Motor types

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Mr.Wizard

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Feb 28, 2015
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For belt grinders VFD on a 3-phase motor is recommended in the Sticky. What are the advantages of this over a DC motor such as one from a treadmill?

What kind of motors are used in woodworking routers? They are impressively compact and have good rated power, but speed range is usually limited to around 2:1 and RPM is high. Are there motors of similar design but different parameters that could be used?
 
Just giving you an electrical engineer's answer: the biggest difference between AC (both synchronous and asynchronous) and DC motors are the absence of brushes; therefore AC motors have close to maximum thermodynamic efficiency (translates to high torque, for instance), have a longer lifetime, and can be built quieter, but RPM is more complex to control. Also, while it doesn't matter for your application, synchronous AC motors can be "turned around" easily, to function as generators.

If you look through the cooling vents of your router or drill, you probably see blue sparks: these are the brushes of a DC motor.

Note that these days, you'll find hybrids, meaning AC motors driven by electronic circuitry, which allows easy control of RPM. More expensive to build for high torque, however, compared to classic AC motors. Some more expensive treadmills in gyms, etc., have motor systems like this.

Roland.
 
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For grinding metal you want a TEFC (Totally Enclosed Fan Cooled) motor .You can let the inside of a woodworking motor have a lot of sawdust inside and it will have no problems but metal dust will ruin a motor if it is not TEFC
 
ferider ferider While I have an electrical engineer's attention may I ask: what about brushless DC motors? These are common in many small devices like computer fans and quadcopters, and they seem to be efficient and robust. There are versions with enclosed coils which should address the point Spalted raised.
 
Mr.Wizard said:
ferider ferider While I have an electrical engineer's attention may I ask: what about brushless DC motors? These are common in many small devices like computer fans and quadcopters, and they seem to be efficient and robust. There are versions with enclosed coils which should address the point Spalted raised.
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Good question. AFAIK, they are all what I called "hybrid" above. Basically a multi-phase AC motor, with an MCU (Motor Control Unit) and inverter in front. Price (and size) of controller/inverter are cheap and can be integrated on a single chip if the motor doesn't have much power. The more torque/current you need (e.g. in the treadmill above), the more expensive it gets.
 
Three phase AC motors are an industry standard, and commoditized. Most of them are built like a tank.

As mentioned above, a sealed construction is extremely important. Both for the motor (TEFC, TENV), and for the VFD. Right now I'm using a KBAC-27, that is a ruggedized, sealed VFD. Some time ago I tried with a cheap chinese VFD, but after adding a sealed box, and figuring out the cooling requirements, it ended up being as expensive as buying the real thing in the first time.

I would stay away from brushed motors, if I were buying. But I would not turned down a free DC motor if one was available.
 
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