Lets talk about wood working motors

[Roger Frazee]

I think we should start a thread and talk about motors. And maybe a cumulative input will help clear up some misconceptions, even some of mine...;)

I think we should also develop some questions that need be answered that will make sense and clear the smoke. This will also help control going left or right when we should be going straight.

What say ye ?? Maybe a myth buster type thread.....:)

I'm sure, not being a professional wood worker myself, that experience is a critical factor in determining what tool you need to do a certain process but I think that leads to.... why that tool has the motor it does?

Questions like

1.) If we have a dual voltage motor for a table saw if we wire it 230 volts... will it really start faster (get up to speed quicker some might say), have more horsepower, cut thicker and harder wood than if the motor was wired 120 volts ... why or why not ??

2.) What is it that we need in a motor to increase our ability to cut wood with larger dimensions (in particular depth of cut) or hardness ?? Ripping ability that kind of thing. Is it as simple as horsepower ?? If so what changes in the motor to give it more horsepower??

3.) If I have 2 motors one USA made to NEMA standard and one Chinese made to IEC standard and these motors are both 2 hp rated could one out perform the other if installed on the same tool?? In other words are there any parameters we must watch out for with a foriegn motor vs a USA motor? Is there anyway to know outside buying it and trying it....:)

4.) What design class of motor is best for a table saw, jointer etc... why?? Never know but I would suspect you need to change them once and a while. In other words I wonder if you can improve a wood working saw/tool but trying a motor of a different design class or change horsepower ??

5.) What is happening to a motor when it overheats or starts to stall...why is it stalling ?? Yes it has too much load ( I'm known for doing this..:() for what it is designed for.... but what is happening to the motor performance causing it to stall ?? We need to answer question 3 to see what is going to be required to put that same load to a saw without issues.

6.) Will a manufacturer under power a saw or other tool ??? Which leads to... could a wood worker know this by researching the motor used on the tool ?? Or is it just experience ?

7.) Are there rules of thumb for a saw motor depending on the process ...ie.. what your wanting to do with the saw..??

8.) Why is it that 230 volt motors require less amperage tha 120 volt motors and are there any advantages to motor performance that are significant.

9.) Is there a difference between a motor wired for 120 volts only or 230 volts only versus a single motor that will give you the choice of wiring it 120 or 230 volts.

10.) If my 10" table saw has a 1.5 hp motor will I improve it's performance if I replace that motor with a 2 hp motor??

FWIW... I'm no motor expert just a lowly wireman that connects the things once and a while so I don't have all the answers. But I've always wanted to see something addressed that maybe simplifies or puts motors for wood working tools in a better lay-mens logic than what a engineer would discuss with another engineer. I realize this will require some technical discussion no matter what we do.

So I'm thinking about asking one question at a time that will develop a discussion related to specific wood working tool motors. For example the question will specify the motor is for a table saw...etc..Then receiving the input for several days so it will sort of be like developing a FAQ on wood working motors.

Stupid idea or one of interest ??

[Chris Friesen]

1) If your wiring is undersized at 120V, you may get some benfit from running at 240V. It will basically never hurt to run at the higher voltage, and you might gain something.

2) When ripping on a tablesaw, a thinner kerf blade and fewer teeth will make for an easier cut and hence allow you to handle larger dimensions.

3) When comparing motors you need to look at the specifications: efficiency, power factor, and current draw. As a secondary comparison look at insulation ratings and duty cycle.

4) Normally for woodworking tools in a dusty environment you want a TEFC motor. In some cases an open drip-proof motor will do fine.

5) Motors will stall if they are asked to perform a task that is beyond their capabilities. Due to some complicated electrical behaviour the motor windings will then heat up.

6) Generally motors are sized relatively equivalently between brands. However, there are some cases where one brand/model is comparatively weak. This is usually mentioned in tool reviews.

7) The bigger the saw the bigger the motor. If you plan on doing a lot of resawing on a bandsaw you may want to go to a saw with a larger-than-usual motor.

8) P=V * I Power (watts) is voltage (volts) times current (amps). If you run at twice the voltage, the same power can be developed with half the current.

9) Generally not.

10) Are you bogging down your current motor? If so, you might be able to improve its performance with a heavier-duty motor. However, I'd suggest trying different blades first. Normally the next step up from a 1.5HP saw would be a 3HP cabinet saw using a toothed belt drive or three V-belts.

