In recent years, many countries have introduced minimum efficiency requirements for electric motors. Since 1 July 2021 in the European Union, the requirements have increased even further. Using more efficient motors and drives is the foundation of an efficient system. Variable speed operation also leads to significant energy savings (in the range of 15-40%). But how about the settings of the drive? Do they have an impact on the power consumption of the electric motor?
I will focus on induction motors and try to answer this question by using some examples from real-life applications, rather than simulations or laboratory-made measurements.
Variable torque vs constant torque
To start, it is important to understand the torque characteristic of the application. We often encounter the terms variable torque (VT) and constant torque (CT). What do they mean? In a drives context, when we say variable torque (VT), we refer to applications with quadratic torque characteristics – that means the torque increases with the square of the speed. These are some of the most common applications such as fans and centrifugal pumps. (Please note that positive displacement pumps have a constant torque characteristic.) Applications with constant torque are applications for which the load is typically not significantly altered by the speed. This includes conveyor belts, hoists, and mixers.
In CT applications, the voltage applied to the motor follows a constant U/f ratio. In VT applications, the torque is reduced at lower speeds. Thus, it is possible to reduce the voltage fed to the motor at speeds lower than nominal, even further than done by the U/f ratio. By reducing the motor voltage, the losses in the motor are reduced and a higher efficiency is achieved. This applies when U/f or VVC+ control cores are selected.
Using CT or VT curves is not always optimal. The main reason is that the load of the motor in reality is different than the theoretical curve. In most situations, the motor is oversized. And then there are situations when the load varies in CT applications. Don’t let the word “constant” fool you – this doesn’t mean the load is constant, it only means the load changes independently of the speed. An example could be an escalator (CT application) where the motor load depends on how many people are using it. If the escalator is empty, the load is different than when the escalator is full of people.
The Automatic Energy Optimization function
The Automatic Energy Optimization (AEO) function can be used for ensuring optimal efficiency, even in the above-stated situations. The AEO reduces the magnetization flux in the motor to the level required by the actual load. Advanced algorithms allow for a good dynamic response. However, there are limitations and this function shall not be used in highly dynamic applications.
How much energy can we save by using the AEO function? The answer to this question is not straightforward. Sometimes the application is already optimal, or close to optimal. In such cases, there is not much to optimize. In other cases, the application is far from optimal. Obviously, in these cases the AEO would bring the biggest benefits.
Real-life application examples
Let’s take a look at how all this works in real life. Measurements have been performed on three motors turning fans, which are part of three air-handling units. All motors have a nameplate power of 2,2 kW. We have measured the motor input power with three drive settings: constant torque (CT), variable torque (VT) and automatic energy optimization (AEO). We have plotted the relative power saving of VT compared to CT, of AEO compared to CT and of AEO compared to VT. The difference between the three motors is the load. Fan 1 takes 1,05 kW at a nominal speed (1500 rpm), that is 48% of the motor's nominal power of 2,2 kW. That means the motor is oversized. In the case of Fan 2, the power at nominal speed is 1,93 kW, resulting in 88% of the motor's nominal power. And in the case of Fan 3, the match is almost perfect with the power at nominal speed being 2,29 kW (104% of nominal).
… and their results
As expected, in all three installations the VT setting results in savings when the speed is lower than nominal. The lower the speed, the higher the relative (not absolute!) savings. In Fan 3, with the best match between motor size and load, the AEO function compared to CT gives a similar level of saving as the VT setting. That is because the VT curve is very close to the optimum. Therefore, there is no saving by selecting AEO instead of VT. In Fan 2, where the match is not so good anymore, the savings at nominal speed are in the range of 0.5 %. At lower speed, the savings are higher – at 75% speed, the savings are in the range of 7-9%. At 50%, the savings are 22-25%. The savings are even bigger in Fan 1 (where the motor power is double the needed power). Here, already at nominal speed, the savings are in the range of 5%. (Who said you can’t save energy with drives running at full speed?) At 75%, the savings are in the range 7-13% and at 50% speed the savings are 25-37%.
Are these savings significant? To answer this question, let’s put these figures into perspective. The average efficiency for a 4 pole 2,2 kW induction motor in the efficiency class IE1 is 82.02%. For IE2 class, the value is 2.5% higher – 85.5%. For IE3, the average efficiency for the same motor is 88.06% – that is 6% higher. In many situations, the gains with the correct drive settings will be at least in the same order of magnitude when exchanging an IE1 motor with an IE3 or even IE4 motor. If you are ready to invest in the highest efficiency motor, why not optimize your application with the appropriate drive settings?
Optimize your performance from day one
As we’ve seen from the examples above, fine-tuning the drive parameter settings right from the start makes good business sense. You can achieve up to 20% annual energy savings, increase uptime by 35% and extend the lifetime of your drive with optimum calibration and configuration. If you’d like to get your hands on savings that are already available to you, you could reach out to your local DrivePro® expert for more information on the DrivePro® Life Cycle Service, Start-up.
Authors: Norbert Hanigowszki, Director of Drives Intelligence, and Ana-Mari Tataru-Kjær, Senior Control Engineer.