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Universal vs Induction motor

#4170 by 8iowa » Sat Jun 09, 2007 5:13 pm

In recent postings on this and other forums I have sensed that there is a lack of understanding regarding the Universal and the Induction type electric motors. There is a big difference, and I hope that the following discussion will help woodworkers to become more informed as they shop for new tools and spend their hard earned cash.

Universal motors have been around for a long time, in small appliances and tools such as vacuum cleaners, circular saws and hand drills. The Universal motor is compact, cheap to produce, and due to it’s high speed (10,000 rpm or higher is normal) can provide a lot of HP in a small package. They can easily be designed to run on AC or DC current. There are numerous downsides. This type of motor has a commutator with brushes, which will eventually wear out and spark. I once had a Sears shop vacuum that with nearly worn out brushes that gave off quite a shower of sparks. Thankfully I didn’t have wood dust in the canister or on the floor. The Universal motor is an electrically inefficient design that draws high amperage per given HP, and in some applications can generate enough heat to cause the motor to burn out when subjected to continuous operation. It is also a very noisy motor. Many people commonly complain that their shop vacuum is the noisiest tool in their workshop. Once this Universal motor gives out – that’s it! Modern Universal motors are not replaceable or repairable - you replace the whole tool.

The induction motor has no brushes. The field coil induces a current in the rotor causing it to rotate almost in synchronization with the current in the field coils. AC induction motors are wound around an even number of poles. Thus a 60 HZ (cycles per second) motor with two poles will run at a nominal speed of 3600 rpm. A four-pole motor has a speed of 1800 rpm, six poles at 1200 rpm,… you get the idea. The actual shaft speed is a little lower due to slippage between the field and the rotor. Thus a two-pole motor usually has a full load speed of around 3500 rpm. The AC induction motor will be considerably larger in size per given HP than a Universal motor, having a lot more copper, aluminum and steel, and it will have larger heavier load bearings – thus it is more costly. It is however more electrically efficient and quieter than a Universal motor, and is designed for continuous duty at full load. The life of the induction motor is much longer. There are induction motors made 75 years ago that are still performing every day. Another advantage is the fact that induction motors have been standardized by NEMA, thus making it possible to easily replace a motor in your machine with another motor made by a different electric motor manufacturer.

I’ll cite some woodworking tool examples; when you visit your local Woodcraft store or one of the “big boxes” you will likely see the Dewalt 735 or the Ridgid TP1300. Both are 13-inch “bench top” planers. The Dewalt costs $550 and the Ridgid is $379. Now, you ask, why would anyone pay $1200 for the Shopsmith Pro Planer? Isn’t the Shopsmith planer “way overpriced”? A close inspection would of course show many additional features in the Pro Planer that are beyond this discussion, but perhaps the biggest difference is in how these planers are powered. Both the Dewalt and the Ridgid have built in direct drive 10,000-rpm Universal motors, whereas the Pro Planer is powered with a 1 ¾ HP two pole induction motor on a separate mount, that with pulleys and belts drive the cutters at 5750 rpm – much easier on bearings. All three planers draw about 15 amps, however, the Pro Planer will deliver significantly greater HP to the cutters due to the motor’s higher efficiency and the fact that the feed is driven separately by a DC motor. It’s interesting to note that neither Dewalt or Ridgid are willing to disclose the HP of their motors in their detailed specifications. If they were to do so it certainly would not enhance the marketability of their product.

With price driving the big retailers in today’s market, more and more stationary tools, including table saws, are showing up that are driven with Universal motors. This may be a perfect example of “you get what you pay for”. The uninformed consumer may not be getting a “good buy” or even a satisfactory tool.

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#4180 by charlese » Sat Jun 09, 2007 9:46 pm

I like your enthusiasm for the subject of electric motors. Have noticed several posts expressing your concerns with motors.

With this knowledge, I might have re-thought my purchase of a Dewalt 735. Got it for $400 at a Lowe's sale. However, as I am now 73 yrs young, and hoarding-ly frugal, My decision probably would have stayed with the Dewalt. When my motor on the Dewalt quits and I have to buy a new one - I'll give you a holler. Mind you, I would have to wear out several motors (probably a half dozen) (don't know if I'l live that long) before I even come close to the price of the SS.