[Bruce Wrenn]

Beware of the 1.5 / 2.0 HP trap. Delta puts motors on their contractor's saws that are rated 1.5 HP on 110, and 2.0 HP on 220. The reason is you can't have a 2.0 HP motor on a 15 amp 110 circuit, so for 110, they rate it at 1.5HP. I have a 1.5 HP Unisaw, that can be run on 110. Doing the math (volts X amps, divided by 746) motor comes closer to 2.5 HP on 220. Plus, some motors (read better, more expensive) are designed to be more efficent.

[Rod Sheridan]

Hi,

1) No difference on a properly sized branch circuit. The motor is really two 120 volt motor windings and they cannot tell if they're in series for 240 volts or in parallel for 120 volts.

2) More horsepower is required to do more work. You can either increase the HP, or decrease the work through feed rate/blade selection/sharpness.

3) The only noticeable differences are in the torque curves, which will occur before rated speed. Once the motor is up to speed, motors of equal HP, service factor and speed will have equal rated torques.

4) A high efficiency, high power factor motor will be most economical to operate from an energy perspective. A Totally Enclosed Fan Cooled (TEFC) motor will keep the dust out of the motor, an important consideration for dusty environments.

5) A motor overheats because it is either generating too much internal heat, or the fan/cooling fins are plugged with dirt.

In the latter, occasionally inspect/clean the motor, in the first case run the motor at correct line voltage and frequency and stop overloading it.

6) No idea, I tend to think that machines are either adequately, or over powered. For example I have a 12" jointer/planer with a 4 HP motor. Who's going to use 4 HP on a jointer or 12" planer???

Similarly I have a 3HP cabinet saw (General 650). How in the world, in a home shop could I ever use 3 HP in a table saw unless I used a spoon as the cutting tool?

7) I owned a 1 1/4 HP 120 V saw before the General. Using a good ripping blade (standard kerf), with reasonable feed rates the saw cut 2 1/4" white oak with no issues.

My 3 HP saw does the same task no better, except for the feed rate.

8) Parallel versus series connection of the windings, see answer #1, no difference in performance.

9) No, however the dual voltage motor require more work in manufacture. (extra leads to bring out for voltage selection).

10) See answer # 7, you will increase feed rate capabilities, if the saw is strong enough for the forces involved.

Regards, Rod.

[daniel lane]

How in the world, in a home shop could I ever use 3 HP in a table saw unless I used a spoon as the cutting tool?


This is a wonderful image in my mind. What a great way to do cove molding! :D



daniel

[Chris Friesen]

How in the world, in a home shop could I ever use 3 HP in a table saw unless I used a spoon as the cutting tool?

A 3/4" dado stack (or molding head) raised to full height in hard maple would probably get pretty close.

[Rod Sheridan]

A 3/4" dado stack (or molding head) raised to full height in hard maple would probably get pretty close.

Yes, but you would have other more pressing considerations such as kick back to worry about.

regards, Rod.

[Roger Frazee]

1) No difference on a properly sized branch circuit. The motor is really two 120 volt motor windings and they cannot tell if they're in series for 240 volts or in parallel for 120 volts.
8) Parallel versus series connection of the windings, see answer #1, no difference in performance.I agree Rod...:)

This is exactly what I was wanting to "get out in the open" and give some logic to disspell the myths about 120 over 230.

If I may I'd like to submit these drawings at the end of this post of typical capacitor start and split phase motors commonly used in the shop.....this is only to support what you have accurately stated... these are from one of my motor maintenance handbooks when I was in my electrical apprenticeship program. These are some of the best drawings that are easy to understand I've come across...note... the thermal protection isn't shown but I don't really think that is necessary to prove your point. These drawings also show CCW and CW configurations. May have to click the drawings a couple times to read them.

To address the belief that the motor will start and get up to no load speed quicker on 240 volts. My understanding is this is true because the locked rotor current is halved vs 120 volts. However I have not found documentation to that effect as of today. It seems logical to me because the start winding appears to always be in series regardless of voltage applied.

[Rod Sheridan]

This is a wonderful image in my mind. What a great way to do cove molding! :D



daniel

Daniel, that's hilarious.........Unless it works then I want half of the royalties.:D

regards, Rod.

[Wayne Cannon]

Chris Friesen and Rod Sheridan had good answers for most of these. As an electrical engineer many years out of school, I'll try to add a few cents worth.

1.) See Rod Sheridan's answer -- no difference in the motor itself. See Chris Friesen's answer -- the halved current at 240 v will reduce the voltage drop for the same size of supply wire. If wiring is properly sized, there should be no difference.

