Transformer Size

[lamby38]

Hi all,
I have a second saw that's not working. Just want to check I have the correct transformer size, can someone check my calcs?

Motor 110v, 24amp
110v*24A = 2.64kVa.
So my 3.3kVa transformer should be enough?
Another point is it say 15amp on the transformer, yet the motor is 24amp. I'm a bit confused.
Any help welcome.
Ty

 

[riveralt]

Now might be a good time to dig out the manufacturers instructions to be sure.

 

[westie101]

Not forgetting the up to 7 times starting current if the motor is DOL. That might just put a hole in the transformer secondary volts

 

[lamby38]

Thanks chaps,
It's a brick saw but I can't find any details online. I never thought about the start-up current for the motor, very good point. It's an eBay purchase, seller says it works and i'll need a 5kVa transformer to drive it. There nothing happening from the 3.3kva tranny, the transformer still works after I tried the saw. This is going to be a learning curve.
Best/easiest/cheaest way to test? If I don't have a 5kva Tranny?

 

[flameport]

Another point is it say 15amp on the transformer, yet the motor is 24amp. I'm a bit confused.

15A is the input current of the transformer at 220V.

 

[lamby38]

Another point is it say 15amp on the transformer, yet the motor is 24amp. I'm a bit confused.

15A is the input current of the transformer at 220V.

How does that relate to the current or load from the unit being powered?

 

[lamby38]

The seller told me I'd need a 5Kva transformer but the unit has a 16amp 110v plug on it.
Yet a 5kva transformer needs a 32amp plug, I think.

 

[Adam_151]

It varies inversely proportional to the voltage step

Eg. Primary amps x primary volts = Y VA
Sec amps x sec volts = Z Va


The VA figures in an ideal world would be identical, however transformer losses mean they are not

So if Z VA = 2640
Assume a bad loss of 10% for demonstation purposes so Y Va is equal to 2640 x 1.1 = 2904

Primary volatage is 230 so 2904/230 = 12.62 Amps on the primary side assuming the 10% loss (its probably a lot less than this)

 

[ban-all-sheds]

It's a brick saw but I can't find any details online.

Is it 110V because it's designed for UK construction sites, or because it's designed for the US market?

It's an eBay purchase, seller says it works and i'll need a 5kVa transformer to drive it.

Presumably he knows what he's talking about, as he used to use it.

There nothing happening from the 3.3kva tranny

Possibly because it can't supply enough current for the motor to start?

If I don't have a 5kva Tranny?

If the saw works you are going to need one, so why not get one?

 

[JohnW2]

If I don't have a 5kva Tranny?

If the saw works you are going to need one, so why not get one?

Or, perhaps better, try to borrow, or even hire, one, to make sure that the saw actually does work when powered by such a transformer - before potentially wasting even more money.

Kind Regards, John

 

[lamby38]

Its 110v for uk construction, it's a big old industrial looking thing so maybe a 5ka would make more sense. I've a mate who has one so just need to organise to borrow. I just thought there would be a way to easily calculate.
Seller says he used a 5kva Tranny with a fly lead to 16amp plug to test, this is the reason I'd assumed okay to use a 3.3kva tranny.
I guess it would work with a 16amp plug but not very wise to use with that plug, it's gonna carry to much current.

 

[JohnW2]

I guess it would work with a 16amp plug but not very wise to use with that plug, it's gonna carry to much current.

Quite. You indicated that the running current is around 24A (and the starting current probably a lot more than that) - so a 16A plug would be appreciably stretched beyond its intended operating conditions. As you say, it would probably work fine, but they don't give 'ratings' to things like plugs and sockets just for the hell of it!

Kind Regards, John

 

[lamby38]

I guess it would work with a 16amp plug but not very wise to use with that plug, it's gonna carry to much current.

Quite. You indicated that the running current is around 24A (and the starting current probably a lot more than that) - so a 16A plug would be appreciably stretched beyond its intended operating conditions. As you say, it would probably work fine, but they don't give 'ratings' to things like plugs and sockets just for the hell of it!

Kind Regards, John

Thanks John.
A 5kva tranny seems to be rated up to 32amps which less tha the 24on the motor plate, I'm still trying to understand this start-up current, is a 5kva tranny going to have enough load to get the motor going. Should have paid more attention in my physics class lol

 

[JohnW2]

Thanks John. A 5kva tranny seems to be rated up to 32amps which less tha the 24on the motor plate ...

I guess you mean 'more', rather than less! In any event 5kVA is around 45A at 110V, so ought to be more than enough for the "24A" running current.

I'm still trying to understand this start-up current, is a 5kva tranny going to have enough load to get the motor going.

No-one can really tell you that (which is why you need to try a 5kVA tranny and see). A transformer will generally be able to cope with a starting current much higher than it can supply continuously as a running current, so it's quite possible that it will start with a 5kVA one - particularly since that's what you've been told!

Kind Regards, John

 

[bernardgreen]

There are two main factors that limit the power that a transformer can cope with.

In simple terms

Resistance heating of the wires in the coils is the one most quoted. Ths determines the size of the wire used to wind the coils.

Saturation of the magnetic core is just as important but seldom mentioned. This determines the size of the magnetic core.

Resistance heating is slow acting and thus will allow the transformer to be over loaded for very short durations such as motor start up currents.

Magnetic saturation is instant and limits the amount of power that can be passed from the input winding to the output winding.

The power limit due to magnetic saturation is ( in decent quality transformers ) several times the rated continuous power set by the resistance heating in the coils. This allows for short duration over loading to occur without saturation occuring. This requires a magnetic core that is larger than is required to transfer the maximum rated power.

Transformers where the magnetic saturation occurs just above the rated maximum will not allow any more than the rated power and will not cope with motor start up currents. This will invariably mean the motor unable to take the power it need to accelerate to speed will take a long time to reach normal speed. This prolonged run up time may damage the motor and/or the transformer.

When the power exceeds the magnetic capability of the core then stray magnetic fields may induce eddy currents in metal around the transformer leading to heating of these parts. The metal case being one of them.