Inductances are not required and the effective input current is reduced.

The drive saves you money and brings additional control functions with it; what's not to like?

Let's not forget about the simple "Energy Saving" feature, standardized on all Invertek drives. This function continuously monitors the load on the drive and if the load is light it reduces the magnetizing current (effectively by slightly reducing the output voltage) in the motor, reducing losses in the motor and drive. The disadvantage of this is a small delay when absorbing a sudden load; in many applications you would not notice it. The savings aren't huge, sometimes quoted as a percent or two, but every little bit helps, doesn't it? Parameter P-06 in E3 (P1-06 in P2 and Eco) enables this function. Figure 3

Fig. 3: Energy saving function

It's worth reiterating the massive savings you can make by mounting a drive on a pump or fan. Frequency converters have now largely replaced other methods of flow and pressure control in ventilation systems, but there are still many energy-saving opportunities waiting to be driven.