...... when buying a one or two horsepower motor for most woodworking machines do you always want a capacitor start/capicator run motor.
The answer is ... it depends. I don't believe that I have a single woodworking machine with a run capacitor (except for my Jet mini lathe where the same capacitor serves as both a start and a run capacitor).
Run capacitors are most commonly found where the motor is operating with a fairly constant load that is very close to its full load capacity. Examples of this would be air conditioning compressors, shop air compressors, and sometimes HVAC air handler motors.
Many of my tools have universal motors (router, sanders,drill, shop vac, etc.) They are used because they are lightweight for the amount of power that they produce. I would not want a hand drill with a heavy AC induction motor.
Some AC motors do not need a start capacitor -- one type is the shaded-pole induction motor. Those motors have very low starting torque, but that is fine where starting torque is near zero. An example is the motor on a portable fan or the motor on a small aquarium pump.
Most of the big machines in my shop use capacitor-start AC induction motors (lathe, table saw, planer, jointer, bandsaw, drill press, dust collector, and radial arm saw). These are most commonly the workhorse motors of stationary shop tools.
If you are buying a motor for a woodworking machine or something that you are building for your shop, the type of housing is more important than looking at capacitors. Shops are dusty and motors do not like dust so stay away from ODP (open-frame drip-proof) motors unless you have a free one and don't care if it gets dust clogged after a few years.The most suitable type for a woodworking shop is a TEFC (totally enclosed fan-cooled) motor. It is completely sealed to keep dust out and the end opposite the shaft will have a small shrouded fan that blows cooling air over the outside of the motor housing. There are also TENV (totally enclosed non-ventilated) motors that do not need an exterior fan, but they will be much more expensive than the TEFC motors. Wash-down duty motors are an example of TENV motors.
Of course, there are three-phase motors that are increasingly finding their way into home workshops thanks to the proliferation of variable frequency drives for machines such as lathes. Three-phase motors do not have capacitors and their construction is simpler than single phase motors so they are less expensive than single-phase motors (in theory, at least). Plain Jane three phase motors are not the best choice for use with variable frequency drives for a lot of reasons that I have mentioned in previous threads. A new (and more expensive) three phase motor designed for the tougher environment of variable speed service is the best choice to survive the high voltage transients, heating, high speed, and low rotor inertia needs of variable speed service.
Cap start motors have more kick to overcome the enertia of being stopped. Cap run motors often run with less amperage draw and therefore run cheaper and cooler. Important: service factor, true horsepower, quality manufacturer. Service factor is the amount of over-load it can take and still live. Higher is better. Horsepower ratings on tools is mostly a lie. A TRUE 1 HP motor on 115 volts pulls ± 11 to 13 amps. There is no way a SEVEN HP vacuum cleaner or a 3 .5 HP router will run on a 20 amp standard household wall outlet.
Bill Turpin in WNC mountains
Thought that I would elaborate a bit on Bill T's comments:
Single phase AC induction motors need something to get them going ... and in the right direction. This "something" is a secondary start winding in series with a capacitor to shift the "phase" (or timing) of the current through the start winding. This provides a rotating magnetic field in the desired direction for the armature to chase around in circles. It also gives the motor some starting "oomph" (a very precise technical term that can't be explained in words). Once the motor is up and running near its full speed, it doesn't need the extra starting torque and it no longer needs to be told which way to rotate.
The way that capacitor-start/capacitor-run motors work is that the start winding is no longer a limited duty start-only winding such as found in capacitor-start motors. It is in the circuit full time and is wound with heavier duty wire just like the main windings. In the case of capacitor-start/capacitor-run motors, the run capacitor switches into the auxiliary winding circuit and takes the place of the start capacitor when it is kicked out by the centrifugal switch. These motors are designed for peak efficiency when operating under maximum load conditions and are therefore often used where a definite purpose motor is required. I can't vouch for the greater efficiency, but Bill T. is probably correct when they are operated for their intended purpose. Under light load conditions, they may not be as efficient as the plain old everyday capacitor start motor. The main thing is to match the motor to the application. Over sizing a motor is a way to waste energy not to mention paying more money for the motor.
Very often shop air compressors use definite purpose motors and in such cases, the motor nameplate may not list a horsepower rating. My cynical perspective on that is some air compressor manufacturers have not been completely straightforward (I'm trying to remember the word that means "not completely straightforward") when it comes to stating horsepower. I presume that the rationale goes something like, "we are looking at the machine as a whole and therefore the true motor HP is not relevant since we have come up with a more creative definition of horsepower that does not involve the laws of physics".
In the case of buying AC induction motors that are not of the "definite purpose" type, the horsepower and other operating parameters listed on the nameplate can be considered truthful and accurate (although some no-name Pacific Rim motors could be slightly suspect).
A caveat about Service Factor: think of it as a safety net and not as something to rely on for continuous operation because the fine print says that the service life of a motor will be substantially reduced from continuous operation above a SF of 1.0.
The actual FL (full load) current of a motor is a function of its efficiency and efficiencies can vary from as low as 60% up to almost 90% for single phase motors. Therefore, one motor may require a FL current of 13 Amps while another may only require 9.5 Amps. The Government has mandated that new motors meet higher efficiency standards, but it appears that existing designs are exempt along with a host of other exemptions that is making the process anything but fast. Motors with higher efficiency is usually synonymous with higher quality motors.
The design of some motors is such that their peak efficiency is reached only at very specific conditions. An example is motors used for squirrel cage blowers in HVAC systems. When they are operated with the squirrel cage in the ducted shroud and a specific range of back pressure, their efficiency is high (which means that the current drawn will be at its minimum value. If the same motor were operated without a load the efficiency is so low that it might actually trip its thermal overload switch (if it has one).
I was going to try to sell Bill T. a 15 HP motor that runs on two AAA batteries, but I see that he is cynical about HP ratings on tools. I can't imagine why.
BTW, for anyone who does not already know -- do not believe any HP claim on a product with a universal motor (that is the type found in drills, routers, weed eaters, electric chain saws, portable sanders, etc.) BTW, Bill T, would you be interested in buying a 5 HP electric toothbrush.?
