Portable generator

What sized portable generator might I need for a 3/4Hp 240 volt cap start motor which drives a 10" circular saw ?

What does the data plate say about FLC/FLA or LRA?

 

Something of an educated guess but I doubt if anything less than a 3kw output would give you a steady enough

supply for easy sawing.

Brian, I don't know that I can answer your question. The plate:-

Crompton Parkinson Doncaster
A.C.Motor BS 170 69H Insl Class E

BS Frame No VPA 5643A
BHP (I think 0.75, but plate is well scuffed up.
RPM 1425
Volts 220 / 240
Amps 5.4 (May be 5.1)
PH 1 Hz 50
Rating CONT.

BobH

I would have thought you would need a generator that will handle the start up surge which can be 3-10 times FLC depending on load. I think 3/4hp is a bit under powered for a 10" saw but maybe it's ok for your application.

Mike

Yes Mike, The saw has done me and my Dad nearly 50 years of service with only one re-wind in 1976 and a new contactor. The saw is all home made and will easily cut 2" planks of soft or hard wood, 3.5" when pushed and certainly seen some service.

I have hooked it up to a 2Kw generator which stalled when it was cold. In fairness, the generator states to only use up to 300 watt devices for the first 20 mins of use. After it warmed up, the saw runs, but down a bit on power - would this tend to burn the motor out ?
BobH

When running, it will take more power than its rating horsepower dependent on the efficiency of the motor - say around 110%.

Starting it, as Mike states, is an entirely different ball game.

If down on power, check the A/C voltage and the frequency of the supply. It may not be even achieving full speed, if the start windings are not being sufficiently energised at start up, so a check on rotational speed would be good, too.

The saw will need 240v to develop full power, if the generator cannot sustain this then the motor will not be happy. I assume it is the engine that needs to warm up to deliver full power as temperature shouldn't really affect the alternator. If the generator can deliver 2kW into an inductive load and will start the motor easily then I would have thought it would be OK. I think you will need to take some measurements to be sure the motor is getting a steady 240v if it can't do this then there is a problem.

Mike

The rule of thumb I have heard is to size the generator at three to four times the power of the motor. That should give enough reserve to cope with the starting surge. Most of these small generators don't really like to be worked too hard anyway.

Another way to do the job is to fit a suitable petrol motor to drive the saw directly. Not so good if you also want to use the same saw indoors as well as outdoors.

One interesting effect to bear in mind is that if the frequency drops due to a heavy load, the motor will take more current, loading the generator more heavily, and hence depressing the frequency even more. I've heard of a case where every fridge motor in a small hospital burnt out due to this effect. That was a somewhat large standby plant of course, but obviously not large enough for the job. A more modern plant might have had an under frequency trip.

John

The best I have used is the 5.5 HP horizontal shaft Honda, or it's clone, I know these work well on a saw bench cutting mainly fire wood, and stored outside all year. This would be a good size motor on a generator, maybe rated 4Kw.

Ian S C

I have a 2.5 kw generator, petrol 4stroke, and cheap, rated at3kw max for short time. This runs an electric chainsaw no probs once warmed up. Chainsaw is rated 1.8kw. It is important to keep all the governor connections etc. clean and adjusted. A 500w 2stroke genny runs drills etc. no probs.

But they are probably brushed A/C or D/C (have a commutator). Not the same as an induction motor. Their speed is dependent on the supply voltage, I believe?

NDIT- you are most likely correct, but I thought it was the power, ie. the wattage used by the tool, that was being discussed ?

OK, I think the 2Kw generator is probably a bit too small for my application. This morning, when everything was cold (+4 deg C), the generator started up Ok and after a few mins, would run the shop vacuum cleaner, but not a small compressor (only one appliance at a time), but would run a 10" angle grinder. After 30mins (specified warm up time on generator), it would not run the 3/4 Hp circular saw even after various attempts, but fine for the compressor. So I would say that I need a minimum of 2.5kw. Or limit the tools that I essentially need to run off the generator.
Thanks for the various contributions.
BobH

Does the saw run ok on a mains supply? If the equipment you mentioned runs ok it seems the generator makes power but is the saw is the only induction motor? As others have posted you need correct voltage and frequency for an induction motor to deliver its rated performance. Many manufacturers big up their equipment with big numbers, compressors often give the swept volume rather than free air delivery and stereo systems used to quote peak music power watts to kid you it would fill the Albert Hall.

Mike

I had a 3kw generator, and it was all but stalled by the starting current of a 1/3hp capacitor start motor. I am a bit dim as far as electricity is concerned, but would suggest you need a 5 or 6kw generator as a minimum.

It also depends on the type of generator. One cannot really beat a rolled steel magnet framed armature generator for motor starting. Stator wound with rotating field and electronic AVR are the usual order of the present day - both cheap and more likely to need a greater size margin to start any particular load.

I’ve yet to have a problem with my Stamford 4.5kVA genny powered by a Hatz diesel, whether cold or otherwise. It will black smoke at 3.5kW but runs 3kW (resistive) loads all day long.

Several decades ago, my first back-up generator was a 1.5kVA genny, powered by a B&S (supposed) 3HP engine. It would only start the freezer motor if one held the governor wide open. Freezer motor was rated at 160W.

Having read all this I will say that you need to multiply your requirements by 3.5. This will give you a 3.5 to 4.5 Kw generator. bare in mind that generated power costs approx 4 x what line power would cost.

You certainly do not want a generator that labours under power, it will get hot and not give you a lot of time to shut down.

Clive

With all the losses within the electrical generation & problems with startup loads what sized petrol/diesel motor would be needed with correctly sized pulley to give correct RPM of the saw blade? Presumably a diesel would be a better power unit.

Seems to me that it would be a far more efficient way of operating the saw. A belt tensioner to disconnect drive could be installed.

Hi there Sam and others. I am looking at a generator as there is other equipment to be used at the remote location, the circular saw just being one of them. It is not an option to convert certain items to independent power units.
BobH