Generator Upgrades

The generator is now wound for 4 poles. If you increase it to 3000 or 3600 rpm, it will produce respectively 100 or 120 Hz.

If it is possible to rewind the generator for 2 poles, you will get 50 or 60 Hz, either of which might be useful.

Or you could use a gear reducer between, which will be noisy and will add various alignment, vibration, and maintenance issues.

Is there any issue with rewinding for 2 poles.Won't i be able 2 get that done from some winding guy?

I think this can be done, but it may depend on the specific generator. If you have access to a rewinding shop, they should be able to tell you, and give you an estimate of cost.

How does that work in theory?

On the bit about the number of poles, the synchronous rpm of motors and generators = 120 x Hz/no. poles.

Hz...120xHz...poles...rpm

60.....7200......2.....3600

.....................4.....1800

.....................6.....1200

.....................8......900

50.....6000......2.....3000

.....................4.....1500

.....................6.....1000

.....................8......750

Because of slip in induction motors, the running rpm's are about 3% less in each case. Thus rated rpm's are something like 3500, 1750, 1165, 875; 2900, 1450, 965, 725; with variations by specific design and manufacturer.

Thank you. The rewinding, is that a rotor or stator thing. Or mechanical work too?

You could just slow the engine down to 1800 RPM. But you will have roughly half the usable horsepower of its 3000 RPM rating.

I tend to agree to a degree, but I don't think the power loss will be anywhere near as much as you say as Diesels have their maximum torque (and power!) at much lower RPMs than a comparable petrol or gas engine.....it is possible that max torque is around 2000 RPM or even less....if true, this will save a lot on diesel fuel usage as .

Guessing a little as the graphic display of the torque and power curves from the engine manufacturer are needed to be 100% certain, but I would suspect that the power loss will only be 15% or maybe even less, and this may still provide enough power to use the generator fully......depending upon the max possible electrical load that the generator can take of course....that also being an indication of the maximum mechanical load it will impose on the engine.

If the generator can impose a mechanical load greater than the engine can handle, it will be relatively easy to include a circuit breaker to drop the generator offline. Also a load/power meter should be attached to see that this load level is not actually reached.

G'day Andy,

It all depends on the "source" of the motor, if its a quality motor like Hatz, then yes the rated horsepower won't fall too much at lower RPM. But if it is one of those chinese copies then it will be lucky to achieve the rated power at maximum rpm, and be a tad wobbly in speed regulation at lower speeds.

Would have been nice to have had all the information on the motor and generator from the OP. Saves guessing a reasonable answer.

You are so right!!

GA, especially for the belt drive idea.

well at 3000 rpm it say max output 8.8 and 3600 rpm 10 hp

Generator is built for 6kw output at 1800 rpm

@ Tobugrynbak: I agree V-belts are the best practical solution, but there is a flaw in the transmission ratio calculation: if the motor has to turn 1.67 times to one turn of the generator it is not the ratio of the DIAMETERS of the pulleys but their CIRCUMFERENCE that matters. Not so?

NO! The ratio of diameters is identical to the ratio of circumferences; the factor π simply cancels.

diameters matter --- and a chain with 2 sprockets works also - even with less loss see my comment lower. success

Well whether you measure a pulley by diameter or circumference matters naught, the result is the same.

Pulleys are usually sold by they're diameter/number of sheaves/ profile of belt.

Now if you have a 100mm circumference pulley driving a 167mm pulley, the ratio will be the same as a 100mm diameter pulley, driving a 167mm diameter pulley. The resultant division(or multiplication) of rpm will be the same. The belt speed will be higher though.

What will bite you though is the ability of a 32mm diameter (100mm Circumference) pulley to transfer sufficient power to drive 54mm diameter (167mm Circumference) pulley for a given load. That's another consideration depending on the application. It will produce the same driving/driven speed ratio however.

My example is based on a readily available components to achieve the desired result for the OP. If you scale up the pulleys further to the same ratio you will improve the power transmission ability and reduce the belt speed, but the driving/driven speeds will still be the same.

There are a quite a few parameters to be considered. to much to go into in a short reply

Cheers

@ Tornado and Tobugrynbak: Thanks for explaining, my mistake...

All Good, that's what we do here.

How about introducing a gearbox?

Already been done, at least twice.

Hmm, according to my yard stick you will need to provide a little bit over 8hp (maybe 8.5hp)to drive your generator for a 6kw power draw.

