Some Basic Motor Insight

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Forum topic by TheWoodenOyster posted 08-29-2014 12:25 PM 1087 views 0 times favorited 18 replies Add to Favorites Watch
View TheWoodenOyster's profile


1275 posts in 1356 days

08-29-2014 12:25 PM

Topic tags/keywords: motor

Hey everyone,

I have had power tools for about 3 years now and I am getting to the point where I want to know more about motors. Specifically, I am talking about the motors that run my table saw, band saw, jointer, planer, drill press, etc. I have some questions that I know some of you have answers to. I’ll try to keep this short. If there is a book anywhere with this sort of stuff in it, I would be glad to hear about it, but I am trying to avoid mumbo jumbo textbooks. I am looking for answers that apply to woodworking motors, not necessarily motors in general, which is probably what I would find in most books.

1. How does one “burn out” a motor prematurely?

2. I think I understand that most motors run at 3450 RPM or 1725 RPM, and at 110 or 220v. I assume the amperage pull is based on these two factors. Assuming that is true, does a 1 HP motor run the exact same as a 3 HP motor until it gets to 1 HP of capacity? And at that point, what makes it max out where a 3 HP motor will continue to increase in HP?

3. I have an old iron powermatic with a 2 HP, 220 motor in it. I might be soon installing a new motor (see question 1…ooops). Could I install a higher HP motor to increase the cutting capacity of my saw? I figure if I am going to put a new 2 HP in, I might as well go with 5 HP for a little extra money.

Thanks again.

-- The Wood Is Your Oyster

18 replies so far

View ChrisK's profile


1794 posts in 2502 days

#1 posted 08-29-2014 12:38 PM

Here is a little light reading:

One answer to question 1: If you allow the motor to bog down, i.e. slow down while pushing wood through the saw, for too long the motor will pull more current than the windings are designed for and the wire insulation overheats and burns through. This can cause windings to short and ‘burn’ the motor out. If the motor is allowed to stop completely, or stall, the windings can overheat very quickly. The proper size circuit breaker can help to prevent this.

As far as up sizing the motor, you need to make sure that all the parts that spin the blade can handle the extra torque of the motor. Going from 2 to 3 hp will most likely be OK. You need to inspect all of the bearings, shafts, belts and such to make sure there are no cracks, splits and the like.

-- Chris K

View ChrisK's profile


1794 posts in 2502 days

#2 posted 08-29-2014 12:45 PM

Here is some easier to read info:

-- Chris K

View Minorhero's profile


372 posts in 2026 days

#3 posted 08-29-2014 01:48 PM

Electric motors unlike combustion motors do not lose horsepower overtime. If your motor is starting up and not smoking then it doesn’t matter how old it is, its still giving you the same hp as when it was new. Thus there is no reason to buy a new motor unless you really just want to spend the money.

That being said, if this is a 30+ year old saw you might want to replace bearings. This is just good maintenance practices and its pretty easy to do.

Another thing to take into account is that if you upgrade to a 5hp motor is whether it can physically fit in your saw. It is pretty much guaranteed to be larger.

View DocSavage45's profile


7656 posts in 2263 days

#4 posted 08-29-2014 05:23 PM

Great information here. Maintenance is also important. How often do you use the saw? Do you have adequate wiring to your shop as more HP requires more current and expense in running the equipment.

Check with manufacturer or supplier to find out if you can upgrade? Is there an upgrade? I thought about and checked out a 5 hp cabinet saw as opposed to a 3 hp saw. Mountings and other parts as stated above can change your thinking. LOL! Staying w/my 3 hp.

Mechanical things are easy to do if you understand mechanical things? LOL!

-- Cau Haus Designs, Thomas J. Tieffenbacher

View shampeon's profile


1705 posts in 1604 days

#5 posted 08-29-2014 06:01 PM

It’s far more common for a typical capacitor start 1 phase motor to have a bad capacitor or starter switch than burned out windings. Capacitors will degrade over time, and various factors accelerate that. A well used motor might have enough dust and gunk to cause bad connections to the starter switch or other contacts. Someone else mentioned bearings, which will eventually seize.

But all of those things can be pretty easily replaced or repaired.

Regarding the voltage and HP of the motors, you have to make sure your circuit can handle the amperage draw of a 5HP motor. That and the size of the motor is really your limiting factor.