2.) What changes in the motor to give it more horsepower??
More current per winding, more windings, and closer tolerances result in stronger magnetic fields within the motor providing greater forces/horsepower. It's a real space and heat design problem.
3.) Are there any parameters we must watch out for [between cheap and quality motors]?
I would suspect that efficient heat dissipation, bearing quality, and balance would be the biggest differences. One motor could run much hotter than another, significantly reducing the percentage of run-time versus cooling-off breaks, and the motor's lifetime.
4.) See Chris Friesen's and Rod Sheridan's answers.

5.) What is happening to a motor when it overheats or starts to stall...why is it stalling??
Each pole in a motor is a pair of electromagnets, all pulling together. When your load is stronger than their combined magnetic attraction forces, the motor will "slip" until the next pole's magnetic field catches. Once it starts to slip, the power drops dramatically.

When the motor is running, there is an internal voltage created that "pushes back", reducing the current draw. However, when the motor is stalled (or just starting up), much more current is drawn with significantly more heat generation. This is why dust collectors, for example, tell you not to start the motor more often than once every few minutes. It's the starting current that contributes most to overheating -- not the normal running current.
6.) Will a manufacturer under power a saw or other tool ???
Sure. It's a profit margin issue. But it's also a reputation issue. Better motors cost more, raising the price of the product and/or reducing profit margin. I think reviews (or actual trials) are the only way for a customer to judge. A 1 HP contractor saw may be fine for cutting 2-by fir, while it may take a 5 HP cabinet saw to cut 12/4 maple rapidly. Some may stall; some may overheat. What is the customer's need?
7.) See Chris Friesen's and Rod Sheridan's answers.
8.) See Chris Friesen's and Rod Sheridan's answers.

9.) Is there a difference between a motor wired for 120 volts only or 230 volts only versus a single motor that will give you the choice of wiring it 120 or 230 volts?
A single-voltage motor can make more efficient use of the physical space inside the motor. I suspect the difference is minimal.
10.) If my 10" table saw has a 1.5 hp motor will I improve it's performance if I replace that motor with a 2 hp motor??
This question has been asked many times. The answer is that generally the rest of the saw has been designed around the forces generated by the given motor. If your model comes with two sizes of motors, I would definitely feel safe switching to the larger of the two sizes -- e.g. 1.0 to 1.5 HP, 1.5 to 2 HP, 3 HP to 5 HP, etc.
There is no such thing as a stupid question.

[Jeff Bratt]

Questions like these are often asked and people are usually quite interested in the answers. So here's my set - to add to, or expand on, or complement other great answers also posted:

1) Dual voltage motor wired for 240V might perform better - IF the 120V circuit is deficient. The most noticeable change will be starting faster, as during start-up is when these motors draw the most current. If the 120V circuit can supply all the current required with negligible voltage drop, then there will be no noticeable performance difference.

2) Horsepower is speed x torque - so more horsepower means faster, stronger, or both. For any cutting tool, this means the tool can take out bigger chunks of wood faster. Or it can handle harder woods. Or turn bigger blades or make fatter dado cuts. Is it required? That all depends... There is lots of stuff that can change inside a motor - better or larger magnetic materials, or larger coils of larger wires, or better insulation that can stand higher temperatures. The basic idea is to get as much of the electrical current as possible to make magnetic fields that turn the rotor. Any electrons that don't go into making magnetism end up making heat - and heat is ultimate limitation when designing motors.

3) There is a precise definition of what horsepower is, and the induction motors we have been talking about are generally rated accurately. Could there be differences between rating systems? Sure, but I haven't seen many problems with these larger woodworking tools. To measure actual horsepower, you would have run a motor on a dynamometer. However, some manufacturer's marketing departments laugh at any realistic definitions of horsepower. Shop vacs and routers - which contain a completely different kind of motor - all have HP claims that are wildly over-stated - you can see my web page about those here (http://home.roadrunner.com/%7Ejeffnann/WoodWorking/Shop/HP/Horsepower.html).

4) There are several types of induction motors with different classes (temperature ratings) of insulation - however all these woodworking tools use basically the same kind of motor. Changing "classes" is not going to help performance. Other types are designed for higher starting torque (for starting under load), for higher stall torque, for continuous operation, or for constant start-stop operation, among other things. Types of motor enclosure relevant to woodworking are open frame, drip proof, or totally enclosed. This depends on the environment where the motor is mounted. Enclosure ratings like "washable, sterilizable, or explosion proof" generally don't apply to woodworking.