It of course depends on what your planned power draw is if your only going to draw 4 kw out of it then you can get away with slightly lower hp from the motor (not RPM though).

I think the belt drive idea is the winner here. It is likely to be way less expensive than a rewind or a gear reduction, and quieter and less maintenance than a gear. I don't have a belt table on me, but I think a single C strand or double B will do the job. The overhung load capacity of the generator drive-end bearing will need to be checked.

1) is this a single bearing, or 2 bearing generator head?

if 2 bearing, the belt drive suggestions are your best choice.

if it is single bearing, it mounts on a "generator taper" engine, and you will have to derate your output to what the engine provides at 1800 rpm

Yosemit3, here an idea, if the generator is single bearing set it up this way. Mount the generator solid to a frame at the shaft end. Take two pillow block bearings run an appropriate size shaft through the bearings with the pulley between the bearings then on the end of the shaft have an engine coupling attached that connects to generator shaft. Just like it, if it was connect to the motor. No side loads on the single bearing. The motor would just have the other pulley as normal. Belt tension can be done several ways motor mounts slot adjustable, not the best. Generator mount with pillow blocks as a unit with bolts in slots, a little better. Best would be tension idler like used in automotive serpentine belt application.

This setup is a little more involved but not much. But it's a way around excess side load on gen with single bearing.

Charles.

Change nothing to the generator; just adjust rotation speed by means of belt or reduction gear drive.

What is stopping from running the 3000 RPM diesel engine running at 1800 RPM. Why you are not able to throttle it down?

Not enough horsepower at 1800 rpm to drive the generator.

Depends only on the engine couple. Some diesels have that power in the lower RPM's.

Your diesel, if build for a generator has most probably a governor, that is the device that regulates the right dose of diesel injection to maintain a constant RPM (revolutions per minute) If you have a couple/or momentum curve of your diesel engine and you do not need the full 10 HP - run the engine to 1800 RPM - Your engine will make less noise and will run smooth. You can also drive the generator with 2 V- belts and pulleys if both shafts ( engine and generator) have 2 or more bearings. I have 3 solutions for you: 1. Run the engine @1800 RPM (change the governor - sometimes only 2 little weights) 2. Run your engine and generator with pulleys and 2 V- belts with diameters: RPM1 X d2 = RPM2 X d1 (or n1Xd2= n2Xd1) (your engine will have the smallest pulley here) 3. Buy a generator @Harbor Freight for $350 and use that, coupled directly and run your engine @3600 rpm.

I've done this modification before. It will work on generators that have two armature bearings and an input shaft. You need to belt drive the generator from the engine with a belt drive. Figure out the pulley sizes and that's it except for fine tuning the engine governor to get 60 Hz. If the generator is not capable of a modification to belt drive listen to the other experts here.

The frequency of the output is determined by the RPM. 60 Hz. generators turn at 1800 RPM. Assuming your engine has a throttle control and a mechanical governor to reasonably maintain the RPM, set it to 1800 RPM you are good.

Al

Direct coupling would not help since it has to do with adjusting the the engine speed that also affects the engine efficiency. Engines available for generators are for single speed that matches the speed of the alternator.

The best option available to you is the speed reducer system which can be either gear box, pulley or sprocket.

In this way the engine performance will not be affected and maximum out put will be delivered from Ur alternator.

Dickson

The power losses are considerable in such systems, going from memory that can easily exceed 10%, that would probably be about the same power loss if the engine is run slower than designed (though the designed speed has not been posted, nor has the exact engine type either!)

Just a thought!!

I think belt n pulley r best.Only thing i have in mind is how much would it reduce power? I mean its 10 hp engine i think if i use it full would be able to draw 6kw and if i use belt system would i be able to draw 5kw? or it be lesser than that?

1800/3000 = 60% of rated speed. As the engine speed is reduced, the hp delivered also decreases. But the torque increases, partially compensating for this. At 60% speed, the engine can still deliver about 70-80% of its full power. We would need to see the engine performance curve to know what this particular one will do.

Another item to consider is that the load on the generator might be only 4-5 kW, even though it is rated for 6 kW. If so, the simple approach of slowing the engine down might very well work, but it won't give much reserve capacity.

A good belt drive may have about 3-5% losses. Thus the engine can still deliver about 9.5 hp. This is a good reserve to have, especially if you will be starting electric motors.