What size transformer?

studdlymurphy

Hi All,
This is a US spec machine so runs on 110v I want it to work in Ireland so need a step up transformer. I just want to know what wattage of transformer to get.
From the plate i calculate 1440watts is this correct and also if that is the case what size transformer should I get 2kw just to be on the safe side?
Any suggestions would be great.
Thanks

Gate Automation

if you get 2kW you will be on safe site

jdesbt

traffos are rated in kva not kw.
your load of 1440watts is aprox 1.8kva so id go bigger than a 2kva traffo especially if its for continious use.

DublinDilbert

The handiest/cheapest thing will probably be a 3kva site trafo, fit a 110v yellow plug onto the appliance.

2011

What is the machine? If it is a motor it may have a high starting current so 2kVA may be it bit small. The synchronous speed may be reduced too as we are 50Hz here and that machine is designed for 60.

By the way it is a step down transformer that you need

Bruthal

jdesbt wrote: »

your load of 1440watts is aprox 1.8kva

How did you get that? If the actual amps the item takes is 12, then the kva is 1.44

Gate Automation

robbie7730 wrote: »

How did you get that? If the actual amps the item takes is 12, then the kva is 1.44

no, 1440W is 1.4 kW and its 1.8 kVA

http://www.dieselserviceandsupply.com/power_calculator.aspx

Bruthal

Gate Automation wrote: »

no, 1440W is 1.4 kW and its 1.8 kVA

http://www.dieselserviceandsupply.com/power_calculator.aspx

If a load is 1.4kw at parity pf, the kva is also 1.4 i would of thought myself.

1.4kw = 1.8kva when the pf = 0.8

If the actual amps of a device is 12 amps drawn at 120v, then the kva to power it at this 10 amps = 1.44

So that calculator you are using, is assuming a pf of 0.8 in a load powered by a generator by the look of it.

If a single phase motor takes 10 amps, and its voltage is 230v, then the kva needed to power it is 2.3kva.

If the motor pf is 0.8, then its kw (forgetting efficiency) is kva x pf = 2.3 x 0.8 = 1.84kw

So from 10 amps, its 2.3kva, but its pf means its kw = 1.84 So its kva can be derived from its actual 10 amps x 230v.

kva = kw/pf
or
kw = kva x pf

So to say 1.44 kw = 1.8kva just like that, is not right. It is only that, if the power factor is 0.8

Thats how i see it anyway.

cast_iron

Gate Automation wrote: »

no, 1440W is 1.4 kW and its 1.8 kVA

http://www.dieselserviceandsupply.com/power_calculator.aspx

That might be correct for generators that they are selling, where they must be assuming a max pf 0.77 for potential machines their customers may have.
It doesn't apply generally though. If it's a purely resistive load, the pf will be 1, and robbie's figures will be correct.
There's a danger of going with google's first answer.

Bruthal

cast_iron wrote: »

That might be correct for generators that they are selling, where they must be assuming a max pf 0.77 for potential machines their customers may have.
It doesn't apply generally though. If it's a purely resistive load, the pf will be 1, and robbie's figures will be correct.
There's a danger of going with google's first answer.

Indeed.

Even without it being a pf of 1, and without knowing its pf, if the appliance takes 10 amps at 120v, then the 1.44 = the kva. The kw will be less than or equal to this, depending on pf.

So anyway, a 2kva transformer should do.

Bruthal

2011 wrote: »

What is the machine? If it is a motor it may have a high starting current so 2kVA may be it bit small.

Id say its a treadmill maybe.

studdlymurphy

robbie7730 wrote: »

Id say its a treadmill maybe.

Correct, it is a treadmill.
Any recommendations on a good place to get transformers that would be suitable?

Bruthal

studdlymurphy wrote: »

Correct, it is a treadmill.
Any recommendations on a good place to get transformers that would be suitable?

Electrical wholesale shops i would imagine have them. They would be like the yellow site transformers like has been mentioned in the thread.

And then put a yellow 110v plug on the treadmill. A 2kva or bigger is what you will want anyway. They will probably be a 3kva one anyway as the smallest one.

Gate Automation

Thx for explain I think good but later jdesbt suggest something else and i follow this way, and that was wrong

DublinDilbert

I think there's a 3kva trafo for sale over on adverts.ie at the minute...

2011

Remember, many site traffos are not designed to be left powered up indefinitely.
Personally I would agree that a 3kVA would be most suitable.

Also your treadmill may rotate slower at 50Hz.
BTW a plug / socket arrangement will require RCD protection.

Bruthal

2011 wrote: »

Remember, many site traffos are not designed to be left powered up indefinitely.
Personally I would agree that a 3kVA would be most suitable.

