Electric motors have come a long way, and it helps to know their characteristics rather than arbitrarily replace faulty units. Bob Harper explains.

When I did my electrical apprenticeship in the 1970s there were three kinds of motor: DC, AC and three phase.

Although the teachers were well versed in motors, they had a particularly tough time when instructing us on the intricacies of all three. Later, after becoming a teacher, I began to understand why.

The fact is that many apprentices rarely see motors. That might seem strange to you – and to me coming from an industrial background where motors were bread and butter.

For apprentices who work solely on domestic jobs, motors are often an appliance that just needs to be to be changed when ‘broken’. A former student was brave enough to observe: “Why do we need to learn about motors? They come with a plug and cord, so you just unplug the old one and plug in a new one.”

I hope some of you are as stunned by the comment as I was. However, I concede that we try to teach a lot of material to people who won’t need it, unless they change their job.

Another problem stems from the belief that there are only the three motor types mentioned. What about universal and split-phase motors – where do they fit?

And are there any true DC motors, other than the Faraday disc? I’ll let that one go for now.

What about more recent inventions or adaptations of electric machines?

This article examines only rotating machines; that is, those with a spindle on bearings. In particular, it covers rotating electric motors (electrostatic motors are avoided).

Attraction and repulsion
Electric motors work on magnetic fields and forces, and magnetic fields attract or repel according to the polarity of magnets.

More precisely, magnetic flux, called ‘lines of force’ when I was at college, wants to take the shortest path (causing magnetic attraction in the field) but spreads out to fill any space (causing magnetic repulsion between parallel fields).

The basic function in an electric motor is to cause the moving part to be repelled from one magnetic pole and attracted to another. Some motors work mostly on attraction, and some work on repulsion, but that won’t cause any drama here.

Stationary fields
As students, we were told that DC motors have fixed field poles and a rotating armature with at least two armature coils, which are switched (commutated).

Three armature coils are the minimum on practical self-starting motors. The purpose of commutation is to establish a fixed (stationary) magnetic field even as the armature rotates that reacts with the stationary poles to result in rotation.

Your teachers may not have mentioned that current in the coils alternates as they pass the brushes, but I trust they understood that it does.

You probably remember being told that the same basic motor will operate on an appropriate AC voltage as well, and will be referred to as a universal motor. The point being that DC or AC supply is not a sufficient reason (or method) for categorising motors.

Rotating fields
Nicola Tesla invented the induction motor more than 120 years ago and it is an excellent example of engineering simplicity. The induction motor has only one moving part, the rotor (including the shaft, laminations, squirrel cage and bearings).

Field windings for a three-phase motor are made in three sets, one for each phase, placed around the stator with one coil set for each pole, per phase set. For example, a two-pole motor would require six coils (Figure 1.

The poles of this motor are formed from four coils connected in series (typically). A pair of poles are opposite each other so the pole set for A-phase are top and bottom, marked as ‘A’ and ‘a’. Pole ‘a’ is connected in the opposite polarity to produce a south pole when ‘A’ is a north pole, and vice versa.

The same goes for B-phase, using ‘B’ and ‘b’, and C-phase using ‘C’ and ‘c’.

Motor winders do this automatically, but the rest of us have to think about it. Going around the stator clockwise we would see the poles as ‘A-c-B-a-C-b’. Motor winders may refer to this as “A,notC,B,notA,C,notB” meaning that every second coil set is reversed.

At the time when A-phase is at maximum positive current flow, a north pole is generated, but either side of it the second poles of B-phase and C-phase (b and c) have half the maximum current flowing in the reverse direction, making them half-strength north poles as well (Figure 2, drawing A).

The same happens with A-phase south pole, ‘a’ and ‘B’ and ‘C’ either side of it. Also note the three-phase waveforms in Figure 3.

Looking just at the three-phase waveforms at the points when any phase is at maximum current flow – three for maximum positive and three for maximum negative – we get the six diagrams for Figure 2.

Note that the poles generate maximum magnetic fields in a way that ‘animates’ the combined fields around the stator in what is known as the rotating field. This is easily seen in an animated image but is difficult to show in a magazine.

A three-phase induction motor works like a transformer to excite a magnetic field in the rotor. In a pure induction motor, that field chases the rotating field and tries to keep up. However, that’s not the theory of interest for this article.

The point is that there are stationary fields and rotating fields, and motors can be categorised as employing one or the other method of making things rotate.

Speed of rotation
An interesting question was raised by a customer who had just bought a complete factory from the United States. He wanted to know whether the motors were compliant in Australia.

Ignoring that they were 120/208V or maybe even 240V or 440V depending on where they came from in the US, the motors were all designed for 60Hz and would run 20% slower in Australia on 50Hz.

It was more of a concern that they might have up to 20% less power due to requiring 20% less iron on 60Hz. Short answer: he would probably have to replace all the motors and all control gear.

Aviation has for some decades used motors operating at 200V three-phase 400Hz, because such a motor requires about 12% as much iron as it would at 50Hz.

Makers of electric vehicles have also caught on, and the best vehicles operate frequencies higher than 50Hz. An electric vehicle can be made with a direct-drive motor that runs from standstill to full speed without needing to change the mechanical drive ratio or engage a clutch.

