1000W transformer to power a 300W tool?

Thread Starter

rambomhtri

Joined Nov 9, 2015
319
Hi, I wanna use an american power tool (wood jigsaw) in a 220V outlet. The jigsaw draws 2.4A and 110V, that's about 300W. I can only use a transformer I guess.

The problem is I have a 100W transformer and a 750W transformer, and I've been told that the power of the transformer should match more or less the power of the tool. If I use the 100W, it's going to get too hot, burn or even get broken, I guess. If I use the 750W T, I'm not sure what's going to be wrong, but I've read I should not use it.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
319
Sorry, I don't remember if it is 1000W or 750W, but I guess it doesn't matter in this case since it's more or less the same problem.

My question is why there's that rule of the power, and second, what can go wrong when you plug a 30W device into a 2kW transformer, to be extreme.
 

oz93666

Joined Sep 7, 2010
737
you should not use the 100W one ...

The 750W is absolutely fine ... that is just the MAXIMUM rating ... Any load below 750W is acceptable

I'm assuming the transformer is 1:2 ratio ....240 V in ...120 V out or visa versa.
 

LesJones

Joined Jan 8, 2017
2,687
There should be no problem using a transformer of a larger power rating than requred by the tool as long as the input and output voltages are correct.

Les.
 

AlbertHall

Joined Jun 4, 2014
10,243
The power rating of the transformer should be at least equal to the power rating of the tool. For a couple of reasons it is preferable if the rating of the transformer is a good margin above the tool rating.

Reasons for a bigger transformer:
Using a transformer (or any electrical/electronic component) at its maximum rating is not a good idea as it will reduce reliability.
Your tool may well draw much more current when you first switch it on. The bigger transformer will be better able to cope with that.

A bigger transformer will be more expensive so your 2kW transformer will cost a lot more (and be a lot bigger and heavier) than necessary for the 30W device, but it will perfectly happily work.
 

Thread Starter

rambomhtri

Joined Nov 9, 2015
319
Oh, really?
I was quite surprised when I first was told that a 300W tool should use a 350-400W transformer, because I couldn't think of anything wrong about using a higher W transformer. I'm pretty sure a few months ago the dependent of an electrical supply shop told this to me, that he didn't recommend the use of 750 or 1000 W transformers to use a 300W tool.
 

Alec_t

Joined Sep 17, 2013
11,526
I'm pretty sure a few months ago the dependent of an electrical supply shop told this to me, that he didn't recommend the use of 750 or 1000 W transformers to use a 300W tool.
Either he's in the wrong trade or he was being kind and trying to save you money :).
 

ian field

Joined Oct 27, 2012
6,539
Hi, I wanna use an american power tool (wood jigsaw) in a 220V outlet. The jigsaw draws 2.4A and 110V, that's about 300W. I can only use a transformer I guess.

The problem is I have a 100W transformer and a 750W transformer, and I've been told that the power of the transformer should match more or less the power of the tool. If I use the 100W, it's going to get too hot, burn or even get broken, I guess. If I use the 750W T, I'm not sure what's going to be wrong, but I've read I should not use it.
The only problem with an over generously rated transformer is; it needs more magnetising current that serves no useful purpose.

If you're going into production; a better suited transformer might eventually save enough electric to pay for itself - for a quick job and put the tools away; don't worry about it.
 

Janis59

Joined Aug 21, 2017
1,204
I have benn troubled with the similar problem vary hardly, as I bought from American soldiers the veeeery cheap and veeery good 2,2 kW Bosch big-size angle-grinder. Say about 1% of real price. But then I realized its 60 Hz and 110 V! The first reaction was create the simple phase-delay triac regulator thus the 110 V average will be at hands. Nope, the 3 kW triac was killed in the seconds. Then I applied the 50 Amp triac - it served about 5 minutes. So, now I am collectioning a large transformer cores, as bigger as better :) :) :)
 
Think about your issue. You plug your shaver into a 1 mega-watt power plant. There are transformers i that link, that reduce the avalaible power to the house. There is a main circuit breaker etc. Briefly, you can draw more power than the circuit is rated for.

