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Unread 07-11-2002, 05:34 PM   #2
John Bridge
Join Date: Mar 2001
Location: Rosanky, Texas
Posts: 68,417
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Andy (Felker/Target Tile Saws) has posted this in the Pro Hangout. In order to have it handy for reference, I\'ve copied it and placed it here. It\'s an excellent article.


The purpose of this post is to provide as much pertinent information as I can about electric motors and tile saws so that anyone contemplating a purchase can make an informed decision. Motor types that might typically be found in tile saws are permanent magnet (DC), universal (AC/DC) and AC (Single Phase). Most of what I discuss here is slanted towards the larger tile saws with 1.5hp and up motors, though everything would also apply to the smaller less powerful motors used on portable tile saws.

For our purposes we should consider a few things when looking at a motor. Horsepower is the most sited factor but it is only one of several that should be considered. A motor’s Service Factor, Code, and Duty, also play important roles in how that horsepower will be applied to the material being cut on the tile saw.

The first thing to remember is that horsepower ratings can be deceiving, very deceiving.
All 1hp or 1.5hp motors are not equal and will not cut tile with the same efficiency or speed. Just thinking about the different physical sizes of motors you have seen that are rated the same horsepower will tell you how this can be true. To truly begin to understand what the horsepower rating means you have to understand how that number is assigned and what factors are considered in assigning it.

Horsepower is Torque x speed / 5250, torque is in lb-ft, and speed is rpm’s. The important thing to note here is that the rated horsepower is calculated at one specific motor speed. (And also pulling a certain assigned amperage, which we will discuss later.) When the motor is working, as in cutting through a tile, the motor speed will likely be different each time, which means the actual horsepower will be different. That is all I am going to say about that because I read it in a book and we don’t have to fully understand it to make a buying decision. If everything is equal, there wouldn’t be much else to say and comparisons of motors would be easy, but, everything isn’t equal so we need to know more.

On the serial number plate of the motor, besides horsepower there are several other factors listed that affect performance significantly. The first we will discuss is called Rating. On tile saw motors you may see ratings listed as Saw Duty, Intermittent or Int with a number after it, such as Int 30, or continuous or cont.

In order for a saw to be rated Continuous Duty, it must be able to pull the rated horsepower all day, every day, without overheating or shutting down. This is the best situation to have, in my opinion. Though you will not cut tile all day everyday, you could, and you won’t put your saw through anything it cannot handle.

To be rated Intermittent Duty a motor must be able to supply the rated horsepower for 5 minutes, 15 minutes, 30 minutes or 60 minutes straight without overheating or shutting down. The minutes will be noted after the intermittent rating, such as “Intermittent 30”. This is still a good option because, cutting tile, you should not put your saw through anything that it couldn’t handle.

The Saw Duty rating, means almost nothing. To be saw duty rated for a certain horsepower, a motor needs to hit that horsepower rating for just a very short time period. An example would be a common table saw used to cut through a 2X4 stud. It does not have to be able to maintain it without overheating or shutting down at all. It just has to hit that horsepower for that second. With these motors it is hard to tell what you are getting. They are usually lower cost, lighter duty motors. They may or may not perform well in tile applications, but there really isn’t a good way to know for sure except for trial and error.

One way to play games with horsepower ratings is to rate a motor as saw duty or intermittent duty with a higher horsepower than could be used for continuous duty. How a motor is rated is up to the manufacturer or OEM of the saw. A motor that might be rated as ¾ hp continuous duty may also be able to be rated 1hp Intermittent 15, or 1.5hp Saw duty. This is legal and in many cases is left up to the OEM to decide how the motor should be rated on their application. A manufacturer may choose to rate a higher horsepower and use a different duty rating in order to advertise the higher horsepower.

Another trick is to rate a motor at high horsepower using a high amperage draw that is not readily available when using the saw. When a motor is rated, the horsepower is given on the serial # plate and the amps that the motor uses to achieve that horsepower are given also. I have seen a small motor saw duty rated at 2.5hp, listing the amps as 30. It would pull 2.5hp, but did it by drawing 30 amps on a 110 circuit. Of course, you cannot easily find a 30 amp circuit breaker on a 110 circuit, so, when the saw was used and put under load, and tried to draw 30 amps to provide 2.5hp, it blew breakers. Now, the manufacturer told the truth. If you could find a way to provide the 30 amps you could get the 2.5hp, you just would not be finding the 30 amp circuit to use in normal everyday tile cutting locations. And even if you could find a 30 amp circuit, all wiring, including any extension cords, would have to sized for 30 amps, which would be 10 gauge or larger. Try to look for a motor with amperage listed under 15 so that it will run on common circuits. Certainly, amp draw should be under 20 amps. Anything higher than this will be hard to run in everyday applications without tripping breakers.

When cutting tile, torque is also important. Some granites and porcelains can be pretty tough to cut, and the higher the torque a motor is able to produce the faster the cut may be made. Motors sold in America have a rating listed under “Code” on the serial number plate that helps to determine the amount of start-up torque a motor has. A letter designates it. The later a letter appears in the alphabet, the higher the torque of the motor. On larger 1.5hp saws, a rating of “J” or better works well. It is possible to have a tile saw with a higher horsepower motor, but lower start-up torque code, which will not cut tile as effectively as the lower horsepower saw with better start-up torque. This is possible because under load the motor with the lower start-up torque will not be able to maintain rpm’s as well and will lose cutting power.

Another rating to check is the Service Factor. The service factor is listed as a number that designates how well a motor can handle being overloaded on a continuous basis. A motor that has a service factor of 1.15 should perform when 15% overloaded without overheating or damaging the motor. When no service factor is listed, the service factor is assumed to be 1.0. However a 1.5 hp with a 1.15 service factor is actually a 1.725 hp motor (1.5 X 1.15). It is good to have that reserve capability in an application like tile sawing.

These are not all the design factors involved or listed on a motor, but they are the most important in tile saw applications, at least in my experience covering over 20 years in the business. (Ouch, that’s way too many!) If you have any questions or comments feel free to make them. My whole purpose here is to pass on information that I have found useful to tile contractors in the past. The better informed you are the happier you will be with your purchase. Much of this information is overlooked when making a buying decision because it is not readily available to the user. Now, hopefully, it is.
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