At $1200 for the SS (I'd have to get a stand alone model 'cause don't want to lift the other) I could buy 3 - count them - 3 Dewalts - the whole machine!. My Dewalt weighs in at 93 lbs. and it is on a rolling stand.

The horsepower of the motor has never been an issue with me. As you know, the "measurement" of horsepower is, and never has been, standardized. Torque is a much more dependable measurement. That's why no one measures it. After all - torque is fixed and comparable. No one wants that! Anyway I always take small cuts and the motor is not strained.

I really have never questioned the Value of the SS Pro planer. It is just too expensive for my use.

My Dewalt makes 179 cuts per inch and that's almost glass smooth. There is no to very little snipe without extension tables. Thickness settings range from 1/8" to 6" with no added jigs or sleds.

I'll agree, it does not have the same quality of motor as the SS, but at these prices I'll settle for the motor with brushes. Maybe several of them.

---

Octogenarian's have an earned right to be a curmudgeon.
Chuck in Lancaster, CA

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#4182 by 8iowa » Sat Jun 09, 2007 11:08 pm

Chuck:

Thanks for your nice comments. By-the-way, AC induction motors are standardized by NEMA (manufacturer's association). Not only is the frame, shaft size and basic mounting dimensions standardized, but performance is standardized as well. Manufacturers have to meet HP, amperage, temperature rise, and class of insulation standards as well. Thus, when you buy an induction motor you reall know what you are getting.

Universal motors are not standardized. Manufacturers can do as they please. All you have to do is look around your shop at routers, sanders, drills, even shop vacs and you can easily see that none of the motors are alike. For some reason performance data on Universal motors is never available. In the case of the Ridgid and Dewalt planers all we know is that the motor is 15 amps and the no load cutter speed is around 10,000 rpm. We don't even know the HP rating or even that the motors are rated for continuous duty. I certainly hope that if your motor fails (and if you have my luck that would be one week after the warranty expires) that you will be able to obtain a replacement and service from the manufacturer. I've yet to find someone as good as Shopsmith.

I don't intend to be the electric motor "expert". However, since they are the source of power for our favorite hobby, spreading a little knowledge around just might help fellow woodworkers make more informed purchasing decisions.

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#4184 by Ed in Tampa » Sat Jun 09, 2007 11:49 pm

8iowa
It seems as late I have been on the other side of your fences and I find myself there again. SORRY!

Actually universal motors have a higher starting torgue than induction motors. To increase the starting torgue most induction motors require starting capacitors.

Also as far a reliability the old generator in cars was nothing more than a universal motor being turned to produce electricity. In that case DC but the same mechanics (bearings and such). Manyb of us long for those generators where all you ever had to do was change the brushes every 100,000 miles or more. You never had to worry about diodes or messing them up when you gave someone a jump.

Your right there is no standard for universal motors sizes in most applications, and that is one of reasons they are often used. They can be made to fit the application, however in cases such as the Ryobi BT3000 and Bt3100 saw the motor was readily available should it needed to be replaced.
In most router/shop vac/home appliance the motor casing and the tool housing are one in the same to cut costs.

Universal motors can run slow or fast and this is another advantage over induction which is hard to get much above 4200 RPM. Far too slow for a router without a lot of gearing which costs money and has energy lost.

The actual determination of an electric motors horsepower is Wattage with one horse equaling approx 750 watts. Therefore a induction motor and a universal motor both using 750 watts should ideally produce the same horse power. However there are factors such as cross windings, bearing quality, armature fit, design and brush(universal only) friction that reduces that. However in most applications if a motor induction or universal are both using 750 in most cases the universal will overpower the induction if for no other reason than starting torque.

Universal motors are usually louder than induction because they are turning so much faster. Turn the two motors at the same RPM and the sound is nearly the same.

Both have their place. I don't want and would not use a induction motor drill. However in a refrigerator I want an induction motor.

