Portable generator

[Speedy Builder5Participant @speedybuilder5]

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

[Speedy Builder5Participant @speedybuilder5]

What size generator for 3/4 Hp motor

[Brian Sweeting 2Participant @briansweeting2]

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

 

[J HancockParticipant @jhancock95746]

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.

[Speedy Builder5Participant @speedybuilder5]

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

[Mike PooleParticipant @mikepoole82104]

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

[Speedy Builder5Participant @speedybuilder5]

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

[not done it yetParticipant @notdoneityet]

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.

[Mike PooleParticipant @mikepoole82104]

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

[John OlsenParticipant @johnolsen79199]

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

[Ian S CParticipant @iansc]

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

[Gordon WParticipant @gordonw]

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.

[not done it yetParticipant @notdoneityet]

Posted by Gordon W on 16/02/2018 10:07:01:

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?

[Gordon WParticipant @gordonw]

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

[Speedy Builder5Participant @speedybuilder5]

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

[Mike PooleParticipant @mikepoole82104]

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

[MikeParticipant @mike89748]

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.

[not done it yetParticipant @notdoneityet]

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.

[Clive HartlandParticipant @clivehartland94829]

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

[Sam Longley 1Participant @samlongley1]

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.

[Speedy Builder5Participant @speedybuilder5]

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

[not done it yetParticipant @notdoneityet]

Posted by Sam Longley 1 on 16/02/2018 18:25:04:

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.

If a 3/4HP electric motor will drive it, then any 3/4HP engine will run it. Power is simply a rate of doing work – it matters not a jot from what the rotary power supply used.

Starting the saw blade with a fuelled engine might present the same problem as starting an equivalent induction motor, but less so if there is the possibility of slipping a clutch or drive belt. But even a slow revving engine with a large flywheel could start it satisfactorily.

Fuel type comes down to cost. My diesel genny currently costs less than twice the cost of grid electricity. But my genny is only mobile, not portable!

SB5 could likely get away with his 2kW generator if the saw blade could be decoupled from the motor, as the surge current at start up would be neither as great nor of the same duration.

[Sam Longley 1Participant @samlongley1]

If the saw blade hit a hard spot momentarily the diesel may have the weight & torque of the flywheel behind it. It may run a little slower.That is why I thought diesel may be better. A small diesel would start from about 1.5hp – Do they make them smaller? mine is quite compact ( & does power a generator). But there are loads of petrol lawn mower engines about rated at higher capacity complete with clutch that might do the job

I appreciate that the OP may want to power a number of units from the one source, but if he has to have a big generator just for the saw then eliminating that from the equation may have allowed him to have a much smaller generator. Easier to move about etc. It depends what get used & moved the most.

Edited By Sam Longley 1 on 17/02/2018 07:54:26

[Speedy Builder5Participant @speedybuilder5]

Slight change of tack. My generator does not have a provision for an earth connection. Should I run an earth to the frame/engine chassis, Should I install an RCD ? As an earth would be as best as I could provide on-site, would an RCD have a chance of working ?
BobH

[Ian S CParticipant @iansc]

My home brew portable gen set uses a 3hp Kawasaki motor to a car alternator, and I can get about 500Watts out of it, I have an inverter for it, but I'm yet to try it on the fridge. The base that the unit is sitting on doubles as a sack barrow, and is rigged as a trailer to be towed by my bicycle. I can boil enough water for a cupper in about 20 minutes

Ian S C

Edited By Ian S C on 18/02/2018 09:28:19

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