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grousedogtom

Hi
I want to run 3 110volt power tools from a generator ,the total load of all 3 tools is 4650 watts,what kva rated generator would i need to run all 3 tools together?

2011

grousedogtom wrote: »

Hi
I want to run 3 110volt power tools from a generator ,the total load of all 3 tools is 4650 watts,what kva rated generator would i need to run all 3 tools together?

4650 watts would be the total out put of the 3 drills.
So if they are 90% efficient then the input (generator output) would have to be about 4650/0.9 = 5167 W.

kVA = kW / Power Factor

For a drill the power factor would be about 0.88

⇒ kVA = 5.167/0.88 = 5.872

Therefore a 6kVA generator would be just about able to do it, but you would be better with a 10kVA unit. Starting currents can be high and you do not want to trip your generator all of the time.

M cebee

i would have thought the 4650 would be the actual usage and not output on tools
If it's just tools prob a 6 or a 7.5 kva 'tool rated' trafo i would think

antlyn

2011 wrote: »

grousedogtom wrote: »

Hi
I want to run 3 110volt power tools from a generator ,the total load of all 3 tools is 4650 watts,what kva rated generator would i need to run all 3 tools together?

4650 watts would be the total out put of the 3 drills.
So if they are 90% efficient then the input (generator output) would have to be about 4650/0.9 = 5167 W.

kVA = kW / Power Factor

For a drill the power factor would be about 0.88

⇒ kVA = 5.167/0.88 = 5.872

Therefore a 6kVA generator would be just about able to do it, but you would be better with a 10kVA unit. Starting currents can be high and you do not want to trip your generator all of the time.

You need to multiply by 0.9 to get the percentage. Dividing has given you a number greater than the total load!

M cebee

the va for genny sizing will be greater than kw

antlyn

Yeah I know was just saying about the equation used

M cebee

my 110v bosch kango is 1350w rated input power,14.5 amps
no pf supplied
-so thats 1595 va

2011

antlyn wrote: »

You need to multiply by 0.9 to get the percentage. Dividing has given you a number greater than the total load!

No.

The power input will always be larger than the power output value unless the unit is 100% efficient (simply not possible).

Remember

Efficiency = (output / input) * 100

as can be seen here:

http://wiki.answers.com/Q/What_is_the_formula_for_calculating_efficiency

2011

M cebee wrote: »

i would have thought the 4650 would be the actual usage and not output on tools

The kW rating of motors is the output AFAIK.

If it's just tools prob a 6 or a 7.5 kva 'tool rated' trafo i would think

This may be ok for the transformer, but the OP asked about the rating of the generator. I am not sure if the OP even requires a transformer, I would simply use a 110VAC generator if it were me.

Transforemer will also introduce a further loss. AFAIK 96-98 % efficiency would typical for a transformer.

M cebee

my bad-didn't read question,it's a 110v genny

Motor nameplate rating is output power so you need efficiency or line current for sizing
Tools and appliances are rated input power

Bruthal

antlyn wrote: »

You need to multiply by 0.9 to get the percentage. Dividing has given you a number greater than the total load!

It hasn`t. His calculation is correct. The generator VA has to be equal to or greater than the kw load. It cant be smaller.

If a motor takes 4kw to run at full load (forget efficiecy for this), and has a pf of 0.8, then the kva to power the 4000 watts is 4000/0.8 = 5000 va.

To reverse the calculation, the generator supplies 5000va, but for a pf of 0.8, only 80% of the amps are doing any work, so the 5000 x 80/100 = 4000 watts. So while the generator is powering the 4kw load, it is outputting 5kva.

Same with the efficiency. If the drill is outputting 1000 watts, and is 90% efficient, it will show as 1111 watts on a watt meter.

1000/0.9 = 1111.

2011

M cebee wrote: »

i would have thought the 4650 would be the actual usage and not output on tools

"Almost all manufacturers stamp the design full-load power output in horsepower on the nameplate"

From this link:

http://eeref.engr.oregonstate.edu/Systems/Motor_Systems/Motors/Performance_Relationships

M cebee

yes for motors
Nameplate Kw/hp is output

my bosch110v kango for example is marked with rated input power

M cebee

the amps will be marked on them anyhow-calculating va from that is simplest way imo

2011

M cebee wrote: »

the amps will be marked on them anyhow-calculating va from that is simplest way imo

+1