-- ian | "You can't stop what's coming. It ain't all waiting on you. That's vanity."

View Crank50's profile


170 posts in 997 days

#6 posted 08-29-2014 07:09 PM

Everyone pretty much covered the burned out and replacement questions so I’ll try to address the “what is the difference between a 2hp and 5hp motor” question.

A motor is determined to have 1hp if it could lift 550 lbs 1 ft in 1 second as the English unit is defined.
Also, the horsepower is equal to approximately 746 watts.

So, since a HP is a unit of power determined by torque(ft lbs) and speed(RPM) you have to allow that if you change one of these units, the other will change inversely. Increase the speed and the torque is reduced to keep the same horsepower. So a higher RPM motor will have a lower torque rating than a low RPM motor of the same horsepower. In either case, as long as the horsepower rating is the same the watts of electrical power consumed will be the same, about 746 watts.

What makes a motor be a 5 HP motor instead of a 2 HP motor? To get to the bigger rating several things are different. The shaft might be larger to handle the greater torque. The diameter of the motor’s armature might be larger to generate that torque. Or, the wires of the windings might be larger and have more wraps in the coils to increase the wattage. The reason I say these things MAY be different is that different things can be changed in different ways to come up with the same results. Everything is a balancing act here. You might use higher temperature rated insulation and get the same power with smaller wire, but it will be running hotter for instance.

Over all I will concur with the suggestion that you probably could go to a 3 HP motor from a 2 HP likely without any problem. Just check everything good, maybe replace the belt with a new one, and be sure the RPM stays the same. If your saw has a magnetic starter it might have some overload protection that would have to be upgraded as well.

And, as said earlier, unless you stalled the old motor or overheated it with a long duration overload or blocked it’s cooling air flow, there is a chance the only thing wrong with it is a sawdust mucked up internal start switch or a bad capacitor. Both of which are easy, inexpensive repairs.

View TheWoodenOyster's profile


1275 posts in 1356 days

#7 posted 08-29-2014 10:06 PM

Wow! Thanks for all of the input guys. Thinking about it now, a 5 HP motor might be sort of speculative, as an English soccer commentator would say. 3 HP on the other hand might be a viable option.

I guess I will have to just turn the saw on to see if I burnt the motor out. I let it cool down last night after it overheated, and will probably be in the shop tomorrow to do the test run. It did bind up for a minute and smoke a bit which scares me, but that could have just been the belt slipping. I just put a new link belt on it a week or so ago.

I’ll check out the bearings out too and see how they look. It is a 1965 powermatic, but I think the motor in it probably isn’t original. It looks too new.

-- The Wood Is Your Oyster

View Grandpa's profile


3256 posts in 2096 days

#8 posted 08-30-2014 03:27 AM

That saw probably came out with a set of belts. Probably 3 belts. Link belts don’t do well replacing matched sets of belts. IF you have a 2 or 3 groove sheave you should buy a matched set of belts. I would look into the motor or have someone clean it and see if that is the problem. It should be a Totally Enclose Fan Cooled. TEFC on the data plate. A good 2 HP motor will cut a lot of large hard wood. Have that motor checked.

View Loren's profile


8159 posts in 3069 days

#9 posted 08-30-2014 03:36 AM

3hp is more than adequate for a 10” cabinet saw
running modern, maintained blades. 5hp is
a benefit on a 12” or 14” saw. It depends on
what you want to do and how much you want
to invest.

View Tootles's profile


780 posts in 1923 days

#10 posted 08-30-2014 03:41 AM

1. Already some good answers about how you “burn out” a motor. It is all about overheating the windings to the point that the insulation breaks down and a short circuit occurs.

2. Maximum speed is dependent only on the frequency of your supply (60 Hz in the US, 50 Hz in many other countires) and the number of “pole pairs” that your motor has – this is the “synchronous” speed. A pole pair is what you find in any magnet. For example a bar magnet would have two poles – one “north” pole and one “south” pole, and this is one pole pair. This means that the same motor runs faster in the US (60 Hz) than it would in Europe or Australia (50 Hz). One of the biggest implications this has is on the cooling of the motor, which brings us back to why motors burn out.

