Petrol Generator question

[breakshift]

Hey guys, I can't for the life of me find out how many poles (or the rated engine rpm, with which I could calculate the number of poles) the following generator has:

**broken link removed**

I'm not very good with electrical machines but I assume it's a synchronous generator, in which case I only need to know one of the two (no. of poles or engine rpm).

Any ideas?

Thanks, Mike.

 

[Diver300]

Why do you want to know?

I think it is highly likely to be 3000 rpm / 2 poles because the engine is quite small, and a quick google of the engine no gives Open Frame Heavy Duty Generator which says 3000 rpm.

Small engines like that generally run a quite high revs. The exception is on some lawnmowers where the speed is kept down to keep the blade tip speed low, to keep them quiet and to stop stones being thrown too fast.

 

[breakshift]

Well I need to know as I'm trying to work out the generator k value (constant which represents the construction of the machine). This is so I can remove the engine supplied with the generator and couple another engine to it, allowing me to use only the generator electrical output to characterise the engine.

Thanks for the reply - I'll ask some mechanical colleagues to verify that for me.

 

[Reloadron]

I agree with Diver300. Looking at your specifications in part:

Specification

Maximum output: 5.5kw

Continuous output: 5.0kw

AC Voltage: 230v/115v

DC Voltage: 12v

Frequency: 50hz

Had the frequency been 60 Hz it would have been 3600 RPM. These are typically 2 pole units.

Frequency = (Number of Poles) X RPM/[120)

50 = 2 * 3000 / 120

Give this link a read for a better understanding.

<EDIT> Remember that when using a different engine the rotational speed must be maintained with some degree of accuracy as it will determine the output frequency and also the output voltage and regulation is based on that rotational speed. </EDIT>

Ron

 

[breakshift]

Remember that when using a different engine the rotational speed must be maintained with some degree of accuracy as it will determine the output frequency and also the output voltage and regulation is based on that rotational speed.

Well actually the aim is to use this generator as a way of measuring the output power of the engine for a range of different RPMs. I want to plot RPM vs torque (keeping output power constant).

Edit: Actually that's wrong, I need power vs RPM.

 

[Reloadron]

Well actually the aim is to use this generator as a way of measuring the output power of the engine for a range of different RPMs. I want to plot RPM vs torque (keeping output power constant).

OK, so you just want to use the generator as a load for an engine. Give the generator a few loads, measure the RPM and Torque (Torque Cell) and plot.

Back to the beginning:

Hey guys, I can't for the life of me find out how many poles (or the rated engine rpm, with which I could calculate the number of poles) the following generator has:

Diesel & Petrol Generators | Portable & Electric Generators & Pressure washers

Two pole, 3000 RPM 50 Hz generator that runs on petrol or for me gasoline.

Similar diesel units are 4 pole 1500 RPM for 50 Hz.

For some reason I remember a thread similar to this several months ago.

<EDIT> I saw your edit. When you say power do you mean watts? </EDIT>

Ron

 

[Diver300]

Well actually the aim is to use this generator as a way of measuring the output power of the engine for a range of different RPMs. I want to plot RPM vs torque (keeping output power constant).

If power is kept constant, there is a strict relationship between RPM and torque, so you can calculate it without bothering with any hardware.

Mains generators will be particularly lousy for a load test, because they are not adjustable and the voltage will vary massively with RPM. The voltage regulation usually depend on the RPM.

It might be cheaper to get a replacement alternator for one of those generators, if you don't want the petrol engine.

Engine testing of often done with water brakes or eddy current dynamometers.

 

[Reloadron]

Engine testing of often done with water brakes or eddy current dynamometers.

That thought crossed my mind also. Loading the engine with a generator is not the best way to go about this and I keep trying to remember that other thread.

Ron

 

[breakshift]

Okay, as I said, I'm not a mechanical guy... let me explain exactly what I'm trying to do.

I need to design and construct an engine test bed. So the idea is to just couple any engine (providing not too powerful) to it, and from that be able to characterise it in terms of RPM vs power.

The engine-under-test will be used in an aircraft to drive a propeller - and so when in flight, the RPM can be measured and the power being delivered to the propeller is known. This can then be used to keep the aircraft in trimmed flight.