Reduced mechanicals mean reduced losses.

Variable-speed drives
If the iron is not a problem, and coil inductance is understood, motors can be used over a range of frequencies, thereby providing a range of speeds.

Late last century, engineers began to recognise that three-phase induction motors would have a flexible speed range if the supply frequency was flexible.

They began to develop variable-speed drives (VSD), also known as variable-frequency drives, using different electronic methods not to be explained here.

VSD advances demanded better motors, designed from scratch to operate over variable frequencies and voltages, and to handle the choppy square wave voltages from early VSDs.

Laminations were made thinner and other core materials developed, winding methods were improved and winding security was boosted to avoid early failure due to a wide range of vibration frequencies.

Essentially, windings are tied down better or impregnated with a material to glue them together in the core.

Most recently the motor laminations are not laminated at all. They are compacted or cast from powdered materials mixed with a resin or ‘sintered’ together to provide the highest operating frequencies with the lowest iron losses. (Remember iron and copper losses?)

Super magnets
Next came the realisation that the squirrel cage rotor, although cheap and simple to manufacture, was stealing much of energy lost in an electric motor.

Adding a permanently magnetised rotor made from modern ‘super’ magnet materials resulted in extracting more power from the same physical size.

Also, though the squirrel cage rotor always results in ‘slip’, the permanent magnet rotor can run at synchronous speed, as though it were a synchronous motor. Slip is caused by the rotor needing to have an alternating frequency to generate current in the windings. At synchronous speed the frequency is zero, or DC, and transformation cannot take place.

BLDC motor
So what have we got now? Is it a three-phase motor with a permanent magnet rotor? Or is it an inside-out permanent magnet DC motor with the magnets rotating and the commutator replaced by electronic switching? If you think about it, they are the same.

Coming from either technology, the brushless DC motor (BLDC) or electronically commutated motor (ECM) is not a DC motor. The power to the electronic circuitry may be DC, AC or three phase.

The BLDC motor is very powerful for its size. Flexible speed is controlled by the switching frequency, thus often not requiring gearing or belts.

So the BLDC has a PM rotor, switched stator with a rotating field running on DC chopped into three phases. How would you categorise it?

It isn’t meant for three-phase connection, isn’t meant for 50Hz or DC, won’t work on DC without the help of electronics and is in fact frequency agile. All I can say is that it works as a rotating field motor.

My argument is that there are two types of electric motor: stationary field and rotating field.

Universal electric motor
Not to be confused with the old ‘series universal motor’ used on sewing machines and electric drills, the BLDC is likely to become the universal electric motor, perhaps some day replacing the ubiquitous three-phase induction motor.

It seems the electronics have come so far that we can expect a motor that performs as a three-phase unit, with low vibration and therefore low noise levels, simple operation or dynamically variable speed. All of this is contained in what was previously the terminal box, albeit now provided with cooling fins.

Quadrature motor
Although no longer necessary, the quadrature motor essentially uses two phases offset 90 degrees, as in most single-phase AC motors.

It has two identical windings, unlike other single-phase motors that often have a high-impedance winding (run winding) and a low-impedance winding in series with a capacitance (start winding).

The windings for either design are placed 90 degrees electrically around the rotor and are supplied by a two-phase quadrature voltage to create the rotating magnetic field.

In the quadrature motor, the AC is switched to supply two phases at a fixed or variable frequency from a single-phase supply with the electronics contained in a box that replaces the terminal box.

Although I believe it must have a capacitor in there, the well-known starting capacitor is not required. I don’t know whether the operating frequency is 50Hz or some other. The technology I was aware of was employed in domestic rainwater pumps.

BLDC application
One application that I understood was taken up by DeWalt but has recently appeared through Milwaukee Power Tools is a power tool that uses a BLDC motor.

I may get some of these details confused between the two brands, as it all comes from memory, so please don’t quote me. The unmarked machine had a direct-drive shaft and a slightly smaller than normal body diameter. It had plenty of torque due to electronic control of frequency and pulse duration and shape.

(The sine wave automatically generated by mechanical generators doesn’t have to be the form used by electronic wave generators, especially on battery-powered devices.)

Full speed is quite high, yet smooth and quiet, and virtually vibration free. Slow speed goes almost to standstill without any sense of ‘lugging’.

There must be some form of feedback, as locked rotor condition does not draw excessive current, and the tool seems to have a high degree of internal protection.

If I remember correctly the battery voltage was unusually high at 48V, possibly to take advantage of high-voltage electronic devices and lower current. Effectively the tool was a three-phase variable-speed motor in the palm of your hand.

Domestic applications
I am also aware of a domestic pump motor that uses a three-phase type of motor, possibly BLDC.

The advantage is that the machine not only runs what is essentially a three-phase motor on a single phase supply but also varies the speed depending on the water draw. It pumps only as hard as it needs to, thereby saving energy.

Many appliances have used BLDC type technology for some time, including the direct-drive washing machines, air-conditioners and refrigerators.

For the typical domestic, commercial or industrial electrician, it’s even more of a case, or perhaps an argument that if a motor is broken, unplug it and plug in a new one. Do you need an electrician to do that?

I suggest you make a real performance out of testing to see whether it’s broken enough to justify replacement.