Now to life span, believe it or not but many many production shops used lunch box planners. I know a company here in town that specializes in custom wooden stairs and they have a super 18 or 22 inch plane but they also have a 12 inch Dewalt. Know which planner gets choosen the most by the employees? The Dewalt. And I assure you it has seen more wood through it than most hobbiest planners see in a lifetime.

I love ShopSmith and I would love to have a ShopSmith planner and had the lunch box planners not come out I would probably have one. But unfortunately within a short time period of Shopsmith releasing their planner Ryobi came out with the first lunch box. The rest is history. Today I own a Delta 22-580 not because I think it is the best out there but because the price was right and I could justify it for what I do. I bought it for 1/4 the price of the Shopsmith standalone planner. I have had it nearly three years and so far it is has worked flawlessly.

Induction and universal motors both have a place. In most cases induction are used for slower more longer run time applications and universal are faster and used where the tools or motor is used in short bursts. Universal are almost always used to bring down motor costs where a compromise will not effect the overall operation of the machine. I think most lunch box planners are proof of concept.
Ed

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#4191 by 8iowa » Sun Jun 10, 2007 10:50 am

Good morning Ed:

I don't have any problem with your comments.

In my discussion I purposly stayed away from technical terms and tried to express the differences between the Universal and Induction motors in the best laymans language I could muster. If I have not done this well I apologize.

You an I both know that few people are aware (or even care) about the relationship between HP and Wattage. It's still commonly thought that the term "horsepower" has a direct relationship to the amount of work a horse can do!! If we get involved in the definition of these terms a lot of eyes will start to glaze over.

Perhaps the relative noise levels of these two types of motors is an important topic for further discussion. Far to many home and professional woodworkers do not wear any hearing protection. Noise levels above 85 decibels accelerate hearing losses. Most of us older guys have already suffered some loss and we need to protect what we have left. Manufacturers of tools don't seem to have much interest in this problem, and if a prospective buyer asks questions he is not likely to get a satisfactory answer. If I have raised awareness to this issue then all my work has been worthwhile.

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#4194 by jcbrowne » Sun Jun 10, 2007 12:53 pm

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.

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#4197 by alancooke » Sun Jun 10, 2007 3:06 pm

Obviously, from reading through this thread, these technical aspects are very important to some and less so to others. Either way , though, I find it darn interesting, so you "Smart, techno guys" please keep telling us about how much better our favorite machine is built!

Alan Cooke

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#4214 by Ed in Tampa » Mon Jun 11, 2007 10:26 am

8iowa wrote:Good morning Ed:

I don't have any problem with your comments.

In my discussion I purposly stayed away from technical terms and tried to express the differences between the Universal and Induction motors in the best laymans language I could muster. If I have not done this well I apologize.

You an I both know that few people are aware (or even care) about the relationship between HP and Wattage. It's still commonly thought that the term "horsepower" has a direct relationship to the amount of work a horse can do!! If we get involved in the definition of these terms a lot of eyes will start to glaze over.

Perhaps the relative noise levels of these two types of motors is an important topic for further discussion. Far to many home and professional woodworkers do not wear any hearing protection. Noise levels above 85 decibels accelerate hearing losses. Most of us older guys have already suffered some loss and we need to protect what we have left. Manufacturers of tools don't seem to have much interest in this problem, and if a prospective buyer asks questions he is not likely to get a satisfactory answer. If I have raised awareness to this issue then all my work has been worthwhile.



8iowa
Your right a lot of the information is just that information. Not all information is good or even needed.
However I think every woodworker should know that 1 horse power is about 750 watts and the maxium horse power for any tool plugged in a common 110 house wiring is between 1 1/2 to 2 horse power not the 3 -5 horse ratings some manufactures label their machines.

As for noise, I believe we are seeing a generation that when they reach the age of 50+ will be deaf. Thanks to mostly to their idea to be cool you must have a car radio that rattles the windows of everything within 500ft. However noise pollution from other sources is often overlooked. Things like leaf blowers, shop vacs, routers, and other machines that didn't effect our ears 30-40 years ago.

However I can remember the screech, chatter and squeal of hand tools as they sawed, hammered, planned and etc before the power tool.

Your right though we have to start thinking more about hearing protection than we do.
Ed

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