Induction versus brush motor starting loads

[WX]

Mr Sibley suggested I do this. As some of you are aware I'm planing to upgrade my poor excuse for a saw bench. I run all my power tools via a 2.6 KVA genny. The saw I am thinking of buying is powered by 1.5 hp induction motor, not the question is. Will my genset run it?

[Ron Williamson]

My rough non-electrician(ie. talking out my ass) math says,
1.5hp/.75(kw conversion factor)=2kw.
I'll assume this to be full load amps(FLA on the motor plate,if you're lucky).

It should work,but you need to be aware that lots of manufacturers puff up the hp and wattage.This is known as 'peak hp' or 'peak output' and can be maintained for a only short time before tripping off or letting out the magic smoke.
Now if the motor mfr. posts bloated hp numbers and the genny mfr. is realistic then you have a bit more cushion.
In my experience,KVA is realistic, where wattage needs a second look.
R

[WX]

Thank you Ron, that is a good starting point for me to ponder.

[Nicholas Carey]

1 hp is 550 ft-lbs / second. Or, in terms that make sense from an electrical perspective, 745.69 watts.

1.5 hp is 1119 watts.

If the motor is, say 75% efficient, then it should draw about 1400 watts. That's 5.8 amps at 240 VAC.

Your 2600 watt generator produces 10.8 amps at 240 VAC (21.7 amps @ 120 VAC).

W = V * A
A = W / V

It would seem then, that your generator theoretically has enough oomph with a healthy surplus, to drive the saw. Reality of course might be different.

The motor on the [prospective] saw should have a manufacterer's plate on it listing its power requirements in Amperes. That's what you need to look at.

Horsepower is a lousy spec for an electric motor:


the claimed horsepower is some measure of output power, not input power.
manufacturers might not lie, exactly, but almost certainly find truth to be extremely elastic.
motors aren't 100% efficient.
Motors draw more power -- sometimes quite a lot more power -- during startup than in operation.

Honda Power Systems has some useful info here (http://www.hondapowerequipment.com/products/generators/content.aspx?asset=gg_wattage) on figuring wattage requirements. They figure the startup load on a 10 inch table saw to be about c. 4500 watts and operating load to be c. 1800 watts.

This page (http://www.sawdustmaking.com/ELECTRIC%20MOTORS/electricmotors.html) suggests that a 1-1/2 hp motor will draw somehere between 13.9 to 19.5 amps at 115vac or 7.0 to 9.7 amps at 230 vac.

somebody else has asked almost the same question here:
http://permanentmagnetgenerator.us/in-electric-generators-what-kva-rating-should-i-use-for-a-1-5hp-electric-motor-connected-to-a-deepwell-pump.html

[Nicholas Carey]

In the US anyway, the electric code requires that every electric motor must have a spec plates containing the essential data needed to replace it. This page (http://ecmweb.com/mag/electric_understanding_induction_motor/) goes into detail about each of the required data:
The NEC states that the motor nameplate must show the following information:


Rated voltage or voltages
Rated full-load amps for each voltage level
Frequency
Phase
Rated full-load speed
Insulation class and rated ambient temperature
Rated horsepower
Time rating
Locked-rotor code letter
Manufacturer's name and address

In addition to this required information, motor nameplates may also include data like frame size, NEMA design letter, service factor, full-load efficiency, and power factor.

Finally, some nameplates may even include data like bearing identification numbers, certification code, manufacturer serial number, and symbols and logos.

[WX]

The only info I have at present is this:
motor 1-1/2hp 240v motor type induction motor speed 2880 rpmI have emailed them but no reply as yet.

[Ron Williamson]

I shouldn't do math that early.
1.5hp x.75
R

[WX]

I get 1.5 hp x .75 = 838.912356 watts

[PeterSibley]

The question is the difference between induction motors and brush motors , not in running loads , but start loads ?

Any help ?
Chuck you there ?

[The Bigfella]

I get 1.5 hp x .75 = 838.912356 watts

Fail.

Try about 1125 watts.

The starting load, in the textbook I looked at is "several times" that 1125 watts.

[PeterSibley]

That "several times" is quite variable in my experience .

[WX]

Fail.

Try about 1125 watts.


That's what you get when you use google as your calculator. :D

[epoxyboy]

An induction motor will typically draw five to six times rated full load current on start, and most gensets will happily cope with a 300% overload for several seconds.
As a saw bench is a fairly low inertia load, and has a flat torque characteristic until you start cutting stuff so I don't see any problem with your setup getting started. I can't comment on how well it will manage under load though.
The company I work for designs and maunfactures induction motor soft start systems 7kW to 20MW, and I recently did a commissioning job on a ship refit in China, where three 1.2MW gensets drive two 900KW motors on a Voith Schneider propeller. Nice Dutch built ship, very nasty Chinese shipyard. Does the phrase bird**** welding mean anythin to anybody!
You also need to bear in mind that the RPM of the induction motor may be substantially different from an equivalent power brush motor - this should be on the nameplate.

Pete

[SV Papillon]

You posted that your gen set is 2.6 Kva not Kw correct. So at 240v single phase you will have about 8.33 amps availble (http://www.dieselserviceandsupply.com/Power_Calculator.aspx#kvatokw)

Your 1.5 Hp 240v single phase saw will draw 7 to 10 amps (http://www.engineeringtoolbox.com/electrical-motor-hp-amps-d_1455.html) depending on efficiancy. This is all assuming we are talking about single phase 3 phase all different. In any case google is your friend Just put in motor power calculator or something like that and after a little trial and error you will get to the right info. The above are quick searches and you can probably find exactly what you are after with a little digging, wikpedia is a great resource too.

Good luck

Jake

[Stiletto]

I have an induction motor on my German Elektra Beckum mitre saw. I notice that it takes longer to get up to speed than brush motor saws, whilst operating torque seems about the same.

Intuitively it feels like there is less initial current draw than the 'big hit' a brush motor has.

I have had it for over 20 years with no maintenance whatsoever on the motor, although sawdust regularly blown out .

If it suits your appllication, an induction motor is a pretty trouble free piece of kit.

[Ron Williamson]

For giggles,I'll go out to the shop this AM and put my amprobe on a coupla machines,then start them.
If I don't get distracted.
My big bandsaw, edge sander and small dust collector are in the neighbourhood of 2hp and have accessible wiring.
The bandsaw and edgesander would be comparable loads,the DC will be way higher.
R

[WX]

Email from Carbatec.

say that you would need at least a 10 kva generator for the
contractors saw. This is to allow for start current.
So much for that idea.

[The Bigfella]

See your Hafco thread... for a suggestion

[WX]

Just got told by Carbatec that my getset is for lighting and small power tools only. Guess I need a bigger genny...hmmm.

[The Bigfella]

I've got an 8.5kvA.... sorta attached to my boat though.

[Ron Williamson]

Soooo...I have an update.
I used an analog amprobe,which is good because a digi would make 88888 as the amps ramp up and down.
Anyway,my 2hp edgesander 230v with a FLA of 9.2
Starting amps,30-ish,running amps 2.9 at 211 VAC
My 2hp dust collector,starting amps about 48,running amps 10 at 211 VAC.
The bandsaw was out of reach(maybe if I had a 10' wingspan).
R

[PeterSibley]

Thanks Ron ....betwwen 4 and 10x running load to start .

[Michael D. Storey]

So what would be involved in some copper wire and poles power source?
Beats the **** out of a genset any ole day.

Jusayin