5) There is a magnetic field inside the motor dragging the rotor around. When the load on the motor exceeds the strength of that magnetic field, the motor will stop turning, or stall. When that happens, the "resistance" of the motor windings drop to a much lower value than when the motor is running - and this means a lot more current flows in the motor windings. The same thing happens when the motor is starting up - it's just that the start-up should happen very quickly. If the motor is stalled or prevented from turning for more that a few seconds, you will either trip a circuit breaker or quickly burn out the motor. Some motors also have a thermal sensor inside that detects heat buildup in the coils, and acts as a secondary defense against damaging the motor.

6) Some manufacturers do under-power tools - ratings and reviews can discuss this. You can compare the specifications of similar tools pretty easily. Tools marketed to the home users are almost always designed to plug into a 120V - 15A circuit. This effectively limits the motor size to 1 1/2 HP - maybe 1 3/4 HP is the motor is very efficient. Small-shop tools or industrial buyers have 240V or 480V or 3-phase power available, and have no such limitations.

7) Bigger is generally better - up to point. For a 10" tablesaw, 5 HP seems at least close to the upper end. However, lots of great woodworking is done with 1 or 1 1/2 HP saws - or totally with hand tools. Power is not everything. (Sorry Tim Taylor.)

8) Power equals current (amps) x voltage. Double the voltage means half the current will produce the same power. All wires have some resistance, so the higher the current, the more power is lost to heating up the wires. This power loss to heating increases as the square of the current, so it is very significant. This is why electrical transmission lines run at thousands (or hundreds of thousands) of volts - to lower the current flowing in the wires.

9) Inside a dual voltage motor, each winding is split in half. For the higher voltage jumper setting, the coils are in series - for the lower voltage, the coils are in parallel. The motor windings literally do not "know" what voltage the motor is connected to - the voltage on the windings is the same for either configuration. A single voltage motor is slightly less complicated by not having split windings and extra jumpers.

10) A higher horsepower motor could improve the performance of a saw. Or the trunions and other mechanical parts could just flex more when connected to higher powered motor. A single v-belt transmits somewhere around 1 HP, so multiple belts, or a flat belt are usually needed in higher HP applications. Answers to that question go way beyond the realm of motors...

Some motor links:
http://ecmweb.com/mag/electric_understanding_induction_motor/
http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee1/bdeee1_1.aspx

[Jeff Bratt]

To address the belief that the motor will start and get up to no load speed quicker on 240 volts. My understanding is this is true because the locked rotor current is halved vs 120 volts.

The starting current or "locked rotor" current is 3-5 times the rated or full-load current. This is why a regular 120V 15A circuit might have some trouble starting a motor, and all the lights dim briefly. Half the current is required for the higher voltage jumper setting of a dual-voltage motor, and 240 volt circuits usually aren't sharing current with other loads. Circuits designed for dedicated motor use have larger wire and are derated to accommodate starting currents. The largest current draw happens during start-up - this is why performance improvements at the higher voltage (if there are any) are most noticable then.

[Roger Frazee]

I don't think I'm going to be able add much to what is already been said. My goal in asking the questions was to get cumulative input from several people wrong or right and sort through it all so that one thread is available for members and those searching that covers a lot of FAQ and corrects some misunderstandings about motors.

Hey.... those guys over on Practical Machinist got some competition here!!!

Anyway hope maybe some more replies show up and answer some of the questions you get from the answers to my questions.

Thanks Jeff for the link to your website. Great information there....


Open question to anyone .....

Wayne this might be a good one for you to explain.

I'm trying to understand this slip thing that Wayne discussed. I know that lower horsepower induction motors of design b generally run about 5% slip. Is this slip percentage the rated rpm at the rated FLA?

For example the synchronuous speed for a 2 pole induction motor is usually 3600 rpm so is it the slip that is the difference between synchronous speed and the rated nameplate speed which is generally around 3450 rpm?

My understanding is that I have increasing torque once I pass the pull up point on a torque curve till I reach the breakdown torque at which the motor is very near synchronous speed or 'peak speed' . At no-load synchronous speed my torque is 0 ft-lbs. Now in order to produce useful torque and hp I have to load the motor and slow the rpm. When I reach the rated fla is that the point on the torque curve that I reach the max slip? Maybe someone has a link to a graph that shows all these relationships. I can't find one the has them all on the same page...:)

[Mike Henderson]

Slip - the difference between the speed of rotation of the magnetic field and the speed of rotation of the rotor - changes with the load on the motor.