Also your treadmill may rotate slower at 50Hz.
BTW a plug / socket arrangement will require RCD protection.

RCD on 110v 55v/55v would seem unnecessary really?

The treadmill motor itself will likely be a permanent magnet one. So the motor itself wont be governed by the frequency. Whether the electronics controlling it are, is another thing. Id say it will work as normal, except maybe the voltage is a little lower from the transformer than the rated 120v.

M cebee

the rules dont require rcd for 110v rlv sockets(reduced low voltage) rules 554.1.4 and 554.6

- you would have to fit 110volt 16amp 10mA rcbo's for each cct on 110volt ,they don't make anything bigger than 16amp capacity in a 110volt 10mA trip

http://www.handyhardware.ie/product/19885/Carroll-Meynell-5003-3-Socket-Intermittent-Transfo

this one for example is rated at 2.5kva continuous duty

or 5kva on a 25% duty cycle


the 3kva 'tool rated' trafo should be ok for the 1.4kva treadmill anyhow

Bruthal

M cebee wrote: »

the rules dont require rcd for 110v rlv sockets(reduced low voltage) rules 554.1.4 and 554.6

- you would have to fit 110volt 16amp 10mA rcbo's for each cct on 110volt ,they don't make anything bigger than 16amp capacity in a 110volt 10mA trip

http://www.handyhardware.ie/product/19885/Carroll-Meynell-5003-3-Socket-Intermittent-Transfo

this one for example is rated at 2.5kva continuous duty

or 5kva on a 25% duty cycle


the 3kva 'tool rated' trafo should be ok for the 1.4kva treadmill anyhow

I wouldnt say a treadmill would have a high duty cycle anyway. Not many would be on it all day. Might be on it a continous hour alright, but the 3kva will be fine id say.

Indieman

Hi all,
I've a similar problem with 3 garage doors. They were there when I bought the house and are top of the range so I'd like to keep them. They run on 110v though and I've them connected to a 3kva transformer constantly.
My question is; the transformer is using 130w of power constantly just to be on, is there a way round this??

DublinDilbert

That 130w is to magnetise and demagnetise the core of the trafo. It will consume this once it's connected to the mains.

You've 2 options;
1) modify / upgrade the systems so they work off 220v.
2) turn the trafo on/off as required, would need mods on the control side of things.

It will need someone with electrical and electronic knowledge. If you post schematics we might be able to give you some ideas. 150w constantly must just eat the electricity...

Bruthal

DublinDilbert wrote: »

That 130w is to magnetise and demagnetise the core of the trafo. It will consume this once it's connected to the mains.

You've 2 options;
1) modify / upgrade the systems so they work off 220v.
2) turn the trafo on/off as required, would need mods on the control side of things.

It will need someone with electrical and electronic knowledge. If you post schematics we might be able to give you some ideas. 150w constantly must just eat the electricity...

€40 euro per bill. A fair whack.

Bruthal

Indieman wrote: »

Hi all,
I've a similar problem with 3 garage doors. They were there when I bought the house and are top of the range so I'd like to keep them. They run on 110v though and I've them connected to a 3kva transformer constantly.
My question is; the transformer is using 130w of power constantly just to be on, is there a way round this??

How do you use the doors, remote control button in hand, or key switch etc?

Indieman

robbie7730 wrote: »

Indieman wrote: »

Hi all,
I've a similar problem with 3 garage doors. They were there when I bought the house and are top of the range so I'd like to keep them. They run on 110v though and I've them connected to a 3kva transformer constantly.
My question is; the transformer is using 130w of power constantly just to be on, is there a way round this??

How do you use the doors, remote control button in hand, or key switch etc?

I have a keypad outside the garage and I use keychain buttons.

I was trying to find info on batteries but no luck, the doors themselves use very little power to tick over 10-20w. Batteries with an ac converter charged for a couple of hours a day was my thinking.
The doors need about 800w to open and close for the couple of seconds???

M cebee

tricky one
it's not ideal to be switching the primary

would it be possible to interlock the doors and fit a smaller trafo

Bruthal

Originally Posted by jdesbt
the original post said 1440watts convert this to kva gives 1.79

The original post assumed multiplying 12 amps by 120v = 1440 watts, which it does if the power factor is 1. And as a rough estimate, this can be taken.

But the reality is, real time amps x volts in an AC circuit = VA, not watts. It will = watts if the pf is 1 though.