You really care about VA or the product of voltage and current. When you buy a generator, the generator has to be rated higher than the induction motor. Usually, the rule of thumb is 3-5x more current. This only has to be applied for a brief instant.

A power tool is usually unloaded when first turned on. This reduces the inrush current.

You can guess this number better if you can accurately measure R of the motor. Take I=V/R as the inrush current.

One rule is "The current in an inductor cannot change instantaneously". So, initially, the line sees the DC resistance of the motor.
 

ian field

Joined Oct 27, 2012
6,539
I have benn troubled with the similar problem vary hardly, as I bought from American soldiers the veeeery cheap and veeery good 2,2 kW Bosch big-size angle-grinder. Say about 1% of real price. But then I realized its 60 Hz and 110 V! The first reaction was create the simple phase-delay triac regulator thus the 110 V average will be at hands. Nope, the 3 kW triac was killed in the seconds. Then I applied the 50 Amp triac - it served about 5 minutes. So, now I am collectioning a large transformer cores, as bigger as better :) :) :)
The motor will run a bit faster because UK mains isn't fast enough to produce the expected inductive reactance. It may impact on total life expectancy - eventually.
 

Janis59

Joined Aug 21, 2017
1,204
RE: Ian field
Somehow I see obvious that reactance is X(L)=2pi()*f*L. As the L is function of load and in free run is more or less constant, then reactance is as bigger as frequency LOWER. Americans are living on strange 60 Hz whilst we and even England inspite of illogic and all-around demolishing Brexit tendency, even inspite of illogic and wrong traffic direction still uses the very correct and round 50 Hz network, thus the reactance You will have BIGGER. Other difference except the network plugs not passing with any of lands in whole world and strange love for fuses into plug instead of wall is 240 Volts in wire, am I true? Other EU uses the 220V or at least 230V. Thus, Voltage indeed gives a bigger spindle turns, but it is boldly compensated by lower frequency. Thus, IF this machine would be designed for 60 Hz 220V then any English Gentlemen at 240V 50 Hz would have the rather normal workabout. But when the winding turn count is designed for 110V, then story is absolutely another - just motor and transformer have identical formula for turns count n=444*U/f/Sc. Thus if we now up to pain well that free run current I(o) in transformer looks like EXPONENT vs voltage, then why to think there is sth other in motor? Of course the machine, inspite being good without any defects, makes FULL SHORT in the 220 or 240 V network in no respect what frequency it has.
 
50 Hz requires the transformer to be heavier. So a 60 Hz device on 50 Hz may heat up a little more. 50 Hz devices on 60 Hz no real problem. There is a difference in speed. You sometimes see 50/60 Hz as a rating. The TS/OP is also using the tool intermittently.

To the OP. Even an unloaded transformer will have a surge current. So, the VA rating of the transformer Matching the VA rating of the circuit makes sense. Now it you had a 100,000 VA xformer on a 1000 VA circuit (Overkill), it might not work. 1500 VA on a 1000 VA would be OK too.

Matching the VA rating of the tool is the most economical.

Remember it is V * A; Volts * Amps, not Power in Watts. Surge current * Voltage has to be handled by the transformer.

The EU thing of a fuse in the plug would be a good thing, but I would not consider it essential in this application.
 

Janis59

Joined Aug 21, 2017
1,204
to KSS: ""The EU thing of a fuse in the plug would be a good thing""
1) It is absolutely nonsense the same way as nonsense is to keep different water pressure in both cold and hot water pipes - its just solely Englishmen problem. NOWHERE in Europe noone uses a fuse in the plug, because then plug becomes very large, clumsy, and expensive. Fuses are in the fuse box somewhere on or in the wall.
2) About the surge current. Ordinary async motor takes average 7x i(nom) in the surge. But I was telling not about this, but about those current taken by free rotating motor at nominal rpm without of any mechanical load or identically any transformer switched in network with no load in the secondary. That is the no-load current what, for concrete example I measured for ferrite transformer of 120 kVA in percantage of I(max) permitted by primary wire. 100V=0,001%; 200V=0,0015; 300V=0,0020; 400V=0,0028; 500V=0,008; 525=0,015; 535=0,05; 545=0,2; 555=0,8; 565=5; 575=99%. Do You now understand what I mean??
 
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