Though they are not explicitly named for their widespread use, universal motors can, in fact, be found just about everywhere. Open any number of household appliances or industrial machines for example, and chances are you will find a universal motor at its core. But if they aren’t named for the seemingly “universal” applications to which they lend themselves, why are they named as such? To put it simply, universal motors are so named because they can run on both alternate current (AC) and direct current (DC), meaning the “universal” namesake refers to their power supply options.
Having said all of that, the purpose of this article is not to delve into the origin of the term, “universal motor”, but rather to explore what makes this type of motor different from others as well as the various pros and cons of using one.
Innumerable as the various types of industrial machinery are, they are principally driven by either a universal motor or an induction motor. For the uninitiated, an induction motor uses electromagnetic induction between the stator and rotor to generate current. Typically, you will find induction motors used to power heavy duty and stationary equipment that is in continuous operation for extended periods of time. In a manufacturing setting, this would include things like large industrial fans, mixers, or air compressors. A three-phase induction motor is most notably used in more rugged industrial applications as they are cheaper, easier to maintain and have proven themselves to be exceedingly reliable and durable over the long term. Common applications include oil extraction mills, cranes, and lifts.
Squirrel-cage induction motor & Universal motor construction
Universal motors, for their part, utilize current in a different manner. As stated above, universal motors operate on AC or DC current. While an induction motor can be single or three-phase, universal motors are single-phase series excited motors. Universal motors do not have a permanent magnet stator but relies instead on a series of field and armature windings. The same current flows through each winding courtesy of the series path, resulting in a similar magnetic field in both windings. The interaction of these fields produces unidirectional torque regardless of the power supply.
A universal motor’s stator consists of a projected pole core and winding with a magnetic field. The rotor of the motor is comprised of a projected pole core, armature winding, a commutator, and a rotating shaft. When compared to induction motors, universal motors are much more lightweight and offer superior torque. Whereas a universal motor can generate a high starting torque, an induction motor needs time to achieve a high torque level and cannot be stopped and restarted in rapid succession. Universal motors are ideal when large amounts of power are required in rapid bursts, with a constant level of torque and at variable speeds. Most people rely on universal motors to run their personal power tools but they are also responsible for driving industrial sanders, drills, and saws on the factory floor.
There are a number of advantages that set universal motors apart. First, there’s cost. The relatively simple construction of a universal motor translates to a much cheaper alternative to induction motors. As mentioned above, universal motors can achieve high levels of torque at variable speeds which opens it up to a greater breadth of applications. Everything from handheld power tools to vacuum cleaners.
As far as motors go, universal motors are both small in size and lightweight; two qualities that lend themselves well to applications that require ease of portability. If all of that wasn’t enough, we must once again reiterate that universal motors can operate using both AC and DC currents.
Like everything else, there are disadvantages to utilizing universal motors. For example, when compared to induction motors, universal motors tend to operate at a louder volume; sound also increases in proportion to the increase in speed. If even slightly unbalanced, a universal motor can generate considerable vibrations which can cause self-damage or damage the piece of equipment it is operating. Since they are less rugged than induction motors, universal motors require more upkeep and maintenance. Two principal universal components, namely carbon brushes and commutators, can wear out over time.
Universal motors are everywhere; and while they may be noisier and require more maintenance than the commonly used alternative, the portability, torque production, and ability to operate on both AC and DC current makes them incredibly useful for many, many applications.
As an Electromechanical Technician, it’s important to know the differences between certain motors, their uses, and advantages as you will encounter them on the job. If you’re looking to upskill or grow your career in electromechanics, check out our Electromechanical Technician program. The program prepares you for the growing electromechanical industry, giving you the in-demand skills, you’ll need to advance in your career.
8iowa,
Thanks for sharing the difference between these two types of motors. It is knowledge like this that we should consider when purchasing any tools for our work, specially if the characteristics of one or the other have any bearing on our demands.
Though the technical aspects presented are not only valid, the truth is that many of us were led to SS as a result of space and time considerations. I would not doubt that the majority of us would choose to set up our workshops with individual self powered tools - but that takes up space which many do not have (even manufacturers).
At my age, though I worked with wood most of my life (staircases and roofs) and had the space to place individual machines in the workshop, the time has come when other considerations rule my purchases and tool selection: space, time and cost.
I no longer have the ample space I would like (it is now a garage/junk deposit/car shed, etc.) and I must consider not only the machines themselves but the workspace required around them. This is the primary consideration that led me to SS. As far as space efficiency it served the purpose I sought. I might even say that every one of my children and friends are impressed with the efficient handling of this aspect and would turn tomorrow to buy a SS if they even had the minimal space required and the money to pay for this benefit. Notice that I need not speak of the high qualities and scope of the Mark V.
With an ever growing home space problem for most younger people (home costs vs surface - apartments/condos/towers, etc) it is no surprise that this is a very powerful consideration when purchasing any tools. Albeit the fact that SS integrates 5 of the most sought tools it still is larger and heavier to handle than the sum of individual components that can be stacked if one purchases the so called "lunch box tools".
The next and probably strongest determinant overall is time. If I am considering a machine that will be used continuosly or even most of the time for production (maybe 20%) I will go heavy/heavy duty/costly and make space for it in the shop. But if it will only be used occasionally then cost becomes of importance so as not to freeze capital (family expectations and needs) in things that are not fully justifiable.
You got me thinking (after the fact of course) about the time (available and stolen) that I have to pursue woodworking. Projects that would be completed in a couple of days now take a couple of weeks. Before it was a full day's work, today it is a couple (when lucky) of hours in the shop (when the climate is favorable).
If I am going to use a machine less than an hour a week (or even a month) and I can replace it fourfold if something should go wrong, the knowledge that I am purchasing a weaker tool hardly figures on the purchase check list. This is unfortunately something to be taken into account when expanding into accessories of our strong reliable SS.
How often do you use your planer/jointer/band saw/etc. in a normal work session? This will indicate when the excellency of SS becomes a determinant in your purchase - after considering flexibility and space handling.
The younger woodworker is forced to consider these aspects sometimes way above the highly true and realistic issues of technical differences between motors. SS has made a proper and perfect selection for us when designing our tool but at the same time must know that as times change the market changes and though technical considerations are of great importance, apparently secondary aspects have presently made their way in to reduce the importance of the things that you and I look for when we dole out our savings (or earnings) for our hobby/pastime/or side line work.
I share my cent worth not in any way belittling the highly valuable knowledge you have shared here. Thanks.