Slip is what induces current in the rotor coils. When there's no load on the motor, the rotor is close to the speed of the rotating magnetic field. There must be some difference (slip) in order to overcome the friction in the motor.

As you load the motor, the slip increases. The rated speed of an induction motor is the speed at full load.

If you look at the torque curve of a typical induction motor, you'll see a peak about 2/3 of the synchronous speed. That peak is outside the normal operating range of the motor. If you load a motor sufficiently to slow the motor such that you reach the peak of that curve, you'll have excessive current in the motor.

Of course, if you do reach that peak (and don't unload the motor) the motor will rapidly slow down (it will slide down the left side of the torque curve).

Mike

[Roger Frazee]

Thanks Mike that makes sense....

After searching I did find a graph that is close to what I was looking for that has the relationships on one page...credit for the graph is bottom right


http://www.engineeringtoolbox.com/docs/documents/651/electric_motor_current_torque.png

[Jeff Bratt]

I'm trying to understand this slip thing ... Maybe someone has a link to a graph that shows all these relationships. I can't find one the has them all on the same page...:)

From the link (http://ecmweb.com/mag/electric_understanding_induction_motor/) I supplied earlier, an illustrative graph for a type B motor...

http://ecmweb.com/mag/405ecm08fig2.jpg

When an unloaded motor starts, it starts from A and goes on through through to point E very quickly. No-load speed is just slightly less than synchronous speed - 3600 rpm for a 2-pole motor, 1800 rpm for a 4-pole motor. As the load on the motor increases, it slows down - which means the slip increases, the current draw increases, and the torque increases, until you reach point D. The motor rated speed, current, torque, and slip are all measured at point D. Increasing the load slows the motor down more, and increases all the other things, heading towards point C. If you increase the load so the motor slows down past point C, the motor stalls.

[Roger Frazee]

Excellent Jeff...... thanks....

[Chip Lindley]

To add minimally to all that has been offered so far, I will leave the technical aspects to others. In my 25 years experience, most problems with machines run on a 15A, 115V circuit at home concern the fact that the circuit has other draws on it besides that contractor-type TS.

IF the 15A circuit were dedicated entirely to the saw, it still may or may not be sufficient. An old Rockwell CS with 15A (1.5hp) never failed to start for me on 115V, but did dim the lights. The saw shared the circuit with lighting and whatever else was plugged into the garage wall sockets. During heavy cuts, the branch circuit breaker tripped often. The overload button on the motor never tripped!

Just the opposite would occur with an *under-powered* saw. During heavy cuts, a *lite* motor on the minimal side of 1hp would trip the motor's overload before the branch breaker were tripped. Consumer CS-type saws are simply not intended to rip 8/4 oak! Consumers rip pine, plywood and mdf.

Practically speaking, if a dedicated 30A/115V circuit must be run to successfully run a 2hp motor on 115V, that motor might as well be switched to 230V, using existing 12ga wire. 230V circuits opens up many possibilities beyond the 1.5hp *barrier*....including running a rotary phase converter to power dirt-cheap 3-phase machinery!!

[Jeff Bratt]

I'll add a little more about motor overloading. If you look at this graph again:

http://ecmweb.com/mag/405ecm08fig2.jpg

you can see there is an area between points D and C where the motor will still operate (at a slower that rated speed), but is driving more than its rated load. These motors have a "service factor" that specifies that the motor can be operated 15%, 25% or possibly 50% above the rated horsepower for short periods of time. The graph in this example shows the stall point C at over 200% of rated horsepower. While the motor will run in this area, the ultimate limitation is heat. More current flowing in the motor windings means much more heat is being generated. The class of insulation used in the windings determines how hot the motor can get before it "burns out". And that is literally what happens - the insulation melts, the windings short-circuit, and the motor is toast. For a totally enclosed motor, you can understand why the cooling fan is important part of the design considerations.

The actual performance of each individual motor may vary slightly, but all should be within their nameplate ratings. And while these ratings are not absolute limits, operating a motor in its "overload area" for extended periods of times will eventually cause trouble. This is why some motors have thermal circuit breakers in them to try and prevent overheating the coils. Also starters or contactors for higher horsepower motors often have "heaters" in them calibrated to the horsepower of the motor being controlled. These monitor the current flowing through the motor and shut it down if overload conditions persist for too long.