Singapore Expats Forum

Voltage Converter (Step-down)

Discuss about computers & Internet. Including mobile phones, home appliances & other gadgets. Read about Windows security risks or virus updates.
User avatar
sundaymorningstaple
Moderator
Moderator
Posts: 35106
Joined: Thu, 11 Nov 2004
Location: Still Fishing!
Contact:

Re: Voltage Converter (Step-down)

Postby sundaymorningstaple » Wed, 07 Dec 2016 2:33 pm

SE, you are the electrical guru here so I've got one for you. I'd also like a recommendation if you have one.

I want to buy a portable site table saw. Like the DeWalt DWE7491RS Jobsite Table Saw for example. It only comes in a 110-120v model (and still keeps the dado blade functionality - UK version cannot do a dado blade). If I understand correctly, the wattage of that unit would be around 1800W (I think) Is the wattage calculated by Amp x voltage? (15A x 120V). For sustained usage here I would need a stepdown transformer in the neighbourhood of at least 2700w (3000W) I think using a 50% increase for startup surge (no soft start) should be sufficient. Or do I even need something that big? I've seen several types of stepdown transformers but the costs for something in the 3000W range is enormous (anywhere from $200 to $1500 SGD). Don't know if I'm looking at the wrong stuff or not. Which is better, fused or Thermal Cutout (I would have thought that the thermal cutout would be better as you wouldn't be caught without fuses. Can I buy a decent one here or is it better from the US (from a safety perspective)? (Shipping is an additional killer cost considering the weight of some of those units)

Recommendations? Do I need a unit that big or would 2000w be sufficient?

User avatar
Strong Eagle
Moderator
Moderator
Posts: 11050
Joined: Sat, 10 Jul 2004
Location: Off The Red Dot
Contact:

Re: Voltage Converter (Step-down)

Postby 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.

User avatar
sundaymorningstaple
Moderator
Moderator
Posts: 35106
Joined: Thu, 11 Nov 2004
Location: Still Fishing!
Contact:

Re: Voltage Converter (Step-down)

Postby sundaymorningstaple » Sun, 11 Dec 2016 12:21 am

Interesting. Thanks, SE. My initial thoughts were a 1.5x over the rated wattage which is where I got my 2700W from. Yeah, I've got a whole workshop back on the farm but never needed to get into the electrical side of transformers so it a new game for me. When I lived in Seletar Camp, all my power tools were local/Asian so were all 220-240 anyway. But for the life of me I cannot find a jobsite table saw over here. (closest is Australia, but it would be cheaper to fly to Aus and pick it up and hand carry it back cause their shipping is worse than the US and Canada (which is worse than the US as well).

What is interesting it that the most expensive transformers seem to be Thermal cutouts and the fused ones decidedly cheaper. No idea why - maybe because all the Thermal ones are made in the US? Maybe I'll tap PNGMK as well as he may know where to buy a decent Transformer that will suit my needs.

User avatar
x9200
Moderator
Moderator
Posts: 9313
Joined: Mon, 07 Sep 2009
Location: Singapore

Re: Voltage Converter (Step-down)

Postby x9200 » Sun, 11 Dec 2016 7:35 am

sundaymorningstaple wrote:SE, you are the electrical guru here so I've got one for you. I'd also like a recommendation if you have one.

I want to buy a portable site table saw. Like the DeWalt DWE7491RS Jobsite Table Saw [..] UK version cannot do a dado blade).

The restriction is because of the EU regulations, but I wonder if this is just that or also some differences in the actual design (i.e. shorter arbor).

User avatar
sundaymorningstaple
Moderator
Moderator
Posts: 35106
Joined: Thu, 11 Nov 2004
Location: Still Fishing!
Contact:

Re: Voltage Converter (Step-down)

Postby sundaymorningstaple » Sun, 11 Dec 2016 3:24 pm

x92, it's the arbor. It is shorter because of EU or UK (not sure which) regulations. The funny part is most of them sold in the UK are still 110-120V so need a stepdown converter as well, but the have a different motor with a short arbor. I already looked into that one thoroughly. That why I'm still considering a US 110V which has the longer arbor and the proper safety covers, and splitters to cover dado cutting.

johnkor
Newbie
Newbie
Posts: 6
Joined: Tue, 01 May 2018

Re: Voltage Converter (Step-down)

Postby johnkor » Tue, 19 Jun 2018 1:10 am

you should purchase a transformer of at least 5,000 watts

User avatar
PNGMK
Governor
Governor
Posts: 5668
Joined: Thu, 21 Mar 2013
Location: Sinkapore

Re: Voltage Converter (Step-down)

Postby PNGMK » Tue, 19 Jun 2018 2:03 am

johnkor wrote:you should purchase a transformer of at least 5,000 watts


You do realize the maximum you can draw from the standard Singapore socket is about 2,500 watts? I agree with uprating but that's over the top. Also what type of transformer? Not one of those shitty auto-transformers from Sim Lim.....
IANAL. IANACPA. IANA Teacher.
Arrested? Lawyer Up
International School job? School website or http://www.ISS.edu
School advice? Avoid for profit schools
Tax advice? CPA
Pay me $5k for 100% success filling in PR form!


  • Similar Topics
    Replies
    Views
    Last post

Return to “Computer, Internet, Phone & Electronics”

Who is online

Users browsing this forum: No registered users and 1 guest