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by Strong Eagle » Thu, 08 Dec 2016 1:02 am
I do hope that PNGMK chimes in here as well. It's one thing to understand the theory of electricity, another to create a practical, usable, and safe wiring environment.
There are several things to consider in deciding what size of transformer to buy. First, the advertised amperage (15) is probably the peak design current draw of the motor under load. When the saw motor is running at full RPM under no load, it will draw very little current. The more you load the motor and slow it down, the more current it will draw until you reach the point that if you were to jam the blade and stop the motor, current could easily rise to hundreds of amps until the circuit breaker blows.
Since I’m sure you have used table saws in the past, you know that as you push lumber through it, you control the speed of the cut because you can sense when the motor is beginning to run too slowly. In my experience, most motors driving power tools can be slowed to where they draw more than their rated amperage for short periods of time… you push the lumber a bit too hard, the motor slows way down, then you immediately let up.
Another thing to consider is that, depending upon the design of the motor, the starting current will be two to five times normal operating current for a near instantaneous period of time.
Conclusion number 1: Although the saw is rated at 15 amps under load, it can certainly draw more for short periods of time when you overload it, and you need to account for that in the size of your transformer so that you are not blowing circuit breakers.
The next thing to consider is the “apparent power” of the motor. As you have stated in your post, the maximum wattage of the motor is 1,800 watts (15A x 120V). For resistive devices like a space heater or electric stove, this is the maximum power that will ever be drawn.
This is not the case for electric motors. In a resistive device, the sine wave for both current and voltage are exactly overlaid on one another over time. In an electric motor, the sine wave for the current leads the sine wave for voltage over time, and this lead increases as the load on the motor increases. The net effect is that although the saw only overall draws 1,800 watts, for each cycle of the sine wave it can have a higher rating. This is why larger motors are rated as “Apparent Power” in VA (maximum volts times maximum amps).
Conclusion number 2: Without getting into intense computations with data from the manufacturer, you should probably guesstimate a VA rating of 1.5 times the wattage, or a VA of 2,700.
My view then, is that you should purchase a transformer of at least 5,000 watts that is capable of taking a temporary surge of more than that. If you look at some of the transformers being advertised, they say 5,000 watts but that is surge, certainly not intended operating load.
Transformers are also inductive devices and because of the laws of electricity, it is guaranteed that there will be electrical losses in them that show up as heat. I would therefore be inclined to purchase a more expensive, high quality device. Properly made transformers use silicon steel, specially coated (not lacquered) for insulation to minimize other losses in the transformers. I suspect the cheap ones may not be as choosy about the construction.
Then again, maybe a cheaper one is OK. After all, you’re not going to be out on a jobsite 8 hours a day so the loads will be intermittent.
As to where to purchase: I think the quality is more dependent upon the brand than where it is purchased… they all seem to be made in Asia anyway. I see that some companies will ship from the USA for $70 to $100 which may still be cheaper than buying in Singapore. I’d consider buying in Malaysia… I don’t know where in JB but there is a huge electronics/electric mall in KL.
Finally, as to fuses or thermal protection… hard to say. You’d want slow blow fuses for sure for the momentary high draws at startup and overload. Thermal protection doesn’t do anything about current overload, except that the assumption is if you are running too much current the transformer heats up too much and the thermal disconnect kicks in. It seems that most of the more expensive units opt for fusing and not thermal protection.