Thread: Generator question

Originally Posted by Rossluck

I've just made up a generator from an old Asian el Cheapo 13 HP Honda copy motor and a Honda 4 KVA generator. My problem is that I seem to have to run the motor at high revs to get up to 50 HZ. The pully was taken from the old motor (Honda 9HP) so the ratio is the same.

Do all generator motors have to run at high revs to run at 50 HZ? I know I can use a smaller pully on the motor, but it's not easy. How much damage can I do running at, say, 40 HZ, which it does easily?

A 2 pole generator (alternator) will need to rotate at a higher RPM than a 4 pole unit. How many poles does your generator have?

Yes, you can damage some equipment by operating them at 40Hz. I would highly recommend that you try to achieve 50Hz.

If you are only running brush type motors it won't make much difference.
But if you want to run induction motors you should get it up to 50Hz.

If it runs easily at 40 Hz use a pulley 25% larger and it should run 50Hz easily.

Originally Posted by echnidna

If you are only running brush type motors it won't make much difference.
But if you want to run induction motors you should get it up to 50Hz.

If it runs easily at 40 Hz use a pulley 25% larger and it should run 50Hz easily.

Just curious Bob, what's the difference you are talking about?


To produce 50Hz, the following applies;

1] 2 pole - 3 000 rpm.
2] 4 pole - 1 500 rpm.
3] 6 pole - 1 000 rpm.
4] 8 pole - 750 rpm.

Thanks for the advice Bob and elkangorito. I just looked up the RPM's of Honda motors and they run at maximum power at 3,600. This means that according to the table produced by elkangorito the motor should run at 3000 RPM without much trouble. I was testing it in the shed and maybe that's why I thought it was screaming. I might test it with a tacho and see what it's actually doing.

But I also would like to hear more about brush motors. If it won't harm them to run at less than 50Hz, maybe I could drop it down a bit when drilling and so on.

Originally Posted by elkangorito

Just curious Bob, what's the difference you are talking about?


To produce 50Hz, the following applies;

1] 2 pole - 3 000 rpm.
2] 4 pole - 1 500 rpm.
3] 6 pole - 1 000 rpm.
4] 8 pole - 750 rpm.

The tables accurate,
but Rossluck wanted 50cps at a lower engine speed so he needs to gear it up so the alternator will run at the correct speed while the engine runs at his preferred speed.

Originally Posted by Rossluck

....This means that according to the table produced by elkangorito the motor should run at 3000 RPM without much trouble. .....

That is for direct drive, if you have pulleys that are different sizes, you will need to factor in the gearing ratio which is what Bob is getting at.

If you can get a shaft drive tacho to measure the speed it will help heaps.

I know that the cheap direct drive gen sets run at high revs & eat fuel.
The ones designed to run at slower revs & use less fuel cost more because of the engineering in them.

Please bear in mind that the speeds I quoted are the speeds at which the "alternator" is supposed to operate to achieve the desired Hz. This may not relate to the rpm of the motor, particularly if gearing is involved as Cliff Rogers said.

Originally Posted by Rossluck

But I also would like to hear more about brush motors. If it won't harm them to run at less than 50Hz, maybe I could drop it down a bit when drilling and so on.

Ross, this has nothing to do with brush vs brushless motors. Your main concern is the equipment you wish to supply with electricity. If you can't produce a 50Hz supply, you are at risk of damaging equipment.

Originally Posted by elkangorito

Ross, this has nothing to do with brush vs brushless motors. Your main concern is the equipment you wish to supply with electricity. If you can't produce a 50Hz supply, you are at risk of damaging equipment.

Fair enough elkangorito. I can't argue.

I changed the pully this afternoon. First I went the wrong way and put a smaller pully on the motor, but when I struggled to reach 50 Hz again I had to sit down and dredge up memories of my push-bike years and the gear ratios. Common sense prevailed and I had a larger pully at hand that I installed.

I think it's about right now. I've been testing it on a one HP bench grinder that sucks a lot of power at startup, and the motor gives a grunt when I flick the switch but handles it without too much trouble. Just guessing, I think I'm running at 50 Hz at about 2000 to 2500 revs. But either way the motor handles it at a moderate and reasonable number of revs.

One more question: what sort of tolerance is allowable, is it say between 48 and 52 Hz?

Originally Posted by elkangorito

Please bear in mind that the speeds I quoted are the speeds at which the "alternator" is supposed to operate to achieve the desired Hz. This may not relate to the rpm of the motor, particularly if gearing is involved as Cliff Rogers said.

Thats what he said he is trying to do. Keep the alternator speed up with lower engine revs.

Just because its a honda copy doesn't mean it will run the same. If you ever lift the heads off a cheap copy the ports will definitely look different, with rough burrs and casting marks. Having said that roughened ports can increase torque at lower revs by creating turbulence and increasing the swirling effect in the combustion chamber and can make the engine more efficient at a lower speed.

Not to be taken as gospel of course but is certainly possible.

Originally Posted by Timmo

Thats what he said he is trying to do. Keep the alternator speed up with lower engine revs.

Just because its a honda copy doesn't mean it will run the same. If you ever lift the heads off a cheap copy the ports will definitely look different, with rough burrs and casting marks. Having said that roughened ports can increase torque at lower revs by creating turbulence and increasing the swirling effect in the combustion chamber and can make the engine more efficient at a lower speed.

Not to be taken as gospel of course but is certainly possible.

Thanks Timmo. I don't actually trust Chondas, but the motor that I'm using has been very reliable on a compressor that I was using on building sites. The only problem I had with it was that the muffler kept coming off, and no matter how hard I tried I couldn't fix it. On the generator I was able to solve this problem by using a long muffler from another motor that I bolted to the frame holding the generator. On top of that, the generator and motor are bouncing on rubber, so that should reduce vibration.

Originally Posted by Cliff Rogers

That is for direct drive, if you have pulleys that are different sizes, you will need to factor in the gearing ratio which is what Bob is getting at.

If you can get a shaft drive tacho to measure the speed it will help heaps.

I know that the cheap direct drive gen sets run at high revs & eat fuel.
The ones designed to run at slower revs & use less fuel cost more because of the engineering in them.

Thanks Cliff. I borrowed my brother's tacho and had it for about two years. Just recently I gave it back to him, and I've never needed it more .

Originally Posted by Rossluck

Thanks Cliff. I borrowed my brother's tacho and had it for about two years. Just recently I gave it back to him, and I've never needed it more .

That is like having a clean out... as soon as you chuck it out, you will find a use for it.

Originally Posted by Rossluck

One more question: what sort of tolerance is allowable, is it say between 48 and 52 Hz?

Frequency tolerance is up to the equipment receiving the frequency. Usually, wire wound equipment like electric motors, heaters etc, are quite tolerant to a slight increase in supply frequency than they are to a decrease in supply frequency. Electronic equipment may not be so tolerant.