But there is no way you can say 1.44kw = 1.79 KVA without knowing what the power factor is. And there is no way something taking 12 amps at 120V can be 1.79kva. It sounds like an online calculator was used, that assumes a PF of 0.8

A 1.44 kw load does not just convert to 1.79 kva like a mile converts to 1.6 km

This 12 amp rating might be the nominal though, and it could vary.

If you want to convert KW to KVA, and vice versa, its this,

KVA = KW/PF
KW = KVA x PF

So as you can see, your answer assumes a PF of 0.8, and also assumes the 12A x 120V = 1440 watts, where as we can only be certain its actually 1440VA.

Edit: Had 10 amps instead of 12 amps throughout post.

2011

M cebee wrote: »

the rules dont require rcd for 110v rlv sockets(reduced low voltage) rules 554.1.4 and 554.6

Yes, you are 100% correct.

I was having a bit of a "moment" and thinking of the plug/socket arrangement upstream of the transformer. This RCD often trips when site transformers are plugged in. My post was not clear, my bad.

2011

robbie7730 wrote: »

The original post assumed multiplying 10 amps by 120v = 1440 watts, which it does if the power factor is 1. And as a rough estimate, this can be taken.

But the reality is, real time amps x volts in an AC circuit = VA, not watts. It will = watts if the pf is 1 though.

But there is no way you can say 1.44kw = 1.79 KVA without knowing what the power factor is. And there is no way something taking 10 amps at 120V can be 1.79kva. It sounds like an online calculator was used, that assumes a PF of 0.8

A 1.44 kw load does not just convert to 1.79 kva like a mile converts to 1.6 km

This 10 amp rating might be the nominal though, and it could vary.

If you want to convert KW to KVA, and vice versa, its this,

KVA = KW/PF
KW = KVA x PF

So as you can see, your answer assumes a PF of 0.8, and also assumes the 10A x 120V = 1440 watts, where as we can only be certain its actually 1440VA.

Agree.
This power triangle calculator illustrates Robbie's point well.

Bruthal

2011 wrote: »

Agree.
This power triangle calculator illustrates Robbie's point well.

(I had typo`s in my post of 10 insted of 12 amps:D)

But yes put the 1.44 kw and the 1.79 kva into the 2 orange boxes in that calculator, and it shows the PF of 0.8, which was obviously assumed in what ever one was used by posters getting 1.44 kw = 1.79 kva

An attempt at explaining it (2011 or another can probably do a better explaining job:)), but anyway, no harm trying it.

The relationship between kw and kva is simple enough. Amps sine wave and volts sine wave out of phase with each other. So if 12 amps is flowing, but its only 80% in phase with the voltage, only 9.6 amps is doing any work, so for kw, its this 9.6 amps by the voltage.

So for 120v, kw would be 9.6A x 120v = 1.152 kw (real power)

But a generator or transformer still needs to supply the 12 amps for the load. So its rating has to be 12A x 120v = 1.44 kva (apparent power), and to calculate suitable generators or transformers, the KVA is all we need to know.

So in this example, knowing the power factor is 0.8, 1.152 kw = 1.44 kva. But of couse for this treadmill, we dont know the pf, and dont need to.

It would be like if you have the 2 sine waves drawn out in their actual layout, and at any point in time, if they are 80% in phase, or pf 0.8, then multiplying amps x volts will only ever equal 80 percent of what they will if the 2 sine waves are perfectly in phase. This is the kw power. So while 12 amps is in the sine wave, over each cycle, only 9.6 is in phase when calculated against the voltage sine wave.

A transformer/generator though, has to be rated to carry the actual RMS amps of 12 amps, not just the ones 80% in phase. So they have to be rated in KVA.

M cebee

2011 wrote: »

Yes, you are 100% correct.

I was having a bit of a "moment" and thinking of the plug/socket arrangement upstream of the transformer. This RCD often trips when site transformers are plugged in. My post was not clear, my bad.

i notice now on the blakely website that they state standard rcbo's are not allowed on 110v rlv

-as they don't have overload protection on both poles

i could have sworn last time i checked they stated they could supply boards with rcbo's


30ma on 110volt circuits is for disconnections times, whereas the 10ma is for shock protection

so i could be wrong there-i used mcb's anyhow

2011

M cebee wrote: »

i notice now on the blakely website that they state standard rcbo's are not allowed on 110v rlv

-as they don't have overload protection on both poles

i could have sworn last time i checked they stated they could supply boards with rcbo's


30ma on 110volt circuits is for disconnections times, whereas the 10ma is for shock protection

so i could be wrong there-i used mcb's anyhow

I had a look at some of our 110V distribution boards at work following your post. This is a brand new large installation and we have DP MCBs on all outgoing circuits, no RCDs or RCBOs.