Does that make sense?

One thing I'm not sure about is how torque comes into this. Obviously power = torque x RPM. Assuming the angle of the propeller blades don't change, will the load torque remain constant during flight? What if the aircraft attempts to climb altitude?

 

[Diver300]

The propellor torque will depend on the rotational speed, the airspeed, and the air density.

The propellor torque will always increase as the rotational speed goes up. The engine torque will only rise a little as engine speed increases from low speeds to maximum torque speed, and will then fall quite fast as engine speed increases.

Just for fun, the engine power may depend on the airspeed, as the air intake pressure may change. The engine power will reduce with air density at altitude.

Obviously, if the engine is throttled back, the torque will fall. If the engine is left on full power, the torque will change a lot when the aircraft climbs. As the aircraft climbs, it slows. With lower air speed, the propellor torque will increase for the same RPM. The engine will slow down, reducing the propellor torque and the engine torque will increase, to the point where the propellor torque is the same as the engine torque.

The faster the climb, the more the engine will slow.

I don't see how knowing the engine power will help keep the aircraft trimmed.

 

[breakshift]

This is how we have been told to do it - I have my reservations with the project too...

I'm finding it very hard to find any literature detailing electrical machine constants, k. Could someone point me in the right direction?

Just looking for any information on it really.

 

[Reloadron]

I knew this sounded familiar and here is your rthread from back in November that you left hanging. How much more do you want to beat this to death? A generator is not a practical load. Whoever is suggesting this has a few things to learn or all of the information is not being presented in both threads you started. You made no mention of aircraft engine in the last thread so what are you fishing for? Would this be a school project? What electrical machine constants are you looking for?

You should have kept this going in your last thread on the same subject.

Ron

 

[breakshift]

Yes both threads I've started are related to the same project - but I am asking distinctly different questions in both threads, which is why I didn't continue the previous thread.

It's a Uni project (Masters year, to be precise), and the supervisor is an electrical/electronic person, and openly admits he has no knowledge of mechanics. So I think half the reason why he wants it done this way is because he knows no other way - and the other half probably because he wanted to include electronic engineers somehow (most of the 40+ people are mechanics).

I'm sorry if my threads have annoyed you, Ron, but the nature of this project is confusing even to me and my colleagues, so I'm trying to explain it as best I can.

Regarding the machine constants:

To be able to use the generator to test engines, the generator itself must be characterised. By this I mean relate the incoming RPM and torque to outgoing current and voltage. The machine constant 'K' is a quantity that describes the construction of the machine and relates all these things:

torque = K x current
RPM = K x voltage

Essentially the K value will allow me to measure what the generator is producing (V and I), and work it back into an accurate measurement of the output of the engine (torque and RPM). Does that make sense?

 

[Reloadron]

It isn't that it annoys me. I just don't see this as viable. The last thread never resolved things and I don't see using a generator useful for this type application. I find it surprising that an electrical/electronic type would come up with this.

Another problem you will face is that a generator is not a 100% efficient machine. On paper one horsepower is 746 watts But that fails to take into account the inefficiency of the generator. For example if I want a 5 KW generator how much horsepower is required? I could say 5 * .746 = 3.73 HP. That was easy. That assumes a 100% efficient generator. In reality small generators like that linked to are likely not even 90% efficient but if we assume it is now we get 5 / .90 * .746 = 7.447 HP.

Unless every factor about the generator itself is known all we get is pretty much useless data. That being just one small problem with this method. In theory if I apply a precise 7,460 watt load to a generator the shaft HP should be 10 HP and we know it won't be close to that. I typically see 8 HP or greater on 5 KW units. Matter of fact I have an 8 HP engine (@ 3600 RPM) in my shed rated at a constant 4 KW output. Everything varies generator to generator based on the efficiency of the unit. There is no magic.

All of that aside, you could place a good rotary torque sensor on the unit, plus a good speed pickup on the unit and load the unit till it is delivering 5 KW monitoring voltage across the load and current to the load and then calculate how much HP is actually being used. I suggest maintaining 3600 (60 Hz.) or 3000 RPM (50 Hz.). You could find a manufacturer of a generator and ask for every bit of available data on that generator.

