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.