Short answer: for the same cutting “load” (up to 1 hp load), the 3 hp motor is probably running slightly faster than the 1 hp motor, but it is not something that you would notice. Long answer, keep reading:

Motors don’t really run at their synchronous speeds, they run a little slower. This is because they don’t actually deliver any torque (i.e. power) at their synchronous speeds, they only deliver torque at speeds slower than synchronous speed. So when you run the saw without cutting anything, the speed is slightly lower than synchronous speed – just enough to overcome the electrical and mechanical losses that occur. As you cut, the speed decreases further and the torque increases proportionally up to the maximum torque. But if you overload the motor and slow it down even more, the torque actually decreases and you will soon stall the motor. A stalled motor stops the fan, which stops the cooling but it still has current flowing to cause heating. That’s a problem.

A 1 hp and 3 hp probably do have a similar speed at which maximum torque is delivered, and it is not much less than the synchronous speed. So for the same cutting load, the 3 hp motor runs fater than the 1 hp motor. Sorry – I did say that it was a long-winded answer.

3. As already stated, the decision of whether to increase the power of the motor will be based largely on mechanical factors rather than simple electrics. Will the saw physically be able to deliver that power without causing damage somewhere? Will the drive belt handle deliver the load without snapping? Will the motor physically fit? Will the bigger motor be able to cool itself adequately? And yes, will your circuit, switches etc handle the extra current? At 220 V, 2 hp draws almost 7 Amps, which goes up to 17 Amps at 5 hp.

Good idea to check if it runs again after letting it cool down. You may have simply tripped an internal thermal overload that is there to prevent the motor from burning out.

If it does not run, it is worth checking the starting capacitor. There is typically a centrifugal switch in the motor to remove the capacitor from the circuit when the motor is running, but if you stalled the motor, the capacitor would have come back into play. It could be that which is damaged.

-- I may have lost my marbles, but I still have my love of woodworking

View TheFridge's profile


5676 posts in 907 days

#11 posted 08-30-2014 04:17 AM

A larger hp motor has bigger winding with a stronger magnetic field producing more torque which translate into more hp. Most motors in the 600-250v class have no difference in insulation of their windings. It’s all in the # of windings and the gauge of the wire in the windings that add up to hp.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View TheWoodenOyster's profile


1275 posts in 1356 days

#12 posted 08-30-2014 05:39 AM

For the record I was milling green pecan and this is a bandsaw. I suppose I should have clarified that in the original post.

-- The Wood Is Your Oyster

View runswithscissors's profile


2127 posts in 1446 days

#13 posted 08-30-2014 07:33 AM

Maybe I’ve got this wrong, but my thinking is that a greater hp motor doing the same work as a lesser hp one (such as ripping a 2” thick piece of oak), will not be stressed as much. In other words, shouldn’t you get greater life out of a higher hp motor if it doesn’t have to work as hard to do the same job?

Which is why I plan, when I get around to it, to replace the 9 amp at 110 volt (supposedly 1 hp) motor on my Performax sander with a 1 hp rated (at 12 amps, 110v) motor, rewiring it to run off 220. (both are 1750 rpm motors). I already have that motor, and am tired of the overload switch tripping frequently on the original, even when taking very light passes.

As I said, maybe my thinking is wacky here. Feel free.

-- I admit to being an adrenaline junky; fortunately, I'm very easily frightened

View DocSavage45's profile


7656 posts in 2263 days

#14 posted 08-30-2014 04:27 PM


As discussed above there may be a number of factors with the existing motor. Shouldn’t be having an overload from light passes? I’d check, bearings, belts and housing for sawdust . Does it have a magnetic switch? Could be a problem. Is the shut down on a ground-fault? I’ve had faulty ground faults trip often. Starting capacitor can also be an issue.

-- Cau Haus Designs, Thomas J. Tieffenbacher

View shampeon's profile


1705 posts in 1604 days

#15 posted 08-30-2014 05:32 PM

So milling green logs on a bandsaw changes things slightly. I don’t know what your capacity is on the bandsaw but I would think a 2 HP motor would be ok if you have the proper blade on it and you weren’t pushing too hard. A typical thin kerf blade won’t work for green lumber. A higher HP motor would be a good call if you’re going to routinely be resawing the full height. But after getting the motor in order, I’d look for a super-low tooth count blade specifically for resawing green lumber.

-- ian | "You can't stop what's coming. It ain't all waiting on you. That's vanity."

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