motor problem

[sawdust1]

I have recently purchased a chainsaw mill which is powered by an electric motor, the seller has had a new motor fitted and wired into an isolation switch by an electrician who said it needed a 35A supply and a 35A type c mcb. The seller has now emigrated to Sweden and i can't get hold of him.
As i'm using the mill away from a mains power source i purchased a Generator to power the mill but the problem i'm having is that when i switch on the mill it starts to spin then trips out the AC breaker on the Generator.
The motor is an Amtecs AMAS 112M4 3.7KW 5HP 19.7A
The generator is a Clarke PG6500DVES 5.5KV 13HP
Will a Type c breaker between the Genny and motor switch cure this ? or some other way ?
Thanks.

 

[Tris]

That sounds like inrush current, if the motor is rated 19.7 amps I think that's about 4.4kw, and it'll need more than that to overcome the start up inertia, so probably well above the 5.5kv available.
I'm no electrician so hopefully someone who can suggest a solution will be along soon

 

[Sideways]

Yes.
The motor will draw 20A under load but it will want 100 to 160A briefly while it gets upto speed. The breaker fitted to the genny should be as big as that is capable of delivering so don't mess with it. It's preventing you from melting the generator windings.
Your genny is probably just too small. It'll run 5.5kW of lights or heating but not 5.5kW of motor.
I bet it sounds like it's been punched in the gut the moment you switch on the mill, if it even gets that far before the breaker trips.

The 35A thing makes no sense to me at all. You'll need 20A minimum and could put in a bigger supply to have a little headroom from the supply but 25A and 32A are the next standard sizes going up.
The type C breaker advice is right.

The breaker doesn't protect the mill, it protects the genny or the fixed wiring that you are plugged into from the mill.

 

[clogs]

sideways,
what would be the biggest motor that size of genny could handle on a red basis......
i have genny that size n it'd be handy to know.......

 

[Tris]

What bar length has it got? I bought a Hyundai chainsaw and a generic mill from eBay for less than £300, allows easy working away from power supplies

 

[sawdust1]

Electrician has just been, everything is quite stiff from never been used in 3 years, so suggested i take it down to the
workshop to wire it into the mains to get everything moving freely with lots of oil before trying the genny again.
Will report back.

 

[Spectric]

The load is a motor and therefore you need to take into account the power factor which will not be 1, assume 0.9 and then that 5.5Kva genny will be to small to run that motor let alone the startup requirements.

230 volt, 5.5kW and 0.9 Pf gives 6Kva and 26 amps running .

 

[Sideways]

sideways,
what would be the biggest motor that size of genny could handle on a red basis......
i have genny that size n it'd be handy to know.......

That's an interesting question as I haven't played a lot with genny's to really know from experience. I've searched a few relevant posts on the IET web forum (electrical engineers), some of the big generator mfrs, and some of the smaller ones to try and get an answer that feels reasonable.

These views might give you food for thought:
On a big 200hp plus generator, advice is never to try and start a motor bigger than half it's capacity. But a smaller genny can't handle that much.
0 to 50% of capacity sounds like a typical "first" load to connect to a genny. Not many will take 100% full load applied in one hit.
Then, motor startup current is very typically 5x full load, so that suggests not connecting a motor more than 1/10 rated generator capacity.
And Spectric's point about power factor is important. Motors are magnetic, inductive things. They require reactive or imaginary power to flow in and out of them as well as real power that gets consumed. The genny has to supply the real plus reactive power which is measured in KVA not kilowatts. This reduces capacity even more.

One online calculator that seems to get copied or linked to recommended a 5.7KVA generator for a 1.5HP motor. That's just 1.1kW
So yeah, about 1/5 of your generator capacity.
Darn !

You'll probably have best luck with low inertia machines like a tablesaw than something where the motor has to accelerate a lot of extra mass.
And once the first motor is running, it may be easier to start a second one. The mechanical inertia and magnetic field stored energy in first motor will help feed the startup demand of a second motor.
Running motors isn't too big a deal, it's getting them going.


Maybe the OP would have better luck if he tried a rental 20kVA generator to see if that works better.



And paraphrasing comments between a couple of qualified engineers : Generators have much higher impedance than the mains, this means that they can't supply much more current than their rated KVA. Not only does this mean that they aren't good at supplying the startup current, they aren't good at supplying the high fault current needed to trip a circuit breaker within the short time required for safety.
So a breaker can't be trusted to work as expected when fed from a generator.
ALWAYS PROTECT THE OUTPUT OF A GENERATOR WITH AN RCD.

 

[Tris]

Just for the sake of curiosity, seeing as there are people here who know infinitely more than I ever will, would a soft start make this possible and is it possible to retrofit one to a motor?

 

[ChaiLatte]

What would be the biggest motor that size of genny could handle on a reg basis...

It is a difficult question to answer without some indication of what the motor is connected to. If you are starting a motor with just a pulley on it, with the main load coming on when you operate, for example, a belt tensioner, that is a different situation to when the motor has a 500kg flywheel attached to it that needs to accelerate from a standstill.

It is the same with the chainsaw mill in the original question: the more load that can be removed from the motor on startup, the lower the inrush current.

The soft starter will lower the inrush current but they are quite expensive to buy. On top of purchase price, it would need proper installation and some kind of enclosure, so you'd have to compare total cost with that of buying a more suitably-rated generator.

 

[Sideways]

A VFD can act as a soft starter and certainly can lower the startup surge presented to the mains. But a VFD is a electronic switching device and doesn't look like a simple resistor to whatever is supplying it. I've seen comments that generators like them even less than they like motors.

 

[Spectric]

A VFD can act as a soft starter

You could use a soft starter which has a fixed frequency but can ramp up the current on startup, but your generator is still on the small side so you still will have problems. I have worked in places where some equipment is safety critical and on loss of power a large stationary gen set will kick in but this was with very large motors and 1000 Kva generators but more importantly is that these systems were always three phase. Single phase motors are not self starting unlike three phase which are and three phase startup current used to be controlled with star-delta starters and now softstarters with bypass contactors so any issues can be resolved. I think the solution to your problem is to forget getting enough power from a generator and to replace the electric motor with a stationary internal combustion engine, the sort of setup you see on the Tv channels from places like America.

 

[sawdust1]

Thanks for the replies, its given me food for thought.
I think scrap the idea of using the Generator i have and look for something bigger and maybe go 3 phase will see
whats around on my budget.

 

[sawtooth-9]

sideways,
what would be the biggest motor that size of genny could handle on a red basis......
i have genny that size n it'd be handy to know.......

Are you talking a three phase genny ?
For three phase motors you usually allow about five times the full load running current for start up - so star / delta starts really help.( basically half the delta start current )
I am running a 4 Kw motor on a 20 KVa genny - but it NEEDS a star start.
To start a 4 Kw motor in delta could use 8.4 x 5 = 42 amps
The genny can supply up to around 30 amps. Hence the star delta start.

 

[sawdust1]

Picked up this beast Saturday, this one should be man enough, what do you think ?
Pictures of the mill as well.