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Paul Clarke (aka monpjc) has been writing technical articles and blogging since 2010. His main interests being embedded electronics including FPGAs as well as software for microcontrollers.
The Rise of the Permanent Magnet Motor
Toward the end of July I read a post about how efficient AC induction motors are and that how using them and reducing their speed will help us all to reduce the greenhouse effect, save money, and the planet. While this is not wrong, I was a little shocked that people still consider AC motors with such high regard when in fact there are much better solutions—solutions in the form of Permanent Magnet or EC Motors.
The article I read started by saying that the U.S. is introducing regulations requiring manufacturers to produce more energy-efficient appliances. However, it’s not just the U.S. enforcing this, the E.U. is also providing regulations for motors to comply to. We are all very aware that being more efficient is not only better for the planet but also for our pockets. Induction motors have claims of being robust, reliable, low cost, and have high efficiency of around 80 percent. It is important to keep in mind, though, that these motors can only run at full speed and hence, some form of speed control is required. This can come in a number of forms where variable frequency drives come to the top of the list more often than not.
AC motors are inherently floored as they contain losses. At the core of motors are large windings that induce a magnetic field into the rotor. The rotor consists of a squirrel cage (for example) where the rotating electrical field generates a magnetic field that interacts with the main field and which generates the rotation. But the rotor will not follow the line frequency exactly and this is called slip. Slip in a motor is a loss and one that can’t be avoided in induction motors. Also, due to the motors design we get losses in the laminations as well as the electrical copper losses in the windings, all of which are required to induce the magnetic field in the rotor.
So ignoring induction motors to start with, let’s consider the benefits of speed control and frequency drives. There are lots of different manufacturers making these, each with their own efficiency figures. Frequency drives in my experience often talk about peak efficiency being the best results you will get. This will often be with a low frequency drive. This is because the switching devices, IGBTs or FETs, are most efficient at lower frequencies. However, electric motors contain lots of copper wires and when current is passed through them, they will move as a result of the magnetic field. The same can also happen to the laminations, when switched at frequency cases it causes the motor to hum like a speaker. This can be very annoying to anyone working around the motor, so controls engineers will use frequency drives at a higher frequency above the human hearing range. This, as you have predicated, will lower the efficiency of the frequency drive.
Below is an example of sound levels recorded between AC (Black Line) and PMM/EC fans (Green Line).
At ebm-papst we see lots of AC motors being used for fan applications, and it is here where we see other inefficiencies due to the load on a motor. Motors have peak efficiencies when running under optimum conditions. But if the speed is reduced or they are used outside their optimum range, then the motor is less efficient. Running an induction motor slower is primarily achieved by creating more slip in the motor. Slip is already a loss in the motor and by inducing more, the motor becomes less efficient. Overall, what can look like a green product can be filled with inefficiencies in the final application.
Permanent Magnet Motors (PMM) already have a fixed and permanent magnetic field. This means that the energy you are using in an AC motor to generate the magnetic field in the rotor is not required in a PMM. This instantly gives PMMs an advantage as you are saving energy already. Just by running the motor at full speed you can see savings between motors. In this example we can see an AC motor with an input power of 110 Watts that is only delivering 50 Watts output. For the PMM in the same situation, the losses would only come to 17 Watts, therefore to generate the same 50 Watts output you would only need 67 Watts, a savings of 43 Watts.
These motors are, however, not like they used to be. Remember the Scalextric cars bombing around a track that would generate lots of RF noise or wear out the brushes? PMM are now typically brush-less motors so this is no longer a problem. They have integrated electronics that allow for power and speed control into the windings.
In effect, the new electronics replace the outdated electronics that would be found in frequency converters. Although, in the case of these motors, the AC supply is regulated and converted to an internal DC supply that is used to run the windings. This means not only do you get the benefit of lowering the losses from PMM, but switching speeds are low and the windings generate no audible noise.
Having integrated electronics also allows for speed control. There for interfaces like 0-10 volts or some form of communication protocol can be used to speed control the motor, allowing PLC direct interfacing.
All new PMM and EC motors are far more efficient, as well as offering the same robustness, reliability, and life as that of an induction motor.