Personally I just don't see using a generator as a viable load for an engine test cell (bed). That is pretty much what most here are trying to convey.

Just My Take
Ron

 

[RCinFLA]

Petrol (gasoline to us Yanks) engines peak out in their HP at higher rpm's, in the range of 3400-3800. If you run a gasoline engine with a 4 pole alternator, at 1500 rpm for 50Hz, you require a much larger engine for the electrical power output.

For an AC electric generator you need a bigger engine then 1 hp per 746 watts. This is to allow for surge loading on the generator without too much drop in rpm, which keeps the electrical output near correct frequency. Roughly the engine HP rating for a generator is 2 HP per 1 kW. A 5kW generator requires a 10 hp engine (or greater).

When I say surge, I don't mean power above generator rating. This is just the ability to correct for moderate load changes within the generator rating range.

 

[breakshift]

Unless every factor about the generator itself is known all we get is pretty much useless data. That being just one small problem with this method. In theory if I apply a precise 7,460 watt load to a generator the shaft HP should be 10 HP and we know it won't be close to that. I typically see 8 HP or greater on 5 KW units. Matter of fact I have an 8 HP engine (@ 3600 RPM) in my shed rated at a constant 4 KW output. Everything varies generator to generator based on the efficiency of the unit. There is no magic.

Well the machine constant K is what describes the generator - so knowing that will allow us to relate the generator output to the mechanical input (or engine output). Like you say, of course the generator is not 100% efficient, but that can easily be measured experimentally and then taken into account later on.

you could place a good rotary torque sensor on the unit, plus a good speed pickup on the unit and load the unit till it is delivering 5 KW monitoring voltage across the load and current to the load and then calculate how much HP is actually being used. I suggest maintaining 3600 (60 Hz.) or 3000 RPM (50 Hz.). You could find a manufacturer of a generator and ask for every bit of available data on that generator.

Due to the compact nature of the gen-set it is not possible to mount a mechanical torque sensor between the engine and the generator, and our budget isn't good enough to go out and buy one anyway. We have a RPM sensor tested and ready to go. So we can at this point measure output current and voltage, and the engine RPM. Torque is a problem, so we are thinking of dismounting the generator and running it as a motor. This way we can feed it with a known current and voltage, and simply measure the torque using whichever method is easiest. Then we have everything we need to find 'K', the machine constant. From there we can move on to generator efficiency etc.

Now we're not sure if the K value will vary with RPM, so we really need to characterise it over a range of RPM. As you can tell I'm not mechanically inclined, but I know that the engine that came with the gen-set doesn't like running at anything other than its rated RPM. But when we come to coupling our aircraft engine to it, this may be different - I'm not sure.

Is this 'K' value issue making any sense to you?

 

[Diver300]

Is this 'K' value issue making any sense to you?

It makes perfect sense. However, it is only constant where the magnetic field is fixed. Any alternator that is intended to be used to generate mains will have some sort of automatic voltage regulator that will vary the magnetic field over a wide range.

 

[Reloadron]

To add to Diver's post and answer your question as best I can:

Now we're not sure if the K value will vary with RPM, so we really need to characterise it over a range of RPM. As you can tell I'm not mechanically inclined, but I know that the engine that came with the gen-set doesn't like running at anything other than its rated RPM. But when we come to coupling our aircraft engine to it, this may be different - I'm not sure.

I can just about guarantee you it will not be a linear curve or straight line equation. It will also vary from unit to unit.

Ron

 

[crutschow]

Regarding a torque sensor, would it be possible to mount the generator so that its frame could freely rotate in operation? Then just mount an arm to the frame with a spring scale to measure the force on the arm. Simple multiplication of the arm length from the shaft to the scale times the measured force gives you torque.

 

[bryan1]

Regarding a torque sensor, would it be possible to mount the generator so that its frame could freely rotate in operation? Then just mount an arm to the frame with a spring scale to measure the force on the arm. Simple multiplication of the arm length from the shaft to the scale times the measured force gives you torque.

Yep on reading thread a prony brake came to mind for measuring the torque and crutschow described it purfectly.........