Boosting a petrol generator with a car battery?

Summary
Want to run an electric motor from a petrol generator but the startup load of the motor is too large. Could a car battery supplement the generator to start the motor?
Hi All,

So i am looking to buy a petrol generator to run a 2.2KW electric motor. I was thinking of getting a 3KW model which under normal conditions this should be ok - but on start up I've been told an electrical motor draws 6 to 10 times the normal load, which is way too high for the generator. So I am wondering if I could could hook up as well a car battery on a 12v- 230v invertor?

My logic is when the motor starts it draws on both the generator and battery, then once started only draws on the generator.

Is this possible and how complex would the wiring be? On a sidenote getting a more powerful generator is not feasible as it would require around 18KW which costs thousands!!!

Many thanks,
 
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You would have to somehow phase the inverter and generator outputs together.

What is the motor spinning when it first starts? A heavy fan? Have a look into soft-start options for your motor.

Is this 240V single phase?

What is the power factor of the motor? This will be written as PF or cosΦ on the rating plate. Most generators will allow for a start-up surge, so you won’t need an 18 kVA (note units and research!). You may, however, need a bit more headroom than 3 kVA for a 2.2 kW motor.
 
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Battery is no good. It can be done but more pain than gain.
In case you want to apply some overkill then you can add a frequency changer into the mix. But that's really an overkill. Better go with what @Guineafowl suggested.
 
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It heavily depends on the motor load and if the motor is for example inverter driven and has a soft start.

As an example my little diy 3kw diesel generator can start and run my portable compressor, the same compressor will pop a 20A breaker (120V) if started when its cold.
 
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I couldn't remember the motor, so quick google image search the compressor is:
1.1Hp continuous, 15A, 120V, 2pole induction machine.

The generator was load tested, will deliver 3kw continuous resistive load (heaters), but is over-fueling a little, anything much above that, the diesel starts to slow down and stall.

If the motor you want to start is 2.2kw mechanical output, then I would say 3kW generator is not enough, but 5-6kW should do the job.
 

jrmichler

Science Advisor
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I once had a gasoline generator rated 4 kW continuous and 4.5 KW peak. It would just barely start my 2 hp air compressor, then easily run the compressor up to its full rated pressure. It would easily start my 1.5 hp table saw. Extrapolating from that, a 2.2 kW motor would need a generator rated for at least 7 kW peak power.

Can you decouple the motor from the load, get it up to speed, then engage a clutch? A clutch can be as simple as a spring loaded idler pulley on a V-belt, such as is done on snowblowers. You would still need a generator larger than 3 kW.
 
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I once had a gasoline generator rated 4 kW continuous and 4.5 KW peak. It would just barely start my 2 hp air compressor, then easily run the compressor up to its full rated pressure. It would easily start my 1.5 hp table saw. Extrapolating from that, a 2.2 kW motor would need a generator rated for at least 7 kW peak power.

Can you decouple the motor from the load, get it up to speed, then engage a clutch? A clutch can be as simple as a spring loaded idler pulley on a V-belt, such as is done on snowblowers. You would still need a generator larger than 3 kW.
^^^ This is why we need to know what the motor is driving. The higher end air compressors have an unloader valve that allows the motor to spin up before working against the pressure in the tank. A kind of mechanical soft-start.

As ever, you need to consider the whole system, not just the electrical side.

Edit: Where’s @jim hardy ? This would be just his cup of tea.
 
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I have to agree - how to fix depends on the load. One " cheat" manually throttle the generator before starting the motor, or any way to get a flywheel on the generator shaft.
A VFD may also help.
But none of this may work on something like a compressor.
 

sophiecentaur

Science Advisor
Gold Member
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3,598
I think the flywheel solution is probably the only practical one. Allow the engine to wind up the flywheel with no load on the generator then apply the load. I would imagine that the generator could probably provide the initial high start current. It puts me in mind of the commentator at a Steam Traction Engine rally who referred to "instantaneous horse power", available from the massive flywheel, which was available when the engine was used as a Ploughing Engine and a large stone was encountered by the ploughshare.
On the whole, though, a bigger engine and generator are really what's called for if you want a reliable system. You could find long term problems with an overheated stator winding.
 
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From a physical point of view, a flywheel would be an elegant solution. However, obtaining a flywheel that somehow fits to a close-coupled genset and ensuring it’s properly balanced and doesn’t overload the running gear is going to be a challenge.

Also, the flywheel giveth, and the flywheel taketh away - assuming the genny is a pull-start, you’re gonna need a thicker start rope!

We haven’t heard much from @hyphagon yet. Any thoughts?
 

sophiecentaur

Science Advisor
Gold Member
22,930
3,598
assuming the genny is a pull-start, you’re gonna need a thicker start rope!
A clutch could help sort that out and the flywheel could be external, on a separate shaft with a flexible linkage. It would need to be well bolted down and (of course) properly balanced. despite that, I am sure the mechanical solution would be the one to go for; the 'battery' idea would just be too problematical and probably very expensive.
I would do a lot of looking on eBay and Gumtree until something becomes available at a reasonable price and the smaller set could always be sold later to help with the cost. [Edit - if you just decide to upscale the installation]
Some people are 'mechanical' and some are not. Some have money to spend and others don't. The solution may or may not exist, depending on which combination the OP fits into.
 
Last edited:

Henryk

Gold Member
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First, you have to find out the source of the problem. Is the motor start-up current due to the load connected or just the motor itself, or both.
One way to deal with the load is to use centrifugal clutch between the motor and the load. It would connect the load only when the motor reaches certain rpm.
If the problem is the motor startup current, you can limit that. One way is using NTC thermistors in series with the motor. They would limit the start up current but provide negligible resistance once hot.
Or use a SCR-based soft start circuit.
 

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