110v transformer question

[Mark A]

Hi chaps,

Would a 110v 2400w 9" grinder run off my 3.3kva 16a transformer, or will the tool draw too much current?


Mark

 

[John Brown]

Since 1 kVA is equivalent to 1,000W (for all intents and purposes...), I'd say you should be fine.
Full disclosure - I'm not an electrician.

 

[Mark A]

That's why I'm not certain.

By my rudimentary grasp of electrickery:

3300VA ÷ 110v = 30amps intermittent use.
2400w ÷ 110v = 22amps.
22 is a bigger number than 16, but smaller than 30.


Still don't know.

 

[DTR]

2400W @ 110V will draw 21.8A, so I'm going with no.

Having said that, 2.4kW is a maximum rating. In practice it may pull far less than that when not heavily loaded. So it may run fine for some jobs, but trip the transformer's overload (assuming it has one :shock: ) if pushed hard.

 

[Mark A]

That's what I'm thinking. Maybe it would require a 32amp transformer to run without issues, which I don't have.

 

[John Brown]

The transformer, as I understand things, increases the current while decreasing the voltage(that's how you can buy an arc welder that puts out 250 Amps and runs from domestic mains). In an ideal transformer, these two factors would be equal and inverse, i.e if you halve the voltage with a transformer you double the current. In the real world there are always losses involved, which manifest themselves as heat(mainly).
Full disclosure - I did used to be a partner in a company that manufactured transformers, but I'm still not an electrician.

 

[Sideways]

Your grinder will be OK.

You need to take the KVA rating and multiply it by the power factor of the motor to find out what motor input power the transformer will supply.

This is because the transformer has to supply more current than the motor will turn into real work.

Power factor is always given on the plate of an induction motor, but rarely on a "universal" commutator motor.

But universal motors generally have a power factor of 0.9 or more so this isn't a big deal for hand power tools.

3300 x 0.9 = 2,970 watts this is more than the 2400 watts your grinder needs.

The grinder will pull a peak current for a few moments while starting but this isn't as bad as an induction motor (peak starting current can be 5x to 7x more than the running current, just for a couple of seconds) so your transformer should cope OK.

 

[MikeK]

Sideways is correct, but I would use a more conservative power factor of 0.8 for inductive loads. Even then, the capacity of the transformer should be fine. Power in must equal power out plus any internal losses. Using kVA or kW for power transfer calculations makes the process a bit easier since the input and output voltage and current values are already factored into the equations.

 

[John Brown]

"Your grinder will be OK."

To be honest, I expect it was the transformer the OP was worried about, rather than the grinder.

 

[Mark A]

Thanks guys, that's reassuring.

The grinder has soft start, so that should minimise the initial power surge, I think?

 

[Sideways]

Thanks guys, that's reassuring.

The grinder has soft start, so that should minimise the initial power surge, I think?

Yes, that will help.

The transformer will be rated at a certain power for intermittent (say 10 or 15 minutes per hour, 30 mins on, 30 mins off, whatever) or possibly continuous usage at a lower rating. Follow these timings to keep your transformer happy. The current flowing through the transformer when the tool is running heats the transformer windings up and at higher loads, the heat is generated much faster than it can get out. It's OK for the transformer to heat up inside for a few minutes as long as it's allowed to cool off. For continual use, you have to limit the heating inside to a lower level that matches the rate at which it can lose that heat to the surrounding air.

 

[deema]

The information you’ve been given is almost right. Any power factor away from 1 means that the power source has to provide more power. Rather than multiplying by the power factor you divide which gives a larger power source required. For large inductive loads / large variations in power factor power correction capacitors are installed into factories to both meet the legal requirements for keeping the power factor within certain limits and also to reduce the power consumed.

 

[Sideways]

I multiplied the transformer power rating by the power factor to get the (lesser) "real" load power that it could support. Not the most common way of writing it but the same thing as dividing your 2400 watt load by a 0.8 or 0.9 pf and comparing with the 3300W KVA rating of the transformer.
Don't worry, it's the same calculation either way you write it and the transformer is still big enough for the job